A N INVESTIGATION OF A POTENTIAL CARRYING C A P A C I T Y OF C O H O A N D CHINOOK S A L M O N IN T H E GEORGIA STRAIT by SCOT A L E X A N D E R M O U N T A I N B.Sc, The University of British Columbia, 1992 A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T OF T H E REQUIREMENTS FOR T H E D E G R E E OF M A S T E R OF SCIENCE in T H E F A C U L T Y OF G R A D U A T E STUDIES D E P A R T M E N T OF Z O O L O G Y We accept this thesis as conforming to the required standard T H E UNIVERSITY OF BRITISH C O L U M B I A July, 1996 © Scot A. Mountain, 1996 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 of ^ o o logjv.^ The University of British Columbia Vancouver, Canada Date YVJ ST* . y P l 0 ! ^ • DE-6 (2/88) A B S T R A C T Stable or decreasing catches in conjunction with increasing hatchery releases have suggested decreasing marine survival rates for populations o f Pacific salmon (Oncorhynchus sp.) in the Georgia Strait. I examined the possibility that a carrying capacity is imposing limits on the populations o f coho (Onchorhynchus kisutch) and chinook (Oncorhynchus tshawytschd) salmon. Two investigations were carried out; the first involved an examination o f the impact that juvenile salmon have on their food supply. The second used a computer model to predict the possible results that a hatchery based fisheries manipulation might produce under different experimental protocols. The feeding study suggested that juvenile salmon might be having, a much greater impact on their available food supply than has previously been suspected. Overall , it was estimated that chinook and coho together consume an average of 4 % to 6 % o f their main foods daily. If these impacts are taken together with those o f other species, this suggests that a carrying capacity might wel l be important. A hatchery manipulation experiment is one obvious way to test for a marine survival l imit as implied by a carrying capacity. Us ing a metagaming approach to model such an experiment, insights were obtained into how it could be performed most efficiently. The results suggest that, depending on the required outcome, it would be advisable to maintain current exploitation rates of both coho and chinook stocks during such an experiment. Other factors that would favor a rapid conclusion to the experiment are extreme as opposed to conservative manipulations, and minimal attempts to rebuild stocks through other means. However, even i f these recommendations are heeded, the model suggests that a hatchery experiment might need to be a long term project. Wi th reductions in hatchery releases as high as 75% every second year, average times to produce conclusive results were on the order o f a decade or more. T A B L E O F C O N T E N T S Page T I T L E P A G E i A B S T R A C T i i L I S T O F T A B L E S v L I S T O F F I G U R E S v i A C K N O W L E D G M E N T S v i i i C H A P T E R 1: G E N E R A L I N T R O D U C T I O N 1 C H A P T E R 2: A D I R E C T I N V E S T I G A T I O N O F F O O D S U P P L Y 6 2.1. I N T R O D U C T I O N 6 2.2. F E E D I N G S T U D Y 7 2.2.1. Materials and Methods 7 2.2.2. Results H 2.2.3. Discussion of Stomach Contents Patterns 17 2.2.4. Feeding Study Conclusions 22 2.3. V I R T U A L P O P U L A T I O N A N A L Y S I S 24 2.3.1. Introduction 24 2.3.2. Materials and Methods 25 2.3.3. Discussion of V P A Results 32 2.4. B I O E N E R G E T I C S •• 36 2.4.1. Materials and Methods 36 2.4.2. Discussion of Results 48 2.5. F O O D A V A I L A B I L I T Y S T U D Y 60 2.5.1. Discussion 61 2.6. S U M M A R Y 67 i i i C H A P T E R 3: M E T A G A M E - A C O M P U T E R M O D E L T O A S S I S T I N D E S I G N I N G A L A R G E S C A L E F I S H E R I E S E X P E R I M E N T 68 3.1. I N T R O D U C T I O N 68 3.1.1. What the "Game" in the Metagame Program Does 70 3.2. M A T E R I A L S A N D M E T H O D S 72 3.2.1. The Bayesian Assessment Method 72 3.2.2. The Stock Production M o d e l 74 3.2.3. Us ing the Metagame 76 3.2.4. The Questions that Were Addressed 83 3.2.5. Simulations Run 84 3.3. R E S U L T S A N D D I S C U S S I O N 85 3.3.1. Manipulation o f Stocking