Collapse of Shark Populations in the Northwest Atlantic
Ocean fishing fleets have expanded rapidly over the last fifty years and many fisheries have been subject to intense exploitation. As a result many commercial fisheries are depressed, and species of tuna, billfishes, sea turtles, and marine mammals have declined to historically low numbers. Shark populations (there are over 350 species of sharks) are vulnerable to overfishing because they have a few young at a time, and it takes years for sharks to become sexually mature.
Long-line fisheries have traditionally been thought of as less destructive and more selective than trawlers, which scoop up everything in their path indiscriminately. An average long-line set has 550 hooks trailing out behind the boat and it is not uncommon for a long-liner to catch more sharks on one set than the target species. Julia Baum and her colleagues at Dalhousie University have analyzed the bycatch of long-liners to get an idea of shark abundance, distribution, and population trends in the Northwest Atlantic1.
This data shows that shark populations have declined dramatically since 1986: hammerheads by 89 percent; white sharks by 79 percent; tiger sharks by 65 percent; thresher sharks by 80 percent; blue sharks by 60 percent; oceanic whitetip sharks by 70 percent; and mako sharks by only 50 percent.
We discussed this important paper with the lead author, Julia Baum.