THE RELATIONSHIP BETWEEN THE SUMMER FOOD OF JUVENILE 



SOCKEYE SALMON, ONCORHYNCHUS NERKA, AND THE STANDING 



STOCK OF ZOOPLANKTON IN ILIAMNA LAKE, ALASKA' ' 



Stephen H. Hoag^ 



ABSTRACT 



The foregut contents of juvenile sockeye salmon in samples taken at night by tow net in 

 the limnetic area of Iliamna Lake consisted primarily of zooplankton and rarely of in- 

 sects. The number of organisms per foregut was correlated with the estimated zoo- 

 plankton density between and 100 m. Cyclops and Bosmina were the dominant zoo- 

 plankters in both foregut and zooplankton samples. The zooplankton hauls contained 

 a greater percentage of calanoid copepods than the fish foreguts. Food selectivity was 

 indicated but appeared to be minimal. Fry (age 0) foreguts contained a lesser percentage 

 of Cyclops and a greater percentage of Bosmina than did yearling (age I) foreguts. 



Juvenile sockeye salmon {Oncorhynchus nerka) 

 spend 1 or 2 years in Iliamna Lake before mi- 

 grating to sea. They occupy the littoral from 

 the time they emerge from the gravel, in late 

 winter or early spring, until mid-July, when 

 they move to the limnetic area where they re- 

 main until migrating to sea in the spring of the 

 following or second year (at age I or age II). 

 A similar change in distribution was found in 

 Lake Aleknagik in the Wood River system (Pel- 

 la, 1968) and is probably common for all juvenile 

 sockeye salmon in the lakes of Bristol Bay. Pre- 

 ious food studies, summarized by Rogers (1968) , 

 indicated that juvenile sockeye salmon feed pri- 

 marily on insects in the littoral and on zooplank- 

 ton in the limnetic area. 



The standing crop of zooplankton is usually 

 used as an indicator of food availability for zoo- 

 plankton feeders. However, differences in size, 

 agility, and visibility of the zooplankters may in- 

 validate this assumption. 



The objectives of this study were: (1) to de- 

 termine the food of juvenile sockeye salmon and 



' Contribution No. 353, College of Fisheries Univer- 

 sity of Washington, Seattle, WA 98195. 



" Work on this study was supported by the U.S. Fish 

 and Wildlife Service, Contract Nos. 14-17-0005-82 (B) 

 and 14-17-0005-129 (B). 



* Formerly, Fisheries Research Institute, University 

 of Washington ; presently with the International Pacific 

 Halibut Commission, University of Washington, Seattle, 

 WA 98195. 



Manuscript accepted November 1971. 

 FISHERY BULLETIN: VOL. 70, NO. 



2, 1972. 



(2) to compare the composition of the diet with 

 that of the estimated standing stock of zooplank- 

 ton during the summers of 1966 and 1967. The 

 diets of fry (age 0) and yearlings (age I) were 

 also compared. The population density of ju- 

 venile sockeye salmon in Iliamna Lake was high 

 during these years as the escapement into the 

 Kvichak River system was exceptionally large 

 (24.3 million fish) in 1965. 



THE ENVIRONMENT 



Iliamna Lake is the largest lake in Alaska, 

 with an area of 2,622 km' and an average depth 

 of 44 m. It empties into the Kvichak River, 

 which flows into Bristol Bay. The Lake was 

 divided geographically into four sampling areas 

 (Figure 1). Areas I and II have a mean depth 

 of 34 m and an even bottom of glacial till. Area 



III is much deeper (mean depth 74 m) and has 

 a highly variable, glacially scoured bottom. Area 



IV is made up of islands and isolated bays and 

 also has a highly variable bottom contour. 



Twenty-nine fishes have been identified in the 

 Kvichak River system (Bond and Becker, 1963) , 

 but only the sockeye salmon is of commercial im- 

 portance. The threespine stickleback, Gaster- 

 osteus aculeatus, the most abundant of the res- 

 ident species, may compete with the juvenile 

 sockeye salmon for food. 



355 



