An evaluation of habitat enhancement and wild fry supplementation as a means of increasing coho salmon production of the Clearwater River, Washington
I examined mainstem habitat enhancement and coho fry supplementation as method for increasing coho salmon (Oncorhynchus kisutch) production in the Clearwater River Basin. I also measured macro- and microhabitat use and behavioral patterns of summer rearing coho salmon in the mainstem. Overall, fry supplementation did not significantly increase wild coho fry or cutthroat trout (O. clarki) emigration. However, a strong (r = 0.94) positive relationship observed in control streams between early summer coho salmon rearing densities and numbers of emigrants was altered in supplemented streams. At low rearing densities, more coho fry emigrated from supplemented than control streams. Emigration rates of wild coho salmon, cutthroat, and steelhead (O. mykiss) were significantly altered in supplemented streams during one year but not the other. Although not statistically significant, coho fry densities (fish/m$\sp2$ pool area) were greater (18-89%) in supplemented than control streams. Wild coho salmon and cutthroat trout densities were not reduced by supplementation. Wild coho fry in supplemented streams were smaller than those in control streams, apparently due to reduced spring and early summer (May to late June) growth.Summer rearing densities (fish/km) of juvenile coho salmon in the mainstem were increased by introducing woody debris bundles and were positively related to debris densities (#/km). However, woody debris introductions did not result in increased migration of coho salmon into overwintering habitats. Coho salmon abundance was greatest in pool habitats containing the largest, most complex woody debris accumulations. Two distinct foraging groups, 'foragers' and 'resters', were observed using mainstem woody debris accumulations. Foragers selected focal positions with faster current velocities and deeper water, and generally were deeper in the water column and farther from woody debris cover than resting individuals. Foraging individuals foraged and displayed aggressive behavior more frequently than resting individuals.Based on the coho salmon mainstem rearing densities and numbers coho salmon emigrating from streams, coho salmon production in the Clearwater River Basin appears to be limited more frequently by insufficient numbers of juveniles to fully seed available habitat than by habitat availability. However, mainstem rearing likely limits coho salmon production during years with adequate escapement in this system.
- Fisheries