Fish populations disrupted by climate change

Altered species distribution, less food and smaller fish: climate change has consequences for fish and fishing. Reducing our greenhouse gas emissions is crucial to the preservation of marine biodiversity.

No matter what efforts we make to reduce our greenhouse gas emissions, the consequences of climate change will be with us through at least 2040-2050. Beyond then, it will depend on how the climate evolves and what measures we take starting now. 

Human activity is responsible for 90% of the CO2 released into our atmosphere. Each year, the ocean absorbs between 30% and 40% of the CO2, which has resulted in increased acidification by 0.1 pH unit and a temperature rise of 1°C since the industrial era. This excess CO2 in seawater also weakens the concentration of calcium carbonate. Crucially, this chemical element is what plankton, corals, mollusks (oysters, mussels, etc.) and many other calcifying marine organisms use to build their shells or internal skeletons.

Rising water temperatures mean less mixing of shallow and deeper ocean layers, which leads to lower plankton abundance (plankton are at the base of the marine food chain) and less atmospheric reoxygenation of the water. In turn, this means less food and less oxygen for fish. And fish are cold-blooded: faced with higher temperatures and more and more frequent heatwaves, fish are forced to accelerate their basal metabolism, which requires more food and oxygen. 

Mediterranean sardines shrink from 15 cm to 11 cm

Recent studies by Ifremer have shown that the spawning period of certain species has been modified due to temperature changes and acidification. Earlier spawning had been observed among populations of sole as well. This shifts larval development earlier than usual so that it no longer coincides as well with the plankton blooms that larva feed on. Research performed on sea bass by Ifremer suggests that acidification could also be contributing to greater egg production, lower-quality eggs and altered reproductive behavior in males. 

Fish growth appears to be affected as well by less nutritious food sources and a generally less efficient food chain. Several studies have noted major changes in the abundance of different species of plankton. The larger species, which are often the most nutrient-rich, seem to be less numerous. As for small deep-sea fish (sardines, anchovies), data shows a decrease in adult length and weight along the French coasts of the Mediterranean: sardines have shrunk from 15 to 11 cm on average, their weight has plummeted from 30 g to 10 g, and individuals over two years old have disappeared. This phenomenon has been observed in the North Sea too, for other species (herring and sprat) with consequences that stretch all the way up to large predatory fish at the top of the food chain.

20% fewer marine animals by the end of the century?

Scientists have been able to estimate the impact of climate change on marine fauna over the long term using modeling tools. These models, which are more and more precise, all predict a significant decrease in marine animal populations (see article in Nature Climate Change). Didier Gascuel, a researcher and professor at the Institut Agro, participated in this work with the Eco Troph model that simulates the abundance of predator and prey species. “In the worst-case scenario, without reducing our greenhouse gas emissions, our results show that the average decrease in marine animal life could reach -20% by the end of the century, and even -30% or -40% in some areas. These uncontrolled developments could tip the whole system into overfishing and draw us closer to an impoverished ocean scenario, particularly where the largest fish are concerned.” 

Hake and mackerel found as far away as Iceland

For European fishermen, this will mean smaller catches and different catches overall. Due to changing environmental conditions, some zones are slowly becoming uninhabitable for fish. On the other hand, new areas—farther north and deeper—may become more and more hospitable. These trends are already under way, with species distributions moving northward. In reality, individual fish are not moving. Local populations decline in the south while other populations flourish in the north. The “tropicalization” of fish communities can be witnessed in the south as an increase in species that prefer warm waters and a decrease in those that prefer cold waters. This phenomenon has already been reported in the Bay of the Somme, the English Channel, the Celtic Sea and the North Sea. Some species also change their habitat mode, like plaice in the North Sea, which are staying in the same area but moving into deeper water.

The entire map of fish population distribution is being altered. Cod is losing ground as abundance lessens in its more southerly habitats, while other species like hake and mackerel are expanding their territory. These latter two species can now be found around Iceland and in the Norwegian Sea. Overall, the trends are troubling and diminished fish abundance looms on the horizon.

These decreases in abundance can be compensated, at least partially, by more careful fisheries management. Adaptation and sustainable management will be necessary for the fishing industry in Europe and around the world in this context.