Reduction Levels, Hypothesis 4 (Marine Carrying Capacity) True 85 3.3.2. Manipulat ion o f W i l d Catch Retention, Hypothesis 4 True 91 3.3.3. Reducing Exploitation Rates, Hypothesis 4 True 93 3.3.4. Increasing Exploitation Rates, Hypothesis 4 True 93 3.3.5. The Other Hypotheses 96 3.3.6. Conclusions 100 C H A P T E R 4: G E N E R A L C O N C L U S I O N S , 102 R E F E R E N C E S 104 iv LIST O F T A B L E S Table Page Table 1. Frequency of occurrence and abundance by weight o f chinook stomach contents for five sampling periods during the summer o f 1993 11 Table 2. Frequency of occurrence and abundance by weight o f coho stomach contents for five sampling periods during the summer o f 1993 12 Table 3. A taxonomic listing of items identified in smolts' stomachs 13 Table 4. Release o f chinook juveniles from S E P facilities 27 Table 5. Chinook salmon: adult hatchery catch contributions broken down by smolt year and catch year....28 Table 6. Chinook salmon; percent o f each hatchery smolt year represented in each catch (representation coefficients) 29 Table 7. Chinook salmon; percent representation o f w i l d smolts in catches, assuming that hatchery smolts survive 80% as wel l as w i l d smolts to be caught as adults 30 Table 8. Chinook salmon; number of adults in each catch year that originated in each smolt year 31 Table 9. Chinook smolts; estimates o f w i l d smolt population sizes derived from catch statistics and representation coefficients 32 Table 10. Prey consumption (cal/m-^/day) vs. abundance estimates (cal/rn-^) 61 Table 11. The parameter settings which defined the manipulations tested 85 v LIST O F F I G U R E S Figure Page Figure 1. Decreasing catches of w i ld coho and chinook in conjunction with increasing releases from hatchery operations have ocurred in the Georgia Strait 2 Figure 2. F l o w chart showing the procedures involved in the direct investigation of the food supply 7 Figure 3. Location o f the stomach sampling cruises on the Georgia Strait 9 Figure 4. Coho: Stomach abundance by weight o f major prey items during the summer of 1993 14 Figure 5. Chinook: Stomach abundance by weight o f major prey items during the summer of 1993 16 Figure 6. Feeding regimes used in the bioenergetics model 23 Figure 7. F low chart showing the procedure followed in the modified V P A 25 Figure 8. Chinook salmon; upper bound estimates of number of smolts entering the Georgia Strait, assuming l 80% ratio o f hatchery to w i ld survival 34 Figure 9. Coho salmon; upper bound estimates of number of smolts entering the Georgia Strait, assuming 80% ratio o f hatchery to w i ld survival 35 Figure 10. Temperature regime used in the bioenergetics model 42 Figure 11. Range of consumption estimates for major prey items (in calories/cubic metre/day) 53 Figure 12. The Metagame main user interface 76 Figure 13. The multitrial plots form shows the patterns in probabilities placed on the various hypotheses....78 Figure 14. The graph control form allows the metagame user to plot up to four graphs, each showing different information 80 Figure 15. The Game Parameters Form shows the various parameters and default values that affect the running o f a metagame 81 Figure 16. The M o d e l Parameters Form shows how the Metagame defines population production under the four different hypotheses 82 v i Figure 17. The proportion of trials that showed the probability of hypothesis 4 being true to be greater than 90% when hypothesis 4 was in fact true 87 Figure 18. The proportion of trials that showed the probability of hypothesis 4 being true to be greater than 90% when hypothesis 4 was true 89 Figure 19. The proportion o f trials that showed a specific l ikelihood o f hypothesis 4 being true 91 Figure 20. The proportion o f trials that showed the probability of hypothesis 4 being true to be greater than 90% when hypothesis 4 was true 95 Figure 21. Chinook: decline in the estimated probabilities o f hypothesis 4 being true when the other hypotheses were true 97 Figure 22. Coho: decline in the estimated probabilities o f hypothesis 4 being true when the other hypotheses were true 98 v i i A C