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Mussel culture in British Columbia : the influence of salmon farms on mussel growth and biochemical… Taylor, Barbara Elan 1990

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MUSSEL CULTURE IN BRITISH COLUMBIA: THE INFLUENCE OF SALMON FARMS ON MUSSEL GROWTH AND BIOCHEMICAL COMPOSITION by BARBARA ELAN TAYLOR B.Sc, University of B r i t i s h Columbia THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (ZOOLOGY) We accept t h i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA J u l y 1990 (c) Barbara E l a n T a y l o r , 1990 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University or Vancouver, Canada Department of DE-6 (2/88) i i ABSTRACT To r e a l i s e the poten t i a l for mussel culture i n B r i t i s h Columbia, mariculture research must i d e n t i f y s p e c i f i c environments and suitable locations which promote maximum growth i n mussels. The present study investigates the possible advantages, through n u t r i t i o n a l enrichment, of salmon farms as si t e s for mussel culture. Mussels were cultured at d i f f e r e n t distances around two salmon farms on the east coast of Vancouver Island (Departure Bay and Genoa Bay). Three parameters of mussel growth: condition index, carbohydrate content, and crude protein content were monitored at 3-6 wk in t e r v a l s from September 1988 to August 1989. Di s t i n c t seasonal differences i n growth were observed, but distance from the farm did not substantially influence mussel growth. Adult mortality and l a r v a l settlement were s i m i l a r l y unaffected. Contrary to prediction, the farms did not increase available food for mussels. Measures of seston and chlorophyll concentration, made concurrently with the mussel c o l l e c t i o n s , indicated that neither a di r e c t contribution of nutrients i n the form of feed and f i s h faeces, nor an i n d i r e c t contribution of waste ammonia to augment phytoplankton production, occurred. This was despite currents flowing, at least part of the time, i n such a d i r e c t i o n as to transport p o t e n t i a l nutrients from the farms to the mussels. i i i TABLE OF CONTENTS Abstract i i Table of Contents i i i L i s t of Tables i v L i s t of Figures v Acknowledgements v i Introduction 1 Materials and Methods 10 Sites and Culture Technique 10 Mussel Sampling 18 Data C o l l e c t i o n 19 1. Shell Length 19 2. Condition Index 20 3. Carbohydrate 22 4. Crude Protein 22 5. Spat Density 23 6. Chlorophyll and Seston 24 7. Currents 26 8. S t a t i s t i c a l Analyses 26 Results and Discussion 28 1. Condition Index 28 2. Carbohydrate 42 3. Crude Protein 48 4. Spat Density 52 5. Chlorophyll 52 6. Seston 55 7. Currents 61 General Discussion 66 References Cited 87 i v LIST OF TABLES 1. Energy reserves and energy budgets of mussels cultured i n Departure Bay and Genoa Bay 85 V LIST OF FIGURES 1. Location of study s i t e s at Departure Bay and Genoa Bay and location of source mussels i n Indian Arm and Okeover Inlet 12 2. Site-diagrams of Departure Bay and Genoa Bay showing the array of stations at the salmon farms 14 3. Long-line system for culture of mussels suspended within protective cages 17 4. Monthly fluctuations i n condition indices of mussels cultured i n Departure Bay and Genoa Bay 30 5. Monthly fluctuations i n dry s h e l l weights of mussels cultured i n Departure Bay and Genoa Bay 35 6. Monthly fluctuations i n dry tissue weights of mussels cultured i n Departure Bay and Genoa Bay 37 7. Monthly fluctuations i n carbohydrate contents of mussels cultured i n Departure Bay and Genoa Bay 44 8. Monthly fluctuations i n crude protein contents of mussels cultured i n Departure Bay and Genoa Bay 50 9. Monthly fluctuations i n chlorophyll concentrations at Departure Bay and Genoa Bay 54 10. Monthly fluctuations i n seston concentrations at Departure Bay and Genoa Bay 57 11. Fluctuation i n t i d a l height and seston concentration i n Departure Bay 60 12. Current-flow d i r e c t i o n i n Departure Bay and Genoa Bay 63,64 v i ACKNOWLEDGEMENTS My f i r s t words of thanks are to my supervisor Dr. Thomas Carefoot for his advice, help, and moral support during the course of the study. I am also indebted to Dr. Glen Jamieson, S h e l l f i s h Section Head, and his s t a f f at the P a c i f i c B i o l o g i c a l Station e s p e c i a l l y Wayne Harling, Steve Head, Dwight Heritage, Anton P h i l l i p s , and Dr. Ian Whyte, for the l o g i s t i c a l support they provided as well as the invaluable experience they lent to the project. I would also l i k e to thank my other committee members, Dr. Bruce Owen and Dr. Timothy Parsons for t h e i r input to t h i s project. Greg Mullen deserves special c r e d i t for his help i n producing the figures. F i n a l l y I would l i k e to credit some special people who lent moral support i n darker moments, who brightened monotonous times, and who were consistently there to lend a hand: Michele Cook, Norma Ginther, Sabine Kalwa, S t i r l i n g Taylor, and Gavin St. Michael. This work was supported through operating grants from Science Council of B r i t i s h Columbia (G.R.E.A.T. Award) and NSERC to Dr. T.H. Carefoot, and by operating funds of the S h e l l f i s h Section of the P a c i f i c B i o l o g i c a l Station, Department of Fisheries and Oceans at Nanaimo. 1 INTRODUCTION C u l t u r e o f mussels f o r food has extended over the past seven c e n t u r i e s (Bardach et al., 1912), and the e d i b l e mussel Mytilus edulis has dominated the world harvest o f these b i v a l v e s . T h i s cosmopolitan s p e c i e s i s endemic t o B r i t i s h Columbia and i s o f t e n abundant i n i n t e r t i d a l and shallow s u b t i d a l zones. A p o t e n t i a l thus e x i s t s f o r mussel c u l t u r e i n B r i t i s h Columbia but, t o r e a l i s e t h i s p o t e n t i a l , m a r i c u l t u r e r e s e a r c h must focus on i d e n t i f y i n g s p e c i f i c environments which promote maximum growth of mussels. Salmon farms are a p o t e n t i a l l o c a t i o n f o r mussel c u l t u r e which warrant i n v e s t i g a t i o n . Wallace (1980) r e p o r t e d t h a t mussels on f i s h - f a r m f l o a t s grew at twice the r a t e o f those from s i t e s without f i s h farms. The present study i n v e s t i g a t e s the p o s s i b l e advantages, through n u t r i t i o n a l enrichment, of c u l t u r i n g mussels i n c l o s e p r o x i m i t y t o salmon farms. N u t r i t i o n a l enhancement f o r mussels growing c l o s e t o salmon farms c o u l d be manifested as an i n c r e a s e i n energy storage products, growth r a t e , and/or c o n d i t i o n index. The l a t t e r i s an assessment of t i s s u e a c c r e t i o n r e l a t i v e t o mussel s i z e and w i l l be e x p l a i n e d i n d e t a i l l a t e r . The f o l l o w i n g i s a b r i e f d e s c r i p t i o n o f energy metabolism i n mussels with an e x p l a n a t i o n o f how n u t r i t i o n a l s t a t u s might be enhanced by food m a t e r i a l s emanating d i r e c t l y or i n d i r e c t l y from a salmon farm. Glycogen and p r o t e i n are c o n s i d e r e d the primary energy storage products of mussels (Gabbott, 1983). In Mytilus edulis the d i g e s t i v e g l a n d i s the s i t e - o f glycogen s y n t h e s i s as w e l l as some st o r a g e . The mantle, however, i s the main s i t e o f glycogen 2 storage, s p e c i f i c a l l y i t s v e s i c u l a r c e l l s which can s t o r e l a r g e amounts of t h i s carbohydrate (Gabbott, 1983) . P r o t e i n i s a l s o s t o r e d as granules i n the a d i p o g r a n u l a r c e l l s o f the mantle's conn e c t i v e t i s s u e matrix (Gabbott, 1983) and i n the d i g e s t i v e g l a n d (Thompson et al., 1974). A l l body p r o t e i n s can be regarded as secondary energy r e s e r v e s , but i t i s u n l i k e l y t h a t a l l have equal c a p a c i t y t o be m o b i l i s e d as an energy source. For example, muscle p r o t e i n s are l i k e l y m o b i l i s e d b e f o r e g i l l p r o t e i n s because of the importance of the g i l l as a f e e d i n g s t r u c t u r e . Slow p r o t e i n t u r n o v e r i n e s s e n t i a l t i s s u e s i s known t o e x i s t i n M. edulis, and p r o t e i n degradation products are r e c y c l e d f o r maintenance of e s s e n t i a l p r o t e i n components (Hawkins, 1985). In Mytilus edulis the accumulation of energy storage products and t h e i r subsequent expenditure f o l l o w s a seasonal c y c l e . S t u d i e s of seasonal changes i n b i o c h e m i c a l c o m p o s i t i t o n of mussels are numerous (de Zwaan and Zandee, 1972; Gabbott and Bayne, 1973; Dare and Edwards, 1975; Widdows, 1978; P i e t e r s et al., 1979; Zurburg et al., 1979; P i e t e r s et al., 1980; Zandee et al., 1980; Hawkins and Bayne, 1985; Emmett et al., 1987) w i t h most work being done i n B r i t a i n and Europe. The c h a r a c t e r i s t i c s of mussel e n e r g e t i c s i d e n t i f i e d i n these s t u d i e s have been g e n e r a l l y a p p l i e d t o M. edulis i n B r i t i s h Columbia, w i t h only d i f f e r e n c e s i n the t i m i n g of the c y c l e b e i ng noted f o r l o c a l p o p u l a t i o n s (Emmett, 1984; Emmett et al, 1987). The seasonal c y c l e of accumulation and expenditure of energy storage products i s c l o s e l y l i n k e d t o the annual gametogenic 3 cycle (Bayne, 1973; Gabbott and Bayne, 1973; Thompson et al., 1974). In B r i t i s h and European populations the demand for metabolic energy decreases i n summer as mussels switch from the energy-demanding gametogenesis of winter/spring to a period of reproductive i n a c t i v i t y (Widdows and Bayne, 1971; Bayne, 1973) . Widdows (1973) and Thompson and Bayne (1974) have confirmed t h i s decrease i n metabolism i n mussels during summer by showing evidence of decreased oxygen uptake by mussels i n t h i s season as compared with winter. Also, at t h i s time, food i s readily available i n the form of abundant phytoplankton. Therefore, energy stores such as glycogen and proteins are accumulated during summer, and there i s a concomitant r i s e i n condition index. At t h i s time, glycogen reserves are accumulated i n both mantle and non-mantle tissues, with highest lev e l s being present i n the digestive gland and mantle (de Zwaan and Zandee, 1972; Gabbott and Bayne, 1973; Emmett, 1984). Proteins also accumulate during summer through processes of tissue growth and granule storage (Bayne, 1973; Dare and Edwards, 1975; Pieters et al., 1979; Emmett, 1984), with the majority of accumulation being through growth of non-mantle tissues (Gabbott, 1983). An increase i n condition index at t h i s time r e f l e c t s increase i n body mass r e l a t i v e to s h e l l s i z e . Following summer's r e l a t i v e l y low metabolic a c t i v i t y , metabolic rate gradually increases over autumn and winter due to high energy demands of gametogenesis (Widdows and Bayne, 1971; Bayne, 1973). The majority of stored glycogen i s used up i n 4 e a r l y w i n t e r . As t h i s d e p l e t i o n i s synchronous with oogenesis and v i t e l l o g e n e s i s i t i s g e n e r a l l y b e l i e v e d t h a t d u r i n g w i n t e r glycogen energy r e s e r v e s are p r e f e r e n t i a l l y a l l o c a t e d t o garnetogenesis (Gabbott, 1975; Bayne et al., 1982). S i m i l a r l y , p r o t e i n s t o r e d as granules i n the mantle i s used t o support gametogenesis at t h i s time (Hawkins, 1985). Since food l e v e l s are low d u r i n g winter, the energy r e q u i r e d f o r h i g h m e t a b o l i c r a t e i s met u s i n g energy r e s e r v e s of p r o t e i n s i n the non-mantle t i s s u e s (Gabbott and Bayne, 1973; Dare and Edwards, 1975). The muscles, adductors and f o o t , show a g r e a t e r decrease i n percentage p r o t e i n d u r i n g w i n t e r months than do other organs (Zurburg et al., 1978; Zandee et al., 1980). The most c o n c l u s i v e evidence f o r a switch from carbohydrate metabolism i n summer t o dependence on p r o t e i n metabolism i n winter comes from Bayne (1973). T h i s author r e p o r t s seasonal d i f f e r e n c e s i n n i t r o g e n e x c r e t i o n whereby wi n t e r mussels under temperature and n u t r i t i v e s t r e s s e x c r e t e l a r g e amounts of ammonia, i n d i c a t i n g e x t e n s i v e p r o t e i n deamination. Mussel p o p u l a t i o n s i n B r i t i s h Columbia appear a l s o t o e x h i b i t a w i n t e r dependence on p r o t e i n s f o r energy metabolism. Lowered l e v e l s of t o t a l body p r o t e i n d u r i n g w i n t e r months (December t o February) have been r e p o r t e d by Emmett (1984) f o r mussels growing i n Departure Bay and B a m f i e l d I n l e t . Thus, i n winter, somatic growth i n Mytilus edulis i s minimal and somatic t i s s u e s may even be m o b i l i s e d t o supply energy f o r maintenance and gametogenesis. T h i s d e c l i n e i s evident i n lower 5 c o n d i t i o n i n d i c e s r e p o r t e d f o r t h i s season (Gabbott and Bayne, 1973; Emmett et al., 1987), and r e s u l t s i n a lower y i e l d o f mussel meat from commercial c u l t u r e o p e r a t i o n s d u r i n g w i n t e r . In some B.C. mussel p o p u l a t i o n s the l e v e l o f energy r e s e r v e s at the time of s p r i n g spawning i s low and t h i s may i n c r e a s e s u s c e p t i b i l i t y o f mussels t o post-spawning m o r t a l i t y . A h i g h summer m o r t a l i t y has been r e p o r t e d f o r B.C. mussels by Quayle (1978), H e r i t a g e (1983), Emmett et al. (1987), and Jamieson (1989) . I n s u f f i c i e n t food i n the post-spawning p e r i o d may t h e r e f o r e be a prime o b s t a c l e f o r mussel c u l t u r e i n B r i t i s h Columbia. Indeed, winter food q u a n t i t y and q u a l i t y have been i d e n t i f i e d as l i m i t i n g f a c t o r s f o r growth of mussels c u l t u r e d on both s i d e s of the A t l a n t i c Ocean (Incze and Lutz, 1980; Wallace, 1980; Rosenberg and Loo, 1983). Mussel-salmon p o l y c u l t u r e warrants i n v e s t i g a t i o n because salmon farms re p r e s e n t a p o t e n t i a l f o r constant food supply t o mussels, even i n the c r i t i c a l post-spawning months. Salmon farms p r o v i d e two p o t e n t i a l enrichment i n p u t s t o the environment. F i r s t , s o l u b l e waste such as ammonium and urea are e x c r e t e d as the by-products of metabolism and these may serve as a n i t r o g e n source f o r blooms o f phytoplankton i n the v i c i n t y o f the salmon farm. T h i s r e p r e s e n t s an i n d i r e c t n u t r i t i o n a l c o n t r i b u t i o n . Second, p a r t i c u l a t e o r g a n i c waste made up of unconsumed f i s h meal and faeces, and c o n t a i n i n g p r o t e i n s , carbohydrates, l i p i d s , and c a r o t e n o i d pigments, r e p r e s e n t s a d i r e c t n u t r i t i o n a l c o n t r i b u t i o n f o r mussels. 