ACCUMULATION OF MERCURY IN FISH FROM THE GRAND RIVER, NEAR GALLATIN MISSOURI

Consumption of fish provides an important source of protein and omega-3 fatty acids but global mercury pollution and methylmercury bioaccumulation in aquatic food chains have led to national and state consumption advisories for many fish species. For example, women who are pregnant, of child bearing age or nursing and children under the age of 13 are advised not to consume more than one serving per week of flathead (Pylodictis olivaris), blue (Ictalurus furcatus) or channel (Pylodictis olivaris) catfish greater than 17 inches in length taken from the Missouri River. Here we report mercury levels (primarily methylmercury) in a sample of 36 channel catfish, 6 flathead catfish and 13 freshwater drum (Aplodinotus grunniens) collected from the Grand River (a Missouri River tributary) in August 2013, near the town of Gallatin. Fish lengths and weights were recorded and muscle samples frozen for determination of total mercury on a Teledyne Leeman Labs Hydra IIC Hg Analyzer. Fish lengths varied as follows: channel catfish 14-33 inches, flathead catfish 20-29 inches, freshwater drum 12.5-20 inches and variation in length explained at least 95% of variation in weight.

Mercury concentrations increased with fish length most rapidly in freshwater drum followed by flathead catfish with channel catfish showing the lowest rate of mercury accumulation. Over 70% of the variation in mercury concentration was explained by variation in length for freshwater drum and flathead catfish but only 20% in channel catfish. Mercury concentrations (ng/g wet weight) varied from 109-482 in freshwater drum, from 99-343 in flathead catfish and from 50-305 in channel catfish. The fact that flathead catfish accumulate mercury more rapidly than channel catfish might be expected given they switch from a diet of invertebrates to one consisting almost completely of fish at a smaller size (10 inches) than channel catfish, which make the switch to mostly fish at 16 inches. What is surprising is the relative rate freshwater drum accumulate mercury given that they feed primarily on benthic invertebrates. All three species accumulate mercury levels that exceed the EPA limit for fish consumption but do so at different lengths. Take home lesson – eat fish but eat small fish.