Exposure of great egret (Ardea albus) nestlings to mercury through diet inthe Everglades ecosystem

We estimated exposure of great egret (Ardea albus) nestlings to mercury in food in the Florida Everglades, USA, by collecting regurgitated food sample s during the 1993 to 1996 breeding seasons and during 1995 measured concent rations of mercury in individual prey items from those samples. Great egret nestlings had a diet composed predominantly of fish (>95% of biomass), tho ugh the species composition of fish in the diet fluctuated considerably amo ng years. Great egrets concentrated on the larger fish available in the mar sh, especially members of the Centrarchidae. The importance of all nonnativ e fish fluctuated from 0 to 32% of the diet by biomass and was dominated by pike killifish (Belonesox belizanus) and cichlids (Cichlidae). Total mercu ry concentrations in prey fish ranged from 0.04 to 1.40 mg/kg wet weight, a nd we found a significant relationship between mass of individual fish and mercury concentration. We estimated the concentration of total mercury in t he diet as a whole by weighting the mercury concentration in a given fish s pecies by the proportion of that species in the diet. We estimate that tota l mercury concentrations in the diets ranged among years from 0.37 to 0.47 mg/kg fish (4-year mean = 0.41 mg/kg). We estimated total mercury exposure in great egret nestlings by combining these mercury concentrations with mea surements of food intake rate, as measured over the course of the nestling period in both lab and field situations. We estimate that, at the 0.41 mg/k g level, nestlings would ingest 4.32 mg total mercury during an 80-day nest ling period. Captive feeding studies reported Elsewhere suggest that this l evel of exposure in the wild could be associated with reduced hedging mass, increased lethargy, decreased appetite, and, possibly, poor health and juv enile survival.