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Water circulation, dissolved oxygen, and ammonia concentrations in fish-net cages Gormican, Stephen Joseph 1989

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Water c i r c u l a t i o n , dissolved oxygen, and ammonia concentrations i n f i s h net-cages By STEPHEN JOSEPH GORMICAN B.Sc, University of B r i t i s h Columbia, 1980 A THESIS SUBMITTED IN PARTIAL'FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTERS OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 198 9 © Stephen Joseph Gormican, 1 9 8 9 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. Department The University of British Columbia Vancouver, Canada DE-6 (2/88) ( i i A b s t r a c t F i s h farming i n the p r o t e c t e d waters of B r i t i s h Columbia i s r e l a t i v e l y new, but has undergone a phenomenal growth i n the l a s t ten years. L i t t l e i n v e s t i g a t i o n has been reported w i t h respect t o c o n d i t i o n s w i t h i n the net-cages employed i n growing salmon at f i s h farms. In p a r t i c u l a r , the r o l e of water q u a l i t y and water exchange has not been examined i n r e l a t i o n to l o c a l m a r i c u l t u r e husbandry p r a c t i c e s and hydrography. The f i r s t part of t h i s study compared water q u a l i t y and water flow i n two l o c a t i o n s , one i n J e r v i s I n l e t w i t h a deep entrance s i l l and the- other i n Sechelt I n l e t which has a shallow entrance s i l l . Marked v a r i a t i o n s i n hydrography occurred between the two s i t e s as a r e s u l t of the d i f f e r e n c e s i n s i l l depth. An i n t e r n a l wave generated at the Sechelt I n l e t s i l l caused d a i l y f l u c t u a t i o n s i n s t r a t i f i c a t i o n and hence water p r o p e r t i e s w i t h i n the net-cages. No such v a r i a t i o n s were observed at the J e r v i s I n l e t s i t e . In the second part of t h i s study, water q u a l i t y and water flow was_ measured i n various l o c a t i o n s i n and near a r a f t of 2 4 net-cages. Generally, i t was found that w i t h i n the r a f t , water flow was diminished i n those cages l o c a t e d downstream of the predominate flow d i r e c t i o n . However, l o c a l topography was thought to have caused marked v a r i a t i o n i n water q u a l i t y and water exchange patterns i n two of the cages. Ammonia concentrations were not observed to exceed reported s u b l e t h a l concentrations at any time, over a 25 h per i o d , at any of the depths sampled, w i t h i n the net-cages. D i s s o l v e d oxygen concentrations d i d , at some depths and times, approach values at which some s t r e s s may be f e l t due to low oxygen. Lin e a r regressions between water q u a l i t y and water speed were not found t o be s i g n i f i c a n t i n most cases. The c o e f f i c i e n t of determinations were low, i n d i c a t i n g that current speed accounted f o r l e s s than 27% of the v a r i a t i o n i n water q u a l i t y . i v Table of Contents Abs t r a c t . . i i Table of contents i v L i s t of t a b l e s v L i s t of f i g u r e s v i Acknowledgements v i i i General I n t r o d u c t i o n 1 Chapter 1 Water q u a l i t y and water exchange at two f i s h farms l o c a t e d i n J e r v i s and Sechelt I n l e t s , B.C 7 Int r o d u c t i o n 7 M a t e r i a l s and Methods 10 Results and Disc u s s i o n 18 Conclusions 38 Chapter 2 Net-cage f i s h farming i n B r i t i s h Columbia: A case study of water c i r c u l a t i o n and water q u a l i t y i n a larg e r a f t of cages..... 39 Int r o d u c t i o n 39 M a t e r i a l s and Methods 40 Results 42 Dis c u s s i o n , 50 Conclusions .55 General D i s c u s s i o n 56 Bib l i o g r a p h y 59 L i s t of Tables 1. Comparisons of t o t a l ammonia i n s i d e cages at the JER and SEC s i t e s . Data were pooled f o r each depth and l o c a t i o n using values c o l l e c t e d over the 25 h sampling programs. Order of means i s as presented under the Comparison heading. (SD=standard d e v i a t i o n , df=degrees of freedom.... 23 2. Comparisons of DO, t o t a l ammonia, and current speed i n s i d e and outside net-cages at the JER and SEC s i t e s . Data were pooled f o r each depth and l o c a t i o n using values c o l l e c t e d over the 25 h sampling program. (SD=standard d e v i a t i o n , df=degrees of freedom) 26 3. Comparisons of DO and t o t a l ammonia i n s i d e cages 23 and 15 at the JER s i t e . Data were pooled f o r each depth and l o c a t i o n using values c o l l e c t e d over the 25 h sampling program. (SD=standard d e v i a t i o n , df=degrees of freedom)..48 v i L i s t of Figures Locations of study s i t e s . The J e r v i s I n l e t s i t e i s designated by a (1) and the Sechelt I n l e t s i t e by a (2) . . > 11 Plan views of the, A) J e r v i s I n l e t (JER) and B) Sechelt I n l e t s i t e s . S t a t i o n s at which water q u a l i t y samples were c o l l e c t e d and p r o f i l i n g performed, are designated by (O)• Current meters were deployed at l o c a t i o n s i n d i c a t e d by (A) . See t e x t f o r d e t a i l s of deployments and sampling p r o t o c o l 12 P l o t s of A) temperature and B) Sigma-t (density - see t e x t f o r explanation) versus time f o r the i n d i c a t e d depths sampled at the J e r v i s I n l e t farm s i t e . Temperature data was c o l l e c t e d hourly using the Hydrolabs instrument at the s t a t i o n outside of cage 23. Sigma-t data was c o l l e c t e d using Interocean S4 current meters suspended simultaneously outside cage 19 (Figure 2) .19 P l o t s of A) temperature and B) Sigma-t (density) versus time f o r the i n d i c a t e d depths at the Sechelt I n l e t farm s i t e . Data was c o l l e c t e d using the Hydrolabs instrument t o p r o f i l e the water column i n s i d e cage 6. Each p o i n t i s a s i n g l e sample - see t e x t f o r e r r o r e s t i m a t i o n 20 P l o t s of A) ammonia versus time and B) current speed f o r the i n d i c a t e d sampling depths at the J e r v i s I n l e t s i t e . S t i p l e d area i n d i c a t e s time of feeding. Each ammonia data p o i n t i s a s i n g l e sample - see t e x t f o r e r r o r e s t i m a t i o n . Current speeds are hourly averages of 6 two-minute sampling episodes 21 P l o t s of A) ammonia versus time and B) current speed f o r the i n d i c a t e d sampling depths at the Sechelt I n l e t s i t e . S t i p l e d area i n d i c a t e s time of feeding. Each ammonia data p o i n t i s a s i n g l e sample - see t e x t f o r e r r o r e s t i m a t i o n . Current speeds are hourly averages of 6 sampling episodes 24 S c a t t e r diagram of t o t a l ammonia versus current speed f o r data c o l l e c t e d at mid-cage depth (6 m) of cage 23 at the J e r v i s I n l e t s i t e . The l i n e i s the l e a s t squares r e g r e s s i o n l i n e w i t h c o e f f i c i e n t of determination (r^) i n d i c a t e d . A t - s t a t i s t i c was used t o t e s t s i g n i f i c a n c e , (p) 28 v i i 8. S c a t t e r d iagram o f t o t a l ammonia v e r s u s c u r r e n t speed f o r d a t a c o l l e c t e d at the mid-cage depth (2 m) i n cage 6 at the S e c h e l t I n l e t s i t e . The l i n e i s the l e a s t squares r e g r e s s i o n l i n e w i t h c o e f f i c i e n t o f d e t e r m i n a t i o n (r^) i n d i c a t e d . A t - s t a t i s t i c was used to t e s t s i g n i f i c a n c e (p) , 29 9. P l o t s o f d i s s o l v e d oxygen v e r s u s t ime f o r the i n d i c a t e d sampl ing depths at the J e r v i s I n l e t s i t e . Each d a t a p o i n t i s a s i n g l e sample - see t e x t f o r e r r o r e s t i m a t i o n 31 10. P l o t s o f d i s s o l v e d oxygen v e r s u s t ime f o r the i n d i c a t e d sampl ing depths at the S e c h e l t I n l e t s i t e . Each d a t a p o i n t i s a s i n g l e sample - see t e x t f o r e r r o r e s t i m a t i o n (p) . . . . ..33 11. S c a t t e r d iagram o f d i s s o v e d oxygen v e r s u s c u r r e n t speed f o r da ta c o l l e c t e d at mid-cage depth (6 m) o f cage 23 at the J e r v i s I n l e t s i t e . The l i n e i s the l e a s t squares r e g r e s s i o n l i n e w i t h c o e f f i c i e n t o f d e t e r m i n a t i o n (r^) i n d i c a t e d . A t - s t a t i s t i c was used t o t e s t s i g n i f i c a n c e 34 12. S c a t t e r d iagram o f d i s s o l v e d oxygen v e r s u s c u r r e n t speed f o r da ta c o l l e c t e d at the mid-cage depth (2 m) i n cage 6 at the S e c h e l t I n l e t s i t e . The l i n e i s the l e a s t squares r e g r e s s i o n l i n e w i t h c o e f f i c i e n t o f d e t e r m i n a t i o n (r^) i n d i c a t e d . A t - s t a t i s t i c was used to t e s t s i g n i f i c a n c e 35 13. P l o t o f c u r r e n t speed v e r s u s depth f o r the p r o f i l e exper iment at the J e r v i s I n l e t s i t e . E r r o r b a r s are one s t a n d a r d d e v i a t i o n ...43 14. V e c t o r diagrams o f d a t a c o l l e c t e d at the i n d i c a t e d l o c a t i o n s i n and around the r a f t o f n e t - c a g e s at the J e r v i s I n l e t s i t e . Depth o f in s t rument deployment was 6 m f o r a d u r a t i o n o f 25 h . Data was c o l l e c t e d s i m u l t a n e o u s l y at each l o c a t i o n . The l e n g t h o f each v e c t o r i s p r o p o r t i o n a l t o c u r r e n t speed, (1 d i v i s i o n = l c m / s e c ) , w h i l e the o r i e n t a t i o n i n d i c a t e s f low d i r e c t i o n . F i g u r e s above diagrams are average speed and ( d i r e c t i o n ) f o r the 25 h sampl ing p e r i o d i l l u s t r a t e d i n each v e c t o r d i a g r a m . . .45 15. Comparisons o f d i s s o l v e d oxygen c o n c e n t r a t i o n s v e r s u s t ime f o r the i n d i c a t e d depths i n cage 23 (O) and cage 15 (•) a t the J e r v i s I n l e t s i t e . . . 