(CN) — Fish provide a critical source of protein for a growing human population, but overfishing has placed many of the world's fisheries on the brink of collapse.
A new study published Thursday in the journal Science suggests the situation is likely even worse than many experts predict. Researchers at the University of Tasmania and the University of Victoria found that models used to estimate fishery stocks tend to overstate the size of fish populations, particularly when it comes to overfished stocks.
The scientists analyzed historical population estimates for 230 fisheries, including most of the world's largest fisheries, and compared newer population estimates for prior years to older estimates for those same years. These newer estimates can be assumed to be more reliable, as they are based on data from a longer period of time, so this can reveal systemic biases in how fish stocks are estimated.
The researchers found that while newer estimates tend to match historical predictions for sustainably fished stocks, previous estimates greatly overstated fish populations for stocks considered overfished.
Extrapolating to current predictions, the scientists estimate that 29% of fish stocks currently considered by the United Nations to be "maximally sustainably fished" are actually overfished. A fishery is considered "collapsed" when its stock has dropped to below 10% of its original numbers. The researchers estimate that 85% more fisheries are collapsed than currently recognized.
University of British Columbia professor Daniel Pauly, who helped write a perspective on the study for the journal, told Courthouse News this problem is due to the number of variables used in the models, some of which are subjective. This can cause experts to overestimate fish populations due to sociopolitical pressure.
"The models are over-optimistic," Pauly said. "They predict that things will fix themselves, and they don't."
The study found that the older estimates at times show population increases where later estimates for the same year reveal there actually were none, so-called “phantom recoveries.”
This is a problem because if fishery managers believe fish populations are recovering when they aren’t, that can lead them to allow more fishing than is sustainable. The study found that 66% of the analyzed stocks had overestimation bias, meaning the full extent of population depletion was not known when management decisions were made.
“The existence of phantom recoveries suggests that many highly managed stocks worldwide could be locked into an overfished state,” the researchers write.
Pauly said climate change further complicates matters.
“For some fish, the higher temperature is good for them when they are young and bad for them when they're old. It’s very difficult to include in the models,” he said. “If you want to consider global warming, which you must, this is more parameters, more uncertainty and more probability that you will be over-optimistic.”
Pauly said the solution is for experts to use simpler models with fewer variables.
The study’s authors emphasize the importance of reliably estimating fish stocks in order to properly manage fisheries.
“Although inadequate precaution can generate short-term catch benefits,” the authors write, “it erodes long-term societal interests through loss of species with immense economic, environmental, cultural, recreational, and spiritual value.”
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