K N O W L E D G M E N T S This thesis would not have been completed without the assistance and support o f many people. I would like to thank my supervisor, Dr. Car l J. Walters, for suggesting, obtaining funding, and providing support for the project. Funding was provided v ia an N S E R C science subvention grant, and through a grant from the Ocean Production Enhancement Network ( O P E N ) . Dr . Walters also conceived and produced the heart o f the Metagame model. Particularly the population production and the Bayesian assessment sub-models remained virtually unchanged from his original version. The staff o f the Pacific Biologica l Station were instrumental in helping me obtain stomach samples from smolts collected in conjunction with their sampling cruises. I am especially grateful to Barb Thompson, Chris Nevi l l e and Dr . Richard Beamish for their assistance. Identification of stomach contents would not have been possible without the longterm and patient efforts o f Martha Haro. Dr . J .D. M c P h a i l also provided some welcome assistance in identifying many specimens o f half digested fish larvae. The ecology graduate students o f U B C provided support throughout the project. Joe DeGis i , Leonardo Huato, and Joel Sawada were always ready with sage advice on any topic. Chris Schell acted as a sounding board for my ideas on many occasions. A n d the spruce grouse club provided me with the humor that was necessary to maintain my perspective. Final ly , thanks to Tom and Diane for getting me started and keeping me going. I couldn't have done it without you. A n d Sara, you 've kept me sane through some crazy times, while making some o f the saner times pretty crazy. Thanks for your support, love, and friendship. v i i i C H A P T E R 1: G E N E R A L I N T R O D U C T I O N The coho (Oncorhynchus kisutch) and chinook (Oncorhynchus tshawytschd) fisheries in the Georgia Strait are among Canada's most economically important natural resources. From 1987 to 1990, the commercial catch o f these two species generated an average of $63 mi l l ion per year ( D F O 1992 a). However, this figure pales in comparison to the income generated by tourism related to the extensive sport fisheries based in the Georgia Strait. Historically, recruitment to these fisheries came primarily from w i l d fish that spawned in the streams and rivers around the Strait. However, in the early 1970's hatcheries supported by the Canadian Salmonid Enhancement Program began to contribute a significant proportion o f fish to the total catch. Since then, the proportion o f hatchery fish in Georgia Strait catches has been steadily increasing, while the w i ld fish proportion has been declining. B y 1992, hatchery fish constituted up to 20% o f the chinook catch (Cross et al. 1991), and half o f the total coho catch (Walters 1993). The increase in hatchery releases and catches has not been accompanied by a proportional increase in the total numbers of fish caught in the Strait. In the case o f coho, overall catches have remained more or less constant over the last thirty years (Walters 1993). However, proportions of w i ld fish in the catch are significantly lower than historical levels . Georgia Strait chinook catches have shown a significant decline from 1978 to 1989 (Cross et al. 1991). This decline in total catches is mirrored almost exactly by the decline in numbers o f w i l d fish being caught. Figure 1 illustrates the increasing hatchery releases, and the concurrent decreasing w i ld catches o f both species. The stable or decreasing catches, in conjunction with increasing hatchery releases into the Strait, suggests a major negative impact on the productivity of w i ld salmon stocks. Concern among Brit ish Columbia's fisheries managers and scientists has led to several investigations of possible causes o f the w i ld stock decline. Initial 1 recommendations from the Department o f Fisheries and Oceans (DFO) for reversing the trend focussed on fishing restrictions, habitat restoration, and continued hatchery production to enhance the failing stocks ( D F O 1992 b). Figure 1. Decreasing catches