Climate change may be reducing fish weight in oceans, study finds

New Delhi: Climate change may be causing more competition for food in oceans, leading to a reduction in the weight of fish, according to a study.

Researchers at the University of Tokyo in Japan found that fish weight in the western North Pacific Ocean dipped in the 2010s due to warmer water limiting food supplies.

They attributed the first period of weight loss to greater numbers of Japanese sardines, which increased competition with other species for food.

"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," Professor Shin-ichi Ito from the University of Tokyo said.

"Climate change can alter the timing and length of phytoplankton blooms, 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," the researcher added.

During the 2010s, 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 researchers said.

The results, published recently in the journal Fish and Fisheries, have implications for fisheries and policymakers trying to manage ocean resources under future climate change scenarios.

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

However, the latest research shows that during the 1980s and 2010s, fish weight in the region changed dramatically.

"We investigated 17 fish stocks -- that is, populations or groups of fish -- from 13 species and found that many decreased in weight during this period," said Ito.

The team, including the study's co-author Lin Zhen, a PhD student, analysed fish weight and biomass data from Japan's Fisheries Agency and the Japan Fisheries Research and Education Agency.

Fish weight refers to individual fish (for example, a 200-gram mackerel), whereas biomass is the total weight of a species or group in an area (for example, 50,000 tonnes of mackerel).

The team looked at long-term data for six fish populations from four species between 1978 and 2018 and at medium-term data for 17 fish populations from 13 species from 1995/1997 to 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 showed two periods of reduced fish body weight, first in the 1980s and again in the 2010s.

The team attributed the 1980s weight decline to an increase in Japanese sardine, which likely led to greater competition for food within and between fish species.

By comparison, in the 2010s, although there was a moderate increase in the population of Japanese sardine and chub mackerel, the team's analysis appeared to show that reduced nutrient supply to the ocean's surface from subsurface layers due to global warming's effect on the ocean, was an influential factor.