Warming oceans have caused a slump of up to 35% in some global fish populations, with over-exploitation of stocks exacerbating the problem, a new study has found.
Many fish species have been unable to adapt to the pace of global warming and scientists recorded an overall 4.1% drop in sustainable catches between 1930 and 2010.
The worst-affected ocean was the Sea of Japan, with a 34.7% reduction in fishery size. The East China Sea saw a drop of 8.3%.
Malin Pinsky, a co-author of the study and associate professor at Rutgers University, US, said: “We were stunned to find that fisheries around the world have already responded to ocean warming. These aren’t hypothetical changes sometime in the future [anymore].”
“It affects not just local economies but global trade and the ripple effects are felt all around the world. I do worry about the East China Sea, the Sea of Japan and North Sea in particular because the declines there have already been so large.”
The East China Sea was suffering from a classic “one-two punch” of global warming and overfishing, he told China Dialogue Ocean, with overfishing reducing population sizes and changing their demographics.
Smaller, weaker and less genetically diverse fish are thought to be more vulnerable to the onset of climate change.
In all, 235 fish populations and 124 species were studied in 38 ecological regions by scientists from Rutgers and the National Oceanic and Atmospheric Administration (NOAA).
Using global fisheries data and ocean temperature maps, the researchers estimated temperature-driven changes to the world’s sustainable catch, a bracket that covers about one third of the reported global numbers.
They found that the five fish species worst affected by overheating oceans were: Atlantic cod (in the North Sea and Irish Sea), sand eel, common sole, saithe (also known as pollock) and haddock.
A smaller number of fish species were positively affected by the balmier marine conditions, including Greenland halibut, Atlantic cod (in the Gulf of St Lawrence, where temperatures were previously colder than optimal for cod), Atlantic herring, sea scallop and northern shrimp.
“Fish populations can only tolerate so much warming, though,” cautioned Olaf Jensen, another study author and Rutgers professor. “Many of the species that have benefited from warming so far are likely to start declining as temperatures continue to rise.”
The new paper says that heated seas are driving changes in ocean circulation and stratification, losses in oxygen concentration and shifts in the distribution and nature of fish populations.
This puts fish stocks under increasing physiological stress, with implications for food availability – a growing concern in coastal developing countries where fish account for up to half the dietary animal protein.
Pinsky said that dwindling sea biodiversity caused by warming oceans could even trigger conflicts, such as the northern European “mackerel wars”.
“When mackerel shifted north into Icelandic waters, they couldn’t agree with the EU how to share access to that fishery,” he said. “It led to overfishing and spilled over into a trade war which appeared to spur Iceland to drop its bid to join the EU.”
The problem can create a feedback loop in which overfishing makes fisheries more vulnerable to ocean warming, while warming then hinders the rebuilding of depleted populations.
Limiting global warming to 1.5C would benefit the fisheries of over 75% of maritime countries
Chris Free, a research leader on the study, said: “We recommend that fisheries managers eliminate overfishing, rebuild fisheries and account for climate change in fisheries management decisions. Policymakers can prepare for regional disparities in fish catches by establishing trade agreements and partnerships to share seafood between winning and losing regions.”
Achieving the Paris Agreement goal of limiting global warming to 1.5C could also make a huge difference, protecting millions of tonnes of global fisheries catch, and benefitting over 75% of maritime countries, according to a separate report published in Science Advances.
Its lead author, Rashid Sumaila, said: “The largest gains will occur in developing country waters, such as Kiribati, the Maldives and Indonesia, which are at greatest risks due to warming temperatures and rely the most on fish for food security, incomes and employment.”
The greatest losses recorded in the Rutgers paper were in the Sea of Japan, North Sea, Iberian Coastal, Kuroshio Current and Celtic-Biscay Shelf regions. By contrast, Labrador-Newfoundland, Baltic Sea, Indian Ocean and Northeast US Shelf regions had the biggest gains.
The next research phase for ocean warming is likely to probe under-explored effects in tropical regions – as well as factors such as ocean oxygen content and acidity.