Herring gulls (
Larus argentatus) in polluted areas on the North American Great Lakes were previously shown to have elevated germline mutation frequencies at minisatellite DNA loci. Airborne or dietary contaminants likely caused induced mutations, but the importance of each exposure type was unknown. Follow‐up experiments with lab mice determined that air pollution significantly induced germline mutations; however, an evaluation of mutations induced by the diet of herring gulls has not yet been conducted. To address this issue, we fed mice a high‐fish diet (58% wet mass) of the most common prey species for herring gulls nesting in Hamilton Harbour, a polluted industrial area on Lake Ontario. We bred the mice and screened pedigrees for germline mutations at expanded‐simple‐tandem‐repeat (ESTR) DNA loci. Mutation frequencies were compared to those in a reference group that was fed fish from Atlantic Canada, and a control group that was fed commercial chow. Germline mutation frequencies were highest in mice fed contaminated fish, but were only marginally or not significantly affected by diet treatment. Statistical power to detect differences among treatment groups was low, and the effect of diet may have more clearly emerged if larger sample sizes were available. Levels of organic pollutants in the fish from Hamilton Harbour were higher than those from Atlantic Canada, but their ability to induce ESTR mutations is unknown. Our findings suggest that a contaminated fish diet may contribute to the elevated germline mutation frequencies observed previously in gulls at this site, but air pollution is likely a more important route of exposure. Environ. Mol. Mutagen. 2008. © 2008 Wiley‐Liss, Inc.