Connecting the Dots: Refining Irregular Blacktip Shark Movement Data

Highly migratory species travel great distances that often encompass multiple political boundaries, which can make management challenging. Blacktip sharks, Carcharhinus limbatus, migrate from mating grounds in Georgia and the Carolinas to overwintering grounds in southeastern Florida. Seventy years ago, only vagrants were reported north of Cape Hatteras, NC. However, it is likely that the migratory pattern has changed as a result of climate change as sea surface temperatures have increased 0.85°C since that time. We instrumented 52 blacktip sharks with acoustic transmitters to determine the current migratory pattern. Collaborative acoustic telemetry networks allow researchers to cost effectively track a large number of animals over great distances but, the resultant detection data are often intermittent due to the irregular spatial orientation of receivers within the arrays. To regularize the temporal component of these data, we employed a correlated random walk model and performed a seasonal hot spot analysis to determine the areas most used by these sharks along the migratory route each season. Great densities occurred from New York to Georgia during the summer while New Jersey to central Florida were populated in fall. Florida to South Carolina coasts were densely populated in winter, while spring densities extended from Florida to North Carolina. Because sea surface in the Gulf of Maine is warming at a disproportionate rate to the global average, the blacktip shark may have effectively maximized its summer latitudinal thermal refuge.