A new study, led by the New England Aquarium, shows that some species of sharks are more resilient than others when it comes to being caught and released in commercial longline fisheries. The results offer important information about the threats they face, which may help scientists better understand how best protect these animals from overfishing or other harmful practices like finning.
The five-year, multi-institutional collaborative study was published in the scientific journal PLOS ONE.
“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 Dr. Nick Whitney, Senior Scientist in the 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.”
The scientists placed tags called accelerometers on the fins of over 300 sharks from five different species that live and die after being caught in commercial longlines– these include sandbar, blacktip, tiger, spinner and bull sharks. The aim was to track their fine-scale movements as well as whether they lived or died once released into waters near Florida Keys or off Louisiana’s coast.
The team used this same technology called “accelerometry” found within Fitbit devices exclusively but with an added purpose–to monitor how big sea life fares when captured by fisherman. While many longliners release their catches, there are some sharks that must be released due to regulations. These laws only work if the animals actually survive after being set free and this can be difficult for researchers studying them because it’s not easy measure how well they fare in nature again.
“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,” said Whitney.
Some animals will die even after release due to stress or injuries sustained during the capture process. The findings of this study showed that for some shark species, like blacktip and spinner sharks, as many as 42-71% percent died in captivity while being watched by researchers; other types such sandbar tigers were much more resilient with only 3%.
This information is crucial to sustainably manage shark populations, as it shows that no-take regulations may be very effective for some species but less so for others.
“Sandbar sharks have been a prohibited species for most fisheries because they were found to be a severely overfished stock,” said Dr. Bob Hueter, co-author of the study, former Director of the Center for Shark Research at Mote Marine Laboratory, and Chief Scientist for OCEARCH. “Our data show that sandbar sharks generally survive catch-and-release in this fishery, so the rules requiring fishermen to release sandbar sharks have no doubt played an important role in this shark stock’s recovery.”
Blood stress is a measure of the animal’s ability to withstand demands placed on it, including injury. This measurement has been previously studied in both captured sharks and those tagged with tracking devices; however no one had ever measured blood stress values for an entire population this way before now.
A 2011 study led by Whitney used those same techniques at MPEEE Institute which found blacktip sharks caught by recreational fishermen showed very different response patterns when tested under lab conditions than wild-caught specimens did – leading them down two entirely separate evolutionary paths as well.
“We found around only 10 percent mortality in our previous study on blacktip sharks caught by rod and reel,” said Whitney. “In this latest study we found that 35 percent of blacktip sharks are dead by the time they’re caught and many more will die after release, producing total mortality of around 62 percent. This shows that bottom longlines are very hard on blacktip sharks, even when the live animals are returned to the water.”