6 E x c r e t e d ammonium and urea can be used d i r e c t l y by phytoplankton and are, i n f a c t , t h e i r p r e f e r r e d forms of n i t r o g e n (M cCarthy et al., 1977). A l t e r n a t i v e l y , these compounds may be o x i d i s e d by blue-green algae i n t o n i t r i t e s and n i t r a t e s b e f o r e b e i n g taken up by phytoplankton ( t h i s n i t r i f i c a t i o n i s probably minimal because Cyanophyceae make up a very l i m i t e d p a r t o f the marine phytoplankton; Lee, 1980). I t i s , t h e r e f o r e , reasonably c e r t a i n t h a t a salmon farm would generate an i n f l u x o f n i t r o g e n i n s e v e r a l d i f f e r e n t forms t o the surrounding seawater. Since n i t r o g e n i s the primary l i m i t i n g n u t r i e n t f o r a l g a l p r o d u c t i o n i n the marine environment (Ryther and Dunstan, 1971; Eppley et al., 1979; C o d i s p o t i , 1989), i t f o l l o w s t h a t salmon farming should augment phytoplankton p r o d u c t i o n and thus i n c r e a s e the p r i n c i p a l source of food f o r mussels. P a r t i c u l a t e o r g a n i c m a t e r i a l s . f r o m f i s h meal and faeces p r o v i d e b a s i c p r o t e i n , carbohydrate,, and l i p i d n u t r i e n t s . Carbohydrates and p r o t e i n s from p a r t i c u l a t e waste are r e a d i l y consumed and d i g e s t e d . Mussels can even u t i l i s e c e l l u l o s e (Crosby and Reid, 1971), a m a t e r i a l o f t e n added t o f i s h f eed as a p a l a t a b i l i t y enhancer, but one which i s not d i g e s t e d by salmon. Some l i p i d s , and s o l u b l e carbohydrates and p r o t e i n , are l i k e l y l o s t as the food p e l l e t s and faeces break down i n seawater. Thus, these n u t r i e n t s have a reduced a v a i l a b i l i t y t o mussels, but those o b t a i n e d are r e a d i l y u t i l i s e d . A l l b i v a l v e s are capable of c o n v e r t i n g o i l s o f the type commonly used i n f i s h feeds (e.g., those c o n t a i n i n g a h i g h percentage of e s s e n t i a l w3 f a t t y acids) t o glycogen ( C a s t e l l and T r i d e r , 1974), and mussels possess a 7 t y p i c a l complement of enzymes f o r d i g e s t i n g carbohydrates and p r o t e i n s . A n t i b i o t i c s and c a r o t e n o i d pigments are l e s s e r c o n s t i t u e n t s of salmon meals, but may p r o v i d e an important n u t r i t i o n a l c o n t r i b u t i o n t o mussels. C a r o t e n o i d pigments are added t o enhance f l e s h c o l o u r of salmon. Recent s t u d i e s by Hertzberg et al. (1988) and P a r t a l i et al. (1989) have i s o l a t e d 19-20 c a r o t e n o i d compounds from the t i s s u e s of M y t i l u s e d u l i s , i n c l u d i n g some v a r i e t i e s used i n salmon d i e t s , and have i n d i c a t e d a c a p a c i t y f o r i n t e r c o n v e r s i o n of c a r o t e n o i d s by mussels. A n t i b i o t i c s are added t o improve the h e a l t h o f salmon by c u r t a i l i n g the growth o f pathogens, but l i t t l e i s known of t h e i r e f f e c t on mussels. There are, however, r e p o r t s of a n t i b i o t i c s c a u s i n g r a p i d s h e l l growth i n mussels (Dey and Bolton, 1978), but the e f f e c t s o f a n t i b i o t i c s on l a r v a l growth and s u r v i v a l appear t o be h i g h l y v a r i a b l e and o f t e n c o n t r a d i c t o r y ( l e Pennec and P r i e u r , 1977). The amount and composition of food wastage i n salmon c u l t u r e v a r i e s depending upon feed composition and feed management p r a c t i c e s employed by farm o p e r a t o r s . Braaten et al. (1983) estimate t h a t 20% o f food goes uneaten by cage-reared salmon. Furthermore, d i g e s t i b i l i t y f i g u r e s from Gowen and Bradbury (1987) suggest t h a t 26% of food eaten by salmon ends up as f a e c e s . Thus, t h e r e i s s u b s t a n t i a l p a r t i c u l a t e waste from salmon farms (equ i v a l e n t t o about 40% of a l l feed g i v e n ) , the bulk of which i s o r g a n i c compounds of n i t r o g e n and carbon. These c o u l d be 8 d i r e c t l y u t i l i s e d by many organisms, i n c l u d i n g mussels l i v i n g i n the v i c i n i t y o f the farm. However, the e f f e c t s of t h i s form o f enrichment are l i m i t e d by the range o f d i s p e r s a l . Most food-wastage p a r t i c l e s , b e i n g more dense than seawater, s i n k i n the immediate v i c i n i t y o f the farm. Gowen and Bradbury (1987), re v i e w i n g the e c o l o g i c a l impact of salmonid farming, r e p o r t e d t h a t the maximum range o f d i s p e r s a l f o r feed d e b r i s i s about 120 m, but t h a t the m a j o r i t y o f p a r t i c l e s s e t t l e immediately around the salmon farm. In comparison, enrichment through m e t a b o l i c by-products should be more widely d i s p e r s e d s i n c e ammonium, urea, and of t h e i r o x i d a t i o n products, n i t r i t e and n i t r a t e are s o l u b l e i n seawater. Gowen and Bradbury (1987) estimate t h a t 68-86% of the n i t r o g e n consumed by salmon i s e x c r e t e d . Thus, a c o n s i d e r a b l e i n f l u x o f n i t r o g e n e x i s t s t o enhance phytoplankton q u a l i t y or abundance i n the v i c i n i t y o f salmon farms. I f mussels are c u l t u r e d c l o s e t o salmon farms they may g a i n a continuous food supply, r e s u l t i n g i n a h i g h e r "scope f o r growth" (energy a v a i l a b l e f o r growth and reproduction) throughout the year, and p o s s i b l y decreased time t o reach market s i z e (50-75 mm). T h i s c o u l d enhance the p r o d u c t i v i t y of mussel c u l t u r e o p e r a t i o n s and would reduce p r o d u c t i o n c o s t s with obvious b e n e f i t t o commercial growers. T h i s study i n v e s t i g a t e d the p o t e n t i a l o f mussel-salmon p o l y c u l t u r e i n B r i t i s h Columbia from the p e r s p e c t i v e o f enhanced mussel growth. Two salmon-farm s i t e s were s t u d i e d , and analyses o f c o n d i t i o n index, glycogen content, and crude p r o t e i n content i n mussels suspended at d i f f e r e n t 9 d i s t a n c e s from the farms, as w e l l as measures of c h l o r o p h y l l and seston c o n c e n t r a t i o n at these same d i s t a n c e s , were used t o assess p o s s i b l e enhancement. 10 METHODS AND MATERIALS Sites and Culture Technique Mussels (Mytilus edulis) were c u l t u r e d at two salmon farms on the east coast of Vancouver I s l a n d , one i n Departure Bay near the P a c i f i c B i o l o g i c a l S t a t i o n , and the other i n Genoa Bay ( F i g . 1). An i d e n t i c a l i n v e s t i g a t i o n o f the p o t e n t i a l o f mussel-salmon p o l y c u l t u r e was c a r r i e d out at each s i t e , although no d i r e c t comparison was intended because o f d i f f e r e n c e s i n environment and o p e r a t i o n at the salmon farms. However, i t was hoped t h a t use of two s i t e s would i n d i c a t e between-site v a r i a t i o n i n mussel c u l t u r e and p r o v i d e more i n f o r m a t i o n on the p r a c t i c a l i t y o f p o l y c u l t u r e w i t h salmon. At the Departure Bay farm the water was 20-25 m deep and the c u r r e n t t r a v e l l e d at a mean speed of 2.7 em's--'- ( i n the p e r i o d monitored) . Temperatures ranged from 2.1 °C i n February t o 18.5 °C i n August; s a l i n i t i e s ranged from 19.0 % i n February t o 32.0 % i n May. In Genoa Bay, water depth was 10-15 m i n the v i c i n i t y o f the farm and c u r r e n t s averaged 1.5 c i r r s - 1 ( i n the p e r i o d monitored). Temperature and s a l i n i t y data were not taken at t h i s s i t e . At both salmon-farm s i t e s mussels were deployed at fo u r s t a t i o n s : 3 m, 15 m, and 75 m from the perimeter of the farm, and at a c o n t r o l s t a t i o n ( F i g . 2 ) . The c o n t r o l s t a t i o n was l o c a t e d such t h a t , as much as p o s s i b l e , the mussels were i s o l a t e d from the farm but not s u b j e c t e d t o markedly d i f f e r e n t environmental c o n d i t i o n s . The s t a t i o n s were s i t e d along a west-east a x i s i n Departure Bay and a 11 F i g u r e 1. L o c a t i o n o f study s i t e s at Departure Bay and Genoa Bay on the east coast of Vancouver I s l a n d , and l o c a t i o n of source mussels i n Indian Arm. and Okeover I n l e t . 13 F i g u r e 2. Site-diagrams of a) Departure Bay and b) Genoa Bay showing the a r r a y o f s t a t i o n s 3 m, 15 m, and 75 m from the farm, and a c o n t r o l s t a t i o n p h y s i c a l l y separated from the farm. 14 15 south-north a x i s i n Genoa Bay. T h i s d i f f e r e n c e was o b l i g a t o r y due t o the t o p o g r a p h i c a l f e a t u r e s of the two s i t e s . Four groups of mussels were hung i n separate cages at each s t a t i o n at each s i t e . The cages were c l o s e d c y l i n d e r s (0.8 X 0.4 m diameter) of vexar mesh (2 cm). Suspended c u l t u r e i s known to e l i m i n a t e b e n t h i c p r e d a t o r s (e.g., crabs and s t a r f i s h ) and enhance growth s i n c e the mussels are a f f o r d e d continuous access t o food. In B r i t i s h Columbia use of p r o t e c t i v e e n c l o s u r e s i s a l s o necessary t o exclude p e l a g i c p r e d a t o r s such as p i l e perch (when the mussels are s m a l l ; e.g., < 14 mm), and s c o t e r ducks (when the mussels are l a r g e r ) . Cage treatment was r e p l i c a t e d f o u r times at each s t a t i o n t o o b t a i n a measure of i n t r a - s t a t i o n v a r i a b i l i t y and t o p r o v i d e back-up i n the event t h a t a cage might be l o s t t o storms, boats, or i n other ways. Each cage was suspended from the f l o a t s o f an anchored l o n g - l i n e set out p a r a l l e l t o the perimeter of the farm at the a p p r o p r i a t e d i s t a n c e , and with i t s c e n t r e approximately 2 m beneath the s u r f a c e ( F i g . 3 ). By c u l t u r i n g at 2 m, f o u l i n g on the cages and mussels by algae, b a r n a c l e s , bryozoans, mussels, sponges, t u n i c a t e s and other organisms was reduced. The i n i t i a l i n t e n t was t o use mussel spat c o l l e c t e d from the bay around each salmon farm. To t h i s end, mussels were o b t a i n e d at the e a r l y j u v e n i l e stage i n June 1988 by suspending t r a d i t i o n a l " c o l l e c t o r s " from the f l o a t s o f the salmon farms. The c o l l e c t o r s were 3-m lengths of well-weathered, 2-cm diameter p o l y p r o p y l e n e rope which had been c o i l e d and weighted such t h a t 16 F i g u r e 3 . L o n g - l i n e system f o r c u l t u r e of mussels suspended w i t h i n p r o t e c t i v e cages. T h i s system was used at a l l s t a t i o n s at both s i t e s . -17 18 they hung i n the top 0.5 m of the water where settlement i s h e a v i e s t ( C h i p p e r f i e l d , 1953). However, these mussel spat were consumed by p r e d a t o r s at both s i t e s b e f o r e the need t o enclose them i n p r o t e c t i v e cages was r e c o g n i z e d . The l o s t animals were r e p l a c e d w i t h mussels from two s i t e s on the mainland coast (see F i g . 1). The mussels u l t i m a t e l y c u l t u r e d i n Genoa Bay came from Indian Arm, and these were c o l l e c t e d by the same method d e s c r i b e d above. In September 1988, 16 of these c o l l e c t o r s with a t t a c h e d mussels were e n c l o s e d i n the cages and deployed at the Genoa Bay s i t e . The Departure Bay mussels were r e p l a c e d i n October 1988 with animals purchased from a commercial grower i n Okeover I n l e t . These mussels had s e t t l e d d u r i n g summer 1988, and had been s t r i p p e d from the l o n g - l i n e s o f an o y s t e r farm and p l a c e d i n 3-m leng t h s o f net t u b i n g known as "socks". S i x t e e n o f these socks were c o i l e d i n s i d e cages and deployed at the s t a t i o n s i n Departure Bay. Thus, the mussel stocks at the two s i t e s d i f f e r e d both i n source and i n attachment s u b s t r a t e w i t h i n the cages. Mussel Sampling Sampling of experimental animals began when the mussels were deployed at each s i t e (September 1988 i n Genoa Bay and October 1988 i n Departure Bay), and continued at 3-6 wk i n t e r v a l s u n t i l August 1989. The more frequent sampling o c c u r r e d i n the summer of 1989 when mussel growth was a n t i c i p a t e d t o be most a c t i v e . Samples were always taken d u r i n g daytime low t i d e . At each sampling time, approximately 15 cm of rope or sock were removed from the stock w i t h i n each cage ( f o r some samples l a t e r i n the 19 study i t became necessary t o use mussels which had migrated t o the s i d e s or bottoms of the cages, but these appeared t o be healthy, f i r m l y attached, and of a s i z e comparable t o ones sampled from other cages at the same s t a t i o n ) . A l l mussels were handled as g e n t l y and as q u i c k l y as p o s s i b l e d u r i n g sampling t o minimise s t r e s s . At sampling times the mussel cages remained out of water f o r 2-10 min depending upon whether or not f o u l i n g growth had t o be removed from the cages. F o l l o w i n g sampling the mussels were taken t o the l a b o r a t o r y and kept f r o z e n at -40 °C u n t i l a nalysed. Data C o l l e c t i o n Four growth parameters were monitored i n the c u l t u r e d mussels: 1) s h e l l l e n g t h , 2) c o n d i t i o n index, 3) carbohydrate content, and 4) crude p r o t e i n content. 1. S h e l l Length S h e l l l e n g t h (maximum a n t e r i o - p o s t e r i o r dimension) was l i k e l y a l e s s u s e f u l measure of t o t a l body s i z e than other measures because of v a r i a b i l i t y , i n the a l l o m e t r i c r e l a t i o n s h i p s between l e n g t h and other s h e l l dimensions. However, i t p r o v i d e d a quick overview o f growth among mussels i n any g i v e n sample as w e l l as a convenient means t o get s t a n d a r d i s e d subsamples from the 15-cm samples taken from each cage. The s h e l l l e ngths of a l l mussels o b t a i n e d from each cage were measured u s i n g e l e c t r o n i c c a l i p e r s . A length-frequency histogram generated from these data i d e n t i f i e d the modal l e n g t h - c l a s s of the sample; mussels of t h i s l e n g t h - c l a s s were then used f o r f u r t h e r a n a l y s e s . Concurrent w i t h measuring s h e l l l e ngths, a s e m i q u a n t i t a t i v e measure of m o r t a l i t y was taken f o r each sample. Moribund and dead mussels were counted, and t h e i r s h e l l l e ngths measured t o determine both seasonal p a t t e r n o f m o r t a l i t y and whether or not m o r t a l i t y was evenly d i s t r i b u t e d throughout the s i z e range o f a giv e n sample. The m o r t a l i t y censuses were, however, not completely accurate because: a) breakdown and l o s s of empty s h e l l s was r a p i d , and b) i d e n t i f i c a t i o n o f dead but not yet decaying mussels i n the f r o z e n s t a t e was d i f f i c u l t . These f a c t o r s may have caused an unde r e s t i m a t i o n o f m o r t a l i t y . 2. C o n d i t i o n Index C o n d i t i o n index was used t o r e l a t e amount of s h e l l t o l i v i n g t i s s u e i n the mussels, thus i n d i c a t i n g t h e i r market q u a l i t y and apparent h e a l t h . C o n d i t i o n index was assessed as: C O N D I T I O N I N D E X = D R Y T I S S U E W T ( D R Y S H E L L W T ) _ 1 (100) The above was chosen because Davenport and Chen (1987), i n a comparison of v a r i o u s methods used t o assess c o n d i t i o n index, found i t t o be accurate and t o employ s t a b l e and e a s i l y measured parameters. C o n d i t i o n index was assessed f o r each o f 10 mussels of modal l e n g t h from the samples taken from each cage at each s t a t i o n and s i t e . The s h e l l and t i s s u e of each mussel were separated and d r i e d t o constant weight at 90 °C i n pre-weighed aluminum weighing pans. P r i o r t o d r y i n g , s h e l l s were s t r i p p e d o f a l l f o u l i n g growths (e.g., byssus threads, b a r n a c l e s , and bryzoans) and a l l byssus threads were removed from the body. T h i s was done f o r the sake of c o n s i s t e n c y because threads were i n a d v e r t e n t l y removed from some mussels d u r i n g sampling. Because mussels were f r o z e n immediately a f t e r sampling, some food remnants r e g u l a r l y remained i n the gut and were i n c l u d e d as t i s s u e weight. A separate i n v e s t i g a t i o n was undertaken t o assess the magnitude of t h i s e r r o r source. Twenty mussels ranging i n l e n g t h from 26-59 mm were suspended near the Departure Bay s i t e and allowed t o feed f o r 24 h r s . These mussels were then p l a c e d i n i n d i v i d u a l 400 ml beakers i n the l a b o r a t o r y and kept without food f o r 18 hr t o allow them to v o i d t h e i r guts. The faeces were f i l t e r e d onto pre-weighed f i l t e r papers (Whatman GF/C) and d r i e d t o constant weight at 60 °C. The dry weight of gut contents i n the mussels was then estimated from the measured weight o f faeces u s i n g an a b s o r p t i o n e f f i c i e n c y o f 7 6% (mean of r e p o r t e d v a l u e s f o r M. edulis at 5-15 °C: Widdows and Bayne, 1971; V a h l , 1973). Each animal's t i s s u e s were a l s o d r i e d t o a l l o w comparison o f e s t i m a t e d dry weight of gut contents to i t s dry t i s s u e weight. R e s u l t s showed t h a t gut contents accounted f o r only 5 + 4 S.D.