Mercury Accumulation in Fish from the La Grande Complex: Influence of Feeding Habits and Concentrations of Mercury in Ingested Prey

Abstract

The total mercury (total Hg) concentrations in ingested prey, as well as the mercury levels in fish, are 2 to 5 times higher in reservoirs than those in natural lakes. Feeding habits of non-piscivorous and piscivorous fish species from the La Grande complex were studied 2 to 4 years after filling (1980 to 1982) and several years later (1992 to 1994). Results showed that non-piscivorous fish from both natural lakes and reservoirs have similar diets. In both environments, lake whitefish shift from a zooplankton dominated diet to one dominated by benthos with increasing fish size. Cisco and longnose sucker feed mainly on zooplankton and benthos, respectively, regardless of size. Thus, our results suggest that the differences in the total Hg concentrations in fish between natural lakes and reservoirs, and the variations over time in reservoirs, are more related to the temporal changes in the concentration of methylmercury (MeHg) in the organisms of the food web than to changes in feeding habits. There are, however, a few exceptions such as large lake whitefish captured below the Robert-Bourassa powerhouse that eat small fish stunned by their passage through the turbines. These small fish represent more than 80% of the stomach content in comparison to only 2% for white-fish captured above the powerhouse. The higher proportion of fish in their diet leads to higher Hg levels in whitefish captured below the powerhouse than those above it. Also, piscivores from reservoirs generally eat more of their congenere or other piscivorous fish, up to 50% of the stomach content volume, than those from natural lakes. This lengthens the food chain which tends to further increases in total Hg levels in piscivores from reservoirs.