47 16. Comparisons o f ammonia c o n c e n t r a t i o n s v e r s u s t ime f o r cages 23 & 15 at the J e r v i s I n l e t s i t e . Depth o f i sampl ing was A) 0.5 m and B) 6 m 4 9 Acknowledgements I would l i k e to thank a l l my committee members f o r t h e i r i n v a l u a b l e advice throughout t h i s p r o j e c t . S p e c i a l thanks goes to Dr. S. Pond f o r as s i s t a n c e i n the f i e l d under l e s s than i d e a l c o n d i t i o n s . Mr. E. Black a l s o played an in v a l u a b l e r o l e i n o b t a i n i n g equipment and funding. My supervisor, Dr. A.G. Lewis provided support throughout t h i s p r o j e c t . F i e l d work would not have been p o s s i b l e without the extremely competent Mr. Jay McNee, and so to him goes a s p e c i a l word of g r a t i t u d e . Anna Metaxas provided encouragement and words of wisdom - always. The Skei f a m i l y , at Sechelt Salmon Farms L t d . , were very k i n d i n p r o v i d i n g accommodation and access to t h e i r farm. I thank Dora Glover at the Sunshine Coast Aquaculture A s s o c i a t i o n f o r everything she d i d to a i d my research. Roderick Farquarson and h i s fa m i l y helped to make the 1988 f i e l d season bearable. A l l the people at Hardy Sea Farms wit h whom I had contact were very h e l p f u l . The N a t i o n a l Research Council (NRC) provided me with two I n d u s t r i a l Research A s s i s t a n t s h i p s (IRAP-H's) to help keep me s h e l t e r e d and fed over the two summers of my program. The Department of Zoology provided teaching a s s i s t a n t s h i p s f o r which I am extremely g r a t e f u l , not j u s t f o r the f i n a n c i a l b e n e f i t s , but al s o f o r the opportunity to gain an a p p r e c i a t i o n of the teaching s k i l l s necessary f o r any academic p u r s u i t , which are so l o s t i n today's p u b l i s h or p e r i s h a t t i t u d e . Last but not l e a s t , I thank Maureen Mann, f o r her patience and support during some very t r y i n g times. 1 General I n t r o d u c t i o n Salmon farming i n net-cages i n the c o a s t a l waters of B r i t i s h Columbia i s a young i n d u s t r y . Cage farming has been p r a c t i c e d i n A s i a and other p a r t s of the world f o r decades (Beveridge, 1984) however, l o c a l p r o l i f e r a t i o n has occurred w i t h i n the l a s t 10-15 years. Black and Carswell (1986) report that e i g h t farms produced 107 tonnes of salmon i n 1985. By 1988, 125 f i s h farms produced over 6,500 tonnes of salmon (Anonymous, 1989) and the B.C. Salmon Farmers A s s o c i a t i o n p r o j e c t s an annual production of over 15,000 tonnes by 1995 (Black and C a r s w e l l , 1986). Rapid growth of the i n d u s t r y has allowed l i t t l e time to assess the e f f e c t of t h i s i n d u s t r y on the environment, nor has there been time to research husbandry p r a c t i c e s f o r l o c a l c o n d i t i o n s and c u l t u r e d species. M a r i c u l t u r e i s known to have a negative impact on the waters i n which i t i s l o c a t e d by lowering d i s s o l v e d oxygen (DO) concentrations and i n c r e a s i n g ambient n u t r i e n t concentrations. In Japan, DO concentrations were observed to have been lowered by as much as 0.5 mg/L by a s i n g l e cage stocked at a d e n s i t y of 22 kg/m^ (Rosenthal et a l . , 1988). In Usui Bay, Japan, s t o c k i n g d e n s i t i e s had to be reduced despite t i d a l currents of 3-4 knots (=154-206 cm/sec) because DO concentrations were f a l l i n g below c r i t i c a l l i m i t s . Oxygen concentrations were reported to vary markedly, w i t h lowest values reported to occur during s l a c k t i d e s and feeding p e r i o d s . Over time the s i t u a t i o n became worse as waste feed and feces increased the b i o l o g i c a l oxygen demand. Ammonia concentrations i n net-cages must al s o be considered due to p o t e n t i a l t o x i c i t y . Ammonia i n the marine environment may e x i s t i n two forms, un-ionized ammonia ( N H 3 ) or ammonium ion (NH^4-) . T o t a l ammonia, the sum of these two forms ( N H 3 + N H 4 4 " ) , w i l l be r e f e r r e d to as ammonia throughout t h i s paper. The p r o p o r t i o n of un-ionized ammonia e x i s t i n g i n the environment i s dependent upon pH, temperature, and s a l i n i t y (Emerson et a l . , 1975). In the marine- environment, excreted N H 3 i s q u i c k l y converted to N H 4 , .'^ and may account f o r 0.01. to 15 percent of t o t a l ammonia' concentrations present (Bower and B i d w e l l , 1978) . F i s h excrete ammonia as the major end-product of p r o t e i n catabolism. Randall and Wright (1987) reviewed the mechanisms of ammonia formation and e x c r e t i o n i n f i s h and concluded that ca. 80% of ni t r o g e n e x c r e t i o n i s i n the form of ammonia. The g i l l s are the major s i t e of ammonia e x c r e t i o n , however the exact mechanism of t h i s process i s unclear. I t i s b e l i e v e d that u n - i o n i z e d ammonia (NH3) passes across the g i l l s by simple d i f f u s i o n processes, while ammonium ion (NH4 + ) ex c r e t i o n r e l i e s upon some so r t of ion exchange process. The t o x i c i t y of un-ionized ammonia to f i s h i s a r e s u l t of a r e v e r s a l of the i n t e r n a l / e x t e r n a l gradient which allows NH3 to re-enter the f i s h across the g i l l s . 3 I t i s a w e l l documented f a c t t h a t ammonia, i f allowed to b u i l d up i n the r e a r i n g waters, may lead to reductions i n stamina, growth r a t e , and r e s i s t a n c e to disease (Burrows, 1964, H i l l a b y and Randall, 1979 and Al a b a s t e r and Lloyd, 1982). These authors a l s o agree that s u b l e t h a l e f f e c t s of ammonia may be induced by long-term exposure to concentrations of >0.025 mg NH3/L ( = 1. 8 (Xmol/L) . The m a j o r i t y of these stud i e s have been conducted i n freshwater systems i n which water i s at l e a s t p a r t i a l l y r e c y c l e d , thereby a l l o w i n g a buildup of ammonia. Information on ammonia l e v e l s r e s u l t i n g from marine f i s h farms i s sparse (Gowen and Bradbury, 1987) . E v r i k et a l . (1985) report that ammonia concentrations around net-cages may be 8-9 times higher than normal l e v e l s . Conversely, Black and Carswell (1986) report that ammonia l e v e l s were v i r t u a l l y i d e n t i c a l at s i m i l a r distances downstream from a c t i v e and i n a c t i v e f i s h farms. Weston (1986) modelled the impact that a t h e o r e t i c a l 250 tonne f i s h farm, l o c a t e d i n the Puget Sound area, would have on the environment. He p r e d i c t e d a r i s e i n t o t a l ammonia concentration of 0.02 mg/L as w e l l as a decrease i n DO of 0.3 mg/L i n the water passing through the s i t e . Weston's (198 6) model i s a p p l i c a b l e t o the waters and c u l t u r e techniques used i n B.C. waters; however i t could be improved with the a d d i t i o n of e m p i r i c a l data c o l l e c t e d l o c a l l y . S u ccessful net-cage f i s h farming r e l i e s upon the flow of water through the cage w a l l s to maintain water q u a l i t y . 4 (For the purposes of t h i s study, water q u a l i t y w i l l be defined i n terms of DO and ammonia concentrations.) Water flow through net-cages may be l i m i t e d by a number of f a c t o r s . Inoue (1972) i n d i c a t e d that the t h i r d cage downstream, i n a s e r i e s of three cages o r i e n t e d p a r a l l e l to the predominant current flow, may experience o n e - t h i r d l e s s water exchange than the most upstream cage. F o u l i n g of the n e t t i n g by mussels and algae may reduce water exchange by as much as 80% (Wee, 1979 and Inoue, 1972) . Wee (1979) a l s o reported that increased time of submergence increases f o u l i n g , but that the rat e v a r i e d w i t h the seasons, and l i f e h i s t o r y of the f o u l i n g organisms. Kennedy et a l . (1977) found t h a t f i s h deaths r e s u l t e d when DO concentrations f e l l to 4.0 mg/L i n a cage h e a v i l y f o u l e d with "a mixture of s e s s i l e marine organisms, plus l i v i n g and dead plankton and d e t r i t u s " . A f t e r net replacement, DO l e v e l s rose to 8.25 mg/L i n the p r e v i o u s l y f o u l e d cage. Water q u a l i t y may vary over a 24 hour p e r i o d . Ammonia e x c r e t i o n e x h i b i t s a strong d i u r n a l p e r i o d i c i t y (McLean and Fraser, 1974 and B r e t t and Zala, 1975) with lowest values o c c u r r i n g before dawn and highest values about 4 h a f t e r the onset of feeding. R e s p i r a t i o n rates are highest j u s t p r i o r to , and during feeding, w i t h the lowest rates o c c u r r i n g at night (Brett and Zala, 1975). Local waters are subject to 4 sla c k t i d e occurrences i n a 25 h pe r i o d , which may f u r t h e r exacerbate ammonia buildup and DO d e p l e t i o n . 