Science writer Erik Stokstad recently interviewed Dr. Rainer Froese (GEOMAR) about the findings of a newly published paper which studied the impact that climate change has had so far on global fisheries. The lead author, Chris Free, is a fisheries scientist who created a computer model of the way fish populations respond to temperature, relying on a large database of scientific assessments, conducted between 1930 and 2010, of stocks that represent roughly a third of global fisheries catch. Free looked for patterns of how these stocks had responded to changes in sea surface temperature.
The study shows warming waters already had an impact on the productivity of global fish stocks however their figures are probably an underestimate because their data are weak on the tropics. Fish in the tropics already live in warm water, so they have likely suffered more from recent temperature rises than have fish in the temperate zone. “Fish there are already with their back to the wall with respect to temperature,” Froese says. “We expect the tropics to be hardest hit.”
According to Froese, a silver lining is that the research suggests well-managed fisheries are more resilient in the face of rising temperature. “We have to stop overfishing to let the gene pool survive, so that [the fish] can adapt to climate change,” he says. “We have to give them a break”, said Froese.
Furthermore, Froese said Free found something else which they only mentioned in passing:
One could think that fisheries-induced selection for small size and early maturity pre-adapts overfished stocks to warming waters. However, the study finds the opposite trend. They speculate that the missing large fish are the cause of this observation, but large fish are oxygen-limited and will not survive in hot water (nice paper by Pörtner showing that for eelpout during a heat wave in the Wadden Sea). It is also not the missing high fecundity of large fish, because the relationship between egg numbers and recruitment is of the hockey-stick type, i.e. no increase of recruitment after reaching a certain minimum amount of eggs. Rather, it seems like the selection for small size and early maturity to survive the high mortality caused by overfishing does not help but hinder the survival of genomes with physiology adapted to less oxygen and higher temperature. The fisheries-induced selection is apparently so strong that it does not tolerate genomes that were pre-adapted to slightly higher than the previously existing temperatures. Natural resilience to environmental changes has been fished out. Could be of interest to dig deeper into this aspect of the paper.
Related post: Warming oceans are hurting seafood supply—and things are getting worse by Erik Stokstad