Although sharks often inspire fear in humans, they have even greater reason to fear us. Nearly one-third of the world's shark species are at risk of extinction, largely due to fishing. A team led by scientists at the University of California, Santa Barbara found that simply requiring the release of captured sharks is not enough to halt their decline. Their study, published in the journal Fish & Fisheries, highlights the need for continued monitoring of shark populations and the adoption of multiple management strategies to protect these important marine predators.
While some sharks are directly targeted by fisheries, the impacts are not limited to these species. "More than half of the sharks caught and killed in fisheries are caught incidentally and then discarded," explained study co-author Darcy Bradley, an adjunct faculty member in the Bren School of Environmental Science and Management and chief scientist at The Nature Conservancy.
Some species are protected by retention bans issued by regional fisheries management organizations, which require fishermen to release individuals they catch rather than retain them. Currently, 17 marine shark species are protected by retention bans to prevent them from being accidentally caught in tuna fisheries.
The research team had a simple question in mind. "Of all the shark species that we know of that are caught in fisheries, how many die when they are caught ashore or shortly after they are released?" said co-lead author Allie Caughman, a doctoral student in the Bren School. They're also curious about how certain regulations affect shark survival.
The authors compiled available data from more than 150 published papers and reports that measured shark mortality at the time of hauling (on board) or shortly after release (post-release). The documents cover nearly 150 species of shark caught using different fishing gear. Using this information, they were able to estimate mortality rates for an additional 341 shark species that were incidentally caught by longlines or gillnets but for which there was no empirical data.
Small sharks and several endangered species are most likely to die after being caught. These include thresher sharks and hammerhead sharks. Mortality rates are also higher for smaller species, those that live in deeper waters, and those that rely on constant swimming to breathe.
"Some species have surprisingly high mortality rates, such as hunting sharks," said Leonardo Feitosa, co-first author of the paper and a doctoral student at the Bren School. "Mortality rates range from 30 to 65 percent. Deep-sea species, such as snorkeling sharks, are also in poor shape, possibly due to trauma caused by extreme pressure changes."
Policy simulations show that release mandates can reduce shark mortality by three times on average, but this is not enough to reduce mortality in already heavily fished shark species, such as mako and silky sharks, to sustainable fishing levels. "A ban is a useful first step in addressing shark overfishing," Bradley said, "but it needs to be complemented by other strategies, such as area-based fishing restrictions, fishing quotas and fishing gear requirements, to maintain populations of many shark species."
A ban is most likely to benefit species with higher reproductive rates, such as blue sharks, bonnethead sharks and angel sharks, as their populations tend to recover more quickly. Blue sharks are actually the most fished species in the world. "While the likelihood of a retention ban on this commercially important species is highly unlikely," Fetosa said, "our findings suggest this could be a relatively simple and effective strategy if necessary to maintain populations."
For other sharks, maintaining healthy populations requires additional strategies. Methods that reduce catch rates in the first place (such as banning the use of wire in longlines) could supplement retention bans. Spatial regulation can also help increase shark populations, such as closing shark nurseries and breeding grounds.
This compilation of studies also highlights the need for more data on mortality in other cartilaginous fishes, such as stingrays, rays and others. "57 percent of the world's endangered cartilaginous fishes are not sharks," Kaufman explained. The team was unable to include these groups in the paper due to a lack of data.
Members of The Nature Conservancy’s team are currently meeting with researchers from the Inter-American Tropical Tuna Commission to collaborate on this issue. This work will help advance and guide these conversations as they work to identify a range of appropriate strategies needed to advance shark conservation.
Compiled from /ScitechDaily