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Vanishing fish, shrinking catches: How overfishing is undermining coastal fisheries

New science shows that overfishing is eroding the sustainability of tropical coral reef fisheries in East Africa, with small-scale fishers losing out on fisheries' productivity as entire species disappear from their catch.

The , now published in Sustainability and led by scientists from WCS and the University of Rhode Island, highlights that fisheries management strategies that focus solely on targeting a few fast-growing, resilient species have failed to compensate for the loss of more vulnerable schooling species.

Their takeaway? Protecting and managing the capture of the whole community of fish in a given fishery is a much better approach than trying to sustainably manage individual target species in very diverse reef fisheries.

By compiling data on fish communities from seven marine parks closed to fishing for as much as 45 years, researchers were able to set historical benchmarks of what maximum biomass and production of tropical reef fish could look like in coastal East Africa. A comparison of fish communities in the oldest parks with those landed in the catch of local fishers revealed a stark loss outside of the parks—less diversity in the fish, and less overall fish catch.

"A key finding of our study is that current catches landed by local fishers are missing a significant portion of the fish that historically would be caught," said Associate Professor Austin Humphries of the University of Rhode Island, an author on the study.

"The disappearance of these key fish—like snappers, unicorn fish, sweetlips, goatfish, and soldierfish—means coastal communities lose food. These diverse species provide essential proteins and micronutrients that aren't easily replaced, threatening both food security and the livelihoods of fishing families."

The study challenges previous management recommendations that suggested fishers should focus on targeting fast-growing species resilient to fishing pressure. The findings reveal that maintaining catches of resilient species alone does not compensate for the lost productivity of disappearing, vulnerable species.

"Many of the vulnerable species lost to fishing were social, schooling fish that often have high natural mortality and contribute significantly to total yields, while the resilient species were solitary," said Jesse Kosgei, Research Scientist with WCS Kenya's Marine Program.

"We found that yields were higher where schooling species were present in local fish catch, which was in managed areas that restrict some types of fishing gear. Reducing use of gillnets and spearguns that capture mostly schooling fish could help rebuild fish populations and sustain future catches."

The study also found that commonly used length-based assessments used to determine the sustainability of a fishery failed to capture the big picture of species losses. As fishing pressure increases, vulnerable species disappear from the catch entirely, making it impossible to measure their sustainability.

"There has been bits of evidence and hope among fisheries biologists that fishing resilient species that are highly productive can compensate for losses of fishing vulnerable species," said the lead author, Dr. Tim McClanahan, Director of Science for the WCS Global Marine Program.

"But our findings show that nearly 50% of potential fisheries production is lost when previously caught species disappear. This finding argues for maintaining all or most species, and not to estimate sustainability from a few resilient target species."

The researchers argue that fisheries management should shift away from a focus on a few productive species and instead prioritize the protection and recovery of diverse fish communities. Reducing the use of gillnets and spearguns, which disproportionately target schooling species, may help prevent further losses.

Sustainability assessments of most fisheries are largely based on the status of the target species, but according to McClanahan, "Because target species are only a portion of the catch, keeping and restoring vulnerable species makes sense from an ecological and food security point of view."