% of the dry t i s s u e weight f o r the range of animals used (which encompasses the s i z e range of most mussels analysed over the course of the s t u d y ) . T h i s does not account f o r the s o l u b l e components of the faeces, however, even i f these amounted t o 25% of the t o t a l f a e c a l weight t h i s would only i n c r e a s e the gut contents t o 8.2% of the dry t i s s u e weight. Thus, i n c l u s i o n of gut contents i n the dry t i s s u e weights of mussels probably r e p r e s e n t e d only a sm a l l e r r o r source throughout the course of 2 2 the experiment, and no c o r r e c t i o n was a p p l i e d t o account f o r t h i s n o n - t i s s u e weight. 3. Carbohydrate Carbohydrate, predominantly glycogen, i s the primary energy storage compound i n mussels (Gabbott, 1976) and, as noted p r e v i o u s l y , a measure of carbohydrate content i s a good i n d i c a t i o n of m e t a b o l i c s t a t e in- b i v a l v e s (Gabbott and Stephenson, 1974). Carbohydrate was assessed as percentage dry t i s s u e weight i n each of 10 mussels from the i d e n t i f i e d modal l e n g t h - c l a s s of the samples from each cage at the 3-m ( c l o s e s t t o the salmon farm) and c o n t r o l s t a t i o n s at each s i t e . I t was expected t h a t any i n f l u e n c e of the salmon farm on mussel growth or energy r e s e r v e would be most evident i n a comparison o f these widely spaced s t a t i o n s . For carbohydrate a n a l y s i s the t i s s u e s of each mussel (with byssus threads removed) were homogenized w i t h a Brinkman P o l y t r o n t i s s u e g r i n d e r and then l y o p h i l i s e d . Carbohydrate content was measured u s i n g the p h e n o l - s u l f u r i c t e s t o f Dubois et al. (1956). The c o l o u r i m e t r i c assay was done i n t r i p l i c a t e f o r each mussel. 4. Crude Protein Measurement of crude p r o t e i n content a f f o r d e d an assessment of b i o a c cumulation of the mussel's secondary energy storage product. The l y o p h i l i s e d mussel t i s s u e remaining a f t e r carbohydrate a n a l y s i s was used t o determine crude p r o t e i n content (thus, only mussels from the 3 m and c o n t r o l s t a t i o n s at each s i t e were a n a l y s e d ) . To have s u f f i c i e n t l y o p h i l i s e d t i s s u e f o r crude p r o t e i n a n a l y s i s , the t i s s u e s of a l l 10 mussels sampled per cage were combined. Each combined sample was analysed i n d u p l i c a t e u s i n g a m o d i f i e d s e l e n i u m - c a t a l y s t K j e l d a h l method (Williams, 1984). N i t r o g e n i n the t i s s u e was converted t o ammonium s u l f a t e by wet-acid d i g e s t i o n u s i n g a mixture of H20 2, H2SO4, and a LiSC^/Se c a t a l y s t which a i d e d i n decomposition of more r e s i s t a n t o r g a n i c molecules. Ammonia i n the a c i d d i g e s t was r e a c t e d i n s e r i e s w i t h s a l a c y l a t e / n i t r o p r u s s i d e and a sodium h y p o c h l o r i t e s o l u t i o n t o y i e l d a green c o l o u r r e a c t i o n which was read at 660 urn i n a Technicon Autoanalyser I I . The n i t r o g e n content of the t i s s u e was converted t o crude p r o t e i n content by: % C R U D E P R O T E I N = % N I T R O G E N (6.25) and expressed as percentage dry t i s s u e weight. 5. Spat D e n s i t y Settlement of mussel spat at each s i t e was used as a f u r t h e r measure of salmon-farm i n f l u e n c e on mussel v i t a l i t y . In May 1989 spat were s t r i p p e d from 5-cm lengths of 2-cm diameter p o l y p r o p y l e n e rope. These ropes were p a r t o f the support s t r u c t u r e of the mussel l o n g - l i n e s and the salmon farms, r a t h e r than b e i n g s p e c i f i c a l l y designed c o l l e c t o r s . Three r e p l i c a t e samples were taken at each s t a t i o n and from w i t h i n a salmon pen at each f a r m - s i t e . The mussels i n these samples were counted and d e n s i t y data compared wi t h r e s p e c t t o d i s t a n c e from the farm. 24 6. C h l o r o p h y l l and Seston Concurrent with each 3-6 wk mussel sample, 6 seawater samples of 0.5 L were taken at each s t a t i o n u s i n g a Van Dorn sampling b o t t l e . The samples were taken from 2 m below the s u r f a c e , the same depth at which the mussels were c u l t u r e d . These samples were used t o determine c h l o r o p h y l l and seston c o n c e n t r a t i o n s at each s t a t i o n at both s i t e s . C h l o r o p h y l l analyses were performed i n t r i p l i c a t e u s i n g a technique adapted from Parsons et al. (1984). M o d i f i c a t i o n s t o the technique i n c l u d e d : 1) s t o r i n g p hytoplankton-bearing f i l t e r s at -15 °C i n g l a s s v i a l s c o n t a i n i n g s i l i c a g e l as a d e s i c c a n t f o r up t o 6 wk, 2) s h o r t e n i n g the time r e q u i r e d f o r acetone-e x t r a c t i o n o f the pigments by s o n i c a t i n g the samples, and 3) measuring pigment c o n c e n t r a t i o n s u s i n g a Perkin-Elmer Spectrophotometer w i t h a path l e n g t h of 1 cm. Admittedly, c h l o r o p h y l l i s not a p r e c i s e measure of the amount of food a v a i l a b l e t o f i l t e r feeders s i n c e the r a t i o o f c h l o r o p h y l l t o carbon v a r i e s c o n s i d e r a b l y w i t h the type and n u t r i t i o n a l s t a t e o f phytoplankton. G e n e r a l l y , however, the dry weight of phytoplankton i s c o n s i d e r e d t o be an order of magnitude g r e a t e r than the weight of c h l o r o p h y l l . Determinations of t o t a l seston c o n c e n t r a t i o n ( t o t a l suspended p a r t i c u l a t e matter) were a l s o done i n t r i p l i c a t e u s i n g a technique adapted from S t r i c k l a n d and Parsons (1972). Seawater samples were passed through a 120 urn screen and then 25 v a c u u m - f i l t e r e d onto pre-weighed m i l l i p o r e f i l t e r s o f 0.22 urn p o r o s i t y . The combination of pore s i z e i n screen and f i l t e r e xcluded zooplankton and l a r g e seston, but r e t a i n e d b a c t e r i a (although t h e r e i s some debate as to whether mussels can feed on b a c t e r i a ) . Thus, a l l p a r t i c l e s w i t h i n the s i z e range f o r f e e d i n g by mussels were measured, as were some p o s s i b l e non-food items. The f i l t e r s were d r i e d t o constant weight at 60 °C and the dry weight of seston measured. Change i n weight due to h y g r o s c o p i c i t y o f the f i l t e r s was e l i m i n a t e d by i n c l u d i n g 3-5 blank f i l t e r s (wetted with d i s t i l l e d water and dried) w i t h each sample group. Seston c o n s i s t e d o f the t o t a l suspended p a r t i c l e s i n a seawater sample and l i k e l y i n c l u d e d some non-food items ( i n o r g a n i c p a r t i c l e s , and o r g a n i c p a r t i c l e s which were, by v i r t u e of s i z e , i n e d i b l e f o r mussels). Therefore, the measured seston c o n c e n t r a t i o n presumably over-estimated a v a i l a b l e mussel food. In c o n t r a s t , the c h l o r o p h y l l c o n c e n t r a t i o n s l i k e l y under-es t i m a t e d a v a i l a b l e food because mussels can feed on d e t r i t u s and p o s s i b l y b a c t e r i a i n a d d i t i o n t o the phytoplankton. To o b t a i n a more d e t a i l e d estimate of the c o n t r i b u t i o n t o seston c o n c e n t r a t i o n made by salmon meal, a s e r i e s o f water samples were taken from the 3-m s t a t i o n at Departure Bay every 2 hrs f o r 24 h r s . These samples were c o l l e c t e d i n November when phytoplankton was s c a r c e . C h l o r o p h y l l and seston were measured as above. 26 7 . Currents For a salmon farm t o c o n t r i b u t e n u t r i e n t s d i r e c t l y t o p o l y c u l t u r e d mussels, the water c u r r e n t would have t o flow f i r s t p ast the salmon farm and then t o the mussels. Current p a t t e r n s were monitored at the two salmon-farm s i t e s u s i n g an Interocean Systems Model 135 c u r r e n t meter at the Genoa Bay farm (February, A p r i l , J u l y , and August 1989) and an AANDERAA t e m p e r a t u r e / s a l i n i t y / c u r r e n t meter at the Departure Bay farm (March-June 1989). The model 135 meter recorded c o n t i n u o u s l y f o r 4-5 d a f t e r each deployment, whereas the AANDERAA meter made r e c o r d i n g s every 15 min throughout i t s deployment. The c u r r e n t meters were s i t e d between the salmon farm and the mussel l o n g -l i n e s . The model 135 meter was kept at 2 m depth (the same as mussel c u l t u r e ) while the AANDERAA meter was i n s t a l l e d at 10 m ( i n accordance w i t h the pre-emptive needs of a s c a l l o p c u l t u r e programme b e i n g conducted by the P a c i f i c B i o l o g i c a l S t a t i o n ) . 8. S t a t i s t i c a l Analyses Growth data, except crude p r o t e i n content, were analysed w i t h 3-way Analyses o f V a r i a n c e (ANOVA) u s i n g the General L i n e a r Models program of SAS ( S t a t i s i c a l A n a l y s i s System) at the P a c i f i c B i o l o g i c a l S t a t i o n . Crude p r o t e i n content was analysed w i t h a two-way ANOVA wit h cage treatment omitted s i n c e samples from the cages were pool e d f o r p r o t e i n a n a l y s i s . A separate ANOVA was run f o r each growth parameter at each farm. A r c s i n e transformed data were used f o r the growth parameters which were measured as percentages ( i . e . , c o n d i t i o n index, carbohydrate content, and crude p r o t e i n c o n t e n t ) . No s t a t i s t i c a l comparisons were made between mussels grown at the d i f f e r e n t s i t e s owing t o environmental d i f f e r e n c e s between Departure Bay and Genoa Bay (e.g., water temperature, freshwater r u n - o f f , f l u s h i n g r a t e , phytoplankton blooms), and owing t o d i f f e r e n c e s between the c u l t u r e programmes at each salmon farm (e.g., feed type, f e e d i n g regime, as w e l l as number, s i z e , and s p e c i e s of f i s h ) . C h l o r o p h y l l and seston data were analysed u s i n g the same SAS package but w i t h 2-way ANOVA 1s. A m u l t i p l e r e g r e s s i o n a n a l y s i s of dry t i s s u e weight, dry s h e l l weight, and s h e l l l e n g t h was done u s i n g the MIDAS programme at UBC. Simple one-way ANOVAs f o r spat d e n s i t y and f o r the 24-hr seston s e r i e s were done u s i n g the UBC G e n l i n programme. 2 8 RESULTS AND DISCUSSION 1. Condition Index C o n d i t i o n index was s i m i l a r i n magnitude and seasonal p a t t e r n at both s i t e s . F i g . 4 i l l u s t r a t e s monthly f l u c t u a t i o n o f c o n d i t i o n i n d i c e s i n mussels c u l t u r e d at each of the fou r s t a t i o n s f o r both s i t e s . I n d i v i d u a l cages at each s t a t i o n have not been d e p i c t e d ; r a t h e r , the cage data were combined f o r each s t a t i o n because t h e r e were no s i g n i f i c a n t d i f f e r e n c e s between mussels c u l t u r e d i n separate cages at the s t a t i o n s (ANOVA: Departure Bay F Q # Q 5 ( 2 ) , 3 , 1 2 7 4 = 1 > 2 ' P=0-62; Genoa Bay F 0 .05 (2),3,1428 = 2.1, p=0.20). C o n d i t i o n i n d i c e s were not s i g n i f i c a n t l y i n f l u e n c e d by d i s t a n c e from the salmon farm i n Genoa Bay (ANOVA: F Q # Q 5 ( l ) , 3 , 1 4 2 8 = 1 * 0 / P=0.39) but were i n Departure Bay (ANOVA: F Q # Q 5 ( l ) , 3 , 1274 = 2 7 * 7 ' P<0.001). However, the l a t t e r i n f l u e n c e was c o n t r a r y t o t h a t hypothesized. Mussels c u l t u r e d at the c o n t r o l s t a t i o n had the lowest mean c o n d i t i o n index (20.3 + 9.4 SD%), but those at the 3-m s t a t i o n (22.3 + 9.9 SD%) were s i g n i f i c a n t l y lower than at both 15 m (23.7+10.0 SD%) and 75 m (22.7 + 9.1 SD%) (p<0.05, Tukey M u l t i p l e Comparison T e s t : TMCT). Des p i t e the s i g n i f i c a n t d i s t a n c e - e f f e c t on c o n d i t i o n index, i t w i l l be seen t h a t other parameters of mussel growth were not s i m i l a r l y i n f l u e n c e d , nor was s u r v i v a l . M o r t a l i t y r a t e s were s i m i l a r at a l l s t a t i o n s w i t h i n a s i t e . V i r t u a l l y no m o r t a l i t i e s were recorded b e f o r e June 1989 and, a f t e r t h i s time, m o r t a l i t y counts rose r a p i d l y , r e a c h i n g almost 29 F i g u r e 4. Monthly f l u c t u a t i o n s i n c o n d i t i o n i n d i c e s o f mussels c u l t u r e d i n a) Departure Bay and b) Genoa Bay at d i f f e r e n t - - d i s t a n c e s from salmon farms (3 m, 15 m, 75 m, and c o n t r o l s t a t i o n s ) d u r i n g 1988-89. Data are expressed as means + SD, combining v a l u e s f o r a l l mussels at a gi v e n s t a t i o n s i n c e cages comprised an homogenous s e t . N = 40 f o r a l l p o i n t s except as i n d i c a t e d (*), where N = 30 due t o l o s s o f a cage. CONDITION INDEX (DRY TISSUE WEIGHT [DRY SHELL WEIGHT]" -1X 100) CONDITION INDEX (DRY TISSUE WEIGHT [DRY SHELL WEIGHT] - 1 X 100) 50% of mussels sampled i n Departure Bay and 40% i n Genoa Bay by August 1989. A l l i n t e r a c t i v e e f f e c t s , w i t h the e x c e p t i o n of s t a t i o n - c a g e at both s i t e s , and month-cage at Departure Bay, were h i g h l y s i g n i f i c a n t , e.g., month-station (ANOVA: Departure Bay F0.05(2),24,1274 = 1 4 - 6 ' P<0.001; Genoa Bay F(). 