of wild coho and chinook in conjunction with increasing releases from hatchery operations have ocurred in the Georgia Strait. Data from Cross et al. 1991. Chinook Salmon (/> T3 1 s S J re £ " z 2 d) § I 3 SI 1980 1982 Year 2 More recently, it has been suggested that the combination o f increasing hatchery releases with stable or reduced returns may in fact suggest the existence o f a carrying capacity limit that imposes an upper threshold on the numbers o f salmon the Strait can produce. Thus, it may be that the decline in wi ld stocks is a result o f competition with hatchery fish for limited resources. Other attempts to explain the decline have concentrated on environmental conditions such as ocean temperatures and pollution. In total, four main hypotheses have been advanced to explain the declines in w i l d coho and chinook abundances (Walters, 1993). These hypotheses are; 1. Overf ishing. 2. Freshwater rearing habitat limitation. 3. Changing oceanographic conditions. 4. Marine carrying capacity. Each o f these explanations has plausible arguments for and against it. The overfishing hypothesis is one that is commonly touted by both the media and D F O . In fact, there is supporting evidence to suggest that a very low proportion o f w i ld fish that recruit to the fishery survives to spawn. However, serving as evidence against this hypothesis is the lack of information relating spawning stock sizes to recruitment. In fact, there are indications that some salmon fisheries have survived under much higher exploitation rates. For example, Fraser River sockeye have been shown to sustain and even increase their populations under fishing impacts that are similar to those experienced by coho and chinook in the Georgia Strait (Walters 1993). Another popular theory is that the decline in w i l d stocks is the result o f a loss o f rearing habitat in freshwater streams and rivers. There are certainly some major impacts on these habitats due to human activities such as forestry, urban development and mining. However, it is not clear whether these activities impact rearing habitat positively, negatively or not at all from the fish's point o f view. In fact, evidence exists that habitat disturbances caused by logging operations may actually be associated with increased smolt production (Holtby 3 1988). Even i f habitat impacts are negatively affecting smolt production, it is highly unlikely that the amount o f habitat destruction that has occurred could account for the large reductions in w i ld abundance (Walters 1993). Further evidence against the overfishing and habitat destruction hypotheses is given by the apparently reduced marine survival rates observed for salmon in the Georgia Strait. These estimates come from coded wire tag ( C W T ) data summarized by Cross et al. (1991). This suggestion of reduced survival implies that the smolt numbers entering the Strait must have stayed the same, or even increased, in order to produce the observed catches. However, this would not be the case i f overfishing or habitat limitation was occurring. I f either o f these hypotheses were correct, then smolt production must have declined, and marine survival must have remained constant or even increased. Unfortunately, the reliability o f the marine survival estimates is questionable, especially for w i l d stocks. Thus, they cannot be taken as definitive evidence against the overfishing or habitat limitation hypotheses. C W T data also provide evidence that any limit on salmon survival is impacting the young fish, in their first year at sea. This conclusion can be drawn from the fact that the proportion o f w i ld one year old "jacks" in the catches has not decreased relative to the older fish (Cross et al 1991). Therefore, there does not appear to have been any reduction in survival between one year o f age and later years. This means that whatever is reducing survival rates is probably acting on the smolts shortly after they go to sea. The two hypotheses that are most consistent with decreased marine survival in the Strait are poor oceanographic conditions, and a marine carrying capacity