As the world’s capacity for plundering the ocean for food grows, our ability to know what level of fishing is sustainable isn’t keeping pace.

Officials in charge of preventing overfishing are often relying on excessively rosy forecasts of the health of a particular fish population, increasing the danger that permitted fishing levels are too high, scientists reported last week in the journal Science.

“It’s vital to bring these issues to the public’s attention,” said Nils Krueck, a fisheries scientist at University of Tasmania (UTAS) who was involved in the new research. “This will hopefully lead to improvements.”

Estimating the size of fish populations is a crucial but tricky task for fisheries managers. The overall number of fish from a sought-after species in a particular area is key to establishing how many fish can be caught without overwhelming the ability for reproduction to keep up. Fish counts from year to year can also indicate whether a population is growing or shrinking, alerting officials to opportunities to allow more fishing or to cut back on a catch before a population crashes.

But coming up with these population estimates is about as easy as, well, knowing how many fish there are in the sea. The number of fish pulled up in nets, either by fishers or research vessels, is just a small sample from a very big and largely invisible world. Fisheries experts rely on complex models to fill in the blanks. These can include more than 40 variables, everything from how quickly a fish species reproduces to the effectiveness of a particular kind of fishing gear.

So how accurate are these estimates? To grade their performance Krueck and colleagues tracked how population estimates for 230 different fisheries around the world changed over time, and how the newest analyses performed when retroactively applied to past estimates of fish numbers. The assumption is that the newest estimate, informed by more data collected over multiple years and models tuned to better match that data, would usually be more accurate.

The results were not encouraging. “We found that the earlier stock assessments were often too optimistic about the number of fish in the ocean,” said Graham Edgar, the study’s lead author and a UTAS marine ecologist.

In two-thirds of the fisheries, the early estimates of a fish population were overly positive. In some cases the inflation was extreme – more than 50% too high in 17% of the fisheries and double the more up-to-date estimate in more than 8% of cases.

These flaws can leave fisheries managers blind to an impending crisis. Population trajectories for twelve additional fisheries would have set off alarms that reproduction wasn’t keep pace, the scientists found. Another 11 fisheries would have been labeled as suffering from a population collapse, a dire condition that can force a halt to fishing.

This excess optimism carried over to estimates about whether an overfished species was showing signs of recovery, a phenomenon the researchers dubbed a “phantom recovery.” That can mislead managers about the effectiveness of their actions. Of the fisheries classified by the United Nations’ Food and Agriculture Organization as “maximally sustainably fished,” 29% instead should be declared “overfished,” the researchers found.

The scientists point to several potential explanations for these flaws. Low-value fisheries tended to have less accurate analyses, suggesting that a lack of attention and data could impair the performance of models. Areas with rapidly warming waters, which can have a dramatic effect on fish populations, were more prone to overstate the health of a fishery.

Then there is the possibility that uncertainties in complex models were overlooked or that certain parameters were tweaked to fit expectations about the outcome.

The new research highlights “the apparent global failure of applied fisheries science to correctly advise managers,” wrote Rainer Froese of Germany’s GEOMAR Helmholtz Centre for Ocean Research and Daniel Pauly of the University of British Columbia in Canada, in a commentary accompanying the paper. Neither was involved in the study.

The new results suggest it might be a good idea to take a more conservative and simpler approach to gauging the health of a fishery, the duo wrote. That would include using more straightforward models and choosing the lower number from a range of possible estimates for the current size of a fishery. Fisheries managers also should be scrutinizing their assessments for evidence of systematic bias.

Even if the new results are more sobering, in the long run they could increase the chances that there are still fish in the sea to count. And to eat.

Edgar, et. al. “Stock assessment models overstate sustainability of the world’s fisheries.” Science. Aug. 22, 2024.

Image:; By nordroden/Adobe Stock