05 (2), 27, 1428 = 14.0, p<0.001), month-cage (ANOVA: Genoa Bay F o.05(2),27, 1428 = 3.6, p=0.01), month-station-cage (ANOVA: Departure Bay F0.05(2),71,1274 = 4 ' 0 ' P<0.001; Genoa Bay Fo.05(2),80,1428 = 4.5, p<0.001). Month-station e f f e c t s may have r e s u l t e d from p o s i t i o n i n g of the s t a t i o n s s i n c e t h i s may have generated s t a t i o n d i f f e r e n c e s which were more pronounced i n c e r t a i n months. For example, at Departure Bay, the 3 m and 15 m s t a t i o n s were s i t e d c l o s e t o the n o r t h shore of an i s l a n d and were u s u a l l y shadowed by i t , the 75 m s t a t i o n was f u r t h e r o f f the n o r t h shore and was l e s s shadowed, while the c o n t r o l was s i t e d on the south s i d e o f the i s l a n d and r e c e i v e d the most s u n l i g h t . An i n f l u e n c e on mussel growth generated by a d i f f e r e n t i a l exposure t o s u n l i g h t would be most pronounced i n summer. S i m i l a r l y , the 3 m, 15 m, and 75 m s t a t i o n s at Genoa bay were s i t e d c l o s e r t o a marina than the c o n t r o l s t a t i o n . With boat t r a f f i c b e i n g h e a v i e s t i n the summer, any e f f e c t o f boat p o l l u t i o n on mussel growth would have been experi e n c e d p r i m a r i l y by the 3 m, 15 m, and 75 m, mussels, and t o the g r e a t e s t degree i n summer. S t a t i o n p o s i t i o n was determined by topography and the n e c e s s i t y t o a v o i d boat t r a f f i c . T o p o g r a p h i c a l f e a t u r e s of the two study s i t e s almost c e r t a i n l y l e d t o d i f f e r e n c e s i n c u r r e n t flow (speed and d i r e c t i o n ) between 32 s t a t i o n s . For i n s t a n c e , at Departure Bay the d i f f e r e n c e i n p r o x i m i t y t o the i s l a n d l i k e l y caused a d i f f e r e n c e i n c u r r e n t flow at the s t a t i o n s s i n c e c u r r e n t near the rock w a l l of the i s l a n d would be slower. A d i f f e r e n c e i n c u r r e n t flow among s t a t i o n s i n Genoa Bay was evidenced by the f a c t t h a t the 3 m, 15 m, and 75 m s t a t i o n s were i c e d - o v e r i n February 1989, but the c o n t r o l s t a t i o n was not. I t was not p o s s i b l e t o monitor c u r r e n t flow at each s t a t i o n s i n c e only two c u r r e n t meters were a v a i l a b l e f o r use and one remained f i x e d at the salmon farm i n Departure Bay. Month-cage e f f e c t s may have been due t o the i n t e r a c t i o n of cages wi t h r e g a r d t o c u r r e n t flow. Mussels i n cages on the o u t s i d e of the l o n g - l i n e may have had access t o a g r e a t e r water flow and t h e r e f o r e more food, because they were i n " f i l t e r i n g -c o m p e t i t i o n " w i t h a mussel p o p u l a t i o n on only one s i d e whereas the two i n n e r cages had a competing p o p u l a t i o n on both s i d e s ( p o s i t i o n e d 1 m away). T h i s f i l t e r i n g c o m p e t i t i o n and any r e s u l t i n g changes i n growth parameters due t o decrease i n food would have been most pronounced at times when c u r r e n t flow was slower (perhaps i n summer when t h e r e was l e s s wind to d r i v e c u r r e n t s ) or when the cages were more occluded by f o u l i n g growth (barnacles, bryozoans, c a p r e l l i d s , macrophytes, e n c r u s t i n g sponges, and t u n i c a t e s s e t t l e d most h e a v i l y i n May-July 1989). While a l l f o u l i n g growth was removed at each sampling time i n an attempt t o minimise the e f f e c t s of f o u l i n g on the cages, o c c l u s i o n of the mesh may have somewhat hi n d e r e d mussel f e e d i n g and thus may have a d v e r s e l y a f f e c t e d mussel growth. Month-s t a t i o n - c a g e e f f e c t s may r e f l e c t a d i f f e r e n c e i n the month-cage i n t e r a c t i o n at d i f f e r e n t s t a t i o n s . Owing t o p a t c h i n e s s of 33 f o u l i n g growth, s t a t i o n s may have e x h i b i t e d d i f f e r e n c e s i n f i l t e r i n g c o m p e t i t i o n i n l a t e s p r i n g and summer when f o u l i n g organisms s e t t l e d and grew on the cages. F i l t e r i n g c o m p etition between mussels w i t h i n a cage may a l s o have been an important i n f l u e n c e on mussel growth. Mussel d e n s i t y w i t h i n each cage would have been ever-changing as mussels moved, sloughed o f f ropes and socks, d i e d , or were c o l l e c t e d , and these changes would have a f f e c t e d the f i l t e r i n g e f f i c i e n c y o f mussels. The month-s t a t i o n - c a g e i n t e r a c t i o n would have been most pronounced f o r the i n s i d e cages which may have experienced g r e a t e r f i l t e r i n g c o m p e t i t i o n at times of low food c o n c e n t r a t i o n or when water flow through the cages was impeded by f o u l i n g growth. These are the most probable e x p l a n a t i o n s f o r the s i g n i f i c a n t i n t e r a c t i v e e f f e c t s i d e n t i f i e d f o r c o n d i t i o n index, and they l i k e l y apply to i n t e r a c t i v e e f f e c t s on other growth parameters, such as dry t i s s u e weight, dry s h e l l weight, carbohydrate content, and crude p r o t e i n content (see l a t e r ) . Monthly changes i n c o n d i t i o n i n d i c e s were h i g h l y s i g n i f i c a n t f o r mussels at both s i t e s (ANOVA: Departure Bay F Q # Q 5 (2), 8,1274 = 269.9, p<0.001; Genoa Bay F Q . 0 5 (2), 9, 1428 = 2 1 8 - 4 / P<0.001). Mussels i n Departure Bay ( F i g . 4a) e x h i b i t e d a s i g n i f i c a n t month-to-month d e c l i n e d u r i n g autumn 1988 and winter 1989 (p<0.05, TMCT). C o n d i t i o n index d e c l i n e d from an extreme h i g h f o r a l l s t a t i o n s i n October (36%) t o a low i n February (15%). T h i s d e c l i n e a c t u a l l y r e s u l t e d from growth of s h e l l ( F i g . 5) while body t i s s u e remained constant ( F i g . 6). S h e l l weight of the Departure Bay mussels ( F i g . 5a) showed s i g n i f i c a n t 34 F i g u r e 5. Monthly f l u c t u a t i o n s i n dry s h e l l weights of mussels c u l t u r e d i n a) Departure Bay and b) Genoa Bay at d i f f e r e n t d i s t a n c e s around salmon farms (3 m, 15 m, 75 m and c o n t r o l s t a t i o n s ) d u r i n g 1988-89. Data are expressed as means + SD, combining v a l u e s f o r a l l mussels sampled at a gi v e n s t a t i o n s i n c e cages comprised an homogenous s e t . N = 40 f o r a l l p o i n t s except as i n d i c a t e d (*), where N = 30 due t o l o s s o f a cage. DRY SHELL WEIGHT (mg) 36 F i g u r e 6. Monthly f l u c t u a t i o n s i n dry t i s s u e weights of mussels c u l t u r e d i n a) Departure Bay and b) Genoa Bay at d i f f e r e n t d i s t a n c e s from salmon farms (3 m, 15 m, 75 m and c o n t r o l s t a t i o n s ) d u r i n g 1988-89. Data are expressed as means + SD, combining v a l u e s f o r a l l mussels sampled at a given s t a t i o n s i n c e cages comprised an homogenous s e t . N = 40 f o r a l l p o i n t s except as i n d i c a t e d (*), where N = 30 due t o l o s s of a cage. 3 7 1 5 0 0 T H ' 1 1 1 1 1 1 1 1 1 1 1 s 0 N 0 J F M A M J J A MONTH O o 3 m O O 15 m A — A 7 5 m D — D Control 1500 T 38 seasonal f l u c t u a t i o n s (ANOVA: Fg #05 (2), 8, 1274 = 2403.9, p<0.001), w i t h month-to-month s i g n i f i c a n c e d u r i n g autumn and winter (p<0.05, TMCT), while dry t i s s u e weight ( F i g . 6a) remained constant f o r the same p e r i o d (ANOVA: F o . 05 (2), 8, 1274 = 3 7 8 * 7 ' p<0.001) b e f o r e i n c r e a s i n g s i g n i f i c a n t l y l a t e r i n s p r i n g 1989 (p<0.05, TMCT). C o n d i t i o n index then remained constant from F e b r u a r y - A p r i l 1989 as s h e l l growth slowed s l i g h t l y (even though s h e l l weight had i n c r e a s e d s i g n i f i c a n t l y , p<0.05 TMCT) and dry t i s s u e weight again remained constant. From F e b r u a r y - A p r i l 1989 s h e l l weight i n c r e a s e d r e l a t i v e t o t i s s u e weight, but without a s i g n i f i c a n t decrease i n c o n d i t i o n index. C o n d i t i o n index was thus only c o a r s e l y r e s p o n s i v e t o r e l a t i v e changes i n s h e l l weight and t i s s u e weight d u r i n g these p e r i o d s , s u g g e s t i n g t h a t i t may be l e s s u s e f u l as an i n d i c a t o r of mussel h e a l t h than i t s common usage might imply. C o n d i t i o n i n d i c e s of mussels c u l t u r e d i n Departure Bay rose s h a r p l y i n the s p r i n g , d u r i n g April-May 1989, as t i s s u e growth (seen as a s i g n i f i c a n t i n c r e a s e i n dry t i s s u e weight, F i g . 6a; p<0.05, TMCT) outpaced s h e l l growth ( F i g . 5a; p<0.05, TMCT). F o l l o w i n g peak spring/summer c o n d i t i o n i n d i c e s ( i n May f o r 3-m and 15-m s t a t i o n s , 30% and 34%, r e s p e c t i v e l y ; i n June f o r c o n t r o l , 25%; and i n J u l y f o r 75 m, 30%) a l l treatment groups once again d i s p l a y e d s i g n i f i c a n t month-to-month d e c l i n e s extending t o the end of the study. The g r e a t e s t d e c l i n e at a l l s t a t i o n s o c c u r r e d July-August 1989, and lowest mean c o n d i t i o n i n d i c e s f o r a l l s i t e s (12%) were recorded i n August. During May-August, s h e l l weight ( F i g . 5a) was s t i l l i n c r e a s i n g s i g n i f i c a n t l y (p<0.05, TMCT), while dry t i s s u e weight ( F i g . 6a) f i r s t h e l d constant i n May-July and then dropped s i g n i f i c a n t l y i n J u l y -August 1989 (p<0.05, TMCT). Thus, Departure Bay mussels e n t e r e d a wasting p e r i o d i n summer ( r e c a l l a l s o t h a t m o r t a l i t y had reached almost 50% by August). Some of the weight l o s s i n e a r l y summer (May-June) may have been due t o spawning. Thick gamete-f i l l e d mantles were observed i n mussels at both s i t e s u n t i l June, i n d i c a t i n g t h a t mussels were i n spawning c o n d i t i o n u n t i l t h i s time. A f t e r June the m a j o r i t y o f mussels had shed t h e i r gametes. The seasonal p a t t e r n o f c o n d i t i o n i n d i c e s f o r mussels c u l t u r e d i n Genoa Bay ( F i g . 4b) d e v i a t e d only s l i g h t l y from t h a t d e s c r i b e d f o r those i n Departure Bay. I n i t i a l l y , i n September-October 1988, th e r e was a s i g n i f i c a n t i n c r e a s e , but from October-A p r i l t h e r e was a s i g n i f i c a n t month-to-month d e c l i n e (p<0.05, TMCT). T h i s d e c l i n e was from a mean ( f o r a l l s t a t i o n s ) o f 41.0 + 8.8 SD% i n October 1988, t o 14.4 + 3.5 SD% i n A p r i l 1989, and r e s u l t e d from dry t i s s u e weight ( F i g . 5b) remaining constant (ANOVA: F 0 > Q 5 (2), 9,1426 = 4 9 2 * 7 ' P<0.001) while s h e l l weight was i n c r e a s i n g s i g n i f i c a n t l y (ANOVA: F o.05(2),9,1425 = 2641.3, p<0.001). In A p r i l - J u n e 1989 t h e r e was a s i g n i f i c a n t month-to-month r i s e i n c o n d i t i o n index and dry t i s s u e weight, and both peaked i n June. Dry s h e l l weight ( F i g . 5a) a l s o i n c r e a s e d s i g n i f i c a n t l y over these months but peaked i n J u l y . C o n d i t i o n i n d i c e s dropped d r a m a t i c a l l y from 40.1 + 13.3 SD% t o 21.8 + 8.2 SD% i n J u l y , and remained at t h i s l e v e l u n t i l the end of sampling i n August 1989. I t was i n t e r e s t i n g t o note t h a t the August c o n d i t i o n index (22.9 +9.9 SD%), d e s p i t e about a 40% decrease i n number, was s i g n i f i c a n t l y h i g h e r than the extreme low of A p r i l (14.4 + 3.5 SD%) when m o r t a l i t y was v i r t u a l l y n i l . T h i s again r a i s e s the q u e s t i o n of u s e f u l n e s s of c o n d i t i o n index as a measure of market q u a l i t y and apparent h e a l t h of mussels. The merit of c o n d i t i o n i n d i c e s which compare amount of body t i s s u e t o s h e l l s i z e and which are p o p u l a r l y used to assess product q u a l i t y i n b i v a l v e s can be assessed by the degree of c o r r e l a t i o n between dry t i s s u e weight and e i t h e r o f two measures of s h e l l s i z e : dry s h e l l weight and s h e l l l e n g t h . The q u e s t i o n has been r a i s e d as t o whether dry s h e l l weight (as used i n t h i s study) or s h e l l l e n g t h i s a b e t t e r c o r r e l a t e w i t h dry t i s s u e weight, s i n c e both have been used f o r mussels (Davenport and Chen, 1987). Data were c o l l e c t e d from 20 mussels randomly s e l e c t e d at the 3-m s t a t i o n i n Genoa Bay. A r e g r e s s i o n of the log - t r a n s f o r m e d v a l u e s was h i g h l y s i g n i f i c a n t (ANOVA o f M u l t i p l e R e g r e s s i o n : F(). 05 (2), 3,198 = 442.6, p<0.001) and was best d e s c r i b e d by the m u l t i p l e r e g r e s s i o n equation: LOG TISS WT = -1.165+0 .250 (LOG SHELL WT) +0 .186 (LOG SHELL LENGTH) C o n s i d e r i n g dry s h e l l weight and s h e l l l e n g t h s e p a r a t e l y , i t i s apparent t h a t dry s h e l l weight i s the s l i g h t l y more accurate p r e d i c t o r o f dry t i s s u e weight (dry s h e l l weight t Q ^ ( 2 ) , l , 1 9 8 = 3.6, p<0.001; s h e l l l e n g t h t 0 . 0 5 (2j1,198 = 2 - 7 ' P = 0-009). Thus, e i t h e r parameter c o u l d be r e l i a b l y used t o estimate body s i z e i n mussels and g i v e n the ease of measurement, s h e l l l e n g t h may be p r e f e r a b l e . The c o e f f i c i e n t of d e t e r m i n a t i o n f o r the m u l t i p l e r e g r e s s i o n was h i g h ( r 2 = 0.82)/ sugg e s t i n g t h a t year-round/ amount of t i s s u e w i l l be r e f l e c t e d w i t h reasonable accuracy/ by s h e l l s i z e . However/ seasonal f l u c t u a t i o n s i n c o n d i t i o n index, which became apparent d u r i n g the study, i n d i c a t e t h a t the accuracy of p r e d i c t i n g dry t i s s u e weight based on e i t h e r dry s h e l l weight or s h e l l l e n g t h i s l i k e l y t o be s e a s o n a l l y i n f l u e n c e d , t o the p o i n t of b e i n g u n r e l i a b l e f o r comparisons between seasons. The nature of i n t e r a c t i v e e f f e c t s of month, s t a t i o n , and cage on dry t i s s u e and dry s h e l l weight at both s i t e s supports t h i s n o t i o n of seasonal i n c o n s i s t e n c y . In a l l cases the month-s t a t i o n i n t e r a c t i o n was s i g n i f i c a n t (ANOVA f o r dry t i s s u e weight: Departure Bay F o.05 (2),24,1274 = 21•6/ p<0.001; Genoa Bay F0.05(2),27/1426 = 1 0 ' l f P<0.001; ANOVA f o r dry s h e l l weight: Departure Bay F Q # Q 5 (2),24/1274 = 3 0 , 2 / P<0.001; Genoa Bay F0.05(2),27/1425 = 3 3 P < 0 . 0 0 1 ) . Other i n t e r a c t i v e e f f e c t s showed a d i s p a r a t e degree of i n f l u e n c e on these two growth parameters. In Departure Bay, dry t i s s u e weight was a l s o s i g n i f i c a n t l y i n f l u e n c e d by the month-station-cage i n t e r a c t i o n (ANOVA: F o.05 (2),71, 1274 = 3.2, p=0.01) but not month-cage (ANOVA: F 0 # 0 5 (2),24,1274 = 1 ' 4 ' P = 1 - ° ° ) ° r s t a t i o n - c a g e (ANOVA: F0.05 (2), 9, 1274 = 3 « 6 / p=0.12). C o n t r a s t i n g l y , the s t a t i o n - c a g e i n t e r a c t i o n s i g n i f i c a n t l y i n f l u e n c e d dry s h e l l weight (ANOVA: F0.05(2) , 9,1274 = 4-6/ p=0.03) but month-cage (ANOVA: F 0 . 0 5 ( 2 ) , 24,1274 = ^•^ / P = 1 « 0 0 ) and month-station-cage (ANOVA: F0.05 (2), 71, 1274 ~ 2 « 4 ' P=0.16) d i d not. In Genoa Bay, dry t i s s u e weight was s i g n i f i c a n t l y i n f l u e n c e d by month-station-cage 4 2 (ANOVA: F Q m 0 5 ( 2 ) , 8 0 , 1 4 2 6 = 3 « 2 / P = 0 . 0 0 4 ) and month-cage (ANOVA: F 0 . 0 5 ( 2 ) < 2 7 , 1 4 2 6 = 3 - 4 , p = 0 . 0 3 ) but not s t a t i o n - c a g e (ANOVA: F 0 . 0 5 ( 2 ) , 9 , 1 4 2 6 = 3 . 4 , p = 0 . 1 4 ) . Dry s h e l l weight, however, was s i g n i f i c a n t l y i n f l u e n c e d by the month-station-cage i n t e r a c t i o n (ANOVA: F o . 0 5 ( 2 ) , 8 0 , 1 4 2 5 = 2 * 8 ' P = 0 « 0 3 ) and s t a t i o n - c a g e (ANOVA: F 0 . 0 5 ( 2 ) , 9 , 1 4 2 5 = 5 . 4 , p = 0 . 0 1 ) , but not month-cage (ANOVA: F 0 . 0 5 ( 2 ) , 2 7 , 1 4 2 5 = 2 . 2 , p = 0 . 7 8 ) . T i s s u e and s h e l l growth responded d i f f e r e n t l y , or at l e a s t t o d i f f e r e n t degrees, t o food a v a i l a b i l i t y as a f f e c t e d by the i n t e r a c t i o n s o f time and l o c a t i o n (see e x p l a n a t i o n s f o r c o n d i t i o n i n d e x ) . Thus, the r e l a t i o n s h i p of t i s s u e t o s h e l l e x h i b i t e d temporal and s p a t i a l i n c o n s i s t e n c i e s . 2 . Carbohydrate T o t a l carbohydrate content, as a percentage of dry t i s s u e weight, was s i m i l a r i n magnitude and seasonal p a t t e r n i n the mussels at both s i t e s ( F i g . 7 ) . Again only s t a t i o n means are shown because no s i g n i f i c a n t d i f f e r e n c e s were i n d i c a t e d among cage means (ANOVA: Departure Bay F Q ^ 0 5 ^ ) , 3 , 6 2 4 = 2 . 4 , P = 0 . 1 2 ; Genoa Bay F o . 0 5 ( 2 ) , 3 , 7 0 9 = 2 . 2 , p = 0 . 