The multi-institutional, five-year study, led by the New England Aquarium and published in the scientific journal PLOS One, found that bycatch mortality is a big contributing factor to shark population declines all over the world. While longline fishermen usually keep the sharks they catch, some species have to be released due to regulations. However, the study shows that the stress and injuries sustained during the capturing process means that some sharks will die after the release.
By tagging sharks, including sandbar, blacktip, tiger, spinner, and bull sharks with accelerometers, the study tracked their movements and looked into whether they lived or died after being caught by commercial longline fisheries and released in the Gulf of Mexico and the Florida Keys.
The study quantified post-release mortality rates by pairing blood-stress physiology with animal-borne accelerometers. Blood samples were taken from the tagged individuals. “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 findings included that post-release mortality (PRM) accounted to 2 to 3 per cent in tiger and sandbar sharks which shows that these species might be much more resilient to the stress of catch and release. “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.”
On the other hand, 42 per cent of blacktip sharks and 71 per cent of spinner sharks will die even after being released alive. 90 per cent of PRM happened within five hours after release and 59 per cent within 2 hours, according to the study. The blood samples showed that most sharks died due to acidosis and increases in plasma potassium levels.
The study shows that no-take regulations might be less beneficial for susceptible species such as spinner and blacktip sharks and will in no doubt help manage shark populations more effectively.
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Photography courtesy of Unsplash.
