- Williams, Amy Sierra
The Graduate School, University of Maine
As harbor seal (Phoca vitulina concolor) populations increase and commercial groundfish stocks dwindle in the Gulf of Maine, it is important to investigate the potential for competition between seals and fishermen. I aged 261 harbor seals from teeth or body measurements and identified prey from the stomach contents of 75 harbor seals caught in sink gillnets in the Gulf of Maine and adjacent waters from 1991 to 1997. Ninety three percent of seals caught in gillnets were less than four years old. Of 24 taxa identified, silver hake (Merluccius bilinearis) was found frequently (70.6% of stomachs), making up 52.1% of the prey items , and 40.8% of the reconstructed biomass. Silver hake, red and white hake, Atlantic cod, squid, and redfish (in IRI rank order) accounted for 77.7% of the reconstructed biomass and 87.4% of the number of prey consumed. Species richness was greatest in summer in northern Gulf of Maine diets (16 species) but more evenly distributed in winter in southern Gulf of Maine diets (13 species). Harbor seals utilized 11 of 22 commercial fish species landed by gillnet fishermen. Using the odds ratio with proportions of mass caught in sink gillnets and proportions of mass in the seal diets taken from the same nets, harbor seals selected silver hake, Atlantic herring, red or white hake, pollock, redfish, and Atlantic cod. Dogfish, monkfïsh, skates, American lobster, and flounder were among some species selected against by harbor seals. The mean length of prey was 22 cm. Harbor seals selected small, juvenile silver hake, red and white hake, Atlantic cod, pollock, and redfish compared to those taken in gillnets. The species and size composition of prey taken by harbor seals differed significantly from sink gillnet catches. Predation by these predominantly juvenile harbor seals had a minimum affect on fish populations targeted by the sink gillnet fishery and seals were not in direct competition with fishermen.