5 C l e a r l y there i s a need f o r a b e t t e r understanding of the i n t e r a c t i o n between the environment and ma r i c u l t u r e as p r a c t i c e d i n B r i t i s h Columbia. The b i o l o g y of the species reared, husbandry p r a c t i c e s employed, and hydrography are unique to l o c a l waters. The problems seen i n Japan may not a r i s e l o c a l l y as'our i n l e t s are much deeper than the Inland Sea of Japan. Conversely, shallow entrance s i l l s to many of our i n l e t s (Pickard, 1975) may r e s t r i c t water exchange or mixing processes to the point where near surface waters are rendered u n s u i t a b l e f o r the p r a c t i c e of m a r i c u l t u r e . The experiments discussed i n t h i s t h e s i s were designed to i n v e s t i g a t e the c o n d i t i o n of water q u a l i t y i n f i s h net-cages l o c a t e d i n the waters of B r i t i s h Columbia. In Chapter 1, a comparison i s made of water q u a l i t y i n two farm s i t e s , one l o c a t e d i n J e r v i s I n l e t which possesses a deep s i l l , and the other i n Sechelt I n l e t which has a shallow s i l l . I t was hypothesized that the presence of the shallower entrance s i l l would r e s u l t • i n marked v a r i a t i o n i n temperature and DO p r o f i l e s , r e l a t i v e to the J e r v i s s i t e . As the f l o o d t i d e progresses across the s i l l , a t u r b u l e n t j e t i s formed which e n t r a i n s waters below the s i l l depth (La z i e r , 1963). These below surface waters may reach anoxic concentrations, and hence c o n d i t i o n s i n the r e s u l t i n g mixed waters may be d e l e t e r i o u s to f i s h h e a l t h . Based on the reviewed l i t e r a t u r e , i t was al s o hypothesized t h a t , over a 25 h p e r i o d (one t i d a l c y c l e ) , ammonia concentrations and/or d i s s o l v e d oxygen concentrations would reach s u b l e t h a l 6 t h r e s h o l d s . A l s o examined was t h e r e l a t i o n s h i p between c u r r e n t speed and water q u a l i t y . I t was b e l i e v e d t h a t i n c r e a s e d c u r r e n t speed would f l u s h t h e net- c a g e s o f ammonia and would a l s o r e p l e n i s h DO c o n c e n t r a t i o n s . F o r m a l l y s t a t e d , t h i s h y p o t h e s i s p o s t u l a t e d t h a t water q u a l i t y ( [ N H 3 ] & [ O 2 ] ) i s d i r e c t l y r e l a t e d t o water exchange. In Chapter 2 t h e J e r v i s I n l e t s i t e was examined i n d e t a i l w i t h r e s p e c t t o w i t h i n s i t e water f l o w p r o p e r t i e s . T h i s farm i s a l a r g e r a f t o f 24 cages and i t was d e s i r e d t o g a i n c o n f i r m a t i o n o f Inoue's (1972) o b s e r v a t i o n s and t o determine i f t h e y a p p l i e d t o l o c a l c o n d i t i o n s . The biomass at t h i s s i t e was 260 to n n e s , which compares c l o s e l y t o Weston's (1986) h y p o t h e t i c a l farm biomass o f 250 t o n n e s . Thus, v e r i f i c a t i o n o f h i s hypotheses r e g a r d i n g t h e impact o f salmon f a r m i n g on t h e environment c o u l d be made. I t was h y p o t h e s i z e d t h a t water f l o w would be d i m i n i s h e d i n cages l o c a t e d downstream o f t h e predominant f l o w d i r e c t i o n r e l a t i v e t o t h o s e cages upstream. Water q u a l i t y was a l s o h y p o t h e s i z e d t o be lower i n cages l o c a t e d downstream o f t h e predominant f l o w d i r e c t i o n . Water exchange as a f u n c t i o n o f depth a c r o s s one net-cage was examined. I t was h y p o t h e s i z e d t h a t any r e d u c t i o n s o f c u r r e n t speed i n s i d e t h e net-cage would be e q u a l o y er t h e depth o f t h e cage. 7 C h a p t e r 1: Water quality and water exchange at two fish farms located in Jervis and Sechelt Inlets, B.C. Introduction The primary source of DO to a net-cage is through water exchange with the surrounding environment (atmospheric input is too slow and the necessary phytoplankton biomass contained within a net-cage insufficient). Any decrease in water exchange is.of utmost concern to the fish farmer. Kils (1979) estimated that oxygen input through the surface (by wind and wave action) and phytoplankton photosynthetic activity, can supply only 0.5% of total oxygen demand. Kils (1979) however, assumed stocking densities of 20-25 kg/m^ and, although not explicitly stated, based oxygen production values on a much lower phytoplankton standing stock than is typically observed for local Pacific waters. Assuming spring concentrations of nitrate in local waters to be 20 (imol/L, a nitrogen to carbon ratio of 10, and a photosynthetic quotient of 1.5 (Williams, 1979), yields a potential increase of 4.2 mg O2/L under, ideal conditions. If even one-quarter of this theoretical standing stock existed, phytoplankton would contribute a significant amount of oxygen to the waters. Conversely, such a bloom, upon death of the phytoplankton and subsequent bacterial respiration, would cause a much greater lowering of oxygen concentration due to the rapid growth kinetics of bacteria. Doubling times 8 f o r b a c t e r i a are i n the order of hours whereas phytoplankton t y p i c a l l y double once per day (Parsons et a l . , 1984a) and so, oxygen increases would occur much l e s s r a p i d l y than decreases. Consequently, f i s h k i l l s may r e s u l t from anoxia caused by b a c t e r i a l r e s p i r a t i o n of a f t e r a bloom. Davis (1975) reviewed the l i t e r a t u r e on DO requirements f o r marine salmonids and concluded that >9.0 mg O2/L assures a high l e v e l of s a f e t y f o r most of the f i s h p o p u lation with few members e x h i b i t i n g symptoms of low oxygen. At the next lower c r i t e r i o n of 6.43 mg O2/L, Davis (1975). suggests that the average member of a f i s h p o p u lation w i l l e x h i b i t symptoms of oxygen d i s t r e s s and t h a t some r i s k i s i n v o l v e d i f exposure to t h i s l e v e l of DO i s allowed to continue f o r more than a few.hours. Occasional phytoplankton blooms may r a i s e oxygen concentrations to the p o i n t of s u p e r s a t u r a t i o n (Holeton, 1979). However, Weitkamp and Katz (1980) reviewed gas bubble disease (GBD) i n f i s h , and found that no n a t u r a l occurrences of GBD due to oxygen alone had been reported. GBD u s u a l l y r e s u l t s from an increase i n t o t a l d i s s o l v e d gas pressure (TGP) most oft e n caused by s u p e r s a t u r a t i o n of water with n i t r o g e n . While concurrent high oxygen l e v e l s may serve to m i t i g a t e the e f f e c t s of GBD, oceanic waters r a r e l y become supersaturated i n n i t r o g e n due to i t s r a p i d i n c o r p o r a t i o n i n t o the n i t r o g e n c y c l e (Parsons et a l . , 1984a). Water exchange i s the only bulk method of ammonia d i s s i p a t i o n . Ammonia uptake by phytoplankton and 9 transformation by b a c t e r i a to n i t r i t e and then n i t r a t e , accounts f o r only a small p o r t i o n due to the large d i f f e r e n c e s i n biomass w i t h i n a net-cage. This study was designed to monitor DO and ammonia l e v e l s i n s i d e and outside net-cages while simultaneously measuring current speed over a 25 h p e r i o d . (The outside-cage sampling was performed as a con t r o l ) . This was to determine f i r s t , i f DO.and ammonia l e v e l s became d e l e t e r i o u s to f i s h h e a l t h . Secondly, i t was to see i f these water q u a l i t y ' parameters c o r r e l a t e l i n e a r l y to water exchange r a t e . This would provide an i n d i c a t i o n of the r o l e of hydrographic c o n d i t i o n s i n determining water q u a l i t y r e l a t i o n s h i p s i n two l o c a t i o n s - J e r v i s I n l e t w i t h a deep s i l l and Sechelt I n l e t w i t h a shallow s i l l . 10 M a t e r i a l s and Methods The l o c a t i o n s of the study s i t e s are. i l l u s t r a t e d i n Figure 1. The J e r v i s I n l e t s i t e ( r e f e r r e d t o as JER) i s loc a t e d near a deep s i l l (385 m) . The Sechelt i n l e t s i t e (designated as SEC) i s l o c a t e d approximately 8 km south of a shallow s i l l . The SEC entrance s i l l has a depth of 14 m at high water (Pickard, 1975). Figure 2 shows a plan view of the s i t e s and cages used i n t h i s study. Cage s i z e v a r i e d between the s i t e s , the JER l o c a t i o n employed cages of 15X15X15 m (length X width X depth), whereas 12X12X5 m net-cages were used at the SEC s i t e . Walkways at both s i t e s , surrounded each net-cage, w i t h the c e n t r a l walkway (oriented on the long a x i s of the r a f t ) ca. 2 m i n width, and the remaining three walkways, ca. 1 m wide. Therefore, except under c o n d i t i o n s of high current flow, adjacent net-cages d i d not make contact. Estimated t o t a l biomass at the JER s i t e was 260,000 kg of p r i m a r i l y Oncorhynchus tshawytscha (chinook salmon) contained i n 24 cages. Stocking d e n s i t y i n the study cage was 4.3 kg/m3 (9843 chinook, average weight 1.4 kg). To t a l biomass at the SEC farm was ca. 156,000 kg, p r i m a r i l y 0. kisutch (coho salmon) and 0. tshawytscha contained i n 28 cages. The net-cage used i n t h i s study was stocked w i t h 4737, 1.1 kg coho at a den s i t y of 7.2 kg/m3. Figure 1. Locations of study s i t e s . The Jervis Inlet s i t e i s designated by a (1) and the Sechelt Inlet s i t e by a ( 2 ) . I24°00' .123° 40 12 Figure 2. Plan views of the, A) Jervis Inlet (JER) and B) Sechelt Inlet sites. Stations at which water quality samples were collected and.profiling performed, are designated by ( Q ) . Current meters were deployed at- locations indicated by ( ). See text for details of depths of deployments and sampling protocol. 