Response of Underyearling Coho Salmon to Supplemental Feeding in a Natural Stream

Abstract

Food rather than space was found to limit the stream production of juvenile coho salmon (Oncorhynchus kisutch) during the summer. The carrying capacity of a small Vancouver Island [British Columbia, Canada] stream was increased 6- to 7-fold above natural levels by augmenting the natural food supply with a daily feeding of thawed marine euphansiids (Euphansiacea). Growth and biomass yield of unfed populations were inversely related to population density. Supplemental feeding canceled the density effect on survival and outmigration, accelerated growth rate and substantially increased the pre-winter lipid reserve, potentially increasing resistance to starvation during winter. During low streamflow of summer, juvenile coho populations may be regulated at biomass levels commensurate with smolt yield in the subsequent spring. The 6-fold increase in potential smolt yield induced by supplenental feeding during summer was nullified by the natural carrying capacity of the stream over winter. Projected smolt yield from the remnant population in late winter following several freshets approximated that expected from natural of production (1 smolt/4 m2). The probability of increasing smolt yield by relieving the natural food limits on density and production during the summer season remains most uncertain unless the carrying capacity over winter of the specific stream is known already to exceed natural levels of summer population to an appropriate degree, or can be increased so that the degree by manipulating environmental factors that determine the carrying capacity over winter, and unless food supply is again augmented in the subsequent spring until the smolts migrate to sea. Attempts to increase carrying capacity over winter by installing artificial refuges were unsuccessful and suggest that the winter behavior of coho is complex and insufficiently understood to allow for effective manipulation aimed at stimulating continued residence in small streams at unnaturally high population densities during winter.