The Role of Dogfish in Seal Diet in the Gulf of Maine

Despite groundfish populations being at historic low abundance in the Gulf of Maine (GoM), predator populations continue to increase. Harbor (Phoca vitulina), harp (P. groenlandica) and gray (Halichoerus grypus) seal populations have increased by 28.7% (1997-2001), 6% (1999-2008) and 8.6% (1999-present), respectively. What are seals eating and how does their diet influence the food web in the GoM? There is anecdotal, but no published evidence, to suggest that seals are eating dogfish, currently an abundant species (SSB=235,000 mt) in the GoM and surrounding areas. We use an Ecopath food web model to explore the potential of dogfish being a primary food source for seals and how they may be impacting the ecosystem. We explore various plausible diets for seals and the implications for other non-prey species in the ecosystem.  In addition, white sharks (Carcharodon carcharias) have increased in abundance, regionally, and shark predation on seals is documented in the southern GoM. We therefore also explore top-down impacts of sharks on seals and lower trophic level fish species by manipulating shark biomass and predation on seals in the food web model. Results of this work will advance our understanding of top-down control and may help to inform fisheries and resource management.