Sharks and rays are among the most threatened marine animals in the world, second only to amphibians in terms of extinction risk. Overfishing — whether targeted or incidental — is their biggest threat, with many caught unintentionally and discarded as bycatch. Even though efforts have been made internationally to reduce wasteful practices like finning and to set catch limits, global shark mortality remains high. Part of the reason is biological: sharks tend to grow slowly, reproduce late, and have few offspring, so even moderate fishing pressure can push populations into decline. Add in weak enforcement, growing global demand for shark products, and you have a recipe for ongoing trouble.
“More than half of sharks that are caught and killed in fisheries are captured incidentally and then discarded,” explained Darcy Bradley, co-author of the study and adjunct faculty at the Bren School of Environmental Science & Management and lead scientist at The Nature Conservancy. That doesn’t mean those sharks survive. Many die before they even make it on board — what scientists call at-vessel mortality (AVM) — or die after being released back into the water, known as post-release mortality (PRM). These deaths aren’t always easy to track, but they matter. Mortality rates vary by species, gear type, and even traits like how a shark breathes. For example, species that need to swim continuously to breathe tend to suffer higher mortality than those that can pump water over their gills. Some gear, like longlines left out too long or trawl nets dragged for hours, increases the chance that a shark won’t survive the encounter. And while AVM can be observed directly, PRM is harder to measure, making it a major data gap in shark conservation.
To reduce shark mortality, many regional and national fishery managers have turned to species-specific retention bans. These rules require fishers to release certain shark species if caught. Since 2010, the number of these bans has grown across the world’s five tuna Regional Fisheries Management Organisations (tRFMOs). Today, 17 oceanic shark species are protected this way, including the oceanic whitetip, which is banned across all regions. The United States has its own bans (e.g., on dusky sharks) but effectiveness varies by gear type and enforcement.
A recent study examined whether these retention bans are actually working. The research team pulled together data from 160 studies covering 147 shark species caught on longlines or in gillnets, using this to build models that predicted AVM and PRM for over 340 species. These predictions were then used to simulate two policy scenarios: one where all sharks were retained and another where retention was fully banned. Smaller species in shallow waters were more likely to die before they were even landed, while deeper-dwelling species often died after being released. When comparing the two scenarios, retention bans reduced fishing mortality for most species… but not all.
On average, the ratio of discarded catch to sustainable catch was about three times higher than what would be considered sustainable. Around 72% of species could still experience up to five times the sustainable fishing pressure and remain within safe limits, assuming retention bans were followed perfectly. But for slow-growing species, the benefits were smaller. For these sharks, even small amounts of bycatch could lead to population declines. Among the 37 shark populations with known mortality data, retention bans could reduce fishing mortality to sustainable levels for about two-thirds, but heavily overfished stocks like some hammerheads or threshers would still be in trouble.
The study also revealed major blind spots in current management strategies. Batoids like rays and sawfishes, many of which are more endangered than sharks, are largely missing from the data. Not to mention that most studies focused on industrial longline fisheries, especially in the Global North, with the United States, Australia, and Portugal dominating the research landscape. Coastal fisheries and small-scale operations — where many threatened species live — are also currently underrepresented. And even in well-studied areas, crucial details like the age or sex of the sharks caught, soak times, or how they were handled on deck are often missing. Juvenile sharks are more vulnerable than adults, and poor handling can mean the difference between survival and death.
These details clearly matter.
So while retention bans are helpful, they’re not a silver bullet. For them to work well, they need to be paired with other measures that actually prevent shark bycatch in the first place. These could include seasonal or area-based fishery closures, gear modifications that reduce shark interactions, or banning the use of wire leaders, which make it harder for sharks to escape once hooked. For example, when Palau banned shark retention and required fishers to switch from wire to monofilament leaders, shark bycatch dropped, and those that were hooked had a better chance of escaping unharmed.
The bottom line is, if we want to conserve sharks effectively, we need to understand how and why they’re dying, and then use that information to reduce deaths before they happen. That means better data, smarter policies, and tools that actually change fishing behavior.
