Collapse of Shark Populations

in the Western North Atlantic

In January 2003, a team of researchers from Dalhousie University headed by Julia Baum published a short paper in the journal Science (Vol. 299, pp 389-392) entitled, "Collapse and Conservation of Shark Populations in the Northwest Atlantic".  In it, they analyzed logbook data of shark bycatch taken by U.S. pelagic longline fleets targeting swordfish and tunas in the western North Atlantic.  Six shark species or species groups were recorded from 1986 onward and eight shark species from 1992 onward.  These data allowed Baum and her team to compare incidental shark catches over a period up to 1.5 decades. 

Baum and her team found that, with the exception of makos (genus Isurus), shark populations of all recorded shark species in the western North Atlantic had declined by more than 50% in the past 8 to 15 years and Scalloped Hammerhead (Sphyrna lewini), White Shark (Carcharodon carcharias), and thresher sharks (genus Alopias) have declined by more than 75% over the past 15 years.  The graph above is another representation of Baum et al's findings.  The species group "Hammerheads" consists mainly of the Scalloped Hammerhead, "Grey Sharks"  includes coastal species of the genus Carcharhinus as well as the pelagic Silky Shark (C. falciformis), "Thresher Sharks" includes both the Common (Alopias vulpinus) and Bigeye (A. superciliosus), while "Mako Sharks" consists mostly of the Shortfin Mako (I. oxyrinchus). 

These results show that overfishing is threatening many large coastal and oceanic sharks in the western North Atlantic.  These sharks should thus be prioritized for conservation efforts, or these species may be extirpated from the region.  The long-term ecological effects of local extinction of so many large coastal and oceanic sharks is unknown, but given their slow maturation and low reproductive rate a cautionary approach seems warranted.  Efforts should thus be made to reallocate pelagic longline fishing effort and/or to reduce shark bycatch in the western North Atlantic.

 
 

ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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