Effects of Hypolimnetic Oxygenation on the Food Resources and Feeding Ecology of Cisco in Amisk Lake, Alberta

Although the ability of hypolimnetic oxygenation to enhance coldwater fish populations depends, in part, on the availability of prey within the hypolimnion, studies that evaluate both fish diets and prey distributions during treatment are rare. This paper compares the diet of cisco Coregonus artedi the dominant coldwater fish in Amisk Lake, Alberta, Canada, with the composition and vertical distribution of prey during the summers of 1989 and 1992 when one basin of this two-basin eutrophic lake was oxygenated; the second basin served as a reference. During periods of thermal stratification, ciscoes in the reference basin were restricted by hypoxia to epilimnetic and metalimnetic waters whereas those in the treated basin distributed into the hypolimnion, where dissolved oxygen (DO) concentrations remained above 1.0 mg/L. Higher DO concentrations enabled ciscoes in the treated basin to feed throughout the water column and incorporate hypolimnetic and benthic prey species into their diets, whereas ciscoes in the reference basin ate primarily epilimnetic prey species. As a result, similarity in the composition of fish diet, which was high in early summer and during fall turnover, decreased through midsummer when differences in hypolimnetic DO concentrations between the two basins were most severe. Although foraging habitat for fish increased during oxygenation, whole-lake fish biomass also increased and resulted in a substantial reduction in per capita food levels; the amount of food in fish stomachs in 1989 was at least 30 times as high as in 1992. These results demonstrate that oxygenation can enhance availability of fish forage organisms in deep water. However, expanded foraging habitat may not always translate into increased food consumption if fish densities increase. Thus, the enhancement of coldwater fish populations during oxygenation will also depend on the nature of density-dependent interactions between fish and their prey.