ADDING NUTRIENTS TO ENHANCE THE GROWTH OF ENDANGERED SOCKEYE-SALMON -TROPHIC TRANSFER IN AN OLIGOTROPHIC LAKE

Snake River sockeye salmon Oncorhynchus nerka, listed under U.S. law a s endangered in 1991 in response to a decline in anadromous adult numb ers, spend their first 1-2 years in Redfish Lake, Idaho, before migrat ing to the sea. To determine how nutrient enhancement might influence phytoplankton, zooplankton, and fish production, we performed fertiliz ation experiments in large enclosures in this oligotrophic lake using juvenile kokanee (lacustrine O. nerka) as analogues for endangered soc keye salmon. Fertilization of the metalimnion substantially increased chlorophyll a (150%), phytoplankton biovolume (75%), primary productiv ity (250%), and zooplankton biomass (200%), and moderately increased f ish growth (12%) over our control enclosures. Community composition of phytoplankton and zooplankton changed little, and water transparency declined less than 15% compared with controls. Thus, we concluded that metalimnetic fertilization could maintain the aesthetic value of thes e lakes while increasing zooplankton food resources for juvenile salmo n. Our results suggest that whole-lake fertilization would aid in the recovery of Snake River sockeye salmon.