1 8 ) . Carbohydrate content f o l l o w e d the same seasonal t r e n d as c o n d i t i o n index, and monthly f l u c t u a t i o n s were again h i g h l y s i g n i f i c a n t (ANOVA: Departure Bay F 0 . 0 5 ( 2 ) , 8 , 6 2 4 = 2 1 2 . 0 , p < 0 . 0 0 1 ; Genoa Bay F Q # Q 5 ( 2 ) , 9 , 7 0 9 = 2 2 9 . 1 , P < 0 . 0 0 1 ) . At both sites-, mussels c u l t u r e d 3 m from the salmon farm had, on average, s i g n i f i c a n t l y h i g h e r percent carbohydrate than those at the c o n t r o l s t a t i o n (ANOVA: Departure F i g u r e 7. Monthly f l u c t u a t i o n s i n carbohydrate as percentages dry t i s s u e weight of mussels c u l t u r e d i n a) Departure Bay and b) Genoa Bay d u r i n g 1988-89. A comparison i s made between mussels c u l t u r e d 3 m from the farm'and at the c o n t r o l s t a t i o n . Data are expressed as means + SD, combining v a l u e s f o r a l l mussels sampled at a gi v e n s t a t i o n s i n c e cages comprised an homogenous s e t . N = 40 f o r a l l p o i n t s except as i n d i c a t e (*), where N = 30 due t o l o s s o f a cage. CARBOHYDRATE CONTENT (% DRY TISSUE WEIGHT) CARBOHYDRATE CONTENT (% DRY TISSUE WEIGHT) 45 B a v F0.05(1),1, 624 = 43.2, p<0.001; Genoa Bay F Q # 0 5 (1), 1,709 = 39.7, p<0.001). However, d e s p i t e t h i s , i t i s q u e s t i o n a b l e whether d i f f e r e n c e s i n mean percent carbohydrate s i g n i f i c a n t l y i n f l u e n c e d mussel growth. Means, which i n c l u d e a l l months, d i f f e r e d by l e s s than 3% at both s i t e s : Departure Bay 14.7 + 8.8 SD% at 3-m versus 12.3 + 6.7 SD% at the c o n t r o l s t a t i o n ; and Genoa Bay 14.1 + 5.9 SD% at 3 m versus 12.6 + 5.6 SD% at the c o n t r o l s t a t i o n . I f t h i s g r e a t e r s t o r e of carbohydrate energy was important t o mussel p r o d u c t i o n , then g r e a t e r v i t a l i t y and hence lower m o r t a l i t y of the mussels at the 3-m s t a t i o n s would have been expected. T h i s was not the case. A l s o , note t h a t at both s i t e s , d u r i n g summer when m o r t a l i t y was h i g h the v a r i a t i o n i n carbohydrate content w i t h i n mussel groups was much h i g h e r than d u r i n g w i n t e r . Increased carbohydrate l e v e l s of mussels grown at 3 m r e l a t i v e t o those at the c o n t r o l o c c u r r e d May-August 1989 at both s i t e s . In B.C. waters, summer i s normally a time of low phytoplankton l e v e l s but of h i g h energy requirements f o r mussels. Therefore, i t may be t h a t d u r i n g the low n a t u r a l food l e v e l s o f summer the salmon farm c o n t r i b u t e d s u f f i c i e n t n u t r i e n t s t o enhance the carbohydrate content of mussels grown c l o s e t o the farm, but t h a t d u r i n g times of h i g h e r n a t u r a l food c o n c e n t r a t i o n ( i . e . , d u r i n g autumnal phytoplankton blooms) t h i s minimal c o n t r i b u t i o n d i d not generate a s i g n i f i c a n t c o n t r i b u t i o n t o mussel n u t r i t i o n . The s i g n i f i c a n c e of the month-station i n t e r a c t i v e e f f e c t (ANOVA: Departure Bay F o.05 (2), 8, 624 = p<0.001; Genoa Bay f Q #Q5(2),9,709 = 8 , 4 / P<0'001) was l i k e l y due t o the i n c r e a s e d carbohydrate content of mussels c u l t u r e d at the 3-m s t a t i o n as compared t o those at the c o n t r o l s t a t i o n . Carbohydrate content was a l s o s i g n i f i c a n t l y i n f l u e n c e d by the i n t e r a c t i v e e f f e c t s of month-cage i n Genoa Bay (ANOVA: F0.05(2),27,709 = 5 , 4 / P < 0 « 0 ° 1 ) , and month-station-cage (ANOVA: Departure Bay F Q tQ5(2),23,624 = 4 * 6 ' P<0.001; Genoa Bay F0.05(2),27,709 = 3.6, p=0.01), but not f o r s t a t i o n - c a g e , nor month-cage i n Departure Bay. The s i g n i f i c a n t i n t e r a c t i v e e f f e c t s on carbohydrate content l i k e l y r e s u l t from the same phenomena as have been p o s t u l a t e d f o r d i f f e r e n c e s i n c o n d i t i o n index, s i n c e t hese growth parameters are i n t e r - r e l a t e d . Mussels i n Departure Bay ( F i g . 7a) demonstrated a s i m i l a r seasonal p a t t e r n of change i n carbohydrate content r e g a r d l e s s o f d i s t a n c e from the farm. The 3-m and c o n t r o l s t a t i o n s e x h i b i t e d a s i g n i f i c a n t month-to-month (p<0.05, TMCT) d e c l i n e i n percent carbohydrate through autumn and e a r l y w i n t e r 1988. In February 1989, carbohydrate reached extreme lows of 3.9 + 1.5 SD% at 3-m and 3.8 + 1.4 SD% at the c o n t r o l s t a t i o n . A f t e r A p r i l 1989 carbohydrate l e v e l s rebounded t o highs of 26.4 + 9.6 SD% at the 3-m s t a t i o n i n May, and 21.2 + 6.1 SD% at the c o n t r o l s t a t i o n i n June. F o l l o w i n g t h i s , l e v e l s r a p i d l y d e c l i n e d t o 14.5 + 3.8 SD% at 3-m and 12.5 + 5.2 SD% at the c o n t r o l s t a t i o n i n August 1989. The monthly p a t t e r n of t o t a l carbohydrate l e v e l s of mussels i n Genoa Bay ( F i g . 7b) resembled t h a t of mussels i n Departure Bay. Again, percent carbohydrate d e c l i n e d from the s t a r t of the study i n September 1988 and reached extreme lows of 3.2 + 0.9 SD% at 3-m and 2.9 + 0.6 SD% at the c o n t r o l s t a t i o n i n A p r i l 1989. Thus, the d e c l i n e was s l i g h t l y more p r o t r a c t e d i n Genoa Bay as compared wi t h Departure Bay. As i n Departure Bay, f o l l o w i n g the s p r i n g low, carbohydrate content rebounded and peaked i n June (17.8 + 2.9 SD% at 3 m, and 15.4 +.5.1 SD% at the c o n t r o l s t a t i o n ) . T h i s summer high, however, was secondary to the i n i t i a l carbohydrate l e v e l recorded i n September which presumably r e f l e c t e d a h i g h l e v e l i n the j u v e n i l e s i n summer 1988. F o l l o w i n g the June 1989 peak, carbohydrate content d e c l i n e d at both s t a t i o n s r e a c h i n g 13.5 + 2.5 SD% at 3 m, and 11.6 + 2.1 SD% at the c o n t r o l , i n August 1989. Two f u r t h e r p o i n t s can be made wit h r e s p e c t t o carbohydrate c o n t e n t s . F i r s t , at both s i t e s the percent carbohydrate o f mussels 3 m from the farm was s i g n i f i c a n t l y h i g h e r than the c o n t r o l group i n October (Departure Bay: tg #05(2),78 = 4 p<0.001; Genoa Bay: tQ.05(2),78 =2.8, p<0.01). However, i n February 1989 t h e r e was no d i f f e r e n c e between s t a t i o n s (Departure B a v ; t o .05(2),78 = °- 4' P>0.50; Genoa Bay: t 0. 05<2).78 = 1 ' 2 ' p>0.20) and i t was only l a t e r i n s p r i n g t h a t mussels at the 3-m s t a t i o n s outpaced t h e i r c o n t r o l , c o u n t e r p a r t s . T h i s suggests t h a t the d i f f e r e n c e between s t a t i o n s at each s i t e was u n l i k e l y t o have been the r e s u l t of a d i f f e r e n c e which e x i s t e d p r i o r t o experimental m a n i p u l a t i o n . Second, the mean percent carbohydrate o f a l l mussels at both s i t e s was s i g n i f i c a n t l y h i g h e r (p<0.05 TMCT) at the end of the study i n August than d u r i n g the s p r i n g -time low but, as a l r e a d y noted, m o r t a l i t y was almost 50% h i g h e r . 48 Thus, the mussels were dying d e s p i t e c o n t a i n i n g an energy s t o r e i n excess of 15% of t h e i r body weight. 3. Crude P r o t e i n In c o n t r a s t t o c o n d i t i o n index and percent carbohydrate, crude p r o t e i n content showed a l e s s pronounced seasonal v a r i a t i o n and the seasonal p a t t e r n d i f f e r e d between s i t e s ( F i g . 8). Since mussels from the f o u r cages at each s t a t i o n were poo l e d f o r a n a l y s i s , only mean v a l u e s f o r r e p l i c a t e analyses on each sample are presented. There appears t o have been no i n f l u e n c e o f d i s t a n c e from the salmon farm on percent crude p r o t e i n at e i t h e r s i t e (ANOVA: Departure Bay F o.05 (1), 1, 70 = P=0.97; Genoa Bay F o.06(1),1,80 = I * 4 8 ' p=0.23). Monthly d i f f e r e n c e s , however, were s i g n i f i c a n t at both s i t e s (ANOVA: Departure Bay F0.05 (2), 8,70 = 1 2 8 . 3 , p<0.001; Genoa Bay F 0 # 0 5 ( 2 ) , 9 / 8 0 = 27.1, p<0.001). The i n t e r a c t i o n of month-station was a l s o s i g n i f i c a n t at both s i t e s (ANOVA: Departure Bay F 0 # Q 5 (2),8,122 = 6 , 8 / p=0.003; Genoa Bay FQ.Q5(2),9,140 = 7 * 0 ' P<0.001). T h i s i n t e r a c t i o n l i k e l y r e s u l t s from the same phenomena as have been p o s t u l a t e d f o r growth parameters i n g e n e r a l (see s e c t i o n on c o n d i t i o n i n d e x ) . Percent crude p r o t e i n i n Departure Bay mussels ( F i g . 8a) remained almost constant at 48% from autumn 1988 t o s p r i n g 1989 at the two s t a t i o n s . R e c a l l t h a t d u r i n g t h i s time dry t i s s u e weight ( F i g . 6a) was a l s o constant, but carbohydrate was d e c l i n i n g ( F i g . 7a). T h i s seems i n d i c a t i v e of a c o n v e r s i o n of carbohydrate t o gametes. A s i g n i f i c a n t d e c l i n e i n percent crude 49 F i g u r e 8. Monthly f l u c t u a t i o n s i n crude p r o t e i n as percentages of dry t i s s u e weight of mussels c u l t u r e d i n a) Departure Bay and b) Genoa Bay d u r i n g 1988-89. A comparison i s made between mussels c u l t u r e d 3 m from the salmon farm and at the c o n t r o l s t a t i o n . Data are expressed as means + SD. N = 4, with each v a l u e r e p r e s e n t i n g a n a l y s i s on the combined t i s s u e s of 10 mussels (where i n d i c a t e d , *, N = 3 due t o l o s s of a cage). o. 2 Ul 5 2 o v-O Q 100 9 0 8 0 7 0 60 5 0 4 0 3 0 2 0 10 0 + + N F M MONTH M o- •O 3 m •• Control .1 g u l x l £2 0 I-3 >-01 OC O O 100 9 0 8 0 7 0 60 5 0 4 0 3 0 2 0 10 0 + + + + + + 4-S 0 N D J F M MONTH H h M J p r o t e i n o c c u r r e d May-July 1989 (p<0.05 / TMCT), such t h a t crude p r o t e i n content reached a low of 36.0 + 2.7 SD% i n J u l y 1989. T h i s may have been a r e f l e c t i o n of gamete r e l e a s e as w e l l as some wasting as mussel c o n d i t i o n d e c l i n e d . By August t h e r e was a s i g n i f i c a n t r i s e i n crude p r o t e i n t o 54.4 + 2.2 SD% (p<0.05, TMCT). T h i s apparent l a t e summer i n c r e a s e i n crude p r o t e i n content may be m i s l e a d i n g , however, based on the r e l a t i v e l y g r e a t e r l o s s o f no n - p r o t e i n t i s s u e s such as carbohydrate, which have a l r e a d y been noted. Genoa Bay mussels e x h i b i t e d even l e s s monthly v a r i a t i o n i n percent crude p r o t e i n ( F i g . 8b) than t h e i r Departure Bay c o u n t e r p a r t s . However, t h e r e was a s i g n i f i c a n t drop i n o v e r a l l means f o r mussels at t h i s s i t e (p<0.05, TMCT), from 51.3 + 5.5 SD% i n September 1988 t o 44.7 + 2.5 SD% i n February 1989. During autumn and winter, as wit h mussels c u l t u r e d i n Departure Bay, dry t i s s u e weight remained steady ( F i g . 6b), but percent carbohydrate dropped s i g n i f i c a n t l y ( F i g . 7b). Presumably, the carbohydrate energy r e s e r v e was being d e p l e t e d at the expense of gametogenesis, s i n c e i t was not f u e l i n g p r o t e i n anabolism and m a t e r i a l was not being l o s t from the body. A f t e r A p r i l crude p r o t e i n rose t o 50.8 + 4.9 SD% and then dropped s t e a d i l y t o an extreme low of 41.9 + 3.1 SD% i n August. T h i s d e c l i n e may have r e f l e c t e d shedding o f gametes as w e l l as a summertime wasting seen i n the decreased dry t i s s u e weight ( F i g . 6b). 52 4. Spat D e n s i t y Spat samples taken at each s i t e were not s i g n i f i c a n t l y d i f f e r e n t with r e s p e c t t o d e n s i t y i n r e l a t i o n t o d i s t a n c e from the salmon farm (ANOVA: Departure Bay F Q . 0 5 ( l ) , 4 , 1 0 = 2.9, p=0.08; Genoa Bay F g # Q 5 ( l ) , 4 , 1 0 = 2.6, p=0.10). Thus, d i s t a n c e from a salmon farm d i d not i n f l u e n c e spat s e t t l e m e n t . 5. C h l o r o p h y l l As w i t h a l l the parameters r e l a t e d t o mussel growth a l r e a d y considered, c h l o r o p h y l l c o n c e n t r a t i o n i n sampled water ( F i g . 9) showed a s i g n i f i c a n t monthly v a r i a t i o n at both s i t e s (ANOVA: Departure Bay F 0.Q5 (2),7,53 ~ 9 0 * 5 ' P <0'001/ Genoa Bay F0.05 (2), 8, 62 = 129.3, p<0.001). Distan c e from the salmon farm was a l s o s i g n i f i c a n t (ANOVA: Departure Bay F Q ^Q5 (l),3,53 = 3.9, p=0.01; Genoa Bay F Q # Q 5 ( 1 ) , 3 , 6 2 = 1 2 . 1 / p<0.001) wit h the d i f f e r e n c e s b e i n g c h a r a c t e r i z e d as f o l l o w s : Departure Bay 3 m < 15 m < 75 m and c o n t r o l , and Genoa Bay 3 m and 75 m < 15 m and c o n t r o l ( s t a t i s t i c a l l y homogenous subsets: p<0.05, TMCT). However, s c r u t i n y of the data suggests t h a t s t a t i o n d i f f e r e n c e s may be l a r g e l y due t o " o u t l i e r s " at the Departure Bay 75-m s t a t i o n i n February 1989 and the 3-m s t a t i o n i n May 1989, and at Genoa Bay f o r the 75-m and 3-m s t a t i o n s i n January 1989. In keeping w i t h t h i s , the month-station i n t e r a c t i o n was a l s o s i g n i f i c a n t at both s i t e s (ANOVA: Departure Bay Fg #05 (2),21,53 = 8.9, p<0.001; Genoa Bay F Q.Q5(2),24,62 = 29.5, p<0.001. The reason f o r such an i n t e r a c t i o n i s u n c l e a r ; however, p a t c h i n e s s i n phytoplankton d i s t r i b u t i o n may have been a c o n t r i b u t i n g f a c t o r . 53 F i g u r e 9. Monthly f l u c t u a t i o n s i n c h l o r o p h y l l c o n c e n t r a t i o n s at the 3 m, 15 m, 75 m, and c o n t r o l s t a t i o n s at a) Departure Bay and b) Genoa Bay d u r i n g 1988-89. Data are expressed as means. N = 3. CHLOROPHYLL CONCENTRATION (mg/L) CHLOROPHYLL CONCENTRATION (mg/L) 55 6 . Seston The c o n c e n t r a t i o n of seston at both s i t e s ( F i g . 10) was not s i g n i f i c a n t l y i n f l u e n c e d by d i s t a n c e from the salmon farms (ANOVA: Departure Bay F Q #Q5(1),3,62 = 1 , D ' P = 0 « 3 9 Genoa Bay F0.05 (1),3,59 = °« 7» p=0.53). There was, however, a s i g n i f i c a n t month e f f e c t (ANOVA: Departure Bay F o.05 (2),7, 62 = !87.5, p<0.001; Genoa Bay Fqt05(2),7,59 = 255.3, p<0.001) which was s i m i l a r at the two s i t e s . Month-station i n t e r a c t i o n was a l s o s i g n i f i c a n t at both s i t e s (ANOVA: Departure Bay F o.05(2),21,62 = 5.9, p=0.001; Genoa Bay F o.05(2),21,59 = 4 * 2 ' P=0.03). As w i t h c h l o r o p h y l l c o n c e n t r a t i o n , the reason f o r such an i n t e r a c t i o n i s u n c l e a r but, again, p a t c h i n e s s of seston d i s t r i b u t i o n s may have been a f a c t o r . Seston c o n c e n t r a t i o n s i n Departure Bay were s i m i l a r at about 13 mg.L - 1 i n a l l months except January ( o v e r a l l mean = 3.0 mg.L -1), J u l y ( o v e r a l l mean = 5.4 mg.L - 1), and August ( o v e r a l l mean = 4.4 mg.L - 1), and these excepted months comprise a s t a t i s i c a l l y homogenous subset (p<0.05, TMCT). The seston c o n c e n t r a t i o n s i n Genoa Bay ( F i g . 10b) demonstrated g r e a t e r monthly v a r i a b i l i t y but had the same seasonal d i s t r i b u t i o n of highs and lows as noted f o r Departure Bay. From 14.4 mg.L - 1 at the s t a r t of the study, seston dropped s i g n i f i c a n t l y (p<0.05, TMCT) t o a low of 1.5 mg.L - 1 i n January, and then rose w i t h almost month-to-month s i g n i f i c a n c e t o 18.2 mg.L - 1 i n June b e f o r e s u b s t a n t i a l l y dropping t o about 2.7 mg.L - 1 i n July-August. 