Effects of Canopy Removal on Invertebrates and Diet of Juvenile Coho Salmon in a Small Stream in Southeast Alaska

Abstract

We assessed changes in availability and consumption of invertebrates by juvenile coho salmon Oncorhynchus kisutch in a small stream in southeast Alaska where patches of dense second-growth riparian vegetation bordering the stream had been removed. Benthic invertebrate populations were assessed during summer 1988 and 1989 with a Hess sampler. Aerial invertebrates were sampled during summer 1989 with wire-mesh sticky traps hung just above the water surface and with floating clear-plastic pan traps. Invertebrate drift was assessed during summer 1989 with nets placed at the downstream end of closed- and open-canopy stream sections. Diets of age-0 and age-1 coho salmon were sampled by flushing stomach contents of fish collected from closed- and open-canopy stream sections. Abundance and biomass of benthic invertebrates were larger in open- than in closed-canopy stream sections and were primarily dipterans, ephemeropterans, and plecopterans. More insects were caught on sticky traps in open than in closed sections on two of four dates sampled, and composition of the catch was primarily dipterans (74% in both closed- and open-canopy sections). Catch rates of invertebrates in the pan traps were significantly higher in closed than in open sections on 12 July and were greater in open than in closed sections on 11 August. No significant canopy effect was detected with regard to dry weight of insects captured in pan traps. Composition of the pan-trap catches was primarily dipterans in both closed and open sections (65% and 72%). Abundance of invertebrates in the drift was significantly higher in closed sections than in open sections on two of four dates sampled; dry weight of invertebrate drift did not differ significantly between canopy types. Dry weight of stomach contents of age-0 and age-1 coho salmon was greater for fish sampled in closed- than open-canopy sections on one of four dates sampled; no significant canopy effect was detected for the other three dates sampled. Aerial insects were more abundant in drift and in diets of age-0 and age-1 coho salmon in closed- than in open-canopy sections. Diet of age-0 coho salmon in both closed and open sections and diet of age-1 fish in closed sections overlapped significantly with composition of the drift. Diet of age-1 coho salmon in open sections, however, did not overlap significantly with drift, an indication of selectivity in feeding behavior. Based on higher abundance of aerial invertebrates above the water surface and increased standing crop of benthic invertebrates that we observed in open- versus closed-canopy sections of Eleven Creek, it appears that canopy removal has the potential to increase the carrying capacity of juvenile coho salmon in streams where populations are food limited.

Notes

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