Patterns of Fish Abundance Associated with a Dredge Disposal Island: Implications for Fish Habitat Enhancement in a Large Reservoir

Abstract

We evaluated trends in fish abundance associated with in-water disposal of dredged material in Lower Granite Reservoir, Idaho–Washington. Fish assemblages were sampled before (1985) and after (1993) construction of a 0.37-ha disposal island to assess local changes in fish community structure, In addition, resident fish abundance was monitored for 5 years (1989–1993) near the disposal island and compared with patterns of fish abundance at other shallow-water sampling stations in the reservoir. Ranked abundance of fishes collected in gill-net samples differed before and after construction of the disposal island. Of 12 common resident fishes in Lower Granite Reservoir, only 6 species were represented in gill-net catches before construction of the island, whereas 11 species were represented in catches after construction. Trophic structure and trends in feeding-guild abundance were not significantly different between disposal and reference stations from 1989 to 1993. However, herbivores and insectivore–piscivores were collected more frequently at reference stations than disposal stations. We hypothesize that variation in average water depths and cover characteristics among stations may be responsible for the observed differences. Observed and expected species richness were not significantly different among disposal stations sampled from 1989 to 1993. Our study suggests that construction of shallow-water habitat with dredged material has increased habitat complexity in Lower Granite Reservoir and has potential as a habitat enhancement technique. Islands constructed from dredged material can reduce local water depth and provide rearing areas for several resident fish species.