limit. It is impossible, with the historical data, to either prove or disprove that an oceanographic influence has caused the decline in salmon survival. Environmental conditions such as water temperature and salinity are constantly changing, and these changes undoubtedly impact the resident species both positively and negatively. In particular, sea surface water temperatures have increased somewhat since the late seventies, and subsequent E l Ninos have been responsible for higher than normal temperatures in the early 1990s. However, because o f the complexity of the interactions in the Strait in response to changing environmental conditions, it is not an enlightening excercise to hypothesize how these conditions might negatively impact salmon survival. 4 The hypothesis that appears to best fit the observations is that o f a marine carrying capacity limit. This hypothesis claims that, due to limited available resources, a restricted number of salmon can be reared in the Strait each year. Thus, the proportion of w i ld salmon in the total stock is being reduced as more hatchery fish claim a share o f the limited total capacity for adult production. The most obvious factor that might impose such a restriction is the limited availability o f food resources. Given that the annual productivity in the Strait must be finite, it is plausible that the Strait can only produce a circumscribed number of individuals o f each resident species. There is another reason that the possiblity of a carrying capacity limit in the Strait should be investigated. O f the four possible hypotheses defining the situation in the Strait, a carrying capacity limitation would be the most easily corrected by fisheries managers. A simple reduction in hatchery outputs to appropriate levels should be enough to improve the survival o f w i ld stocks in the Strait. Therefore, it is important to analyze the Georgia Strait fisheries for any evidence that the hatchery program may be the direct cause o f the destruction of w i l d salmon stocks. The focus of my study was two-fold. Initially, I attempted to investigate the food supply o f Georgia Strait smolts to see whether or not food could be imposing a carrying capacity l imit on production. The second part o f my project involved using a computer model to help design a hatchery based experiment that could aid fisheries managers in differentiating which of the four hypotheses is governing the current situation in the Georgia Strait. The concept o f this experiment is to directly manipulate total hatchery smolt production on a large scale, to determine whether marine survival rates respond positively to hatchery smolt reductions as predicted by the carrying capacity hypothesis. 5 C H A P T E R 2: A D I R E C T I N V E S T I G A T I O N O F F O O D S U P P L Y 2.1. I N T R O D U C T I O N This chapter details my attempt to determine whether or not there is evidence of a food limited carrying capacity for coho and chinook smolts in the Georgia Strait. The investigation proceeded via five steps (Figure 2). The first step was a detailed analysis o f the main food items being taken by smolts in the Georgia Strait. The stomach contents o f nearly 600 smolts were excised and identified in order to determine what prey were preferred. Upon completion of the feeding study, a modified Virtual Population Analysis was performed, using existing catch statistics for Georgia Strait fisheries. This analysis provided an estimation of the total numbers o f fish o f various sizes present in the Strait over time. Having obtained detailed information on what the smolts were eating, and how many o f them were present, a bioenergetics model was used to combine this information with smolt growth data. This model provideds an estimation o f the total food consumption needed to produce the coho and chinook populations in the Georgia Strait. These food requirements were apportioned out to specific food items in appropriate proportions as indicated by the feeding study. The final step was to compare the estimates of the amount o f major food items being consumed with estimates o f food