An Analysis of Gray and Harbor Seal Strandings in Cape Cod, Massachusetts from 1999 to 2012.

In recent years, the number of gray seals, Halichoerus grypus, inhabiting Massachusetts has increased dramatically. In Canada and Scotland, similar increases in gray seal abundance have been accompanied by a concomitant decrease in numbers of sympatric harbor seals, Phoca vitulina. It has been hypothesized, although not confirmed, that interspecific competition between the two species led to a decrease in harbor seal populations. The present study addressed the question of whether gray seals and harbor seal populations are interacting in Cape Cod, MA by analyzing stranding data from Cape Cod in the period from 1999 to 2012, provided by the International Fund for Animal Welfare (IFAW). Gray seal strandings increased 2.9% annually from 1999 to 2012. Harbor seal strandings, on the other hand, increased 8.5% annually until 2004, after which the increased stopped and the number of strandings varied interannually. Strandings were best predicted by species, age class, and month, although age class only affected harbor seal seasonal stranding trends. Seasonal trends in the number of gray and harbor seal strandings caused by human interactions were significantly different. These differences in the stranding record suggest that the two species are interacting ecologically in Cape Cod.