13 3 0 m JER cage 2 3 ^ Q A OUT N o ycage 15 A A A L cage 2 y A 0 U T S cage 6 cage 10 14 Sampling was c a r r i e d out i n July-August i n 1987 (SEC) and 1988 (JER). During these months temperatures reached t h e i r annual peak and the f i s h were expected to reach t h e i r highest metabolic r a t e . A Hydrolabs® Surveyor® I I probe was used to p r o f i l e the water column f o r temperature, s a l i n i t y , pH, and d i s s o l v e d oxygen. Hydrolabs® claims a c a l i b r a t e d accuracy of ±0.05°C, 0.2 ppt, and 0.2 mg/L f o r T, S, and DO, r e s p e c t i v e l y . C a l i b r a t i o n of the instrument before and a f t e r each f i e l d use i n d i c t e d that temperature and pH remained w i t h i n ±2% of t h e i r i n i t i a l values, and c o n d u c t i v i t y w i t h i n ±3%. However, the d i s s o l v e d oxygen sensor d r i f t e d by a maximum of -10%. In situ DO measurements were always performed w i t h i n a 25 h p e r i o d and t h e r e f o r e the instrument d r i f t would presumably account f o r l e s s than 1% e r r o r during the sampling i n t e r v a l . Moreover, d i f f e r e n c e s , not absolute values were important i n t h i s study, and so, DO readings were considered acceptable. Samples f o r ammonia determination were c o l l e c t e d in situ using e i t h e r a diaphragm pump or a 2 L N i s k i n b o t t l e . C o l l e c t e d samples were e i t h e r processed on s i t e (1987-SEC) or f i l t e r e d through precombusted Whatman GF/F gl a s s f i b e r f i l t e r s and frozen f o r subsequent l a b o r a t o r y a n a l y s i s (1988-JER). Ammonia determinations i n e i t h e r case followed methods o u t l i n e d i n Parsons et a l . (1984b) which converts a l l ammonia to the ammonium i o n . Values reported w i l l be expressed as t o t a l ammonia measured - the sum of NH4+ and 15 N H 3. Tables i n Bower and B i d w e l l (1978) were consulted to determine the pr o p o r t i o n of un-ionized ammonia present i n the water column given the in situ T, S, and pH. No more than 5% of t o t a l ammonia e x i s t e d i n the un-ionized ammonia (NH3) form i n any part of t h i s study. Interocean S4® current meters (7) were employed at the JER s i t e whereas Aanderaa RCM-4® current meters (4) were used at the SEC s i t e . Both instruments are i n t e r n a l l y s e l f -r e c ording f o r T, S, pressure and current s p e e d / d i r e c t i o n , with programmable sampling d u r a t i o n and frequency. The S4 i s a true vector-averaging instrument with an accuracy of ±2% of the reading f o r current speed and ±2.0 degrees d i r e c t i o n . Without c o r r e c t i o n s accuracy i s ±0.5-0.6 ppt f o r s a l i n i t y and ±0.2 °C f o r temperature, f o r a t o t a l e r r o r of ±0.6 i n sigma-t. (Sigma-t i s a shorthand n o t a t i o n f o r expressing water d e n s i t y and i s defined as den s i t y [kg/m3] minus 1000.00, i t i s usual to omit the u n i t s (Pond and P i c k a r d , 1986)). C o r r e c t i o n s were not made as absolute values were not r e q u i r e d and d i f f e r e n c e s between depths were much l a r g e r than the e r r o r . Claimed accuracy fo.r the RCM-4 instruments i s ±1 cm/sec (speed), ±5 degrees ( d i r e c t i o n ) , ±0.05°C (temperature), and ±0.025 mS/cm ( c o n d u c t i v i t y ) . The S4 current meters were programmed to sample f o r 2 min every 10 min. The RCM-4 instruments, were set to sample once every 10 minutes. Current meters were suspended simultaneously i n s i d e and outside net-cages v i a a s e r i e s of aluminum poles, Viny® 16 f l o a t s , and polypropylene rope. In a l l deployments, the instruments were p o s i t i o n e d 3-3.6 m from adjacent nets to ensure that they d i d not become entangled i n the nets during periods of high current flow. Depths of deployment v a r i e d with each experiment conducted at the JER s i t e . Experiments concerned with d e t e c t i n g flow patterns across the s i t e used instruments deployed at 6 m. Determination of flow as a fu n c t i o n of depth r e q u i r e d instruments to be deployed at 2, 5, and 8 m i n s i d e the net-cage and at 2, 5, 8, and 11 m outside the cage. Current meters were deployed at 2 and 4 m only at the SEC s i t e . The l o c a t i o n s of deployments i s i l l u s t r a t e d i n Figure 2. Sampling of the water column was performed over a 25 h pe r i o d i n order to inc l u d e the f u l l t i d a l c y c l e . V e r t i c a l p r o f i l e s i n s i d e and outside the net-cages were taken once every hour, while d i s c r e t e samples f o r ammonia determination were c o l l e c t e d every 1.5 h at the JER s i t e and every 1.0 h at the SEC s i t e . At both l o c a t i o n s , samples were c o l l e c t e d over a continuous 25 h sampling program. Ten r e p l i c a t e ammonia samples were c o l l e c t e d and analyzed to determine the e r r o r of the sampling p r o t o c o l . Near surface samples w i t h a mean of 0.35u.mol/L . t o t a l ammonia showed a standard d e v i a t i o n of 0.19. During the f r e e z i n g process, ammonium concentrations of 2, 4 and 8 umol/L were a l t e r e d by 2% of t h e i r o r i g i n a l values whereas standards of 1 |imol/L v a r i e d by 65%. The high v a r i a b i l i t y at low concentrations, r e s u l t i n g from the f r e e z i n g process, 17 perhaps accounts f o r the s u b s t a n t i a l v a r i a t i o n of the r e p l i c a t e samples. Data f o r each depth and l o c a t i o n were pooled f o r each 25 h sampling p e r i o d i n order to allow comparisons of ammonia l e v e l s , DO and current speed i n s i d e and outside of the net-cages. A Student's t s t a t i s t i c was computed using I STAT (Perlman, 1986) data a n a l y s i s programs. Regression a n a l y s i s was al s o performed (using |STAT software) to determine i f a r e l a t i o n s h i p e x i s t e d between water q u a l i t y parameters and water flow r a t e s . D i f f e r e n c e s and re g r e s s i o n r e l a t i o n s h i p s were judged to be s i g n i f i c a n t at the 95% l e v e l (p<0.05). 18 Results and D i s c u s s i o n The water column at the JER s i t e i s s t r o n g l y s t r a t i f i e d as i n d i c a t e d by both the temperature and d e n s i t y s t r u c t u r e (Figure 3) . At the SEC s i t e , s t r a t i f i c a t i o n v a r i e s over the 25 h sampling p e r i o d (Figure 4). The p e r i o d i c i t y i n s t r a t i f i c a t i o n i s due to i n t e r n a l waves generated at the shallow s i l l , passing through the net-cages, and modifying the d e n s i t y s t r u c t u r e by causing convergences and divergences i n the upper l a y e r s of water. As these i n t e r n a l waves are generated t i d a l l y , a semi-diurnal p e r i o d i c i t y i s evident i n the density and temperature p r o f i l e s . The expected d a i l y p a t t e r n of ammonia production i s a peak l a t e i n the afternoon and an ebb near dawn as reported by McLean and Fraser (1974) . This p a t t e r n i s apparent only at 6 m at the JER s i t e (Figure 5) w i t h peaks at 17:30 and an ebb at dawn. There was no c l e a r p a t t e r n observed between current speed and ammonia concentrations, i . e . an increase i n ammonia was not always preceded by, or found to occur during periods of slower water speed (Figure 5). On t h i s p a r t i c u l a r morning, the automatic feeders were empty and thus, while feeding u s u a l l y commenced at 05:30 ( s u n r i s e ) , no feed was s u p p l i e d u n t i l 08:00, upon the a r r i v a l of the farm s t a f f . The f i s h may have been con d i t i o n e d to r e c e i v e feed at 05:30, and so, were induced i n t o ammonia production by 19 F i g u r e .3. P l o t s of A) temperature and B) Sigma-t (den s i t y -see t e x t f o r explanation) versus time f o r the i n d i c a t e d depths sampled at the J e r v i s I n l e t farm s i t e . Temperature data was c o l l e c t e d h o u r l y u s i n g the Hydrolabs instrument at the s t a t i o n o u t s i d e of cage 23. Sigma-t data was c o l l e c t e d u s i n g Interocean S4 c u r r e n t meters suspended s i m u l t a n e o u s l y o u t s i d e cage 19 (Figure 2 ) . -A O o <D D i _ CD C L E B L T 24 22 20 "18 16 14 12 1.0 16:00 3 0 • • 1 mete r O——-O 6 meters A A 1 1 meters -•-i=k ' n o- -o^o-o- •o- - o — o - o — o — o — o — o - C) - A — A — A — A — A — A — A — A — A - - A - - A — A — A -22 :00 04 :00 10:00 16:00 2 5 -D £ 20 in 15 + 8 meters 5 meters 2 meters 10 16 :00 + + 22 :00 . 0 4 : 0 0 Time of day 10:00 1 6:00 2 0 Figure 4 . Plots of A) temperature and B) Sigma-t (density) versus time for the indicated depths at the Sechelt Inlet farm s i t e . Data collected using the Hydrolabs instrument to profile the water column inside cage 6 . Each point is a single sample - see text for error estimation. A 21 15-1 1 1 1 : 1 1 18:00 2 3 : 0 0 0 4 : 0 0 09 :00 14 :00 19:00 Time of day 21 Figure 5. Plots of A) ammonia versus time and B) current speed for the indicated sampling depths at the Jervis Inlet s i t e . S t i p l e d area indicates time of feeding. Each ammonia data point i s a single sample - see. text for error estimation. Current speeds are hourly averages of 6 two-minute sampling episodes. A 16:00 22:00 ' ' 04 :00 10:00 16:00 B 5 - r — — o CD co \ E o TD CD CO CL CO -•—> c CD i _ k_ o 4- -3 -1 --O O 6 m 0 16:00 1 22:00 — • • — l h -04:00 Time of day 10:00 1 6:0.0 c 22 su n r i s e , or by the noise made by the empty feeders. The sharp drop at 10:00 then, may have been the r e s u l t of ammonia e x c r e t i o n r a t e f a l l i n g because of i n s u f f i c i e n t time to process the feed o f f e r e d at 08:00. At the JER s i t e , ammonia concentrations at 0.5 m d i d not f o l l o w the expected p a t t e r n (Figure 5). Moreover, values were s i g n i f i c a n t l y lower than those at 6 m (Table 1). I t i s suggested that the uptake of NH 4 + by algae attached to the near surface p o r t i o n of the JER net-cage may cause the lowering of ammonia concentrations r e l a t i v e to those at 6 m. Furthermore, i t may be that these 15 m deep cages, combined with a lower s t o c k i n g d e n s i t y , allow the f i s h to escape warmer temperatures near the surface, which would have the e f f e c t of concentrating ammonia l e v e l s at depth. Expected ammonia concentration patterns at the SEC s i t e are not apparent at any of the depths sampled (Figure 6) . The observed t r e n d i s of increased ammonia l e v e l s over the 25 h sampling p e r i o d , while current speed appears to have decreased (Figure 6) . I t w i l l be shown however, th a t t h i s r e l a t i o n s h i p i s not s t a t i s t i c a l l y s i g n i f i c a n t . The o v e r a l l t rend of i n c r e a s i n g ammonia over the 25 h p e r i o d may be due to the sampling procedure d i s t u r b i n g the f i s h . McLean and Fraser (1974) reported that ammonia concentrations rose n o t i c e a b l y i n r e a r i n g ponds a f t e r a disturbance such as removal of some. f i s h . Table 1. Comparisons of total ammonia inside cages at the JER and SEC sites. Data were pooled for each depth and location using values collected over the 25 h sampling programs. Order of means is as presented under the Comparison heading. (SD=standard deviation, df=degrees of freedom) Location Comparison 1st mean (SD) 2nd mean (SD) tc a l c t-test (df) P total JER ammonia 0.5 vs 6m 0.9 (1. 