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.
Males dominate in highly fished areas
He says males were predominant in the area where longline fishing, when averaged over a 55-year period, was historically higher. Female mako sharks dominated in the area with lower historical fishing activity.
"If high fishing activity occurs in key areas where, for example, the majority of a population aggregate for feeding or mating opportunities there is a potential for increased rates of decline," the paper says.
Recent studies suggest the population of pelagic sharks has dropped by as much as 80% in as little as 15 years, the paper says.
Sims says the finding has implications for assessing fisheries effects on shark populations.
"Complex structuring coupled with region-specific fishing activities may have disproportionate effects on difference components of shark populations," he says.
Shark biologist Dr John Stevens at CSIRO Marine and Atmospheric Research says sexual segregation in sharks and rays is a "common phenomenon".
Stevens, whose work was cited by the authors, says differential exploitation of the sexes is one part of the problem in declining shark populations.
"The main problem is overexploitation and trying to encourage fishing on a sustainable basis," he says.
Stevens says there is a lack of long-term data on which to model shark populations and their declines.
Any management of over-fishing needs to be done at a global level to be effective, he says.
"Because the population structures of these sharks are so complex they can't be managed in isolation," he says. "Australia is just one small part of their range."
Stevens says any regulation Australia may introduce can only be enforced within its territory and international fishing fleets could just sit outside the "200-mile zone" and do whatever they wanted.