Changes in population structure, growth, and biomass of cisco (Coregonus artedi) during hypolimnetic oxygenation of a deep, eutrophic lake, Amisk Lake, Alberta

Although most hypolimnetic oxygenation studies are designed to enhance cold-water fish populations, production-related responses have rarely been evaluated. We examined changes in density, biomass, and growth rate of cisco (Coregonus artedi) in Amisk Lake during 1989-1991, when one of the basins received hypolimnetic oxygenation, and in the nearby, untreated Baptiste Lake during 1990-1991. Variations in monthly density and biomass estimated using hydroacoustics suggested that as summer progressed, cisco in Amisk Lake responded behaviorally to hypolimnetic oxygenation by migrating from the hypoxic reference basin into the treated basin where oxygen concentrations remained high; by summer's end, the treated basin supported a density and biomass of fish that were twice as great as in the reference. Whole-lake fish density increased almost fivefold and biomass tripled in Amisk Lake during treatment, but there were no parallel changes in Baptiste Lake, where the hypolimnion remained anoxic each summer. Growth rate and condition of cisco in Amisk Lake decreased during treatment compared with pretreatment data, suggesting a density-dependent relationship with biomass; no coincident change in growth rate occurred in Baptiste Lake. These results suggest that hypolimnetic oxygenation can be an effective technique to enhance cold-water fish production, especially if combined with other management practices, such as regulated fish exploitation.