Ocean's fastest shark at risk of being 'overfished'

Overfishing is putting the ocean's fastest shark at risk, according to a recent research.

The study, using satellite tracking by researchers from Nova Southeastern University's Guy Harvey Research Institute (GHRI), the University of Rhode Island and other colleagues, shows that the fishing mortality rate of the shortfin mako in the western North Atlantic is considerably higher than previously estimated from catches reported by fishermen.

These data suggest that this major ocean apex predator is experiencing overfishing, raising serious concerns about whether the current levels of fishery catches in the North Atlantic are sustainable.

"Traditionally, the data obtained to determine the rate of fishing mortality, a key parameter used to help gauge the health of shark stocks, has depended largely on fishermen self-reporting any mako sharks they may have caught," said senior author Mahmood Shivji. "The challenge is that not all fishermen report the same way or some may underreport or even not report their mako shark captures at all, so the these catch data are known to be of questionable reliability."

Shivji said that near real-time tracking of mako sharks using satellite tags and directly seeing how many were captured allowed researchers to bypass the dependency on self-reporting by fishermen.

"Using satellite tags for makos and possibly other fished species can be a time-efficient way and a fisheries-independent tool for gathering useful fisheries-interaction data, including answering fundamental questions about the levels of fishing survival and mortality," said lead author Michael Byrne.

Byrne added that the tracking data also showed that these mako sharks entered the management zones of 19 countries, underscoring how critical it is for countries to work together closely to manage and conserve these long-distance oceanic travelers. When the researchers began to gather, compile, disaggregate and review the data, the results were startling.

An unexpectedly high proportion, 30% of the 40 satellite tagged sharks, were captured in fisheries. After modelling the probability that a mako shark would survive a year without being captured (a 72% chance) and calculating the fishing mortality rates, researchers determined that the rate at which shortfin makos were being killed in fisheries was actually 10 times higher than previously believed.

The study is published in the Journal Proceedings of the Royal Society B.

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