0) 3.5 (3. 0) 3.508 (34) 0 .001 (|imol/L) total SEC- ammonia 0.5 vs 2m 3.5 (1. 9) 3.4 (2. 1) 0.107 (32) 0 .915 (u.mol/L) 0.5 vs 4m 3.5 (1. 9) 3.2 (1. 7) 0.580 (32) 0 .566 2m vs 4m .3.4 (2. 1) 3.2 (1. 7) 0.435 (32) 0 .666 24 Figure 6. Plots of A) ammonia versus time and B) current speed for the indicated sampling depths at the Sechelt Inlet s i t e . Stipled area indicates time of feeding. Each ammonia data point i s a single sample - see text for error estimation. Current speeds are hourly averages of 6 sampling episodes. A 10 C H 1 1 1 1 1 18:00 23 :00 0 4 : 0 0 0 9 : 0 0 14:00 1 9 : 0 0 Time of day 25 There were no s i g n i f i c a n t d i f f e r e n c e s found among ammonia samples c o l l e c t e d at 0.5, 2, or 4 m (Table 1) at the SEC s i t e . The higher s t o c k i n g d e n s i t y , and shallower cages employed at t h i s s i t e are thought to c o n t r i b u t e to the equal d i s t r i b u t i o n of ammonia w i t h i n the net-cage. The JER s i t e 6 m ammonia concentrations were al s o the only ones t o show s i g n i f i c a n t v a r i a t i o n between pooled samples from i n s i d e and outside of the net cage (Table 2). Current meter data i n d i c a t e that the predominant current flow impinged on the cage sampled at the JER s i t e (Figure 14, which i s discussed i n Chapter 2) from the outside during a large p o r t i o n of the t i d a l c y c l e , while the sampled cage at the SEC s i t e r e c e i v e d much of i t s water through adjacent cages. The l o c a t i o n of the SEC s i t e w i t h i n a semi-enclosed bay (Figure 2) caused the formation of gyres w i t h i n the farm which changed i n speed and l o c a t i o n over the t i d a l c y c l e . Thus, because the outside sampling s t a t i o n was subject to waters that had passed through an u n c e r t a i n number of other net-cages i t was considered meaningless as a c o n t r o l . Surface ammonia samples c o l l e c t e d p e r i o d i c a l l y from near the shore averaged 0.8|lmol/L (S.D.=0.7, n=13) and ranged from 0.3 to 2.9|imol/L. These samples i n d i c a t e d that ammonia enrichment was not confined to the f i s h cages. Background ammonia concentrations are considered t o be <1.0 umol/L f o r l o c a l waters (Dr. P.J. Harrison, DOUBC, pers. comm.). Table 2. Comparisons of DO, t o t a l ammonia, and current speed i n s i d e and outside net-cages at the JER and SEC s i t e s . Data were pooled f o r each depth and l o c a t i o n using values c o l l e c t e d over 25 h sampling program. (SD=standard d e v i a t i o n , df=degrees of freedom) Location Depth (m) Parameter INSIDE mean (SD) OUTSIDE mean (SD) t c a l c t - t e s t (df) p JER 1 DO 9.5 (0. 8) 9.4 (0. 6) 0.2 94 (34) 0 . 771 21 J u l y , 6 (mg/L). 7.9 (0. 5) 8.3 (0. 5) 2.501 (34) 0 . 017 1988 11 . 6.5 (0. 2) 6.5 (0. 2) 0.381 (34) 0 .706 t o t a l 0.5 ammonia 0.9 (1. 1) 0.5 (0. 5) 1.272 (34) 0 .212 6 (|imol/L) 4.4 (2. 6) 1.7 (1. 6) 3.187 (26) 0 .004 6 current 2.0 (1. 3) 2.9 (2. 5) 3.838 (300) <0 .001 speed (cm/sec) SEC 0.5 DO 9.1 (0. 3) 9.0 (0. 3) 1.403 (50) 0 .167 6 August, 1 (mg/L) 9.0 (0. 3) 8.9 (0. 2) 0.278 (50) 0 .782 1987 • 2 8.8 (0. 4) 8.8 (0. 4) 0 .572 (50) 0 .570 3 8.6 (0. 6) 8.6 (0. 5) 0.143 (50) •'0 .887 4 8.3 (0. 7) 8.4 (0. 6) 0.236 (50) 0 .815 t o t a l 0.5 ammonia 3.7 (1. 8) 2.9 Cl. 2) 1.486 (31) 0 . 147 2 ((imol/L) 3.5 (2. 0) 3.7 (1. 8) 0.302 (32) 0 .765 4 3.1 (1. 8) 3.4 (1. 5) 0.436 (32) 0 .666 2 current 8.3 (5. 4) 7.2 (4. 4) 0.776 (48) 0 .441 4 speed ' 6.6 (3. 6) 5.6 (3. 1) 1.042 (48) 0 .303 (cm/sec) CTl Un-ionized ammonia ( N H 3 ) l e v e l s peaked at ca. 0.6|imol/l (=0 . 008 mg N H 3/L) at both l o c a t i o n s . This value i s w e l l below the 0.025 mg N H 3/L reported by Al a b a s t e r and Lloyd (1982) to be t o x i c to salmonids on a long-term exposure b a s i s . L e t h a l l e v e l s are reported to begin at concentrations of 0.2 mg N H 3/L (Alabaster and Lloyd, 1982). Lin e a r regressions of ammonia and current speed were not s i g n i f i c a n t at e i t h e r s i t e (Figures 7 & 8) . The slopes are i n the p r e d i c t e d d i r e c t i o n , however they are not s i g n i f i c a n t l y d i f f e r e n t from zero. The i n t e r a c t i o n of the t i d a l c y c l e (four current r e v e r s a l s i n 25 h) and the d i u r n a l p e r i o d i c i t y of ammonia production and u t i l i z a t i o n make t h i s a very complex r e l a t i o n s h i p , and perhaps a l i n e a r model i s too s i m p l i s t i c . At the SEC s i t e , the i n f l u e n c e of the i n t e r n a l t i d e , may have added considerable noise to the data by a l t e r n a t e l y i n c r e a s i n g and decreasing ammonia concentrations by causing convergences and divergences. D i f f e r e n c e s i n DO between the i n s i d e and outside of the net-cages were s i g n i f i c a n t only at the JER s i t e , and only at 6 m depth (Table 2) . This f i n d i n g perhaps gives f u r t h e r weight to the argument that the bulk of the f i s h are l o c a t e d i n the mid-depths of the cage, hence the greater d e p l e t i o n of DO at t h i s depth. Again, the l o c a t i o n of the sampled cage at the SEC l o c a t i o n makes i n s i d e / o u t s i d e comparisons meaningless. Figure 7. S c a t t e r diagram of t o t a l ammonia versus current speed f o r data c o l l e c t e d at mid-cage depth (6 nv) of cage. 23 at the J e r v i s I n l e t s i t e . The l i n e i s the l e a s t squares r e g r e s s i o n l i n e w i t h c o e f f i c i e n t of determination (r^) i n d i c a t e d . A t - s t a t i s t i c was used t o t e s t s i g n i f i c a n c e (p). 29 F i g u r e 8 . S c a t t e r d iagram o f t o t a l ammonia v e r s u s c u r r e n t speed f o r d a t a c o l l e c t e d a t the mid-cage depth (2 m) i n cage 6 at the S e c h e l t I n l e t s i t e . The l i n e i s the l e a s t squares r e g r e s s i o n l i n e w i t h c o e f f i c i e n t o f d e t e r m i n a t i o n ( r 2 ) i n d i c a t e d . A t - s t a t i s t i c was used t o t e s t s i g n i f i c a n c e (p) . 12 1 0 - -o E 8 -'c O E E D £ 2 + 6 -4 -0 Regress ion l ine: r 2 =0 .08 p=0 .26 A . A A • A A A A ~ • A A A A i i i J .... A A - — i 1 1 1 1 0 4 6 8 10 12 14 16 . Current Speed ( c m / s e c ) 18 20 D i u r n a l p e r i o d i c i t y of DO at the JER l o c a t i o n (Figure 9) i s t y p i c a l of a phytoplankton-driven DO regime at 1 m. Near surface waters (1 m) e x h i b i t e d photosynthetic production of DO during the d a y l i g h t hours, with a DO sag oc c u r r i n g during dark hours when r e s p i r a t i o n by f i s h , phytoplankton, and other organisms i n the water column dominated. The l a r g e s t sag however, occurred at 10:00 and i t was thought that the metabolism of feeding f i s h may have had some i n f l u e n c e as they moved up i n t o the near surface waters to feed. DO values at t h i s s i t e d i d ' o c c a s i o n a l l y reach l e v e l s at which some f i s h would begin to e x h i b i t symptoms of .low oxygen s t r e s s , p a r t i c u l a r l y at 11 m. P r o t e c t i o n l e v e l B (Davis, 1975) i s set at 6.43-9.00 mg O2/L, and exposure to the minimal value i s considered to pose some degree of r i s k i f extended f o r more than a few hours. Caveat: instrument e r r o r may have been as much -10% i n DO readings. A recorded concentration of 6.5 mg O2/L may t h e r e f o r e a c t u a l l y have been as high as 7.2 mg O2/L (but was s t i l l w i t h i n l e v e l B). To avoid prolonged exposure to low DO, the f i s h could have moved to near surface waters. P h i l l i p s (1985) i n d i c a t e s that farmed Salmo gairdneri Richardson (rainbow t r o u t ) may a l t e r t h e i r p o s i t i o n w i t h i n a net-cage to avoid temperature extremes. He argues f u r t h e r that aggregation of f i s h i n one p o r t i o n of the cage should not be considered under-u t i l i z a t i o n of the a l l o c a t e d space, but rat h e r , a p r o v i s i o n Figure 9. Plots of dissolved oxygen versus time for the indicated sampling depths at the Jervis Inlet s i t e . Each data point is a single sample - see text for error estimation. 15 1 3 -• D i m O 0 6 m A A 1 1 m E c 1 -| CD cn X O "D 9 CD _> O CO 5 7 E3 O-\ A o - c r / O o A - A - A ' A ^ A — - A o V * o - o o--[] A " A ^ A ^ A ^ A — A — A - A — A - A , - A - A 16:00 2 2 : 0 0 04 :00 10 :00 16:00 Time of day 32 of a -refuge from unfavourable c o n d i t i o n s . Thus, i t i s c l e a r , that any e v a l u a t i o n of water q u a l i t y w i t h i n a net-cage must incl u d e sampling over the complete depth of the cage as the f i s h may b e h a v i o u r a l l y s e l e c t any p o r t i o n of the cage i n which to r e s i d e . DO concentrations at the SEC s i t e v a r i e d g r e a t l y over the 25 h sampling p e r i o d (Figure 10) . The e f f e c t of the i n t e r n a l wave passing through the s i t e i s apparent i n the data (Figure 10). For example, DO l e v e l s at 4 m a r e seen to r i s e from <7 mg/L to near 9 mg/L the during the p e r i o d 22:00 to 03:00. The only p l a u s i b l e explanation f o r t h i s increase of DO during the night,- i s convergence of the water column, caused presumably by the passage of an i n t e r n a l wave through the net-cage. Lin e a r regressions of DO and current speed were not s i g n i f i c a n t at the JER s i t e (Figure 11) but were s i g n i f i c a n t at the SEC l o c a t i o n (Figure 12). In both cases, the slope i s i n the p r e d i c t e d o r i e n t a t i o n , that i s , as speed increased, DO concentrations increased. However, at the SEC s i t e , an r ^ value of 0.16 i n d i c a t e s that only 16% of the variance i n DO can be a t t r i b u t e d to current speed. As i n the case f o r ammonia and current speed, the i n t e r a c t i o n of b i o l o g i c a l (photosynthesis and r e s p i r a t i o n ) and p h y s i c a l processes cannot be described by a simple l i n e a r f u n c t i o n . A n a l y s i s of more data, c o l l e c t e d over the f u l l monthly range of t i d e s , may help to c l a r i f y t h i s r e l a t i o n s h i p . 