Distribution, Abundance, and Growth of Juvenile Sockeye Salmon, 



Oncorhynchus nerka, and Associated Species in 



the Naknek River System, 1961-64 



ROBERT J. ELLIS' 



ABSTRACT 



The Naknelt River system contains eight interconnected and generally biologically discrete basins, 

 each with a different ratio of spawning grounds to rearing area for sockeye salmon, Oncorhynchus nerka, 

 and different densities of juvenile sockeye salmon and associated species offish. Juvenile sockeye salmon 

 and other pelagic species were sampled with tow nets at night. Sockeye salmon were the most common and 

 abundant species in all basins, followed by threespine sticklebacks, ninespine sticklebacks, and pond 

 smelt. Eighteen other species of potential competitor or predator fish were present. 



In the summers of 1961 to 1963, juvenile sockeye salmon in the pelagic areas had a characteristic 

 pattern of abundance for the entire system: abundance (catch per tow) of age increased from early 

 summer to midsummer and then declined to late August. The abundance in late August varied about 

 threefold and, in genera!, was independent of variations in the number of parents from 1960 to 1963. 



In July the abundance of age fish in each basin was proportional to the amount of known contiguous 

 spawning ground, but by late August this relation no longer existed. This change was at least partly due to 

 migration of the age fish — generally from basins of greater abundance of fish to those of lesser abun- 

 dance. The larger and faster growing fish were the first to migrate. Not all basins were involved in these 

 migrations. 



The production of sockeye salmon smolts in the Naknek system is relatively stable. At least three 

 major factors probably contribute to this stability: (1) the presence of several major spawning units or 

 races in widely separated spawning grounds of different types, (2) the presence of several connected lakes, 

 and (3) the migratory behavior of juvenile sockeye salmon during their first summer. 



A mechanism which prevents the population of juvenile sockeye salmon from exceeding some upper 

 limit is not apparent in the Naknek system. A reduction in growth in areas of high density was not 

 apparent in the Naknek system in 1961-64 and apparently did not occur in 1957-65. Many kinds of 

 predators on juvenile salmon are present but probably are not limiting production of smolts. 



The data on abundance and growth of juvenile sockeye salmon and the distribution of the escapement 

 and spawning grounds indicate that it should be possible to increase the production of sockeye salmon in 

 the Naknek system. Two of the major basins. North Arm and Brooks Lake, which constitute about 35% of 

 the system, are now producing juveniles at very low levels. North Arm appears to suffer from too little 

 spawning area, whereas Brooks Lake appears to have adequate spawning area but too few spawners. 



Three factors in the biology of juvenile sockeye salmon of the Naknek system are of special signifi- 

 cance to the managers of the resource and should be investigated in any effort to enhance the production 

 of sockeye salmon in the Naknek system: ( 1 1 the abundance of smolts each spring is fairly constant for the 

 system as a whole and not closely related to the abundance of the parents or, from 1961-64, even to the 

 original abundance of age fish; (2) the apparent growth of juvenile sockeye salmon and potential 

 competitor species is not related to the abundance of these fish in any lake of the Naknek system; and (3) 

 two major lakes, constituting about iS% of the rearing waters, do not receive age sockeye salmon from 

 other basins and are supporting relatively few sockeye salmon. 



The question of what escapement of adult sockeye salmon is needed to ensure full production of 

 juveniles is considered. The present study indicates that escapements in the range of 600,000 to 1,000,000 

 fish, as recommended by other studies, would probably fully use the present combination of spawning and 

 rearing areas without danger of overburdening the food supply. 



The Naknek River system — the Naknek River and 

 tributary lakes — is one of several major producers of 

 sockeye salmon, Oncorhynchus nerka, in Bristol Bay, 

 Alaska. The annual commercial value of the catch of 

 sockeye salmon from the Naknek system has varied in 

 recent years from a few hundred thousand to more 

 than a million dollars, and the ultimate goal of fishery 

 research here is to stabilize the production at the 



'Auke Bay Fisheries Laboratory, National Marine Fisheries Ser- 

 vice, NOAA, Auke Bay, AK 99821. 



higher or even increased levels. As biologists learn 

 more of the life history of sockeye salmon, it becomes 

 increasingly evident that although most stocks (races) 

 have the same general life history, each stock has 

 unique characteristics that are determined by the 

 biological and physical environments in which each 

 stock evolved. It is the interaction between these 

 characteristics and the environment that makes some 

 stocks more productive than other stocks in the same 

 year and some years more productive than other years 

 for the same stock. 