56 Figure 10. Monthly fluctuations i n seston concentrations at the 3 m, 15 m, 75 m, and control stations at a) Departure Bay and b) Genoa Bay during 1988-89. Data are expressed as means. N = 3. SESTON CONCENTRATION (mg/L) SESTON CONCENTRATION (mg/L) 58 The 24-hr seston s e r i e s , c a r r i e d out i n l a t e November when phytoplankton was s c a r c e ( c h l o r o p h y l l c o n c e n t r a t i o n was e s s e n t i a l l y z e r o ) , was an attempt to measure the seston component d e r i v e d from the p a r t i c u l a t e o r g a n i c waste of salmon c u l t u r e without e x c e s s i v e background i n f l u e n c e from phytoplankton as would e x i s t i n summer. The r e s u l t s of sampling over a complete t i d a l and f e e d i n g c y c l e are p r e s e n t e d i n F i g . 11. D a i l y f l u c t u a t i o n i n seston was h i g h l y s i g n i f i c a n t (ANOVA: F0.05 (2),11,23 ~ 10.8, p<0.001) and seemed t o p a r a l l e l both the t i d a l and salmon-feeding c y c l e s . However, i t i s l i k e l y t h a t the seston f l u c t u a t i o n s were responding t o e f f e c t s of the f e e d i n g c y l e and not the t i d a l c y c l e f o r the f o l l o w i n g reason. A comparison of c u r r e n t v e l o c i t y c o l l e c t e d over a 4-d p e r i o d w i t h the AANDERAA meter (see next s e c t i o n ) and t i d a l c y c l e showed an absence of c o r r e l a t i o n between the two. Therefore, s i n c e any i n f l u e n c e of t i d e on seston would presumably have operated through t i d a l l y d e r i v e d changes- i n c u r r e n t speed, an i n f l u e n c e of t i d e on seston can l i k e l y be d i s m i s s e d . Seston c o n c e n t r a t i o n thus appears t o have f l u c t u a t e d i n tandem wit h f e e d i n g at the salmon farm which o c c u r r e d h o u r l y d u r i n g d a y l i g h t . However, the s e v e r a l - h o u r l a g i n d e c l i n e of seston c o n c e n t r a t i o n a f t e r c e s s a t i o n of f e e d i n g i n the evening appears incongruous with the r a t e of water flow away from the farm. The e n t i r e volume of water i n t o which the feed was added would have passed the sampling s t a t i o n w i t h i n one hour a f t e r f e e d i n g (based on mean c u r r e n t speed of 2.7 c n r s - 1 f o r Departure 59 F i g u r e 11. F l u c t u a t i o n i n t i d a l h e i g h t and seston c o n c e n t r a t i o n as measured 3 m from the salmon farm i n Departure Bay. Arrows at the bottom of the graph re p r e s e n t (by p o s i t i o n ) f e e d i n g times at the farm and (by s i z e ) the r e l a t i v e input of feed ( t a l l arrows rep r e s e n t approximately 50% more feed)/ while h o r i z i n t a l l i n e s at the top of the graph i n d i c a t e s t a t i s t i c a l l y homogenous subsets of seston c o n c e n t r a t i o n s (p<0.05/ TMCT). SESTON CONCENTRATION (mg/L) TIDAL HEIGHT (m) o 61 Bay over the monitored p e r i o d ) . Yet, seston d i d not reach a low u n t i l 8-10 h a f t e r f e e d i n g had stopped at 1730h. In t h i s regard, i t should be noted t h a t the farm's seston c o n t r i b u t i o n would have been comprised o f a l l p a r t i c u l a t e o r g a n i c waste of salmon c u l t u r e ( i . e . , f eed p a r t i c l e s and f a e c e s ) . Thus, g i v e n a gut-passage time of 24 h f o r salmon e a t i n g t y p i c a l p e l l e t t e d d i e t s (Fange and Grove, 1979), the p r o t r a c t e d d e c l i n e i n seston was reasonable, s i n c e i t may have r e f l e c t e d the presence o f faeces i n the water. The steep i n c r e a s e i n seston which o c c u r r e d a f t e r f e e d i n g resumed at dawn may have, i n t u r n , r e f l e c t e d the presence o f p a r t i c l e s o f salmon meal i n the water. 7. Cu r r e n t s Data on c u r r e n t d i r e c t i o n at both farms are pr e s e n t e d i n F i g . 12. These pie-diagrams show the a p o r t i o n i n g o f c u r r e n t flow among the fo u r primary and fo u r secondary compass d i r e c t i o n s . In Departure Bay ( F i g . 12a), c u r r e n t d i r e c t i o n was most f r e q u e n t l y eastward, with northeastward b e i n g the next most frequent d i r e c t i o n o f flow, i n a l l months. The frequency at which c u r r e n t flowed i n these two d i r e c t i o n s was g r e a t e r than a l l other d i r e c t i o n s combined i n A p r i l and May. The e x i s t e n c e o f such a predominant eastward flow, r a t h e r than a counterbalanced c u r r e n t flow (with r e s p e c t t o frequency o f d i r e c t i o n ) , suggests t h a t c u r r e n t flow i n Departure Bay was not d i r e c t e d p r i m a r i l y by t i d a l changes. B a s i n topography and winds must have operated t o mai n t a i n the eastwardly flow. The eastwardly flow, however, 62 F i g u r e 12. Diagramatic p r e s e n t a t i o n of the a p o r t i o n i n g o f c u r r e n t flow (as percentage of the monitored period) t o the fo u r primary and f o u r secondary compass d i r e c t i o n s at the two farm s i t e s . C u r r e n t s i n Departure Bay (a) were monitored c o n t i n u o u s l y from March-June 1989 while those i n Genoa Bay (b) were monitored i n t e r m i t t e n t l y over 4-d p e r i o d s i n each of February, A p r i l , J u l y , and August 1989. Insets i n d i c a t e the p o s i t i o n i n g o f treatment s t a t i o n s (3 m, 15 m, and 75 m) around the farms: Departure Bay s t a t i o n s extended i n a l i n e 10° n o r t h of e a s t ; Genoa Bay s t a t i o n s were i n a l i n e extending n o r t h from the farm. N N W NW/^" " 5 / 7 23 [ 33 r Sv. L \ 6 sw \ ^ ^ 5 s F E B R U A R Y W ^ \ NE /10 c " ~ >\ 2 5 \ 0.7 ^ 36 \ o . 0 3 2 / 0.4 s APRIL w 10 ^ \ N E / 2 y v . 2 9 \ 0.7 42 \ 2 10y/ SW s J U L Y • 75 m N E W ^ 11 N W / ^ N NE 52 swV^ BE s A U G U S T N W-• 15 m • 3m FARM 65 should have been b e n e f i c i a l t o mussel-salmon p o l y c u l t u r e at t h i s s i t e , s i n c e l o n g - l i n e s were s i t e d almost due east of the farm. At Genoa Bay ( F i g . 12b), c u r r e n t a l s o flowed most f r e q u e n t l y eastward i n a l l m o n i t o r i n g p e r i o d s . However, d u r i n g the February p e r i o d of monitoring, eastwardly flow was counterbalanced by a westward flow f o r 23% of the time. During a l l other m o n i t o r i n g p e r i o d s the combined frequency o f east, n o r t h e a s t , and n o r t h c u r r e n t s was over 80%. A l s o , eastward flow seemed t o become i n c r e a s i n g l y more frequent d u r i n g A p r i l - A u g u s t 1989. I t again seems apparent t h a t , as i n Departure Bay, t i d a l changes i n Genoa Bay were not the primary f o r c e i n f l u e n c i n g the frequency of c u r r e n t d i r e c t i o n . Note, however, t h a t u n l i k e Departure Bay, the predominance of eastward and northeastward flow i n Genoa Bay was not i d e a l f o r mussel-salmon p o l y c u l t u r e , s i n c e l o n g - l i n e s were s i t e d almost d i r e c t l y n o r t h from the farm. 66 GENERAL DISCUSSION Salmon farming i s the most e x t e n s i v e form of aquaculture i n B r i t i s h Columbia. In 1989, 217 salmon l e a s e s were h e l d , w i t h 125 b e i n g a c t i v e (B.C. M i n i s t r y of A g r i c u l t u r e and F i s h e r i e s : L i c e n s i n g S t a t i s t i c s March, 1990), and these produced 12,385 tonnes of cage-reared salmon (B.C. Salmon Farmer's Annual Report, 1989) . I n t e n s i v e c u l t u r e of salmon i n c o a s t a l waters generates s u b s t a n t i a l amounts of waste (Gowen and Bradbury, 1987), a form of o r g a n i c enrichment comparable with domestic sewage and e f f l u e n t from wood pulp and seaweed p r o c e s s i n g , as w e l l as duck farming, which are known t o p r o v i d e n u t r i e n t s t o organisms i n the surrounding waters (Pearson and Rosenberg, 1978). Such p r o v i s i o n of n u t r i e n t s has r a i s e d the i s s u e of p o t e n t i a l p o l y c u l t u r e of mussels and salmon. However, i n the present study growth, c o n d i t i o n index, carbohydrate content, p r o t e i n content, and spat settlement, a l l showed no c o n s i s t e n t or s u b s t a n t i a l i n c r e a s e w i t h d e c r e a s i n g d i s t a n c e of mussels from a salmon farm. Thus, a v a i l a b i l i t y o f enrichment from salmon c u l t u r e w i t h regard t o mussels comes i n t o q u e s t i o n . For p a r t i c l e s t o be eaten by mussels they must be l e s s than 120 urn d i a ; p a r t i c l e s l a r g e r than t h i s are r e j e c t e d (Reid, 1982). The 24-h seston sampling s e r i e s i n Departure Bay i n d i c a t e d t h a t salmon-feeding i n c r e a s e d the c o n c e n t r a t i o n of p a r t i c l e s ( i n the range f o r f e e d i n g mussels: 0.2-120 urn) from 1 t o 4 mg.L-1. Thus, a d d i t i o n of farm-generated seston would appear t o have doubled the a v a i l a b l e food f o r mussels. However, seston c o n c e n t r a t i o n s i n the present study were never above 20 mg.L-1, and an order of magnitude l e s s than seston c o n c e n t r a t i o n s which can be e f f i c i e n t l y f i l t e r e d by mussels (Widdows et al., 1979). In the present study, then, salmon farms probably c o u l d not have g r e a t l y enhanced the mussels' n u t r i t i o n through a d d i t i o n of seston food as p r e d i c t e d . The low i n c r e a s e i n seston c o n c e n t r a t i o n a s s o c i a t e d with salmon-feeding noted i n the present study seems anomalous t o r e p o r t e d p r o d u c t i o n of o r g a n i c waste by.other salmon farms. In r e v i e w i n g s t u d i e s on the e c o l o g i c a l impact of salmon farming, Gowen and Bradbury (1987) i n d i c a t e d t h a t 40% of the a d m i n i s t e r e d f e e d became o r g a n i c waste i n the form of uneaten p e l l e t s and f a e c e s . Furthermore, Brown et al. (1987) showed t h a t settlement of p a r t i c u l a t e o r g a n i c waste s t r o n g l y i n f l u e n c e d s p e c i e s number, biomass, and d i v e r s i t y of b e n t h i c fauna i n the f i r s t 15 m around a f i s h farm, and were a l t e r e d , a l b e i t t o a l e s s e r degree, f o r up t o 120 m. The salmon farm s e l e c t e d f o r study by Brown et al. (1987) was l o c a t e d i n a S c o t t i s h sea l o c h where mean c u r r e n t speed was 4 c r r r s - 1 and depth was 20 m. These s i t e c h a r a c t e r i s t i c s are comparable with those of Departure Bay and Genoa Bay, and w i t h most salmon farms i n B.C.. The study of Brown et a l . (1987) i n d i c a t e d t h a t mean s i z e of sediment p a r t i c l e s d e p o s i t e d around the salmon farm was w i t h i n the range f o r f e e d i n g by mussels, c o r r o b o r a t i n g t h a t some farms, at l e a s t , generate seston a v a i l a b l e t o f e e d i n g mussels. A p o s s i b l e e x p l a n a t i o n f o r the a p p a r e n t l y low seston i n c r e a s e d e s p i t e salmon-feeding may be t h a t the water samples were taken 3 m 68 o u t s i d e , r a t h e r than from w i t h i n , the farm. Perhaps p a r t i c u l a t e o r g a n i c wastes were f i l t e r e d out by mussels on the nets of the farm and, thus, never reached the experimental animals. Based on known pumping r a t e s o f mussels (Vahl, 1973) and the d e n s i t y o f mussels on the net-pens of, f o r example, the Genoa Bay farm (estimated t o be 2*cm" 2), the e n t i r e volume of a t y p i c a l salmon pen (5.0 m X 5.0 m X 6.0 m) c o u l d t h e o r e t i c a l l y be f i l t e r e d every 3 minutes by the mussels on the net-pens. Because each pen w i t h i n a farm i s e n c i r c l e d by a c u r t a i n o f net and the e n t i r e farm i s e n c i r c l e d by a p r e d a t o r net, and depending upon the d e n s i t y o f mussels on these, salmon farm nets may rep r e s e n t a screen w i t h s u b s t a n t i a l c a p a c i t y f o r f i l t e r i n g p a r t i c u l a t e o r g a n i c waste. In f a c t , the p o p u l a t i o n o f mussels on some net-pens can be s i m i l a r i n d e n s i t y t o a t y p i c a l mussel bed, and Dame and Dankers (1988) urge t h a t such p o p u l a t i o n s be viewed as systems r e s p o n s i b l e f o r s u b s t a n t i a l uptake o f suspended sediments, phytoplankton, o r g a n i c carbon, and f i x e d n i t r o g e n , as w e l l as e x c r e t i o n o f s i g n i f i c a n t amounts of ammonium and o r g a n i c phosphate. Any i n f l u e n c e of the salmon farm i n terms of seston c o n c e n t r a t i o n would, i n any event, have been overshadowed by seasonal i n f l u e n c e s on t h i s parameter. Seasonal changes i n seston c o n c e n t r a t i o n were s i m i l a r i n some r e s p e c t s t o those of the measured growth and energy-storage parameters. Winter and summer lows i n seston corresponded w i t h d e c l i n i n g c o n d i t i o n index and carbohydrate content i n the same season at both f a r m - s i t e s (although i t i s no t a b l e t h a t these d e c l i n e s continued f o r th r e e 69 months a f t e r seston c o n c e n t r a t i o n s had begun t o r i s e i n s p r i n g , see F i g s . 4-7). Dry s h e l l weight, however, i n c r e a s e d over winter d e s p i t e the seston low. Thus, only the summer lows i n seston corresponded with s t r i c t f i d e l i t y t o a m e t a b o l i c a l l y poor s t a t e i n the mussels. P o s s i b l y the winter "low" i n seston a c t u a l l y r e p r e s e n t e d a c o n c e n t r a t i o n of food t h a t was s u f f i c i e n t t o s u s t a i n mussels at t h i s time o f year, when met a b o l i c a c t i v i t y was low. Phytoplankton p r o d u c t i o n , l i k e seston, a p p a r e n t l y was not augmented by the salmon farms, d e s p i t e an e x p e c t a t i o n t h a t i t would be based on nitrogenous e x c r e t i o n by salmon. Since approximately 68% of n i t r o g e n consumed by farmed salmon i s e x c r e t e d (Gowen and Bradbury, 1987), and the m a j o r i t y o f t h i s , e x c r e t i o n i s ammonium (Rychly, 1979), t h e r e should have been ample p o t e n t i a l f o r phytoplankton enrichment by the farms. Ammonium and urea e x c r e t e d by c u l t u r e d salmon may be o x i d i s e d or, as a p r e f e r r e d route, d i r e c t l y u t i l i s e d by phytoplankton (M cCarthy et al., 1977; G i l b e r t et al., 1982a; 1982b). Ammonium can a l s o be absorbed r a p i d l y by marine phytoplankton ( G i l b e r t and Goldman, 1981). I t seems c l e a r t h a t salmon farming should enhance the growth p o t e n t i a l o f phytoplankton by making n i t r o g e n a v a i l a b l e t o primary producers and, through t h i s , enhance growth of mussels. There are s e v e r a l p o s s i b l e ' e x p l a n a t i o n s f o r l a c k of i n c r e a s e d c h l o r o p h y l l c o n c e n t r a t i o n d e s p i t e a n i t r o g e n i n f l u x generated by the salmon farms. For example, Gowen and Bradbury (1987) have suggested t h a t primary p r o d u c t i o n can be enhanced but not l e a d t o i n c r e a s e d phytoplankton c o n c e n t r a t i o n because of quick removal by g r a z e r s . Yet, t h e r e would be no a priori reason t o t h i n k t h a t i n the present study experimental mussels were somehow excluded from such g r a z i n g . Another p o s s i b i l i t y i s t h a t u t i l i s a t i o n o f ammonium and urea e x c r e t e d by salmon may be l i m i t e d by r a p i d f l u s h i n g time (Gowen et al., 1983). At farm s i t e s where f l u s h i n g i s r a p i d , phytoplankton may not remain i n the v i c i n i t y l o n g enough t o c a p i t a l i s e on the h i g h c o n c e n t r a t i o n s of nitrogenous n u t r i e n t s . A l t e r n a t i v e l y , d e s p i t e h i g h c o n c e n t r a t i o n s o f nitrogenous n u t r i e n t s , phytoplankton may be l i m i t e d by other environmental c h a r a c t e r i s t i c s such as low temperature, low l i g h t i n t e n s i t y , or low l e v e l s of other r e q u i r e d n u t r i e n t s (such as m i n e r a l s , v i t a m i n s , and o r g a n i c compounds of n i t r o g e n and carbon). The e x i s t e n c e o f seasonal changes i n c h l o r o p h y l l c o n c e n t r a t i o n d u r i n g the study (see F i g . 