Counting the number of fish in the ocean may well be one of science’s toughest jobs. It also produces a crucial tool governments use to protect marine ecosystems that feed millions of people across the world.
Fish stock assessments work a lot like climate models. Scientists gather a wide range of data from fish catches, like age and weight, and track environmental conditions, like the temperature of the sea, and use mathematical models to estimate the health of fish populations. The analysis is then used to make recommendations to governments.
But as fishery scientists often say, counting fish is a lot like counting trees, except that they move and you can’t see them.
In fact, many assessments of the health global fisheries may be too optimistic, according to a new study published Thursday in the journal Science.
The researchers analyzed data from 230 of the most important fisheries in the world between 1980 to 2018. On average, the study found, assessments overestimated fish stocks by 11.5 percent. The findings, the researchers said, highlight that governments need to take more precautions to protect fish populations.
Though overfishing has historically imposed enormous harm to marine ecosystems around the world, since roughly the 1980s many countries, including the United States, have gotten a lot better at sustainably managing fisheries.
On average, the fish stocks across the world that the study examined were at healthy levels, though some overfished populations, like the blue king crab, are still struggling to recover.
Optimism about the state of fisheries was most pronounced in ecosystems that the study’s researchers considered overfished. According to the study, assessments of these struggling fisheries tended to project recoveries of fish populations that never fully materialized.
“We’ve worked across all of these stocks and see this signal that tells us we’re basically overestimating, on average, how much is out there,” said Amanda Bates, a marine ecology professor at the University of Victoria, in Canada, and one of the authors of the study. “And we’re making management decisions based on that.”
The study’s findings divided scientists. Some considered the study to be welcome scrutiny of one of the world’s key instruments to assess the health of marine ecosystems. Others criticized it as a flawed interpretation of a long recognized problem in fish assessments.
“A lot of the things they say you need to do are already in place,” said Ray Hilborn, a professor of fishery science at the University of Washington. Many fishery managers, he explained, already look back at historical trends to correct for a possible tendency to over or undercount fish populations.
Boris Worm, a professor of marine conservation at Dalhousie University, in Nova Scotia, who wasn’t part of the study, said the study breaks new ground in revealing a bias toward optimism.
Steven Cadrin, a marine scientist at the University of Massachusetts Dartmouth, who has worked on fish stock assessments for decades, said the study’s findings are “invalid,” partly because the researchers used the most recent stock assessments as a benchmark for accuracy. More recent tallies of fish populations may be just as prone to error, he said.
Underlying the disagreement among scientists was a recognition that estimating the number of fish in the ocean is incredibly hard.
The pressure to get it right comes from many directions. “Fishery management is always a balance between conservation, food security, fishing jobs and economics,” Dr. Cadrin said.
To make up for such complexity, fish counting methods can include more than 40 different parameters, such as fish size and age, information about the health of underwater plants and how many fishing boats are out at sea. Each parameter needs to be interpreted and comes with its own level of uncertainty.
More efficient fishing practices may mean fish boats can catch more fish even if there are fewer fish in the sea. And rising sea temperatures could mean some fish will change their ocean-roaming habits.
Climate change is increasingly the wild card in the fish-counting equation, and some scientists suspect that temperatures, not fishing practices, may be behind the struggle of some populations to recover.
Many fish-counting methods assume fisheries will continue producing roughly the same number of fish over time. But, as the ocean warms, those assumptions have been increasingly called into question.
The fisheries included in the study have some limitations. The study’s researchers examined fish assessments with ample data, which are largely done by some of the wealthiest countries in the world, such as the United States, Canada and Australia.
Still, some of the most troubled fisheries, in countries like India and Indonesia, don’t do thorough fish stock assessments, Dr. Hilborn said.
“The global fisheries problem,” he said, “is that about half of the world’s fish stocks are simply not assessed.”
