Dispersal, Density-Dependent Growth, and Survival of Stocked Steelhead Fry in Lake Superior Tributaries

Abstract

When fry of steelhead Oncorhynchus mykiss were stocked at single sites in Lake Superior tributaries during June, fish did not disperse uniformly to fill available habitat. By the end of the growing season, upstream dispersal was minimal and downstream dispersal patterns depended on numbers surviving. When survival was low (1988), density declined linearly for up to 6.5 km downstream. When survival rates were highest (1989), densities appeared to be at carrying capacity for up to 3.5 km below the stocking site; densities farther downstream declined linearly, and the maximum dispersal distance was 10.9 km. Mean weight of young-of-the-year (age-0) fish was density dependent, and overwinter survival was contingent upon the fish achieving a threshold weight. As density of age-0 fish increased (among sites), the density of fish achieving the threshold weight or larger increased to a maximum. Fluctuations in survival between years and streams were probably caused by variable mortality associated with transportation and stocking and by factors such as high water temperature and flooding.