Ammon News - Warmer water means smaller plankton - the microorganisms fish feed on - float to the surface. This means fish get less nutrition from what they eat, researchers found.

The University of Tokyo researchers analysed the individual weight and overall biomass of 13 species of fish, including mackerel, anchovy and sardines, looking at long-term data for six fish populations from four species between 1978 and 2018.

Seawater temperature data between 1982 and 2014 were also studied to see if changes in the ocean's surface and subsurface layers may have had an impact.

The results, published in Fish and Fisheries, showed two periods of reduced fish body weight, first in the 1980s and again in the 2010s.

This initial weight drop was originally attributed to greater numbers of Japanese sardine, which increased competition with other species for food.

However, further analysis revealed that the effect of climate change warming the ocean appears to have resulted in more competition for food, as cooler, nutrient-dense water could not easily rise to the surface.

The findings in the Pacific support previous research in other areas of the world which has found that trophy fish caught in fishing competitions are also getting smaller, and that smaller species of fish are also increasing in number at the expense of bigger ones.

Professor Shin-ichi Ito of the University of Tokyo said: 'With higher temperatures, the ocean's upper layer becomes more stratified, and previous research has shown that larger plankton are replaced with smaller plankton and less nutritious gelatinous species, such as jellyfish.

'Climate change can alter the timing and length of phytoplankton blooms, explosive growth of microscopic algae at the ocean's surface, which may no longer align with key periods of the fish life cycle.

'The migration of fish has also been shown to be affected, in other studies, which in turn impacts fish interaction and competition for resources.'

According to the UN Food and Agriculture Organisation, in 2019, the western North Pacific accounted for almost a quarter of the global total of fish caught and sold.

The team add that their results have implications for fisheries and policymakers trying to manage ocean resources under future climate change scenarios.

Professor Ito said: 'Fish stocks should be managed differently than they were before, considering the increasing impact of climate-induced conditions.

'The situation fish experience is much more severe than decades ago. If we cannot stop global warming, the quality of fish may decline.

'So, we need to take action so that we can enjoy a healthy ocean and delicious fish.'

Daily Mail