availability. Due to a lack of directly relevant food availability data, abundance and production estimates for the major prey items were obtained from historical oceanographic studies o f the Georgia Strait and nearby coastal environments. This step was the constraining factor in my ability to draw firm conclusions about the carrying capacity in the Strait. Because of this difficulty, the results o f the feeding study may best be used to guide further research. Nevertheless, the final result o f this procedure was a comparison of the amount o f food being eaten by coho and chinook with the amount o f food apparently available to them. This comparison was examined to ascertain whether or not it suggested the existence of a carrying capacity limit. 6 Figure 2. Flow chart showing the procedures involved in the direct investigation of the food supply. Results of the feeding study and VP A were used in the bioenergetics model. Bioenergetics results were compared with abundance estimates to produce an estimate of overall exploitation. Feeding Study Used: Stomach dissection and historical data. Produced: Information on preferred food items and early marine growth rates. \ VPA Used: Data on hatchery releases and catch-at -age for wild and hatchery fish. Produced: Estimates of numbers of smolts necessary to produce observed catches. Bioenergetics Used: Computer bioenergetics model of coho and chinook growth and physiology. Produced: Amount of calories of each food type necessary to produce observed growth. Food Availability Study Used: Data on nekton abundance and productivity from historical studies. Produced: Values to compare with bio-energetics output. Final Product Estimation of the impact young coho and chinook have on the available food supply. 2.2. F E E D I N G S T U D Y This section describes the stomach content analysis o f the salmon smolts. This procedure was carried out in order to gain a better understanding o f the food items that are important to young coho and chinook in the marine environment. It was necessary to obtain this information in order to allow a comparison o f the foods the salmon were eating with the available amounts. 2.2.1. Materials and Methods In the summer o f 1993, juvenile coho and chinook salmon were collected in conjunction with the Georgia Strait Juvenile Salmon Survey being carried out by the Pacific Bio logica l Station in Nanaimo, B . C . Fish were sampled from four separate cruises, on M a y 25 through 28, June 14 through 17, June 22 through July 9 and July 5 through 8. Three o f the cruises followed a preset series o f transects that crossed the strait, and extended from the Fraser River plume (123° 23 ' W latitude, 49° 2 ' N longitude) in the south to Qual icum Bay(124° 37 ' W latitude, 49 7 ° 26 ' N longitude) on the northern end (Figure 3). Fishing on the fourth cruise was concentrated in the area o f the Fraser River plume. Sets were generally from 30 to 60 minutes in length, with the shorter time interval being used when the nets were f i l l ing more quickly. Most o f the sets occurred during daylight hours. 25 o f the sets on the Fraser River Plume Cruise occurred at night. F ish were caught from the charter fishing vessel Qualicum Producer, using a dual beam trawl design. Two nets, each with a mouth opening o f approximately nine metres circumference, were trailed from outriggers located amidships . The nets were trailed off the stern, well clear o f the wake of the vessel. Sampling extended from the surface to a depth o f 6 to 7.5 metres. Mesh size on the cod end o f the nets was 2.5 centimetres. A liner with a mesh size o f 1.5 centimetres was used in the nets. Salmon were rapidly sorted from the catch and identified as to species. They were then measured, and either frozen or stored in 10% formalin for later analysis in the lab. Subsampling The first three cruises, which covered the entire strait, were divided geographically into six main areas depending on their depth and proximity to land (Figure 3). For every cruise, up to 30 fish o f both species were randomly sampled from the catch from each area. If fewer than 30 of a species were caught in an area, then all o f that species from that area were analyzed. For example, if, during cruise one, 75 chinook were caught in area three, then 30 were randomly selected to have their stomachs excised. If only 12 coho were caught in the same area, al l o f their stomachs were analyzed. For the Fraser River Plume cruise, chinook were randomly drawn from a total o f 84 separate sets. Since only 21 coho were caught on the entire cruise, all o f their stomach contents were analyzed. 