Some sharks struggle with catch and release
Some shark species are extremely resilient to the stress of being caught and released while other species are much more likely to die, shark researchers found in a New England Aquarium-led study.
The five-year study was of more than 300 sharks from five different species: sandbar, blacktip, tiger, spinner and bull sharks.
Some animals will die even after getting released because of stress or injuries from the capture process. Findings of the study showed that for some shark species, like blacktip and spinner sharks, as many as 42% to 71% of sharks will die even after being released alive. Other sharks, like sandbar and tiger sharks, were much more resilient, with only 3% or less dying after release.
“We set out to do what very few studies had done previously — put electronic tags on a large number of sharks and collect blood samples from the same animals that we tagged,” said Nick Whitney, senior scientist in the New England Aquarium’s Anderson Cabot Center for Ocean Life and lead author of the study.
“We did this to get an idea of how well we could predict their fate based on stress indicators in their blood,” he added.
The scientists placed tags called accelerometers, the same technology found in a Fitbit, on the fins of more than 300 sharks.
The technology helped the researchers track the sharks’ fine-scale movements and, most importantly, whether they lived or died after being caught on commercial longlines and released in the Gulf of Mexico and Florida Keys.
Although longline fishermen typically keep the sharks that they catch, some species must be released due to regulations. Such rules are only effective if the sharks actually survive after they are released, which is difficult to measure.
“The assumption behind no-take regulations is that the shark will swim away and live out its normal life after it’s released, but we know that for some sharks, that’s not true,” Whitney said.
The data from this study shows that no-take regulations may be very effective for some species but less so for others.
While many studies have measured blood stress values in captured sharks and others have tagged sharks to tell whether they lived or died, very few previous studies had measured blood stress values in the same animals that they tagged.
A 2011 study led by Whitney used this same combination of techniques and found a very different result for blacktip sharks caught by recreational fishermen.
“We found around only 10% mortality in our previous study on blacktip sharks caught by rod and reel,” Whitney said. “In this latest study we found that 35% of blacktip sharks are dead by the time they’re caught and many more will die after release, producing total mortality of around 62%. This shows that bottom longlines are very hard on blacktip sharks, even when the live animals are returned to the water.”