Warmer Oceans May Lead to Smaller Fish, Study Finds

sea life
Fish swim off the coast of Peru. Jeff Rotman / Stockbyte / Getty Images

As climate change causes the temperatures of the world’s oceans to increase, the fish calling those warmer oceans home could become smaller, scientists say. This is due to warmer oceans having less oxygen, and bigger fish requiring more of it. This could lead to issues for the fishing industry, and to food insecurity.

A team of international researchers, led by Renato Salvatteci from the Center for Ocean and Society at the Christian-Albrecht University of Kiel, analyzed a section of ocean floor sediment from between 116,000 and 130,000 years ago, during the last interglacial period when the planet was about two degrees Celsius warmer than it is now and contained less oxygen. The core of sediment studied had been collected by a research vessel in 2008 from the Humboldt Current system, which lies off the coast of Peru. The team’s paper, “Smaller fish species in a warm and oxygen-poor Humboldt Current system,” was published in Science.

“There are many lines of evidence saying that a warm ocean with less oxygen will drive [the] shrinking of the fishes of the world,” Salvatteci told Mongabay. “But this is the first one [with] empirical evidence showing smaller fishes in the last interglacial, a period warmer than today.”

The researchers found that, while more recent sediment deposits are dominated by the bones of anchovies, during the warmer interglacial period about 60 percent were of smaller species, including goby-like fish about half the size of anchovies, reported Science. According to Earth.com, the goby-like fish were better able to survive in water with less oxygen due to the surfaces of their gills being larger in comparison to the size of their bodies. Other smaller species included those typically found in deeper water, including Panama lightfish (Vinciguerria lucetia) and blue lanternfish.

Salvatteci and the other researchers said that the ocean conditions and fish communities in the Peruvian waters are moving toward those of the last interglacial period, which could mean a threat to global fish supplies, Mongabay reported. In addition to the warmer waters posing a threat to larger fish species, industrial fishing creates added pressure, said Arnaud Bertrand, study co-author and scientist at the French National Research Institute for Sustainable Development.

“Additional pressures from fishing could also be affecting fish size and communities, so it has been hard to tell if it is the warming climate that is affecting fish size or fishing pressure,” Bertrand said in a joint interview with Salvatteci with Mongabay. “But during the interglacial period, this fishing pressure wasn’t present.”

The anchovy industry in Peru had its highest annual catch of over 13 million metric tons back in 1971, according to Oceana. While the fishery remains one of the largest in the world, since then catches have decreased, with a yearly average of four to eight million metric tons, Mongabay reported.

“Nobody knows when we are going to reach a tipping point, but… the peak of anchovy production in Peru has already passed,” Salvatteci told Mongabay. “Maybe there will be years with huge anchovies in the ocean, but the trend is to lower anchovy biomass in the ocean in front of Peru, because of warming [and] because of the oxygenation. But we don’t know when that will happen.”

A shift in ocean temperatures similar to those in the study could not only affect marine biodiversity, but it could also threaten fisheries worldwide, reported Eurasia Review.

In a related perspective, Moriaki Yasuhara, an associate professor at the School of Biological Sciences at The University of Hong Kong, and Curtis Deutsch, professor of geosciences and the High Meadows Environmental Institute at Princeton University, wrote, “The findings of Salvatteci et al. are the latest addition to the emerging evidence that a warmer future will alter ecological communities in tropical oceans, which disproportionately affect developing countries, where reliance on small-scale fishing is especially high,” Eurasia Review reported.

A professor of marine conservation biology at Canada’s Dalhousie University, Boris Worm, who wasn’t involved in the research, told Mongabay, “[The study’s] findings support the prediction that warmer waters will hold smaller fish that may be better adapted to future conditions, but less likely to feed the world.”

Cristen Hemingway Jaynes is a writer of fiction and nonfiction. She holds a JD and an Ocean & Coastal Law Certificate from University of Oregon School of Law and an MA in Creative Writing from Birkbeck, University of London.

EcoWatch Daily Newsletter