8 Figure 3. Location of the stomach sampling cruises on the Georgia Strait. Three of the cruises followed the transects shown. These were further subdivided into areas as shown. The fourth cruise concentrated on the Fraser River plume. 9 t o Stomachs were excised from preserved fish in Nanaimo, and stored in 10% formalin for transportation back to Vancouver. Once in the Vancouver lab, stomachs were blotted dry and weighed on an electronic balance to the nearest 1 x 1 0 " ^ grams. Stomachs were then dissected, and the contents were emptied, rinsed, and examined under a dissecting microscope. The contents were identified and sorted into taxonomic categories. After being sorted into taxonomic groupings, the items in each group were blotted dry and weighed separately. The empty stomachs were also blotted and weighed. In total, stomachs from 575 juvenile salmon were analyzed. O f these, 335 were chinook and 240 were coho. After the stomach contents o f each fish were quantified individually, fish were pooled into five groupings. Each group consisted o f fish that had been caught within four day (ninety-six hour) time periods. This allowed a temporal comparison of their diet compositions over the summer. 2.2.2. Results For each time period, the average frequency of occurrence and the average numerical abundance by weight of each prey item was calculated (Tables 1 and 2). Table 1. Frequency of occurrence and abundance by weight of chinook stomach contents for five sampling periods during the summer of 1993. Italics show sample size. Prey May 25-28 (109) June 13-16 (93) June 21-24 (41) June 26-29 (28) July 4-7 (54) FO(%)* AW(%)* FO(%) AW(%) FO(%) AW (%) FO (%) AW (%) FO (%) AW (%) Fish larvae 22.9 12.6 26.9 12.5 48.8 17.4 32.1 13.3 33.3 14.0 Digested matter 38.5 23.1 72.0 25.6 97.6 42.9 82.1 33.2 75.9 41.1 Insecta 78.0 59.4 38.7 11.1 46.3 5.7 42.9 12.4 33.3 19.3 Gammaridean Amphipods 21.1 1.4 18.3 3.7 2.4 0.4 0.0 0.0 3.7 0.3 Hyperidean Amphipods 0.0 0.0 5.4 0.1 4.9 0.0 7.1 0.4 9.3 0.6 Cancer sp. larvae 27.5 1.6 40.9 12.8 65.9 28.6 64.3 21.6 55.6 23.6 Porcellanid larvae 0.9 0.9 40.9 33.3 4.9 2.2 50.0 19.1 1.9 0.6 Euphausiacea 1.8 0.1 3.2 0.9 0.0 0.0 0.0 0.0 1.9 0.2 Other identifiable matter 14.7 0.1 23.7 0.1 26.8 2.9 7.1 0.0 27.8 0.2 * Frequency of occurrence (FO) indicates the number of nonempty stomachs in which the prey item was present in any amount. Abundance by weight (A W) indicates the percent of the total weight of stomach contents constituted by the prey item. 11 A n y prey item that consistently made up more than 5% o f the diet (by weight) was considered to be a major prey item. These included fish larvae, terrestrial insects and cancer sp. larvae for coho, and the same items, with the addition o f porcellanid crab larvae, for chinook. The average percent abundance by weight was plotted for each o f these items for each species (Figures 4 and 5). Table 2. Frequency of occurrence and abundance by weight of coho stomach contents for five sampling periods during the summer of 1993. Italics indicate sample size. Prey May 24-27 (89) June 13-16 (7/) June 21-24 (//) June 26-29 (8) July 4-7 (51) FO(%)* AW(%)* FO(%) AW(%) FO(%) AW (%) FO (%) AW (%) FO (%) AW (%) Fish larvae 50.6 21.4 40.8 11.2 63.6 16.8 62.5 22.5 21.6 5.2 Digested matter 82.0 33.2 81.7 27.5 81.8 18.6 75.0 18.4 80.4 18.3 Insecta 92.1 34.8 39.4 10.8 72.7 10.3 0.0 0.0 5.9 0.3 Gammaridean Amphipods 49.4 4.3 5.6 0.1 9.1 1.3 0.0 0.0 2.0 0.0 Hyperidean Amphipods 1.1 0.1 19.7 2.2 18.2 2.2 0.0 0.0 17.6 1.9 Cancer sp. larvae 65.2 5.7 74.6 45.8 81.8 50.9 87.5 59.1 94.1 74.1 Porcellanid larvae 0.0 0.0 4.2 0.8 0.0 0.0 25.0 0.0 