Potential role of spiny dogfish in gray and harbor seal diets in the Gulf of Maine

摘要

ABSTRACT: Harbor seal Phoca vitulina and gray seal Halichoerus grypus populations have increased by 28.7% (1997-2001) and 8.6% (1999-present), respectively, in the Gulf of Maine. Here, we investigated what seals are eating and how their diet influences the food web in the Gulf of Maine. There is suggestion, but little evidence, that seals are eating spiny dogfish Squalus acanthias, currently an abundant species (standing stock biomass = 235000 t) in the Gulf of Maine and surrounding areas. We used an Ecopath food web model to explore the potential of dogfish being a primary food source for seals and how this may be impacting the ecosystem. We explored 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 Gulf of Maine. We therefore also explored top-down impacts of white shark predation on seals and lower trophic level fish species. Our results suggest that (1) dogfish could be contributing to the recovery of seal populations by providing an alternative prey base, (2) there is a tipping point between 10 and 20% of dogfish in seal diet that elicits a linearly increasing trend in seal biomass over time, and (3) seals as prey will have a continued impact on the projected increase in white shark biomass. This work advances our understanding of top-down control and may help to inform fisheries and resource management.