Shortfin Mako Shark Fishing Mortality Much Higher Than Thought

A new study reports that the mortality rate of shortfin mako sharks due to fishing in the western North Atlantic is higher than previously estimated from catch reports.

The report, published online at phys.org and done using satellite tracking by researchers from Nova Southeastern University's Guy Harvey Research Institute (GHRI), the University of Rhode Island and more, suggests the species is experiencing overfishing.

The study shows 30 percent of the 40 satellite-tagged sharks were captured. After modelling the probability that a mako shark would survive a year without being captured — a 72 percent 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 tracks of the tagged mako sharks, including the ones captured, can be viewed online on NSU's GHRI shark tracking website.

"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 Mahmood Shivji, Ph.D., senior author of the study and director of the NSU's GHRI. "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."

The study used near-real-time tracking of mako sharks from satellite tags. Directly seeing how they were captured bypasses the dependency on anglers self-reporting the catches and details. This could be used for other fished species as a time-efficient way to gather useful data "about the levels of fishing survival and mortality," said Michael Byrne, Ph.D., the paper's lead author and postdoctoral fellow at NSU's GHRI when the study was done.