Mercury Contamination of an Introduced Generalist Fish of Intermediate Trophic Level

Mercury contamination is a global issue because mercury concentrations in aquatic systems are influenced by both natural and anthropogenic pathways. Here, liver and muscle total mercury (THg) concentrations in black crappie Pomoxis nigromaculatus from three boreal lakes in southeastern Manitoba, Canada, were related to age, morphology and physiological traits to better understand the dynamics of mercury accumulation in an introduced generalist fish species. These THg concentrations were then compared to black crappie mercury concentrations in other Canadian water bodies and to mercury concentrations in other freshwater fishes in southeastern Manitoba. Age and size had strong positive correlations (P < 0.001, r ≥ 0.60) with muscle mercury concentrations. No evidence of acute point source contamination or physiological impairment in black crappie was found in the study area. Analysis of liver THg revealed the possible impacts of seasonal and ontogenetic differences in diet on exposure. Furthermore, THg analysis of liver and muscle tissue showed how generalist foraging may curb the progressively greater mercury exposure and resultant physiological consequences expected from ontogenetic diet shifts in black crappie. Although there appeared to be temporally varied levels of mercury exposure (i.e., liver THg) by sex, there was no sex effect observed in long-term mercury accumulation in the muscle. Black crappie bioaccumulated less mercury at age than primary piscivore species in the region. These results will help foster a better understanding of mercury biomagnification within a region impacted by legacy mercury.