Sharks die at an incredible speed, mainly for fishing. Retention bans may be helpful

Despite the fear they may inspire humans, sharks have far more reasons to fear us. Nearly a third of sharks are threatened by extinction around the world, mainly as a result of fishing.

A team led by researchers at UC Santa Barbara discovered that the obligation to release captured sharks is not sufficient to prevent the continued decline of these important marine predators. These findings are listed in Fish Fisheries and highlight the importance of combining different strategies for monitoring shark populations and managing numbers.

Some sharks are targeted in the fishing industry, but pressures have grown beyond these species. “More than half of the sharks caught and killed in the fishing industry are caught by chance and then discarded,” explained Darcy Bradley, co-author of the study of the lead scientist at the Bren School of Environmental Science and Management, a refuge.

Some species are protected by a retention ban issued by local fisheries management organizations. Currently, 17 marine shark species are covered with a no-holding to protect them from accidental catches of tuna fisheries.

The team had a simple question in mind. “For all the shark species we know, how many people are dead by the time they land because they are caught in the fishing industry or just after they are released as a result of capture?” said co-star author Ally Corman, a doctoral candidate at Brenschool. They were also interested in how certain regulations affected the survival of sharks.

The authors matched available data from more than 150 published articles with reports measuring shark mortality immediately following delivery (vascular) or release (post release). This literature spanned almost 150 different shark species captured by different fishing gear. Using this information, they were able to estimate the mortality rates for an additional 341 shark species that were accidentally captured by longline or guilnet, but whose empirical data were not available.

Small sharks and some endangered species were most likely to die after being caught. These included threshing machine sharks and hammerheads. There was also high mortality rates for smaller species, species that occur in deeper waters, and species that rely on constant swimming to breathe.

“In some species, such as Smoothhound Sharks, which ranged from 30-65%, mortality was surprisingly high,” said Leonardo Faytosa, a doctoral candidate for the Bren School doctoral program.

Policy simulations showed that retention bans could reduce shark mortality on average by three times, but this was not sufficient to reduce mortality against sustainable fishing levels for already intense fishing species such as macaws and silky sharks.

“The ban on retention is a useful first step in dealing with overfishing of sharks,” Bradley said.

The ban is most likely to benefit faster reproductive species, such as blue sharks, bonnessads and angel sharks, as the population tends to recover faster. Blue sharks are actually the most heavily caught species in the world. “While it is very unlikely that a ban on holding such commercially important species would be implemented, our results show that if this becomes necessary to maintain the population, this could be a relatively simple and impactful strategy,” Faitosa said.

For other sharks, additional strategies are required to maintain healthy populations. Methods to reduce catch rates from the start, such as banning the use of steel wires on long lines, were able to complement the ban on holding. Spatial regulations can also help strengthen shark populations, such as closing shark nursery and puppy lots.

The assembly of this study also highlights the need for more data on mortality rates in other cartilage fish, such as stintrays, skates, and cimara. “57% of cartilage fish that are extinct and threatened in the world are not sharks,” explained Caughman. The team was unable to include these groups in their paper due to lack of data.

Members of the Nature Conservancy team are currently meeting with science staff on the Inter-American Tropical Tuna Committee to work together on the issue. This work will help advance and inform these dialogues as you work to identify the suite of appropriate strategies needed to advance shark conservation.