3.9 0.0 Euphausiacea 0.0 0.0 5.6 1.3 0.0 0.0 0.0 0.0 0.0 0.0 Other identifiable matter 15.7 0.6 25.4 0.3 18.2 0.0 25.0 0.1 37.3 0.1 * Frequency of occurrence (FO) indicates the number of nonempty stomachs in which the prey item was present in any amount. Abundance by weight (A W) indicates the percent of the total weight of stomach contents constituted by the prey item. Taxonomic identification o f prey items in the fish was limited due to the digested nature o f many of the stomach contents. Because o f this, most o f the contents were grouped into fairly broad categories, generally not proceeding beyond the level o f class or order. However, when a wel l preserved specimen was encountered, attempts were made to classify it as specifically as possible. Table 3 is a taxonomic list o f all the items that were found in the stomachs of both species o f juvenile salmon. Detailed identification of items to the species level is presented where possible. However, the species presented should be considered as examples, and not a complete list. For the more specific taxonomic groups (family, genus, and species) the inclusion of one group does not imply that other groups were not consumed. They may have been present, and simply not identified due to their advanced state o f digestion. 12 Stomach Content Change Over Sampling Period: Coho O n examination o f the food items taken by coho at different dates over the summer (Figure 4), two readily apparent changes in diet composition are seen. Early in the summer, terrestrial insects made up a large proportion o f the coho diet (35% by the end of May) . Table 3. A taxonomic listing of items identified in smolts' stomachs P H Y L U M C O E L E N T E R A T A Class Hydrozoa P H Y L U M N E M A T O D A (as parasites) Anisakis sp. P H Y L U M A R T H R O P O D A Class Crustacea Subclass Ostracoda Conchoecia sp. Subclass Copepoda Order Calanoida Acartia sp. Epilabidocera sp. Candacia sp. Order Cumacea Lamprops sp. Order Isopoda Gnorimosphaeroma sp. Order Amphipoda SubOrder Gammaridea Elasmopus sp. Stenothoides sp. Stenothoides burbanki Suborder Caprellidae Suborder Hyperiidea Hyperia sp. Hyperiella sp. Hyperiella macronyx Order Euphausiacea Euphausia pacifica Order Decapoda Suborder Reptantia Section Brachyura Family Cancridae Cancer sp (larvae). Family Grapsidae Hemigrapsus nudus (larvae) Hemigrapsus sp. (larvae) Section Anomura Fami ly Porcellanidae (larvae) Class Arachnida Class Insecta Order Coleoptera (beetles) Order Col lembola (springtails) Order Diptera (flies) Family Tabanidae (larvae) Order Hymenoptera (wasps) P H Y L U M M O L L U S C A Class Cephalopoda Loligo sp. P H Y L U M C H O R D A T A Class Osteichthyes (larvae) Family Clupeidae (herrings) Clupea harengus pallasi (pacific herring) Family Scorpaenidae (scorpionfishes) Sebastes sp. Fami ly Hexagrammidae (greenlings) Family Ammodytidae (sandlances) Ammodytes hexapterus Family Salmonidae (salmonids) Family Pleuronectidae (righteye flounders) P L A N T M A T T E R Phaeophyta (brown algae) Fucus sp. I N O R G A N I C M A T T E R Wood Plastics Fishing Line 13 A s the summer continued and insects became less important, brachyuran crab larvae became increasingly important, making up as much as 74% of the coho diet by the end of the sampling period. Larval fish comprised the other main constituent o f the juvenile coho diet. These appeared to be an important food item throughout the sampling period. Whi le the amount o f fish larvae being eaten varied somewhat by date, they never made up less than 5% o f the total diet by weight at any time. Figure 4. Coho: Stomach abundance by weight of major prey items during the summer of1993. Italics indicate sample sizes. Fish larvae Terrestrial Insects 36, £ 3 0 o> '55 * sr n 8 r C (0 •o I 1 ' n to s 10 Oi u V c D_ O • 0 1SM=y 4-Jtn 12-Jm Z X l r 28-Xn 43 „ 35 .c OJ | 30 >. » 2 5 u | 20 c i 15 n llllllll S 10 u 0) o- 5 :#^ lllflliSK«-. I_ t - 0 6JJ 14JJ 1M/ty 27-My 40n 12-Jin ZkJm 2SOn 6 JJ Date Decapod Larvae „ 7 0 jz CT) >. u TJ C Jl30-TtSrt^ kkold 0.2 0.0 Y o i (Iron end ol historical data) Probability Count 0 10 20 30 «0 SO 60 70 Fidelity