Mako sharks killed at far higher rate than earlier estimates

Research has important implications for managing shark harvest

Todd McLeish

Mako shark tagged by URI researcher Brad Wetherbee and
colleagues from Nova Southeastern University.
(Photo by George Schellenger)

Brad Wetherbee and his research team have been capturing and tracking the movements of mako sharks since 2004, and more than 25 percent of those affixed with satellite transmitters have been caught and killed by commercial or recreational fishermen.

That mortality rate is more than 10 times the rate estimated by the international body responsible for managing the world’s mako shark fishery and far higher than is sustainable.

Wetherbee, a shark researcher at the University of Rhode Island, along with Mike Byrne and other colleagues at the Guy Harvey Research Institute at Nova Southeastern University, published a paper in the journal, Proceedings of the Royal Society B, documenting the mortality of the sharks they have been monitoring. 

They hope it will influence the fishery managers to take steps to reduce the catch of mako sharks.

“Makos are caught in all kinds of fisheries all around the world – gill netters, long liners, commercial, recreational,” he said. “They’re the shark everyone wants to catch because they’re good to eat – like a shark version of swordfish. But if our results are anything close to the true mortality rate, then they’re in trouble.”

Wetherbee admits that his results may not be reflective of the mortality the sharks face everywhere, and he said that there are some people who think that makos are being fished sustainably. But he also believes it would be irresponsible not to report the mortality rate of his study specimens.

“The fishery managers are faced with a lack of data about mako mortality,” Wetherbee said. “But based on our experience, the sharks are being killed at a much higher rate than they’re estimating, which means overfishing is probably occurring.”

Wetherbee and his colleagues catch as many as 20 mako sharks each year – though some years they catch far fewer – off the coast of the mid-Atlantic states, the Yucatan Peninsula of Mexico, and Rhode Island. Each one is affixed with an electronic tag that provides data for approximately one year about the daily movements of the sharks.

“So we know where they are in near-real time,” he said. “When they’re caught, we can follow them right to shore to someone’s dock or their house. We were surprised how often that was happening.”

His tagged sharks have been caught and killed by fishermen in the waters off Canada, Cuba, Mexico, Portugal, New Zealand and along the East Coast of the United States.

Last summer, Wetherbee made a public plea to fishermen in southern New England when one of his tagged sharks was tracked to local waters just as a number of shark fishing tournaments were scheduled. 

He asked anyone catching a mako shark with a satellite tag to release the animal unharmed. The shark survived the tournament season but was killed by fishermen off North Carolina a few months later.

Wetherbee said that those responsible for managing the mako shark fishery are expected to issue an updated stock assessment this fall, and he hopes they will take into consideration the results of his research. 

He also hopes that new policies will be proposed to reduce the number of mako sharks caught in the commercial and recreational fisheries.

“I’m not sure what they’ll do, but I hope they at least recognize that however they’re currently keeping track of mako shark mortality doesn’t appear to be very accurate,” he said. 

“Our data should at least help them get a better idea of what’s going on and give them more information to manage the population.”

Wetherbee and his colleagues also believe that the use of satellite tracking data for estimating shark mortality is a novel methodology that may be useful in other fisheries.

“Using electronic tags to learn the fate of individuals in a fishery is a pretty new way of estimating mortality,” said Mahmood Shivji, director of the Guy Harvey Research Institute. “But there’s no mistaking when a tag is reporting from shore that the shark is dead. It’s a known fate, as opposed to the estimates currently used. There’s promise for researchers to use the same technology on other species for estimating mortality.”

The research paper can be found at http://rspb.royalsocietypublishing.org/content/284/1860/20170658.