Sharks fall foul of fishing bias

The decline in shark populations may be the result of sexual discrimination, a team of European marine scientists says.

In the latest Proceedings of the Royal Society B journal, the researchers show sexual segregation of sharks in the open ocean could be a major contributor to population declines.

Senior author Dr David Sims, of the School of Biological Sciences at the University of Plymouth, says their study found a "striking" level of sexual segregation among the mako shark (Isurus oxyrinchus), in the South Pacific Ocean.

For the study Sims and colleagues used data collected from a commercial Spanish longline fishing vessel targeting swordfish in the southeast Pacific Ocean between December 2004 and March 2005.

A total of 264 male and 132 female mako sharks were captured as bycatch with males occurring predominantly in the western area of the survey region and females dominating in the east.

The paper says based on length measurement 84% of the males were considered to be adult compared with 13% of females.

The researchers say this indicates size, as well as sexual, segregation.

Sims says the "sexual line in the sea" for the mako corresponds with a marked change of intensity in longline-fishing activity.
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