Grey Seal and Harbor Seal Interactions
Since grey seal were extirpated in the 17th century, harbor seals have had exclusive use of haul-out
spaces historically shared by the two species. Multiple studies have indicated that sharing haul-out spaces with grey seals has affected harbor seal behavior. Grey seal presence has been shown to increase the frequency with which harbor seals respond aggressively to one another. Furthermore, harbor seals on the peripheries of harbor seal haul-out groups are displaced when the haul-out sites are shared with grey seals (Murray: 2008, 35). In addition, studies have indicated that male grey seals are “clearly dominant” in interactions between the two species, leading harbor seals to avoid confrontations with their larger neighbors (Renner: 2005). These studies suggest that the growing gray seal population will increasingly place pressure upon harbor seal populations.
Grey Seal and White Shark Interactions
As grey seal populations in Cape Cod grow, people have expressed a concern that white sharks,
drawn by the possibility of feeding on seal colonies, will increase in number around Cape Cod. In recent history, white sharks have not commonly preyed on gray seals in the North Atlantic due to, “the lack of strong spatial overlap between white sharks and large pinniped colonies” (Skomal: 2012, 406). However, it is possible that before the extirpation of gray seals in the North Atlantic in the 17th century, they made up a significant portion of white sharks’ diets. Following the decline of the grey seal, white sharks were forced to find alternate food sources such as dead cetaceans. The recent resurgence of grey seal populations has been accompanied by a significant increase in white shark counts off the coast of Cape Cod (411), suggesting that as seal populations continue to grow, more sharks will utilize gray seals as a food resource.