Grey seal populations have been increasing since the early 1990s, particularly around Cape Cod and surrounding islands. While white sharks are a known predator of grey seals, interactions between the two species are not well understood. This project gathered data on the local and international movements of grey seals and their co-occurrence with white sharks in Cape Cod.

Grey seals observed in U.S. waters are part of a more extensive stock ranging from Northern Canada to the southern mid-Atlantic. While most of this population resides in Canadian waters, sightings and telemetry data for animals tagged on Sable Island indicated transboundary movement.

Dr. Greg Skomal and a team of researchers at the Massachusetts Division of Marine Fisheries used the Ocean Tracking Network’s (OTN) global receiver infrastructure to tag and catalogue great white sharks to estimate the population size and abundance in Cape Cod. Initial results showed an increase in white shark populations alongside the increasing grey seal population. The most surprising result was the broad age range of sharks—including juveniles—which were not previously known to feed on seals.

Researchers hope that an increased understanding of how and when white sharks prey upon seals will enhance and inform public safety measures.

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