As the world's ability to plunder the oceans for food grows, our ability to understand sustainable fishing levels has not kept pace.
Officials tasked with preventing overfishing often rely on overly optimistic projections about the health of certain fish stocks, raising the risk that permitted catches will be set too high, scientists reported last week in the journal Science.
“It's really important to bring these issues to the public's attention,” says Nils Krug, a fisheries scientist at the University of Tasmania (UTAS) who worked on the new study. “Hopefully this will lead to improvements.”
Estimating fish populations is an important and difficult task for fisheries managers. The total number of fish of popular species in a particular area is key to determining how much fish can be caught without overwhelming their reproductive capacity. Annual fish counts can also indicate whether a population is growing or declining, giving authorities the ability to decide whether to increase or decrease catches before the population crashes.
But coming up with these population estimates is as easy as knowing how many fish are in the ocean: The numbers hauled into nets by fishermen and research vessels are just a small sample of a vast, largely invisible world. To fill in the blanks, fisheries experts turn to complex models that can include more than 40 variables, from how quickly fish species reproduce to the effectiveness of certain types of fishing gear.
So how accurate are these estimates? To assess their performance, Crook and his colleagues tracked how population estimates for 230 different fisheries around the world changed over time, and how the latest analyses performed when applied retrospectively to historical estimates of fish abundance. More recent estimates, based on more data collected over multiple years and models tuned to better fit that data, are typically expected to be more accurate.
The results were not encouraging: “We found that previous stock assessments have often been overoptimistic about the number of fish in the ocean,” said Graham Edgar, a marine ecologist at UTAS and lead author of the study.
In two-thirds of the fisheries, earlier estimates of fish abundance were excessively high. In some cases, the inflated estimates were extreme: in 17% of fisheries, they were more than 50% high, and in more than 8% of cases, they were double the most recent estimates.
These shortcomings could keep fisheries managers from realizing the impending crisis: The scientists concluded that population trends in 12 more fisheries would raise warnings that fisheries were not keeping up with reproduction, and 11 more would be labeled as experiencing population declines severe enough to force a closure of the fishery.
This excessive optimism carries over into estimates of whether overfished fish species are showing signs of recovery, a phenomenon the researchers call “phantom recoveries.” This can lead managers to be misled about the effectiveness of their actions: 29 percent of fisheries classified by the United Nations Food and Agriculture Organization as “maximally sustainably exploited” should be declared “overfished,” the researchers found.
The scientists point out several possible explanations for these flaws: lower-value fisheries tended to have less precise analyses, suggesting that lack of attention and data could undermine the model's performance; areas with rapidly warming waters, which can have dramatic effects on fish populations, were more likely to overestimate the health of fisheries.
In doing so, complex model uncertainties may be overlooked or certain parameters may be adjusted to fit expectations about the outcomes.
The new research highlights “a clear, global failure of applied fisheries science to adequately advise managers,” Rainer Froese of the GEOMAR Helmholtz Centre for Ocean Research in Germany and Daniel Powley of the University of British Columbia in Canada, who were not involved in the study, wrote in an accompanying commentary.
The new findings suggest that it may be best to take a more conservative and simplistic approach to assessing fisheries health, the authors write. This would involve using simpler models and choosing the lower end of the range of possible estimates for the current size of the fishery. Fisheries managers should also scrutinize assessments for evidence of systematic bias.
Even if the new findings are more grim, in the long term they could increase the likelihood that there are still fish in the ocean to count — and eat.
Edgar et al., “Stock assessment models overestimate the sustainability of global fisheries,” Science, August 22, 2024.
Image: nordroden/Adobe Stock
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