Adding Nutrients to Enhance the Growth of Endangered Sockeye Salmon: Trophic Transfer in an Oligotrophic Lake

-Snake River sockeye salmon Oncorhynchlls nerka. li sted under U.S. law as endangered in 1991 in response to a decline in anadromous adult numbers, spend their first 12 years in Redfi sh Lake, Idaho, before migrating to the sea. To determine how nutrient enhancement might influence phytoplankton , zooplankton, and fi sh production, we performed fertilization experiments in large enclosures in thi s oligotrophic lake using juvenile kokanee (lacustrine O. nerka) as analogues for endangered sockeye salmon . Fertilization of the metalimnion substantially increased chlorophyll a (150%), phytoplankton biovolume (75 %), primary productivity (250%), and zooplankton biomass (200%), and moderately increased fi sh growth ( 12%) over our control enclosures . Community co mposition of phyloplankton and zooplankton changed little. and water transparency declined less than 15% compared with control s. Thus, we concluded that metalimnetic fertilization could maintain the aesthetic value of these lakes while increasing zooplankton food resources for juvenile salmon . Our results suggest that whole-lake fertili zation would aid in the recovery of Snake River sockeye salmon. The importance of food resources in controlling the biomass of organisms in a given trophic level has been a dominant theme in community ecology for several decades . These bottom-up effects have been predicted from theory (Fretwell 1977; Oksanen 1988; Liebold 1989; Power 1992), described from empirical relationships (McQueen et al. 1986), and tested with whole ecosystem experiments (McNaughton 1977; Langeland 1982; Peterson et al. 1993). Adding nutrients to enhance ecosystem productivity has been practiced for thousands of years and forms the foundation of agricultural and aquaculture practices. In fisheries sciences, Nelson and Edmondson (1955) demonstrated 40 years ago that lake fertilization increases production of commercially exploited stocks; lake fertilization is now commonly used in Alaska and Canada to increase growth and production of juvenile sockeye salmon Oncorhynchus nerka (Stockner 1981 , 1987; Stockner and Hyatt 1984; Kyle 1994). Although bottom-up effects from nutrients are measurable, our ability as ecosystem managers to predict the magnitude of these effects is weak (Power 1992). Much current ecological debate centers on determining when bottom-up effects prevail over top-down effects in defining ecosystem function (Carpenter and Kitchell 1988, 1992; 1 Present address of corresponding author: U.S. Fish and Wildlife Service, Columbia River Fi sheries Program Offi ce, Suite I, Vancouver, Washington 98665, USA ; phaedra..budy @mail .fw s.gov 19 DeMelo et al. 1992; Matson and Hunter 1992). Primary production has been enhanced in many studies in which nutrients have been added to aquatic systems (Liebold 1989). In some cases, however, intense selective predation on primary consumers can weaken or obliterate the effects of nutrient additions, resulting in a strong top-down control of ecosystem function (Carpenter and Kitchell 1993). Omnivory or diet switching by consumers also can reduce trophic transfer resulting in " middle-out" effects (Polis et al. 1989; DeVries and Stein 1992). Finally, reproductive limitation of key species can uncouple the trophic link between primary producers and consumers (Koenings and Burkett 1987). In spite of the well-known effects of nutrient additions, we know of no instance in which ecosystem fertilization has been used as a management tool to assist with the recovery of endangered populations. In this paper, we examine the potential for using lake fertilization to enhance growth and survival of endangered sockeye salmon in Redfish Lake, Idaho. Snake River sockeye salmon would probably benefit from an increase in juvenile size in making the long migration ' from the freshwater nursery lakes to the Pacific Ocean (> 1,400 km), a journey that formerly took less than 10 d but now can take over 2 months due to delays associated with dam passage on the Columbia and Snake rivers. Similarly, predation on juvenile salmon ids is thought to be highly size-specific, with small increases in growth rate translating to large differences in survival (Luecke et al.