Biological effects of repeated fish introductions in a formerly fishless lake: Diamond Lake, Oregon, USA

Abstract

Biological responses to a formerly fi shless lake in the Cascade Range, Oregon (USA) were assessed through monitoring of recent changes and paleolimnological techniques to assess earlier changes. We used un- published fi sheries reports, sediment cores, and available published and unpublished water quality data to evaluate changes to the lake. Diamond Lake has undergone four periods of fiintroductions in the 20 th century. Rainbow trout (Salmo gairdneri) were annually released from 1910-2006, except for 1949 and 1954. Tui chub (Gila bicolor), an omnivorous cyprinid, were introduced in the late 1930s and the late 1980s, presumably as discarded live bait. Diamond Lake was treated with rotenone in 1954 which successfully eradicated the tui chub. The introductions of trout caused relatively modest changes in water quality and lake biota, whereas the introductions of tui chub caused major increases in cyanobacteria, changes in diatom community composition, reduction in transparency, increases in the proportion of rotifers, major reduction in benthic standing crop, and virtual elimination of amphipods, gas- tropods, and other large-bodied invertebrates. The unintended biomanipulations demonstrate the importance of an omnivorous cyprinid in promoting a series of biological responses throughout the lake food web.