Prey Selection in Gulf of Maine Harbor Seals (Phoca vitulina) in Relation to Fish Abundance and Fish Mercury Concentrations

Understanding the current role of harbor seals in the Gulf of Maine ecosystem requires broad knowledge of harbor seal interactions with the marine community. The research reported here is a contribution to that basic knowledge. First, the study explored the hypothesis that harbor seals forage selectively on preferred prey species without regard to prey species abundance. Second, mercury concentrations in top predators often exceed levels predicted from their consumption of mercury contaminated prey. The study tested the hypothesis that selective foraging on prey fish with elevated mercury concentrations increases mercury exposure in piscivorous harbor seals. This four year study, 2000 through 2003, centered on the large harbor seal haulout site at Mt. Desert Rock, which lies 28 km south of the central Maine coast. Fecal analysis defined the summer diet of harbor seals in the central Gulf of Maine in relation to prey species abundance and age class frequency. Prey fish abundance estimates were based on data from the Maine Department of Marine Resources Inshore Trawl Surveys (2000 - 2003). Mercury concentrations determined for harbor seal prey species were used to predict the dietary exposure of harbor seals to mercury. Foraging harbor seals selected for three primary prey species, silver hake (Merluccius bilinearis), Acadian redfish (Sebastes fasciatus), and Atlantic herring (Clupea harengus), and seals selected for particular age classes within those three primary prey species. There was no relationship between prey selection and total prey species abundance. Instead, foraging seals shifted prey consumption in response to the relative abundance of the preferred prey fish age class(es). Harbor seals in the central Gulf of Maine did not select for fish having elevated mercury concentrations. Mean total mercury concentrations in the species and size of prey fish ingested by harbor seals were at or below 30 ng THg/g w.w. These concentrations were an order of magnitude lower than levels typically associated with mercury-induced behavioral changes in fish. Mercury concentrations in harbor seal fur reflected seal exposure to mercury from ingested prey fish having average mercury concentrations