Growth and Survival of Sockeye Salmon 



Introduced into Ruth Lake after Removal 



of Resident Fish Populations 



By 



WILLIAM R. MEEHAN, Fishery Biologist 



Alaska Department of Fish and Game 



Juneau, Alaska 



ABSTRACT 



Sockeye salmon ( Oncorhynchus nerka ) in three lakes on Afognak Island, 

 Alaska, were studied. Ruth Lake was treated with rotenone to remove resident 

 fish. Midarm Lake, which had no salmon, and Little Kitoi Lake, which has a small 

 run of sockeye, were used as controls to compare survival and growth of introduced 

 fry and natural fry with survival and growth of introduced fry in Ruth Lake. Other 

 factors that might influence sockeye production, such as plankton and bottom fauna, 

 were also considered. 



In general, growth and survival of fry and biological productivity were greater 

 in the treated lake. Growth and survival decreased as fry densities increased. 



INTRODUCTION 



Young sockeye salmon generally spend at 

 least a year in fresh water in a lake below 

 the spawning area--usually before migrating 

 to sea. Most sockeye spawninthe inlet streams 

 to the nursery lake, although a few may spawn 

 on lake beaches that have suitable waterflows 

 deep in the gravels. Fry emerge from these 

 beds directly into the nursery areas. In a few 

 locations where sockeye spawn in an outlet, 

 the fry migrate upstream to the nursery lake. 



The phase of the life cycle in the lake may 

 be the most important to sockeye production 

 because it takes place in the most limiting 

 environment in the life-history sequence. 



It may be desirable to control the numbers 

 and growth of the population in the lake so 

 that most fish migrate to sea after 1 yr. 

 rather than remain an additional year or more. 

 The "holding-over" of juveniles an additional 

 year or more could increase intraspecific 

 competition for food and even lead to preda- 

 tion by the older salmon on younger fish. 

 Foerster (1938a), Barnaby (1944), and Koo 

 (1955) demonstrated that the smaller fish tend 

 to hold over an additional year before migrat- 

 ing seaward. The mortality during this extra 

 year of fresh-water residence may override 

 any benefits from the increased marine sur- 

 vival of these fish when they migrate at a 

 larger size. Hence, longer lake residence may 



result in poorer survival to adulthood than 

 would be obtained from an earlier seaward 

 migration of larger numbers of smaller young 

 salmon. 



On the assumption that the lacustrine en- 

 vironment is a major factor in the production 

 of sockeye, biologists of the Alaska Depart- 

 ment of Fish and Game began studying the 

 extent to which production of sockeye could 

 be increased by controlling reproduction and 

 providing more suitable rearing areas for 

 juveniles before they nnigrate to sea. Factors 

 considered were: the optimum density of fry 

 recruitment for a given lake; the basic pro- 

 ductivity of the lake (particularly the quality 

 and quantity of food available to the young 

 salmon); and the effects of other resident fish 

 on the fresh-water production of sockeye. The 

 primary aim of this phase of the investigation, 

 however, was to study the survival and growth 

 of sockeye fry introduced into a lake in which 

 all potential competing and predatory fishes 

 had been destroyed. Removal of predators and 

 competitors from the environment should 

 create nnore favorable opportunities for sur- 

 vival and growth of juvenile sockeye. 



The importance of predation on juvenile 

 salmon has been mentioned often in the litera- 

 ture. Brett and McConnell ( 1950), for example, 

 attributed poor survival (0.4 to 1.1 percent) 

 of juvenile sockeye salmon in Lakelse Lake, 

 British Columbia, to heavy predation by 



