Kokanee Population Density and Resulting Fisheries

Catch rate and effort for and yield of kokanee Oncorhynchus nerka in seven lakes and reservoirs in Idaho and Oregon were related to fish density. The relationships were not linear. As kokanee density increased from less than 10 to 30–50 fish/ha. values of all three fishery characteristics tended to increase. Little or no improvement occurred at higher densities. and yield appeared to decline at densities above 50 fish/ha. The domed shape of' these relationships was explained by a model linking density-dependent length at age and size-dependent catchability: as density increases. anglers encounter kokanee more frequently, but fish length declines at high densities, reducing catchability; low densities should allow fast growth and high catchability, but reduced populations could he subjected to depensatory and potentially destabilizing mortality. This model and its empirical Support indicate that management to increase the density of kokanees likely will not produce a proportional benefit for anglers, and the quality of a fishery may even decline.