A commercial fisherman views his boat’s pollock catch in Newlyn, England, March 7, 2021.. Hugh R Hastings/Getty/Inside Climate News
This story was originally published by Inside Climate News and is reproduced here as part of the Climate Desk collaboration.
In the world’s waters, fish are making a quiet, biological retreat. The once simple rules of the ocean—grow larger than potential predators—are being rewritten as temperatures reach record highs. Desperate to survive, fish are hitting the fast-forward button on life in a biological shift that will soon impact what ends up on dinner tables globally.
“There are simply no real winners here.”
Fish are getting smaller and dying at higher rates as they adapt to warming waters, researchers warn in a report released Thursday in the journal Science. This evolutionary change will reduce global fish yields by one-fifth under current warming predictions, and up to 30 percent in high-emissions scenarios.
This will trigger potentially irreversible evolutionary processes, shaking up entire ecosystems and food webs, with consequences for the billions of people who rely on seafood for protein—a demand expected to increase.
“What I found frightening about this work was that it was difficult to identify winners and losers—there are simply no real winners here,” said Craig White, the study’s co-author and an evolutionary physiologist at Monash University in Australia. “The combination of warming and evolution was always bad for fisheries.”
Fish mortality rates have already been rising as waters warm. Although fisheries management often assumes fish are evolutionarily inert when it comes to overcoming such environmental changes, this is false. Instead, fish are maturing at a younger age and at a smaller size to improve their chances of surviving long enough to reproduce, according to the report.
Fishery yields were already expected to reduce by 14 percent when global temperatures reach 2 degrees Celsius above pre-industrial levels. However, when incorporating evolutionary impacts, the researchers’ new model predicts this reduction worsens to 22 percent.
For the Alaska pollock—a key species for human consumption in North America—this would equate to a reduction of half a million metric tons harvested per year.
“This is a loss of over 1.1 billion meals of high-quality protein per year as a consequence of the effects of global warming on just one species,” said David Reznick, a professor of evolutionary ecology at the University of California, Riverside, who was not involved with the study but co-wrote a new piece about it in Science. “Climate change represents an immediate threat to the earth’s capacity to sustain human life.”
Decades of decreases in size, age at maturity and abundance in species like Atlantic salmon and Baltic cod appear to validate the model’s predictions. In total, the life histories of nearly 3,000 species of fish were tested to corroborate the model’s accuracy.
“What we can’t do is assume that species will evolve their way out of trouble in a way that suits us.”
Researchers noted impacts will vary by geography. Freshwater systems are predicted to warm more than oceans and will therefore see the most severe size reductions.
There will also be consequences beyond harvesting. “Much of what happens in the ocean in terms of who eats whom is based on body size: Big things eat smaller things,” said Joseph Travis, a biologist and former dean of Florida State University’s College of Arts & Sciences. If the size of harvested species decreases, they will become vulnerable to predation by other fish, said Travis, who co-wrote the Science piece about the study.
“The entire ecosystem could be thrown into an alternative configuration as the system moves past its tipping point,” said Travis, highlighting the example of the reconfiguration of Canada’s western Scotian shelf in the late 20th century. Here, the average size of 53 top predators—like cod and haddock—dropped 40 percent in 40 years. As a result, former prey increased by 300 percent as they became predators for young cod.
Increases in fish death frequency from disease, deoxygenation, or overfishing will only add further pressure. “If people try to compensate for smaller fish and less revenue per fish by harvesting more fish, then the problem worsens quickly,” said Travis, warning of potential stock depletion. “The net effect, in the long run, will be less protein available.”
“If humans, as predators, cause the fish to evolve, as do predators in natural ecosystems, then they also cause changes that will not spring back to their former state,” said Reznick. Indeed, as fish decrease in size, populations are losing the genetic variations that encode large bodies. And, as ecosystems shift, populations might be locked into new food chain states they cannot reverse.
“What we can’t do is assume that species will evolve their way out of trouble in a way that suits us,” said White, highlighting that effective climate policy could preserve roughly 18 million metric tons of fishery yields each year.
His message to policymakers is clear: While fish can adapt to survive, the only way to protect people who rely on fisheries for their protein and their livelihood is to reduce warming.
