Releasing Fish Doesn't Hike Population But Threatens Ecosystem

Hokkaido University recently announced a study finding that releasing fry doesn't increase fish populations. In Japan's northernmost prefecture, the release of farmed salmon and trout into rivers is popular, but is this practice pointless? Takumi Taniguchi reported for Mainichi Shimbun.

Photo Insert: Surveys of populations from 1999 to 2019 showed that up to 240,000 masu salmon were released annually, but populations did not increase.

The paper, "Intentional release of native species undermines ecological stability," was published online in the US journal Proceedings of the National Academy of Sciences on Feb. 7.

It is a study by four researchers, including Hirokazu Urabe, 51, chief researcher at the Salmon and Freshwater Fisheries Research Institute of the Hokkaido Research Organization (HRO), which has been studying the density and species of masu salmon and other fish in 32 Hokkaido rivers.

Surveys of populations from 1999 to 2019 showed that up to 240,000 masu salmon were released annually, but populations did not increase. In one river where between 100,000 and 150,000 masu salmon were released between 1999 and 2015, the population peaked at 1.5 fish per sq m in 2007 and declined to below 0.5 per sq m in 2013.

In one river, between 100,000 and 200,000 fish were released between 2003 and 2009, but the density dropped from 0.5 in 2009 to 0.1 in 2010, and then to almost zero in 2015.

The researchers found that in rivers where large numbers of masu salmon were released, both the density of masu salmon and of other species of fish inhabiting the river tended to decrease.

The research team believes that when rivers with limited "environmental carrying capacity" for food and shelter are subjected to large-scale releases that exceed that capacity, competition for survival intensifies and the natural reproduction of masu salmon is suppressed, leading to a decline in the population regardless of the species of fish.

In short, the stability of the ecosystems was undermined. The tendency of released fish to grow larger than those in the wild may also be a factor in intensifying the competition for survival.

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