33 Figure 10. P l o t s of d i s s o l v e d oxygen versus time f o r the i n d i c a t e d sampling depths at the Sechelt I n l e t s i t e . Each data p o i n t i s a s i n g l e sample - see t e x t f o r e r r o r e s t i m a t i o n . 1 8 : 0 0 . 2 3 : 0 0 0 4 : 0 0 . 0 9 : 0 0 1 4 : 0 0 1 9 : 0 0 ' Time of day Figure 11. S c a t t e r diagram of dissoved oxygen versus current speed f o r data c o l l e c t e d at mid-cage depth (6m) of cage 23 at the J e r v i s I n l e t s i t e . The l i n e i s the l e a s t squares r e g r e s s i o n l i n e w i t h c o e f f i c i e n t of determination ( r 2 ) i n d i c a t e d . A t - s t a t i s t i c was used t o t e s t s i g n i f i c a n c e (p). Current Speed ( c m / s e c ) 35 Figure 12. S c a t t e r diagram of d i s s o l v e d oxygen versus current speed for. data c o l l e c t e d at the mid-cage depth (2 m) i n cage 6 at the Sechelt I n l e t s i t e . The l i n e i s the l e a s t squares r e g r e s s i o n l i n e w i t h c o e f f i c i e n t of determination ( r 2 ) i n d i c a t e d . A t - s t a t i s t i c was used t o t e s t s i g n i f i c a n c e (P) • 1 1 10 E c cu X O a> j> o CO CO Q 9~ 8 -7 Regress ion line: r 2 = 0 . 1 6 ' p = 0 .04 " A A A * --_ A _ _ _ ^ — —1 A A A A 1 1 1 1 —1 — _ i _ 1 1 1 0 6 8 10 12 14 Current Speed ( c m / s e c ) 16 18 20 36 Current speeds were g e n e r a l l y higher at the SEC l o c a t i o n than the JER l o c a t i o n (Table 2) . This may be due, i n p a r t , to the presence of mussels f o u l i n g the nets at the JER s i t e to a depth of ca. 4 m. The SEC s i t e had r e c e n t l y replaced, clean nets. The r e s t r i c t i o n of the t i d a l waters through Skookumchuk Narrows, over the entrance s i l l to the Sechelt I n l e t , r e s u l t s i n a "turbulent j e t " ( L a z i e r , 1963) . This "turbulent j e t " serves to a c c e l e r a t e the near surface waters and thus current speeds are higher near the s i l l than at the head of the i n l e t . No such event occurs over the deep s i l l at the entrance to J e r v i s I n l e t - where the JER s i t e was l o c a t e d . I t i s suggested that p r o x i m i t y of the Sechelt s i t e to a shallow s i l l has put i t under the i n f l u e n c e of i n t e r n a l waves generated by t i d a l a c t i o n at the s i l l . These long-p e r i o d waves propagate along d e n s i t y i n t e r f a c e s , causing convergences and divergences i n the near surface waters (Pickard, 1954). The t i d a l component of these waves can be observed i n the twice d a i l y s t r a t i f i c a t i o n changes seen i n the DO, Sigma-t, and temperature p l o t s . I m p l i c a t i o n s of these events to f i s h farmers are many. As the time of d a i l y t i d a l events progresses about one hour every day, the time of occurrence of these events a l s o progresses. The temperatures throughout the cages may o c c a s i o n a l l y become harmful to the f i s h i f , f o r instance, a convergence occurred during an e x c e p t i o n a l l y warm p e r i o d when surface temperatures reached dangerous l e v e l s (greater than 21 °C 37 fo r chinook; Caine et a l . , 1987). A r a p i d increase i n temperature may s t r e s s the f i s h , p a r t i c u l a r l y i f the change i s >5 °C w i t h i n a 12 h p e r i o d (Caine et a l . , 1987) . A r a p i d increase i n temperature may a l s o cause gas su p e r s a t u r a t i o n to occur which may lead to f i s h k i l l s (Weitkamp and Katz, 1980) . Water mass convergences a l s o mean that any harmful phytoplankton i n the water column may become concentrated, which may r e s u l t i n f i s h k i l l s at concentrations that would, i n the absence of an i n t e r n a l wave, have been harmless. E a r l y warning networks of phytoplankton watches must th e r e f o r e add a s a f e t y f a c t o r i n t o t h e i r abundance estimates i f i n t e r n a l waves are present i n t h e i r area. M i t i g a t i o n of harmful e f f e c t s imposed by i n t e r n a l t i d e s , or unfavorable near-surface c o n d i t i o n s , may be brought about by using deep net-cages, which extend below the depth a f f e c t e d by the i n t e r n a l wave. By employing lower s t o c k i n g d e n s i t i e s f i s h farmers may allow the f i s h to b e h a v i o u r a l l y s e l e c t the most favourable p o r t i o n of the water column. 38 Conclusions: D e f i n i n g water q u a l i t y as the concentration of DO and ammonia present, allows the f o l l o w i n g conclusions to be made based on data c o l l e c t e d at two farm s i t e s during the course of t h i s study. D i s s o l v e d oxygen concentrations were observed to reach l e v e l s low enough to have caused s t r e s s to some members of the f i s h p opulation, at depths of 6 and 11 m at the J e r v i s I n l e t s i t e and at a l l depths sampled at the Sechelt I n l e t s i t e . Un-ionized ammonia l e v e l s d i d not exceed the accepted c r i t e r i o n set f o r avoidance of s u b l e t h a l e f f e c t s (0.025 mg NH3/L) at e i t h e r l o c a t i o n , at any depth sampled, over the 25 h sampling pe r i o d s . The presence of a shallow s i l l near the Sechelt I n l e t s i t e , versus a deep s i l l near the J e r v i s I n l e t s i t e , created marked d i f f e r e n c e s i n hydrography between the two l o c a t i o n s . The shallow s i l l , generated i n t e r n a l waves which, upon t h e i r passage through the farm s i t e , modified the de n s i t y s t r u c t u r e . Subsequent changes were al s o observed i n temperature and DO p r o f i l e s over the depth of the cages, and i t was proposed that measures, such as i n c r e a s i n g cage depth and lowering s t o c k i n g d e n s i t i e s , must be taken by the farmer to m i t i g a t e the p o t e n t i a l impact of these changes upon f i s h s tocks. 39 Chapter 2 : Net-cage f i s h farming i n B r i t i s h Columbia: a case study of water c i r c u l a t i o n and water q u a l i t y i n a l a r g e r a f t of cages I n t r o d u c t i o n This p o r t i o n of the study was undertaken to observe the e f f e c t s of the combined p e r i o d i c i t y of b i o l o g i c a l and p h y s i c a l phenomena over a 25 h p e r i o d , i n various p o s i t i o n s and depths w i t h i n a 260,000 kg f i s h farm. Sampling i n v o l v e d three p a r t s or experiments. Part A, was to measure the water flow as a f u n c t i o n of depth i n a stocked net-cage which had been submerged f o r ca. 30 days. This experiment would a l s o i n d i c a t e whether or not a h o r i z o n t a l v e l o c i t y shear was present, which would d i c t a t e the placement of the current meters f o r the second phase. Part B examined the water flow patterns w i t h i n the s i t e . L a s t l y , i n Part C, water q u a l i t y (DO and ammonia) across the s i t e was monitored i n a 25 h sampling 'program which overlapped concurrent water flow measurements as much as was l o g i s t i c a l l y p o s s i b l e . This l a s t program was designed to t e s t the hypothesis that water q u a l i t y would be poorer i n those cages which were l o c a t e d downstream i n the predominant flow d i r e c t i o n . 40 M a t e r i a l s and Methods The JER s i t e p a r t i c u l a r s and equipment/methods used i n t h i s study are described i n the M a t e r i a l s and Methods s e c t i o n of Chapter 1. As i t was d e s i r e d to sample during a p e r i o d of minimal t i d a l flow, sampling was scheduled during neap t i d e s (moon in' 1st quarter on J u l y 21, 1988). Sampling p r o t o c o l i s o u t l i n e d below: Part A Three current meters were deployed at 2, 5, and 8 m depth i n s i d e net-cage 6, while 4 current meters were simultaneously deployed at 2, 5, 8, and 11 m outside net-cage 19, on the north side of the r a f t (Figure 2) . Current meters were programmed to sample f o r 2 min i n every 10 min p e r i o d , and remained i n p o s i t i o n f o r 25 h. Part B Based on the f i n d i n g s of Part A, current meters were deployed at 6 m i n the l o c a t i o n s shown i n Figure 2. Sampling rat e was as per Part A. Due to l o g i s t i c s , the p e r i o d of simultaneous records f o r each instrument was 20:00 20/07/88 to 21:00 21/07/88, which overlapped the water q u a l i t y sampling program of Part C f o r a p e r i o d of 4 h. The d a i l y procession of t i d e s was 1 h f o r t h i s l o c a t i o n , and hence, v a l i d comparisons of water q u a l i t y and water flow were made by s h i f t i n g the time s c a l e a p p r o p r i a t e l y . 41 Part C Water q u a l i t y sampling was done i n s i d e and outside cage 23 as w e l l as i n s i d e cage 15 every 1.5 h f o r a 25 h pe r i o d , beginning at 16:00 21/07/88 and ending 17:00 22/07/88. Samples f o r ammonia a n a l y s i s were c o l l e c t e d at 0.5 and 6 m depth, while the Surveyor® I I was used to measure temperature, pH, s a l i n i t y , and DO at 1, 6, and 11 m. 42 Results Weather c o n d i t i o n s throughout these experiments remained c l o u d l e s s w i t h v i r t u a l l y no wind. As a r e s u l t of these n e a r l y i d e a l m e t e o r o l o g i c a l c o n d i t i o n s , the seas remained calm which meant that no adjustments to the data were necessary to account f o r wind d r i v e n c u r r e n t s . Part A Figure 13 summarizes the f i n d i n g s of the p r o f i l e experiment. Each bar on the graph i s the average current speed over 25 h which c o n s i s t e d of 150-two minute recording episodes. The lack of any strong v e r t i c a l v e l o c i t y shear i s i n d i c a t e d by the s i m i l a r i t y of current speed at a l l depths outside the net-cages. Current speed at 2m, i n s i d e cage 6, i s reduced by 65% r e l a t i v e to that at the northern current meter l o c a t i o n . At depths greater than 5 m, current speed i n s i d e and outside the net-cages i s n e a r l y i d e n t i c a l , with a s l i g h t l y higher speed i n s i d e at 5 m and a s l i g h t l y lower speed i n s i d e at 8 m depth. SCUBA d i v e r s reported that heavy f o u l i n g of the nets by mussels occurred to a depth of about 4 m. Figure 13. P l o t of current speed versus depth f o r the p r o f i l e experiment at the J e r v i s I n l e t s i t e . E r r o r bars are one standard d e v i a t i o n . 