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Even though sharks evoke fear in many, the reality is that these creatures have more cause to be wary of humans. A significant portion—nearly one-third—of shark species are currently facing threats that could lead to extinction, predominantly due to fishing practices.
Research spearheaded by the University of California, Santa Barbara, reveals that simply enforcing catch-and-release mandates for sharks will not suffice to halt the dwindling populations of these vital marine predators. Their findings, shared in the journal Fish & Fisheries, underscore the necessity for systematic monitoring of shark populations and employing multifaceted approaches to effectively manage their numbers.
Although certain shark species are targeted directly by commercial fisheries, the issue extends beyond those explicitly sought after. “More than half of the sharks caught in fisheries are incidentally captured and subsequently discarded,” noted Darcy Bradley, the study’s co-author and an adjunct faculty member at the Bren School of Environmental Science and Management, who also serves as a lead scientist at The Nature Conservancy.
While retention bans are in place for specific species, enforced by regional fisheries management organizations to mandate the release of caught individuals, these measures alone might not be sufficient for shark protection. As it stands, 17 species of oceanic sharks fall under such retention bans to guard them against incidental capture during tuna fishing.
The research team aimed to answer a crucial question: “For all shark species known to be captured in fisheries, what proportion dies by the time they are brought onboard or shortly after being released due to capture-related stress?” posed Allie Caughman, the co-lead author and a doctoral candidate at the Bren School. They were also interested in assessing the impact of specific regulations on shark survival rates after release.
The study involved the review of data from over 150 academic papers and reports measuring shark mortality rates either during capture or shortly after release. This extensive literature review encompassed approximately 150 different shark species captured by various fishing methods. Utilizing this data, the researchers estimated mortality rates for an additional 341 shark species that were incidentally caught via longlines or gillnets, despite lacking direct empirical data.
Findings indicated that smaller sharks and some threatened species are particularly susceptible to mortality following capture, with thresher and hammerhead sharks being notably at risk. Smaller species, especially those inhabiting deeper waters or needing constant movement to breathe, exhibited higher mortality rates.
“We found unexpectedly high mortality rates for certain species, including smoothhound sharks, with rates ranging from 30 to 65%,” remarked Leonardo Feitosa, another co-lead author and doctoral candidate at the Bren School. Deep-water species, such as sleeper sharks, also demonstrated poor survivability, likely due to stress resulting from drastic changes in pressure.
Modeling scenarios suggested that implementing retention bans could theoretically decrease shark mortality by as much as three times on average. However, this alone would not suffice to lower mortality levels to sustainable fishing thresholds for heavily exploited species like mako and silky sharks. “While retention bans are a positive initial measure against shark overfishing,” Bradley emphasized, “they must be supplemented with additional strategies—such as area-based fishing regulations, catch limits, and specific gear requirements—to ensure the survival of many shark populations.”
Species with quick reproductive cycles, like blue sharks, bonnetheads, and angel sharks, are likely to benefit the most from such bans, given their capacity for faster population recovery. Notably, the blue shark is the most heavily fished species globally. “Although it’s improbable that retention bans will be introduced for commercially significant species like blue sharks,” Feitosa stated, “our findings indicate that such measures could serve as a straightforward yet effective conservation strategy if the need arises.”
Ensuring the resilience of other shark species will necessitate a broader suite of strategies. Initial measures to reduce catch rates, such as prohibiting steel wire on longlines, could significantly aid in combination with retention bans. Spatial regulations, including the closure of vital shark nurseries and breeding grounds, could further bolster shark populations.
The compilation of this study also brought to light the significant data gap regarding mortality rates for other cartilaginous fishes, including stingrays, skates, and chimeras. “A striking 57% of cartilaginous species facing extinction globally are not sharks,” Caughman noted, indicating that the team was unable to analyze these groups due to insufficient data.
Team members from The Nature Conservancy are currently engaging with the scientific staff of the Inter-American Tropical Tuna Commission to address these critical issues. Their collaborative efforts aim to refine and promote essential strategies for advancing shark conservation initiatives.
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