9) suggests t h a t phytoplankton p o p u l a t i o n s were probably more i n f l u e n c e d by seasonal c h a r a c t e r i s t i c s than by salmon farms s i n c e t h e r e would have been no seasonal f l u c t u a t i o n i n p r o d u c t i o n of nitrogenous waste by salmon. The monthly f l u c t u a t i o n s i n c h l o r o p h y l l c o n c e n t r a t i o n corresponded only s l i g h t l y w i t h monthly f l u c t u a t i o n s i n growth and energy r e s e r v e s of the mussels. In Departure Bay the lowest c h l o r o p h y l l c o n c e n t r a t i o n (November 1988) was synchronous w i t h d e c l i n i n g c o n d i t i o n index and carbohydrate content. However, i t was noted t h a t at t h i s time s h e l l growth o c c u r r e d and dry t i s s u e weight was constant. Therefore, d e s p i t e s c a r c i t y of phytoplankton i n November/ the mussels were m e t a b o l i c a l l y a c t i v e , producing s h e l l and c o n v e r t i n g carbohydrate. In g e n e r a l , from t h e i r deployment i n autumn 1988 t o s p r i n g 1989, Departure Bay mussels were a p p a r e n t l y u n a f f e c t e d by f l u c t u a t i o n s i n phytoplankton, but t h i s was r e v e r s e d i n summer 1989 when c o n d i t i o n , growth and energy r e s e r v e s d i d seem t o c o r r e l a t e with summer f l u c t u a t i o n s i n phytoplankton. In Genoa Bay t h e r e was on l y s l i g h t c o r r e l a t i o n of c h l o r o p h y l l c o n c e n t r a t i o n w i t h c o n d i t i o n , growth, and energy r e s e r v e s , but here i t was i n w i n t e r . In summer, mussel growth at Genoa Bay appeared t o be l e s s r e l a t e d t o phytoplankton f l u c t u a t i o n s , although c h l o r o p h y l l c o n c e n t r a t i o n rose i n May 1989, c o i n c i d e n t a l with a s p r i n g r i s e i n a l l growth parameters. Thus, even though salmon farms have the p o t e n t i a l t o augment n u t r i t i o n of p o l y c u l t u r e d mussels v i a l i b e r a t i o n of wastes, other f a c t o r s such as temperature, l i g h t i n t e n s i t y , presence of n u t r i e n t s other than n i t r o g e n compounds, presence of competing f i l t e r f e eders, and type of salmon feed, may operate t o reduce the e f f e c t . C l e a r l y , i n c r e a s e d growth of mussels cannot be i n s u r e d simply by s i t i n g mussel c u l t u r e s c l o s e t o salmon farms. The importance of s i t e t o the success of mussel c u l t u r e has been r e p e a t e d l y emphasised i n i n v e s t i g a t i o n s of the i n f l u e n c e of environment and g e n e t i c s on mussel growth and s u r v i v a l (Freeman and D i c k i e , 1979; D i c k i e et al., 1984; Widdows et al., 1984; M a l l e t et al., 1986; 1987a; 1987b; Swarbrick et al., 1990). R e c i p r o c a l t r a n s p l a n t s o f mussels by M a l l e t et al. (1987b) have indicated, t h a t genotype i s p r i m a r i l y r e s p o n s i b l e f o r m o r t a l i t y . T h i s c o n t r a d i c t s the r e s u l t s of D i c k i e et al. (1984) who have r e p o r t e d t h a t growth r a t e s of mussels from the same stock i n two environments d i f f e r only s l i g h t l y , w h i le m o r t a l i t y d i f f e r e n c e s are marked and are the p r i n c i p a l determinants of net p r o d u c t i o n . Therefore, whether through an e f f e c t on growth r a t e or m o r t a l i t y , the r e s u l t i s the same: environment s u b s t a n t i a l l y i n f l u e n c e s p r o d u c t i o n of mussels. Thus, t o reap b e n e f i t from mussel-salmon p o l y c u l t u r e , both salmon-farm s i t e and source of mussels should be c a r e f u l l y s e l e c t e d . T r i a l c u l t u r e s t u d i e s may be the o n l y e f f e c t i v e means t o e s t a b l i s h the p o t e n t i a l growth r a t e of a s p e c i f i c mussel source i n a g i v e n area (Jamieson, 1989). Furthermore, as a t r i a l e f f o r t , the experiments at Departure Bay and Genoa Bay may be i n d i c a t i v e ' of a problem a s s o c i a t e d w i t h mussel source, s i n c e m o r t a l i t y of the t r a n s p l a n t e d stocks was high, and of s i t e , s i n c e the farms d i d not f u l f i l l t h e i r p r e d i c t e d p o t e n t i a l t o augment mussel growth. Although present r e s u l t s cannot advocate mussel-salmon p o l y c u l t u r e on the b a s i s of any n u t r i t i o n a l advantage f o r mussels, t h e r e s t i l l may be l o g i s t i c a l advantages t o t h i s p a i r i n g . For example, salmon farms may p r o v i d e s t r u c t u r a l support f o r d e p l o y i n g l o n g - l i n e s f o r mussel suspension. There may a l s o be economic advantages t o h a r v e s t i n g two crops from one l e a s e . Conversely, other sometimes negative aspects of c o u p l i n g o f mussel and salmon c u l t u r e should be i n v e s t i g a t e d b e f o r e such p o l y c u l t u r e i s commercially implemented. A primary concern i s p o t e n t i a l t r a n s f e r of pathogens and p a r a s i t e s between mussels and salmon. Disease i s an important concern i n a l l c u l t u r e o p e r a t i o n s because spread o f pathogens i s enhanced by i n c r e a s e d d e n s i t y which i s a hallmark of animal husbandry. Disease concerns take on new dimension i n p o l y c u l t u r e because the s p e c i e s might respond d i f f e r e n t l y t o p o t e n t i a l pathogens. Such d i f f e r e n t i a l response has been noted w i t h b a c t e r i a , Vibrio spp., which are p o t e n t i a l l y harmful t o salmon and known t o be harboured i n the hind-guts of mussels, Mytilus edulis desolationis, i n hi g h e r c o n c e n t r a t i o n than i n e i t h e r stomach or surrounding seawater (Bouvy and D e l i l l e , 1987). Contact w i t h mussel faeces may i n c r e a s e the r i s k o f V l f c r i o - i n f e e t i o n i n salmon. Another p o t e n t i a l r i s k i s the p o s s i b i l i t y f o r mussel d i s e a s e s t o be t r a n s m i t t e d t o salmon. At l e a s t one d i s e a s e has been i d e n t i f i e d f o r n o r t h e a s t P a c i f i c mussels (Jamieson, 1989), a tumorous c o n d i t i o n o f b l o o d c e l l s known as hemic n e o p l a s i a which can be found i n 60% o f post-spawning mussels i n B.C. (Bower, 1989) and 70% o f mussels from Puget Sound ( E l s t o n et a l . , 1988). As of yet th e r e has been no i n v e s t i g a t i o n o f t h i s d i s e a s e with r e s p e c t t o salmon. A c o n t r a s t i n g c o n s i d e r a t i o n i s the p o t e n t i a l f o r d i f f e r e n t i a l response which may a c t u a l l y b e n e f i t salmon; t h a t i s , mussels may be able t o f i l t e r and d i g e s t b a c t e r i a r e s p o n s i b l e f o r BKD ( b a c t e r i a l kidney disease) (Evelyn et a l . , unpubl. d a t a ) , thus p o s s i b l y r e d u c i n g r i s k o f i n f e c t i o n . In any case, thorough i n v e s t i g a t i o n o f d i s e a s e r i s k s i s necessary t o assess the p o t e n t i a l c o u p l i n g o f mussel and salmon c u l t u r e . Thorough i n v e s t i g a t i o n o f p a r a s i t i c r i s k s i s a l s o necessary. Mytilocola intestinalis, a p a r a s i t i c copepod, i s o c c a s i o n a l l y found w i t h i n d i g e s t i v e t i s s u e s , and u n i d e n t i f i e d protozoans and Nematopsis sp. 74 have been observed i n the lumen of kidneys i n B.C. mussels. Although Emmett (1984) r e p o r t e d t h a t low frequency of occurrence of these p a r a s i t e s tended t o reduce t h e i r e f f e c t on mussel p o p u l a t i o n s / t h e r e have been no s t u d i e s c>f t h e i r p o s s i b l e e f f e c t on salmon. S i m i l a r l y / the e f f e c t o f salmon p a r a s i t e s on mussels has not been i n v e s t i g a t e d . I n t e r e s t i n aquaculture has a l s o been shadowed by concerns o f water q u a l i t y e f f e c t s . Both salmon c u l t u r e (reviewed i n Gowen and Bradbury, 1987) and mussel c u l t u r e (Galkina et al., 1982; Larsson, 1985; Dame and Dankers, 1988) have been s t u d i e d i n t h i s regard, but never i n the context o f p o l y c u l t u r e . In t h i s context, the c h i e f concern r e g a r d i n g salmon c u l t u r e has been one of e u t r o p h i c a t i o n , and the p o s s i b i l i t y t h a t t h i s process would favour mussel c u l t u r e formed the b a s i s o f the present study. R e c i p r o c a l e f f e c t s o f t h i s type o f p o l y c u l t u r e can be e x t r a p o l a t e d from Larsson's (1985) i n v e s t i g a t i o n o f i n f l u e n c e o f mussel c u l t u r e on water q u a l i t y . T h i s 21-month-long study showed t h a t l o n g - l i n e s o f Mytilus edulis (about 160 tonnes wet-weight o f mussels) d i d not g i v e r i s e t o measurable changes i n c o n c e n t r a t i o n of i n o r g a n i c n u t r i e n t s and oxygen i n the water mass p a s s i n g through the c u l t u r e , but ammonia-nitrogen sometimes doubled and phosphorus quadrupled. Dame and Dankers (1988) a l s o r e p o r t e d s i g n i f i c a n t r e l e a s e o f ammonium and phosphorus from n a t u r a l mussel beds. Thus, s i n c e salmon are known t o be h i g h l y s e n s i t i v e t o ammonia ( I n t e r n a t i o n a l Programme on Chemical Safety, 1986), i t may be necessary t o s i t e mussel-salmon p o l y c u l t u r e s i n areas w i t h 75 h i g h f l u s h i n g r a t e s t o prevent e x t e n s i v e mussel c u l t u r e from n e g a t i v e l y i n f l u e n c i n g water q u a l i t y f o r salmon. The present study has p r o v i d e d i n f o r m a t i o n not only on the p r a c t i c a l i t y of mussel-salmon p o l y c u l t u r e , but a l s o on the seasonal c y c l e of accumulation and expenditure of energy storage products by B.C. mussels. P a r t i c u l a r l y of i n t e r e s t are apparent d i f f e r e n c e s w i t h r e s p e c t t o energy metabolism, growth, and c o n d i t i o n between B.C. mussels a n d . t h e i r much-studied European c o u n t e r p a r t s . These d i f f e r e n c e s may be important to the success of mussel c u l t u r e o p e r a t i o n s i n B.C.. A primary d i f f e r e n c e between energy storage i n mussels c u l t u r e d i n the present study and t h a t r e p o r t e d f o r European p o p u l a t i o n s r e l a t e s t o the c y c l i n g of glycogen. In the present study, glycogen was r e p r e s e n t e d i n the measure of t o t a l carbohydrate. Since the m a j o r i t y of carbohydrate i n mussels i s s t o r e d as glycogen, and s i n c e p r a c t i c a l l y a l l other carbohydrates are accounted f o r as b l o o d sugars (whose l e v e l r e p r e s e n t s the balance between m o b i l i s a t i o n of glycogen from the d i g e s t i v e gland and mantle and u t i l i s a t i o n o f glucose by body t i s s u e s ) , the o b t a i n e d measure of t o t a l carbohydrate was thought t o p r o v i d e a u s e f u l assessment of carbohydrate energy r e s e r v e . In support o f t h i s , Dare and Edwards (1975) r e p o r t e d t h a t v a l u e s f o r glycogen content i n Mytilus edulis were i n c l o s e agreement with i n d i r e c t estimates o f t o t a l carbohydrate. In European p o p u l a t i o n s of M y t i l u s edulis d e s c r i b e d by Dare and Edwards (1975), P i e t e r s et al. (1979), Zurburg et al. (1979), and Zandee et al. (1980)/ glycogen was lowest i n A p r i l , rose i n A p r i l - J u n e t o 30-40% dry weight, and remained at t h i s l e v e l b e f o r e d e c l i n i n g s l o w l y t o an A p r i l low i n the f o l l o w i n g year. In B.C. p o p u l a t i o n s , both i n the present study and those of Emmett (1984) and Emmett et al. (1987), glycogen was again lowest l a t e winter, rose s t e e p l y i n s p r i n g r e a c h i n g 15-30% dry weight, remained e l e v a t e d through summer, and then began a steep d e c l i n e w i t h some s l i g h t c o u n t e r a c t i o n i n October. Thus, where European mussels a p p a r e n t l y spend 5 months at 30-40% glycogen content b e f o r e s l o w l y expending t h e i r glycogen s t o r e ( l i k e l y d u r i n g gametogenesis: Bayne et a l . , 1975; Bayne et al., 1982; Peek et al., 1989), B.C. mussels peak at only 15-30% glycogen content and maintain t h i s l e v e l f o r only 3 months b e f o r e r a p i d l y exhausting t h e i r s t o r e i n August, months b e f o r e the onset of gametogenesis (Emmett et al., 1987 r e p o r t e d t h i s as b e g i n n i n g i n December). The e a r l y d e c l i n e and r e s u l t i n g extreme low of carbohydrate content e x h i b i t e d by B.C. mussels i s not i n d i c a t i v e of a l a c k of carbohydrate metabolism d u r i n g autumn and winter, nor of an i n a b i l i t y t o undergo gametogensis because energy r e s e r v e s are i n s u f f i c i e n t . Rather, i t may be t h a t a carbohydrate r e s e r v e i s not accumulated because the energy a c q u i r e d through autumn-winter f e e d i n g i s used up i n maintenance requirements and gametogenesis. R e c a l l t h a t w i n t e r mussels i n the present study ( F i g . 7) d e p l e t e d t h e i r carbohydrate content without changing dry t i s s u e weight or p r o t e i n content. At t h i s time they were l i k e l y producing gametic t i s s u e s (Emmett et a l . (1987) have a l s o r e p o r t e d gametogenic a c t i v i t y i n mussels d u r i n g autumn-winter). T h i s p a t t e r n c o n t r a s t s w i t h t h a t of energy accumulation and a l l o c a t i o n noted f o r European mussels. In these mussels, u s u a l l y the e n t i r e body i s i n v o l v e d i n accumulation of glycogen d u r i n g summer (de Zwaan and Zandee, 1972) and t h i s energy r e s e r v e i s h e a v i l y r e l i e d upon to meet met a b o l i c demands d u r i n g t h i s season (Bayne, 1973). Then, i n autumn-winter, remaining energy r e s e r v e s (glycogen and p r o t e i n ) are converted t o the l i p i d and p r o t e i n c o n s i t u e n t s of the gametes (Gabbott, 1975; P i e t e r s et al., 1979; Zandee et al., 1980; Kluytmans et a l . , 1985). .Thus, d e s p i t e n o t a b l e d i f f e r e n c e s i n seasonal c y c l e s of energy r e s e r v e s , B.C. and European mussels engage i n gametogenesis and spawning at about the same time. T h i s s i m i l a r i t y i n t i m i n g i s not s u r p r i s i n g s i n c e mussels are capable of wide adjustments i n t h e i r spawning times. By v i r t u e of t h e i r a b i l i t y t o a l t e r gametogenic r a t e (Bayne, 1975) and spawning p a t t e r n (Newell et al., 1982) i n response to environmental c o n d i t i o n s , mussels can a p p a r e n t l y synchronise t h e i r r e p r o d u c t i o n w i t h optimal c o n d i t i o n s f o r l a r v a l growth and s u r v i v a l . Reproductive s t r a t e g y i s known to be a p l a s t i c c h a r a c t e r i s t i c i n mussels (Lowe et al., 1982; Newell et al., 1982; Bayne et al., 1983; Rodhouse et al., 1984; Hawkins et al., 1985). These authors r e p o r t e d t h a t most p o p u l a t i o n s of Mytilus edulis e x h i b i t e d a " c o n s e r v a t i v e " r e p r o d u c t i v e s t r a t e g y where energy storage products were f i r s t accumulated then expended i n gametogenesis but t h a t i n c r e a s e d food q u a l i t y and q u a n t i t y , as w e l l as i n c r e a s e d age, l e d t o a change t o an " o p p o r t u n i s t i c " s t r a t e g y where gametogensis was f u e l e d d i r e c t l y by n u t r i e n t i n t a k e . B.C. mussels, however, seem t o e x h i b i t a more o p p o r t u n i s t i c s t r a t e g y i n t h a t energy storage seems t o be 78 synchronous w i t h gametogenesis (carbohydrate content i n c r e a s e d i n May-June which i s g e n e