Mercury contamination of fish in the Ojibwa diet: I. Walleye fillets and skin-on versus skin-off sampling

During the past two years, walleye (Stizostedion vitreum) have been collected and prepared into skin-off fillets and submitted for total mercury analysis. The survey included 105 fish from 18 lakes in 10 counties in northern Wisconsin and the upper peninsula of Michigan. Fourteen lakes yielded walleye fillets with greater than 0.5 ppm mercury, and six lakes yielded samples in excess of 1.0 ppm mercury. Fourteen fish were collected in the spring and prepared as fillets ground up as either skin-on or skin-off samples. The difference in Hg was significant (T₁₄=?3.26,p=0.006) with skin-on fillets, resulting in an approximately 10% decrease in mercury concentrations. Results of this study suggest that by leaving the skin on the sample, mercury concentrations will be reported 10% lower than if the skin is removed. Obviously, consumption advisories based on skin-off samples could provide more protection for Ojibwa people eating the spring harvest of walleye. In the fall, the difference in Hg samples between skin-on versus skin-off, was less and not statistically significant. However, removal of the skin would be expected to underestimate lipophilic organochlorine burdens and may not be appropriate for fish species where PCBs, DDT, and chlordanes are the major concern. Fall data for 67 fish from 26 lakes in 9 counties are also reported.