44 Part B Current speed and d i r e c t i o n over the 25 h sampling p e r i o d f o r each of the current meters i s i l l u s t r a t e d i n Figure 14. Each current meter contains i t s own magnetic compass, which ensures that the h o r i z o n t a l a x i s f o r each vector diagram has the same east-west o r i e n t a t i o n . However, due to the p e r i o d i c swinging of the r a f t upon i t s mooring l i n e s , o r i e n t a t i o n of the r a f t ' s long a x i s r e l a t i v e to the h o r i z o n t a l a x i s of the ve c t o r diagrams i s v a r i a b l e . Nevertheless, v i s u a l observations of reference p o i n t s on the neighbouring i s l a n d i n d i c a t e d that the r a f t r o t a t e d no more than 10 degrees over a t i d a l c y c l e . The twice d a i l y r e v e r s a l of the t i d e i s seen i n the records provided by the instrument deployed north of the r a f t (Figure 14). Average diminution of current speed across the s i t e from north to south i s approximately 30%. Cages on the southern side of the r a f t ( i . e . cages 2, 6, and, 10) e x h i b i t slower current speeds than adjacent northern cages. D i r e c t i o n a l changes i n flow are evident i n a l l cages r e l a t i v e t o the northern outside reference current meter. The greatest d i r e c t i o n a l change i s i n cage 15, where the t i d a l s i g n a l i s masked by a p e r s i s t e n t westward flow. Current speed i s the highest (ave. 5.3 cm/sec) i n cage 15, with the next cage south, cage 10, al s o e x h i b i t i n g some of the same p a t t e r n of d e v i a t i o n away from t y p i c a l t i d a l i n f l u e n c e both i n speed and d i r e c t i o n . 45 Figure 14. Vector diag indicated locations in the Jervis Inlet s i t e . 6 m for a duration of simultaneously at each is proportional to cur while the orientation above diagrams are ave 25 h sampling period i rams of data collected at the and around the raft of net-cages at Depth of instrument deployment was 25 h. Data was collected location. The length of each vector rent speed (1 division = 1 cm/sec),' indicates flow direction. Figures rage speed and (direction) for the llustrated in each vector diagram. 46-Part C P l o t s of d i s s o l v e d oxygen concentrations measured i n s i d e cages 23 and 15 f o r the 25 h data c o l l e c t i o n p e r i o d are shown i n Figure 15. D i u r n a l p e r i o d i c i t y i s seen i n the 1 m p l o t s i n s i d e both cages. The lowest DO values occurred between 01:00 and 06:00 except f o r one sample at 20:30 i n cage 15. (Sunset occurred at 20:50 and sunrise at 05:30). DO values f o r both cages were higher near the surface than at 6 or 11 m (Table 3). The only s i g n i f i c a n t d i f f e r e n c e observed between the two cages was at 6 m depth, where cage 23 showed greater values than cage 15. The p a t t e r n of ammonia concentration changes over 25 h i n both cages was s i m i l a r f o r the surface sampling depth (Figure 16). Peak values occurred at 10:00 and 05:30 f o r cages 23 and 15 r e s p e c t i v e l y . No c l e a r p a t t e r n of minimum ammonia values f o r surface waters was apparent. The d a i l y time course of ammonia concentration was even more i r r e g u l a r at 6 m depth. Values are higher at 6 m than near surface waters. There were .no s i g n i f i c a n t d i f f e r e n c e s i n ammonia concentrations, at e i t h e r depth, between cages 23 and 15 (Table 3) . Figure 15. Comparisons of d i s s o l v e d oxygen concentrations versus time f o r the i n d i c a t e d depths i n cage 23 ( O ) and cage 15 ( • ) at the J e r v i s I n l e t s i t e . CP c CD CD >s X O TJ CD > O CO CO Q 1 0 - 6 meters 9 8 7 6 1 1 T 1 o 4- 11 meters 8 + 7 6 -o--o . • - - § . > V -O 5 16:00 22 :00 04 :00 Time of day 10:00 16:00 Table 3. Comparisons of DO and t o t a l ammonia i n s i d e cages 23 and.15 at the JER s i t e . Data were pooled f o r each depth and l o c a t i o n using values c o l l e c t e d over the 25 h sampling program. (SD=standard d e v i a t i o n , df=degrees o f freedom) Location Depth Parameter Cage 23 Cage 15 t - t e s t (m) mean (SD) mean (SD) ^calc (df) p JER 1 DO 9.5 (0.8) 9. 3 (0 .8) 0 . 682 (34) 0 .500 21 J u l y , 6 (mg/L) 7.9 (0.5) 7. 1 (0 .6) 4 .358 (34) <0 .001 1988 11 t o t a l 6.5 (0.2) 6. 4 (0 .3) 1 .382 (34) 0 . 176 0.5 ammonia 0.9 (1.1) 0. 9 (0 .8) 0 .059 (34) 0 . 953 6 (|imol/L) 4.4 (2.6) 6. 1 (2 .5) 1 .791 (27) 0 .085 49 Figure 16. Comparisons of ammonia concentrations versus time for cages 23 & 15 at the Je r v i s Inlet s i t e . Depth of sampling was A) 0.5 m and B) 6 m. A o E 3 o 'c o E E D ~o •*-> O 16:00 22 :00 04:00 10:00 16:00 B 10 + o E D ' c O E E D "5 -t-> o 16:00 22 :00 04:00 T ime of day 10:00 16:00 50 Dis c u s s i o n •The p r o f i l e experiment (Part A) confirmed Inoue's (1972) and Wee's (1979) f i n d i n g s that nets used i n c o n s t r u c t i o n of net-cages may r e s t r i c t water flow up to 80%, depending upon the degree to which they were fouled. I t was found that current speeds were diminished by 65% at the depths where the net was f o u l e d by mussels and macro-algae, however at depths below the f o u l i n g , current speeds were w i t h i n ±11% i n s i d e and outside of the net-cage. In f a c t , current speed was ca. 11% greater i n s i d e the net-cage than outside at 5 m, perhaps due to the swimming behaviour of the f i s h . Inoue (1972) found that Seriola quinqueradiata (hamachi) may cause a r o t a t i n g current of 1-3 cm/sec when the ambient current speed f a l l s below 4 cm/sec. Salmonids have a l s o been reported to maintain a c i r c u l a r swimming pa t t e r n i n net-cages, the d i r e c t i o n of which was maintained regardless of t i d e , season, or age of the f i s h ( S u t t e r l i n et a l . , 1979). Inoue (1972) i n d i c a t e d that t h i s behaviour would r e s u l t i n water exchange being maintained through the net^-cage even during periods of l i t t l e or no t i d a l flow. O v e r a l l p atterns of flow diminishment across the s i t e are i n general agreement w i t h the p r e d i c t i o n s of Weston (1986) . Flow was reduced ca. 30% on the south side of the r a f t r e l a t i v e to the north, which compares w e l l w i t h Weston's (1986) p r e d i c t i o n of 50% blockage. Because n e i t h e r was the d i r e c t i o n of current perpendicular to the r a f t , nor could the i n f l u e n c e of b i o f o u l i n g be a c c u r a t e l y measured, 51 these f i g u r e s are considered to be i n reasonable agreement. The s u r p r i s i n g f i n d i n g i n t h i s part of the study, was the apparent d i r e c t i o n a l s h i f t and o v e r a l l greater current speeds observed i n cages 15 and 10. Aside from the doub t f u l s p e c u l a t i o n that the swimming behaviour of the f i s h enhances water v e l o c i t y i n these cages, the most l i k e l y reason f o r t h i s apparent anomaly i s that l o c a l bottom topography i n f l u e n c e d the t i d a l flow p a t t e r n s . The l o c a t i o n of the r a f t i n an area of many bottom i r r e g u l a r i t i e s and i s l e t s may put i t p a r t i a l l y under the i n f l u e n c e of topographic s t e e r i n g or an eddy (Pond and P i c k a r d , 1986) . E i t h e r of these phenomena would a l s o produce the observed anomalies i n current speed and d i r e c t i o n across the r a f t by s u b j e c t i n g various p o r t i o n s of the r a f t to d i f f e r e n t flow streams. The dynamics of d i s s o l v e d oxygen over both depth and time are t y p i c a l f o r the J e r v i s I n l e t area. H i s t o r i c a l values (DOUBC, 1985 and 1986) i n d i c a t e that DO ranges from 6.1 to 7.9 mg/L f o r the summer months at 10 m, which compares w e l l with measured values of 6.5 mg/L at 11 m. The near surface DO v a r i e d d i u r n a l l y , presumably with the d i u r n a l p e r i o d i c i t y of photosynthesis and r e s p i r a t i o n as explained i n Chapter 1. Cage 15 DO values were lower than those of cage 23 i n s p i t e of the greater current speed .in the former. Stocking d e n s i t i e s were s i m i l a r i n both cages, but the f a s t e r current speed i n cage 15 may n e c e s s i t a t e a greater swimming speed and hence, metabolic rate by the 52 f i s h . An increase i n metabolic r a t e would increase the rate of oxygen consumption as w e l l as increase the rate of ammonia production (McLean and Fraser, 1974). Both these changes i n water q u a l i t y were seen i n cage 15 r e l a t i v e to cage 23, which experienced slower water speeds. This f i n d i n g was s u r p r i s i n g , since i t found i n Chapter 1 that increases i n water flow would increase DO and decrease ammonia concentrations. Perhaps the i n a b i l i t y to detect a c o r r e l a t i o n between water q u a l i t y and water speed i n t h i s case, i s due to a balance between f l u s h i n g r a t e and metabolic r a t e . As current speed increased, f l u s h i n g rates increased but metabolic rates would have a l s o increased and thus, l i t t l e change i n water q u a l i t y was observed. The l o c a t i o n of these two cages