r a l l y a time of spawning f o r B.C. mussels). A comparison of B.C. mussels with t h e i r European c o u n t e r p a r t s w i t h r e s p e c t t o p r o t e i n ( reported t o be the second most important energy storage product: Gabbott, 1983), shows l e s s d i f f e r e n c e than noted f o r carbohydrate. In European p o p u l a t i o n s , p r o t e i n content f l u c t u a t e s over the course of the year (40-60%, Thompson et al., 1974; 45-75%, Dare and Edwards, 1975; and 30-52%, Zandee et al., 1980). P r o t e i n content o f B.C. mussels showed s i m i l a r f l u c t u a t i o n s (36-54% i n Departure Bay and 40-54% i n Genoa Bay, present study; 33-65%, Emmett, 1984) . Thus, both degree of v a r i a t i o n and a c t u a l v a l u e of p r o t e i n content are s i m i l a r i n European and l o c a l mussels. The c y c l i n g of energy r e s e r v e s has an obvious i n f l u e n c e on growth and, through t h i s , on c o n d i t i o n index. There seems t o be a common p a t t e r n i n both Europe and B.C.. C o n d i t i o n index, l i k e glycogen content, remains h i g h f o r a longer p e r i o d a f t e r i t s i n c r e a s e i n the s p r i n g ( A p r i l ) i n European mussels than i s the case i n B.C. ones. In f a c t , Zandee et al. (1980), i n a study of mussels i n the Dutch Wadden Sea, recorded h i g h e s t c o n d i t i o n i n d i c e s i n J u l y and August. In c o n t r a s t , mussels c u l t u r e d i n Departure Bay and Genoa Bay had lowest c o n d i t i o n i n d i c e s i n l a t e summer. Regardless of geographic l o c a t i o n , temperate mussels are g e n e r a l l y i n poorest c o n d i t i o n d u r i n g F e b r u a r y - A p r i l . At t h i s time, European mussels are completing gametogenesis (Bayne et al., 1975; Bayne et al., 1982; Peek et al., 1989) as are B.C. ones (Emmett et al., 1987). Thus,.to summarise, B.C. mussels d i f f e r from those o f Europe i n t h a t they e x h i b i t two lows i n c o n d i t i o n index: one o c c u r r i n g i n e a r l y s p r i n g , presumably concurrent w i t h gametogenesis, and a second i n l a t e summer, concurrent w i t h h i g h m o r t a l i t y . Notwithstanding the extended p e r i o d of low carbohydrate content, poor c o n d i t i o n index, and s i m i l a r p r o t e i n content, B.C. mussels have a f a s t e r growth r a t e than t h e i r c o u n t e r p a r t s i n temperate European waters. Mussels c u l t u r e d i n Departure Bay and Genoa Bay reached market s i z e (50 mm) i n 14 months. T h i s compares f a v o u r a b l y w i t h 8-15 months r e q u i r e d i n Spain (Korringa, 197 6) and 12-14 months r e q u i r e d i n New England (Incze and Lutz, 1980), s l i g h t l y improves on the "two summer seasons" r e q u i r e d i n most of the North Sea (Wallace, 1980) as w e l l as A t l a n t i c Canada (Incze and Lutz, 1980), and i s much f a s t e r than the "3-6 summer seasons" r e q u i r e d w i t h i n the A r c t i c c i r c l e (Wallace, 1980; Theisen, 1973). Such d i f f e r e n c e s must, t o some extent, be due t o geographic v a r i a t i o n s i n seasonal temperature, s a l i n i t y , and a v a i l a b i l i t y o f food; nonetheless, i t remains c l e a r t h a t , d e s p i t e short-comings i n terms o f energy r e s e r v e s , B.C. mussel c u l t u r e compares f a v o u r a b l y with t h a t i n other areas of the world: the time t o ha r v e s t i s one of the s h o r t e s t anywhere. Market s i z e f o r c u l t u r e d mussels i s d e f i n e d by s h e l l l e n g t h , and growth-rate data are most o f t e n p r e s e n t e d as i n c r e a s e i n s h e l l l e n g t h over time. However, s i n c e r e l a t i v e t i s s u e weight governs the market q u a l i t y o f mussels, i t i s important t o know 80 the r e l a t i o n s h i p of these two parameters. H i l b i s h (1986) has i n d i c a t e d a h i g h degree of c o r r e l a t i o n between monthly i n c r e a s e i n dry t i s s u e weight and s h e l l l e n g t h (r 2=0.97) i n A t l a n t i c mussels, as has the present study f o r B.C. mussels (r 2=0.82). From t h i s i t i s apparent t h a t growth r a t e can be e q u a l l y w e l l -d e f i n e d by dry t i s s u e weight or s h e l l l e n g t h . However, t h i s h i g h c o r r e l a t i o n must be q u a l i f i e d i n l i g h t o f o b s e r v a t i o n s of uncoupled growth of t i s s u e and s h e l l (Kautsky, 1982; a l s o present s t u d y ) . Kautsky (1982) r e p o r t e d t h a t t i s s u e growth sometimes preceded s h e l l growth i n B a l t i c Sea p o p u l a t i o n s o f Mytilus edulis, w h i l e the p r e s e n t study showed t h a t s h e l l growth . sometimes preceded t i s s u e growth ( f o r example, O c t o b e r - A p r i l mussels i n Departure Bay i n c r e a s e d from 32-43 mm without a concommitent r i s e i n dry t i s s u e weight, and over the same p e r i o d Genoa Bay mussels i n c r e a s e d from 28-40 mm without a s i g n i f i c a n t change i n dry t i s s u e weight). D e s p i t e t h i s q u a l i f i c a t i o n , which may p e r t a i n only i n autumn-winter, the p o i n t remains t h a t , based on r a t e of i n c r e a s e i n s h e l l l e n g t h and the c o r r e l a t i o n between s h e l l l e n g t h and dry t i s s u e weight, B.C. mussels grow f a s t e r than mussels s t u d i e d and c u l t u r e d i n Europe. Another d i f f e r e n c e between European and B.C. mussels which r e l a t e s t o c u l t u r e , i s t h a t European mussels i n c r e a s e i n dry t i s s u e weight i n s p r i n g and summer, and then l o s e t i s s u e i n autumn and winter, but without s u f f e r i n g the h i g h m o r t a l i t y seen i n B.C. p o p u l a t i o n s . Mussels i n the present study i n c r e a s e d i n dry t i s s u e weight i n s p r i n g and began to waste i n l a t e summer, s u f f e r i n g about 50% m o r t a l i t y by summer's end. The magnitude of 81 t i s s u e l o s s was s i m i l a r i n both European and B.C. p o p u l a t i o n s , but h i g h m o r t a l i t y seems s p e c i f i c t o the P a c i f i c n o r t h e a s t and r e p r e s e n t s a s e r i o u s o b s t a c l e f o r c u l t u r e i n t h i s r e g i o n . With r e s p e c t t o t i s s u e l o s s and m o r t a l i t y , Kautsky (1982) r e p o r t e d t h a t Af. edulis i n the B a l t i c Sea r o u t i n e l y s u r v i v e d 30-50% l o s s of dry t i s s u e weight (an extreme i n d i v i d u a l s u r v i v e d 78% l o s s ) and Bayne and W o r r a l l (1980) r e p o r t e d t h a t recovery from a 25% l o s s of t i s s u e s was r o u t i n e f o r 3 - y r - o l d mussels i n the E n g l i s h Channel, while 6 - y r - o l d mussels r e g u l a r l y r e covered from a 68% l o s s . In comparison, Departure Bay mussels l o s t 43% of t h e i r dry t i s s u e weight and recovered p o o r l y (50% m o r t a l i t y ) , while Genoa Bay mussels showed almost as poor a recovery (40% m o r t a l i t y ) from a 29% l o s s of t i s s u e . Some of t h i s l o s s r e p r e s e n t s the emission of gametes, but another p o r t i o n must owe t o m o b i l i s a t i o n of non-r e p r o d u c t i v e t i s s u e t o meet energy demands of maintenance metabolism. Such m o b i l i s a t i o n f i r s t becomes necessary d u r i n g spawning i n s p r i n g and c a r r i e s through t o autumn. Mussels cease f i l t r a t i o n d u r i n g spawning which u s u a l l y spans a 3-4 wk p e r i o d ( C h i p p e r f i e l d , 1953). T h i s " f a s t i n g " n e c e s s i t a t e s m o b i l i s a t i o n o f body t i s s u e s s i n c e i t occurs when carbohydrate r e s e r v e i s at a low and metabolism i s h i g h ( i n c r e a s e d temperature i n summer induces m e t a b o l i c s t r e s s i n mussels: Widdows and Bayne, 1971; Bayne et al., 1978; Incze and Lutz, 1980). Therefore, mussels l i k e l y e n t e r a p e r i o d of n e g a t i v e "scope f o r growth" d u r i n g spawning, a s t a t e which continues through summer and which p o s s i b l y imposes m o r t a l i t y such as t h a t observed i n the present study. Summer m o r t a l i t y i s well-documented f o r mussels i n the nort h e a s t P a c i f i c (Quayle, 1978; He r i t a g e , 1983; Emmett, 1984; Skidmore and Chew, 1985; Emmett et a l . , 1987), the northwest A t l a n t i c (Incze et al., 1980), and the nor t h e a s t A t l a n t i c (Worrall and Widdows, 1984). Summer m o r t a l i t y was i n v e s t i g a t e d by W o r r a l l and Widdows (1984) i n a mussel p o p u l a t i o n from the E n g l i s h Channel which e x h i b i t e d reduced scope f o r growth f o l l o w i n g s p r i n g spawning. Non-predatory m o r t a l i t y peaked one month a f t e r spawning when me t a b o l i c c o s t s were h i g h and energy r e s e r v e s were at a minimum. There may have been a c o r r e l a t i o n between i n c i d e n c e of m o r t a l i t y and r e p r o d u c t i v e e f f o r t i n t h i s p o p u l a t i o n , s i n c e mussels i n l a r g e r s i z e c l a s s e s e x h i b i t e d both h i g h e r m o r t a l i t y and h i g h e r r e p r o d u c t i v e e f f o r t . In another i n v e s t i g a t i o n by Incze and Lutz (1980), m o r t a l i t i e s on r a f t -c u l t u r e d M. edulis i n Maine were thought t o have been caused by reduced food r a t i o n d u r i n g m e t a b o l i c s t r e s s caused by h i g h temperatures. In the present study, summer m o r t a l i t i e s o c c u r r e d when the mussels* mean carbohydrate content was at an i n t e r m e d i a t e l e v e l . Thus, i t appeared t h a t mussels d i e d d e s p i t e b e i n g e n e r g e t i c a l l y competent. T h i s prompted q u e s t i o n i n g of the r e l a t i o n s h i p between energy r e s e r v e s and energy budgets i n mussels at the time of worst m o r t a l i t y i n August, as compared with A p r i l when carbohydrate r e s e r v e s were lowest and m o r t a l i t y was i n s i g n i f i c a n t . A comparison of A p r i l and August mussels i n t h i s r e g a r d was made by combining p h y s i o l o g i c a l c h a r a c t e r i s t i c s and environmental parameters, some measured i n t h i s study and some 83 taken from the l i t e r a t u r e (data shown i n Table 1). The main p o i n t s of i n t e r e s t from the t a b l e are: 1) mussels c u l t u r e d at the 3-m s t a t i o n of both s i t e s possessed a p o t e n t i a l l y l o n g e r - l a s t i n g carbohydrate r e s e r v e i n A p r i l as compared w i t h August, 2 ) August mussels e x h i b i t e d a n e g a t i v e scope f o r growth (with energy d e f i c i t s o f 2-3 k J ' h - 1 ) , but i n A p r i l Departure Bay mussels had a p o s i t i v e scope f o r growth, and Genoa Bay mussels were only m a r g i n a l l y n e g a t i v e i n t h i s regard, and 3) t h e r e f o r e , although the mussels had a g r e a t e r p r o p o r t i o n of carbohydrate i n August r e l a t i v e t o A p r i l , they were e n e r g e t i c a l l y d e f i c i e n t f o r reasons o f i n c r e a s e d metabolism due to h i g h temperature and p o s s i b l y low food c o n c e n t r a t i o n . The r e s u l t s of the present study thus acc o r d w i t h those of W o r r a l l and Widdows (1984), and Incze et al. (1980), who a t t r i b u t e d summer m o r t a l i t y i n E n g l i s h Channel and northwest A t l a n t i c mussels, r e s p e c t i v e l y , t o energy d e f i c i e n c y . However, summer m o r t a l i t y c r i p p l e s the p r o d u c t i o n of mussels i n B.C. t o a much g r e a t e r extent than i n e i t h e r A t l a n t i c r e g i o n . Another d i f f e r e n c e between l o c a l and European mussels r e l a t e s t o maximum s i z e : M. edulis c u l t u r e d i n B.C. r a r e l y exceed 60 mm (Heritage, 1983; Emmett et al., 1987), but i n the northwest A t l a n t i c commonly reach 70 mm (Freeman and D i c k i e , 1979), and on the A t l a n t i c coast of Spain a t t a i n 80-90 mm (Korringa, 1976). In Spanish waters i n c r e a s e d growth may be due t o warmer temperatures, but the northwest A t l a n t i c and no r t h e a s t 84 Table 1. Energy r e s e r v e s and energy budgets of mussels c u l t u r e d i n Departure Bay and Genoa Bay at the 3-m s t a t i o n . Data — p r e s e n t e d are f o r mussels of the mean weights and carbohydrate contents f o r A p r i l and August 1989. 8 5 Culture Characteristic Total dry tissue weight (mg) CHO content (% dry wt) , Equivalent CHO reserve (kJ) Water temperature (°C) Seston concentration (ng/L) Energy Metabolism Estimated V 0 2 (ml 02/h) 2 -Longevity of CHO reserve (h) Energy Budget Departure Bay April August Genoa Bay April August 149 4.8 0.117 10 13.4 37 1.00 360 15.5 0.934 17 4.4 283 0.44 173 3.2 0.927 10 11.4 43 0.59 480 13.5 1.085 17 5.4 354 0.31 ENERGY IN (kJ/h) ENERGY OUT (kJ/h)3 SCOPE FOR GROWTH (IN - OUT, kJ/h) 0.137 0.112 0.025 0.135 2.147 -2.012 0.115 0.158 -0.043 0.194 3.561 -3.367 1 The e n e r g y c o n t e n t o f known w e i g h t o f c a r b o h y d r a t e r e s e r v e was c a l c u l a t e d f r o m a g e n e r a l i z e d h e a t o f c o m b u s t i o n o f c a r b o h y d r a t e s , 1.68 k J - k g - 1 ( C h u r c h a n d P o n d , 1982). 2 V0 2 was c a l c u l a t e d u s i n g summer a n d w i n t e r e q u a t i o n s f o r o x y g e n c o n s u m p t i o n b a s e d on d r y t i s s u e w e i g h t f r o m B a y n e (1973) c o r r e c t e d t o t h e a p p r o p r i a t e t e m p e r a t u r e u s i n g Q^ Q v a l u e s f r o m Widdows (1973). 3 T h e s e v a l u e s w e r e c a l c u l a t e d u s i n g a g e n e r a l i z e d o x y c a l o r i f i c c o n v e r s i o n f o r c a r b o h y d r a t e o f 20.9 k J - L 0 2 - 1 ( C h u r c h a n d P o n d , 1982). * V a l u e s f o r "ENERGY I N " a r e d e r i v e d f r o m t h e p r e s e n t d a t a on s e s t o n c o n c e n t r a t i o n c o n v e r t e d t o e s t i m a t e d e n e r g y c o n t e n t f r o m d a t a i n Widdows et al. (1979), a n d c a l c u l a t e d f r o m e s t i m a t e s o f a b s o r p t i o n e f f i c i e n c y a t v a r y i n g f o o d c o n c e n t r a t i o n a n d o f f i l t r a t i o n r a t a b a s e d on a n i m a l s i z e f r o m d a t a i n Widdows (1978). 5 V a l u e s f o r "ENERGY OUT" a r e c a l c u l a t e d u s i n g V0 2 v a l u e s f r o m g e n e r a l i z e d o x y c a l o r i f i c c o n v e r s i o n , f o r m u l a t e d f o r M. edulis e a t i n g a m i x e d a l g a l d i e t (Widdows a n d H a w k i n s , 1989) o f 20.1 k J - L 0 2 _ 1 . 86 P a c i f i c are s i m i l a r i n temperature. Thus, v a r i a b i l i t y i n growth i s not caused by temperature alone. Some of the i d e n t i f i e d d i f f e r e n c e s between European and B.C. Mytilus edulis (Heritage, 1983; Emmett et al., 1987; Swarbrick et al., 1990; present study) may r e l a t e t o d i f f e r e n c e s i n l i f e h i s t o r y s t r a t e g y . In comparison t o M. edulis i n E u r o p e , . l o c a l p o p u l a t i o n s seem t o have a s h o r t e r l i f e span and h i g h e r r e p r o d u c t i v e output at an e a r l i e r age (gonad development occurs as e a r l y as 1-2 months a f t e r settlement i n l o c a l mussels: Suchanek, 1981) . Many p o i n t s of d i s t i n c t i o n between p o p u l a t i o n s of Mytilus edulis i n B.C. and other geographic regions have been made; such d i f f e r e n c e s may be c r i t i c a l t o the o v e r a l l success of l o c a l c u l t u r e o p e r a t i o n s . Notably, p r o d u c t i o n of M. edulis i n B.C. seems p o t e n t i a l l y more l i m i t e d by summer m o r t a l i t y , and the r e l a t i o n s h i p between spawning and seasonal changes i n energy r e s e r v e s i n l o c a l p o p u l a t i o n s i s a l s o d i s t i n c t (Emmett et al., 1987). 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