Factors Affecting Survival of Kokanees Stocked in Lake Pend Oreille, Idaho

Abstract

In 1987-1992, stockings of age-0 kokanees Oncorhynchus nerka in Lake Pend Oreille, Idaho, were evaluated to determine the contribution of hatchery fish to the age-0 population and their postrelease survival to autumn trawling, and to identify factors affecting survival of stocked fish. Fish were stocked at five locations in June or July and sampled with a midwater trawl near the end of their first growing season (late August–early September). Postrelease survival of hatchery kokanees averaged 23%, differed significantly among years, and ranged from 5 to 43% for individual release groups. From 1988 through 1991, survival of fry stocked in July was significantly higher than those stocked in June, but we could not detect a difference in survival among stocking locations. Survival of hatchery-reared kokanees was correlated with mean total zooplankton density during May–October, total zooplankton density at the time of stocking, density of Diaphanosoma sp‥ density of Daphnia spp‥ and size of Daphnia spp. High densities of zooplankton during 1985–1992 were associated with rapid development of the metalimnion (by June) and high numbers of May and June degree-days. Stocking of kokanees in Lake Pend Oreille did not restore the population to historical levels, and hatchery releases may only provide short-term benefits. We recommend stocking in July, but June stocking may be appropriate if zooplankton densities are sufficiently high to allow good survival of the fish.