w i t h i n the r a f t , coupled with the observed flow d i r e c t i o n s i n d i c a t e s another tenable e x p l a n a t i o n . Cage 15 i s the t h i r d cage from the eastern end of the r a f t , and appears to r e c e i v e water from the two eastern-most adjacent cages throughout most of the t i d a l c y c l e . This flow regime advects DO-depleted, ammonia-loaded, waters from the eastern cages, i n t o cage 15. At the western end of the r a f t , the predominant current flow was from the outside of cage 23, thereby advecting water that has not already been through surrounding net-cages and was consequently of higher water q u a l i t y . In order to compare ammonia enrichment of the water column observed i n t h i s study w i t h p r e d i c t e d values, a reference to background l e v e l s was needed. Sampling 53 performed outside cage 23 could be considered to be upstream of the r a f t f o r at l e a s t part of the t i d a l c y c l e . In f a c t , average ammonia concentrations were lower outside than i n s i d e the cages (by 0.4 and 2.7|imol/L at surface and 6 m depths. r e s p e c t i v e l y ) . Weston (1986) p r e d i c t e d a r i s e i n t o t a l ammonia concentration of 0.02 mg/L (=1.4 (xmol/L) i n waters passing through a farm of s i m i l a r biomass. Values much greater than p r e d i c t e d were p e r i o d i c a l l y observed w i t h i n the net-cages. However, Weston was p r e d i c t i n g the e f f e c t by the whole 250,000 kg farm, and i t i s do u b t f u l , because of mixing by t i d a l a c t i o n , and the r a p i d uptake of ammonium by phytoplankton ( G l i b e r t and Goldman, 1981), that these p r e d i c t e d values would be observed at any great distance from the farm i t s e l f . The only s i g n i f i c a n t d i f f e r e n c e i n DO i n s i d e and outside of the cages was observed at 6m i n Cage 23 (Table 2) . At t h i s depth the 25 h average d i f f e r e n c e was 0.4 mg O2/L, which compares favourably w i t h Weston's (1986) p r e d i c t i o n of a 0.3 mg O2/L decrease. I t i s suggested that enrichment of the environment i n ammonia i s o c c u r r i n g .within t h i s farm s i t e . Cruise data (Dr. P.J. Harrison, unpublished data) c o l l e c t e d on 28 J u l y , 1987 near A c t i v e Pass, i n the S t r a i t of Georgia, B.C., i n d i c a t e that ammonium values never exceed 0.47(imol/L at 12 m depth. Moreover, a s t a t i o n over the Iona I s l a n d sewage o u t f a l l , y i e l d e d values up t o 9.42(imol/L ammonium. C l e a r l y , h y p e r n u t r i f i c a t i o n of the water column i s oc c u r r i n g w i t h i n 54 the farm s i t e , but even outside of cage 23, values were never as high as i n s i d e , i n d i c a t i n g that d i l u t i o n i n the surrounding waters i s r a p i d . Black and Carswell (1986) found that ammonia values downstream of a f i s h farm which was not i n use, but fo u l e d w i t h mussels, were s i m i l a r to values measured at the same distance downstream from a productive farm. This f i n d i n g would make i t very d i f f i c u l t to assess the true impact of f i s h farming unless a s i t e deplete of f o u l i n g organisms could be found (e.g. immediately a f t e r net c l e a n i n g ) . Increases i n biomass, length and c h l o r o p h y l l a content of the green alga Cladophora glomerata have been a t t r i b u t e d to increased n u t r i e n t s near f i s h farms i n the B a l t i c Sea (Ruokolahti, 1988). In the c o a s t a l waters of B.C., n u t r i e n t d e p l e t i o n i s often the l i m i t i n g f a c t o r i n r e g u l a t i n g phytoplankton blooms (Harrison et a l . , 1983) and so any input of ammonia to the environment may serve to increase the frequency and magnitude of blooms. No evidence of i n c r e a s i n g bloom frequency has been reported f o r B.C. waters due to salmon farming, however, i n t e n s i v e f i s h c u l t u r e i n the Inland Sea of Japan has been suggested to be a source of d i s s o l v e d organic matter, r e s p o n s i b l e f o r i n c r e a s i n g the occurrence of t o x i c red t i d e causing phytoplankton, Gymnodinium type-'65 and Chattonella antiqua (Nishimura, 1982). 55 Conclusions: Water flow and water q u a l i t y v a r i e d with depth i n s i d e net-cages, although they appeared to be unrelated. Observed d i f f e r e n c e s i n water flow were due to the presence of mussels on the upper 4 m of the net-cage. Good agreement was found between pu b l i s h e d and observed values f o r current diminution r e s u l t i n g from f o u l i n g . Observed d i f f e r e n c e s over depth, i n DO and ammonia concentrations, were speculated to be a r e s u l t of f i s h behaviour and metabolism i n conjunction with water flow p a t t e r n s . I t was found that the use of current flow patterns outside of the r a f t to p r e d i c t w i t h i n r a f t water flow patterns was sometimes misleading. I t was speculated that l o c a l topography i n f l u e n c e d w i t h i n s i t e flow patterns to such a degree that i n some downstream cages, higher current speeds were experienced than those upstream. For c e r t a i n l o c a t i o n s w i t h i n the r a f t , the r a f t d i d i n f l u e n c e water exchange i n the p r e d i c t e d f a s h i o n . Comparisons made using the data c o l l e c t e d from the two outside s t a t i o n s , i n d i c a t e d slower current speeds on the downstream side of the r a f t r e l a t i v e to upstream. Observed v a r i a t i o n s i n d i s s o l v e d oxygen and t o t a l ammonia concentrations were i n agreement wi t h the p r e d i c t i o n s of Weston (1986). 56 General Discussion While some of t h i s study i n v o l v e d comparison of c o n d i t i o n s w i t h i n net-cages i n B.C. to those i n Japan and Europe, the lack of previous data f o r l o c a l farming c o n d i t i o n s and p r a c t i c e s made t h i s a necessary f i r s t step. Results were found to be comparable to the previous stud i e s with r e l a t i o n to diminishment of current speed, u t i l i z a t i o n of DO, and build-up of ammonia w i t h i n a f i s h farm net-cage. From the p e r s p e c t i v e of the f i s h farmer, t h i s i nformation may a i d i n determination of r a f t c o n f i g u r a t i o n , net-cage depth, s t o c k i n g d e n s i t y , and geographic l o c a t i o n . The i n f l u e n c e of hydrography should always be considered i n s e l e c t i n g a s i t e (Landless and Edwards, 1976). Moreover, the presence of eddies, topographic s t e e r i n g , and i n t e r n a l waves through a s i t e must al s o be taken i n t o account. I f discovery of any of these t o p o g r a p h i c a l l y d e r i v e d problems occurs a f t e r the farm s i t e i s e s t a b l i s h e d , m i t i g a t i o n may be as simple as employing deeper nets w i t h lower s t o c k i n g d e n s i t y , to all o w the f i s h to i n h a b i t the l e a s t p h y s i o l o g i c a l l y s t r e s s f u l p o r t i o n of the water column ( P i c k e r i n g , 1981). In the case of d i f f e r i n g current speeds w i t h i n the r a f t , r e o r i e n t a t i o n of the r a f t w i t h respect to ambient current patterns may a l s o increase water exchange. In net-cages at the J e r v i s I n l e t farm, temperatures v a r i e d markedly w i t h depth, w i t h the near surface waters remaining too warm f o r optimal growth (Caine et a l . , 1987). However, the use of 15 m deep nets,' allows a refuge f o r the 57 f i s h below the thermocline, thereby decreasing stress, and hence, increasing growth rate. The concept of a refuge within the net-cage also applies to avoidance of harmful phytoplankton species, such as the diatom Chaetoceros convolutus and the d i n o f l a g e l l a t e Heterosigma akashiwo, which have • k i l l e d farmed salmon in B.C. (Pennell, 1988). Current husbandry practices during blooms of these species, includes cessation of feeding and lowering of the nets deeper i n the water, i n the hopes of keeping the f i s h out of the near surface waters, where these blooms generally occur (Harrison et ,a l . , 1983) . No previous studies on the diurnal p e r i o d i c i t y of water quality parameters in marine net-cages were found i n the l i t e r a t u r e . This study f i l l s a gap in the knowledge of these events and, as well, indicates a future d i r e c t i o n of research. Water quality monitoring programs, mandatory by law (Anonymous, 1988), may benefit from information gained in t h i s study in terms of selecting the most appropriate time and location within a s i t e for sampling. By sampling i n the middle depths of a net-cage, during slack water, and at least 4 h after feeding, the highest concentration of ammonia should be found. DO readings should be taken during slack water, and also over the entire depth of the net-cage, preferably just before sunrise, to ensure that the most c r i t i c a l values are monitored. Sampling i n t h i s manner w i l l provide the most useful data to both the farmer and the monitoring agency. Future work should i n c l u d e a t i m e - s e r i e s a n a l y s i s of water q u a l i t y i n and around a f i s h farm under c o n t r o l l e d c o n d i t i o n s . A t i m e - s e r i e s a n a l y s i s would allow i d e n t i f i c a t i o n of the p e r i o d i c i t y of the i n d i v i d u a l components (e.g. t i d a l exchange, r e s p i r a t i o n , and photosynthesis) and thus provide the data needed f o r modelling the environmental impact of f i s h farming. Two s i t e s were v i s i t e d during t h i s study which v a r i e d considerably i n d a i l y husbandry p r a c t i c e s . Further i n v e s t i g a t i o n of these f a c t o r s (e.g. feeding schedule, s t o c k i n g d e n s i t y , and feed type) i s needed to determine the r o l e they may have i n i n f l u e n c i n g water q u a l i t y . 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