Blue sharks ride the North Atlantic gyre, moving northeast with the Gulf Stream and returning south and west along the eastern side of the basin. Conventional tagging programmes running since the 1960s recorded thousands of recaptures, including individuals covering more than 5,000 kilometres between release and recapture. Similar large-scale movements occur in the Pacific and Indian Oceans.
The migrations are strongly sex-segregated. Adult females and males are often found in different parts of the ocean and at different latitudes, and mating leaves distinctive bite scars on females, whose skin is roughly twice as thick as a male's.
All that mobility does not confer safety. Blue sharks are the dominant bycatch of pelagic longline fleets targeting tuna and swordfish, with tens of millions estimated caught annually. Because catch reporting is patchy and stock boundaries are unclear, how hard the global population is actually being hit is still contested.