One Planet News: Beyond jaws - cutting edge research into shark mortality
Why fishing is killing too many sharks
By Annette J Beveridge
Who doesn't remember the dramatic music from the film Jaws as the shark moved in for the kill? While the film, which first aired in 1975, captivated audiences, it also created fear and hatred for these magnificent animals.
We may fear sharks, but they have more to fear from us.
Nearly one-third of sharks are threatened with extinction globally, and this is mainly due to fishing.
A study led by researchers at UC Santa Barbara discovered that mandates to release captured sharks are not sufficient to prevent the continuous decline of these important ocean predators. The findings were published in Fish & Fisheries, and shine a spotlight on the importance of monitoring shark populations and the use of different strategies for managing their numbers.
Co-author of the study and adjunct faculty at the Bren School of Environmental Science & Management and lead scientist at The Nature Conservancy, Darcy Bradley said: "More than half of sharks that are caught and killed in fisheries are captured incidentally and then discarded."
While some sharks are deliberately targeted through fishing, the pressure on shark conservation goes way beyond this.
Current shark retention bans require fishermen to release a shark rather than to keep it. There are 17 oceanic shark species covered by the ban currently which aims to prevent them from incidental catch in tuna fisheries.
But the study had a simple question to ask.
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Doctoral candidate at Bren School and Co-lead author, Allie Caughman, said: “For all shark species that we know are caught in fisheries, how many are dead by the time they are landed or soon after release as a result of capture?"
To answer this, the authors collated available data from more than 150 published papers and reports that have measured shark mortality upon hauling (at-vessel) or soon after release (post-release). The literature spanned nearly 150 different shark species caught by different fishing gears and using this information, it was possible to estimate mortality rates for an additional 341 shark species which were incidentally captured by longlines or gillnets.
Researchers found that small sharks and several threatened species were the most likely to die after being caught. Mortality rates included thresher sharks and hammerheads.
Smaller species were at risk, as well as those sharks preferring deeper water or relying on constant swimming to breathe.
Doctoral candidate at Bren School and co-lead author, Leonardo Feitosa, said: "Mortality was surprisingly high for some species such as smoothhound sharks ranging from 30 to 65%."
Deep-water species, like sleeper sharks, also fared poorly, likely due to the trauma of the extreme pressure change.
The simulations revealed retention bans were three times more likely to reduce shark mortality but it would not be enough of an improvement for heavily-fished sharks.
Bradley said: “Retention bans are a beneficial first step towards addressing shark overfishing, but they need to be complemented with other strategies, such as area-based fishing restrictions, catch quotas and fishing gear requirements to sustain populations for many shark species."
Bans are most likely to benefit species with faster reproductive rates - like blue sharks, bonnetheads and angel sharks - because their populations tend to recover faster.
Methods to reduce catch rates - such as banning the use of steel wire on longlines could complement retention bans. Spatial regulations could also help bolster shark populations, such as closing off shark nurseries and pupping grounds.
