Climate change has significantly impacted fish populations across different levels within the ecosystems. At the population level, it affects the seasonal timing of ecological events, like reproduction. At the population level, many fish have shifted towards deeper waters and higher latitudes in search of suitable habitats. At the organism level, the reduction in the body size of fish has been proposed as the third universal impact of global warming.
Challenge in Fishery Industry
Seafood is one of the iconic and important elements of Japanese cuisine, especially in their traditional food, like takoyaki, sushi, and grilled mackerel. However, the self-sufficiency of seafood in the country has been shown to decline over the past decades.
Local fisheries experience different challenges, from increasing costs, changing consumer preferences, labor shortages, reduced sales, and lower prices. It is also likely that the major threat comes from global warming.
The eastern coast of Japan is bordered by the western North Pacific Ocean, a highly productive marine region. According to the 2019 report of the U.N. Food and Agriculture Organization, almost 25% of the global supply of fish caught and sold came from the western North Pacific. In a new study, however, it was found that fish weight in the region dramatically changed during the past decades.
Fish Weight Crisis
In the research "Fish weight reduction in response to intra- and interspecies competition under climate change," experts from the University of Tokyo investigated 17 fish stocks from 13 species. Led by Professor Shin-ichi Ito from the Atmosphere and Ocean Research Institute, the research team analyzed fish weight and biomass data from Japan's Fisheries Agency and the Japan Fisheries Research and Education Agency.
Together with Lin Zhen, Ito looked at long-term data for six fish populations from four species between 1978 and 2018. They also studied medium-term data for 17 fish populations from 13 species from 1995/1997 to 2018.
The research also includes an investigation of seawater temperature data between 1982 and 2014 to determine if the changes in the surface and subsurface layers of the ocean have a significant impact. It was found that the reduction of fish body weight happened in two periods, first in the 1980s and again in the 2010s.
The weight decline during the 1980s was attributed to increased Japanese sardines, possibly leading to greater competition for food within and between fish species. Although there was a moderate increase in the Japanese sardine and chub mackerel population in the 2010s, the analysis revealed that reduced nutrient supply to the ocean's surface was an influential factor.
Researchers blame global warming for failing to deliver enough nutrients from the ocean's sub-surface to surface layers. The upper layer of the waters becomes more stratified with higher temperatures. As a result, smaller plankton and gelatinous species that are less nutritious, like jellyfish, are replaced by smaller plankton and gelatinous species.
As the length and timing of phytoplankton blooms are altered by climate change, they no longer align with key periods of the fish life cycle. Fish migration has also been affected, harming fish interaction and resource competition.
It hopes that the results of their study will help fishery managers and policymakers understand the changes in the oceans around Japan. He believes that fish stocks should be managed differently than before, considering the increasing effects of climate change.
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