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Spectators observe the Chinese sturgeon in critical danger of extinction at the Beijing Aquarium. (Credit: Wikimedia)
The critically endangered Chinese sturgeon may be even worse than we thought, reports a team of researchers from the China Institute of Water Resources and Beijing Hydropower Research.
These ancient fish are anadromous, which means that they live in the ocean and migrate into rivers to spawn in fresh waters. For the Chinese sturgeon, this quest for annual spawning allows them to go up the Yangtze River in China. At least that was the case.
Once threatened by overfishing, these fish experienced a remarkable recovery after the ban on their commercial fishery in 1983. But in recent decades, sturgeons have been confronted with a new threat: gigantic hydroelectric dams on the Yangtze River. block access to their spawning grounds.
Hydroelectric dam
The sturgeons of the past may have traveled nearly 3,000 kilometers to reach one of 19 possible spawning sites. But dam after dam has now blocked their path, starting with the Gezhouba Dam in 1981, which shortened their path by 1,175 kilometers and left only a spawning spot for the fish.
The new research, released today in Current biology, quantifies the effects of Yangtze dams on endangered fish.
Scientists estimate that the initial habitat loss of Gezhouba reduced the total number of available spawning grounds to 24% of what was previously available for fish.
Along the Yangtze River in China, four large hydroelectric dams are now blocking Chinese sturgeon from their spawning grounds. (Huang & Wang / Current biology)
On top of that, they found that when this reduction occurred, the development of fish reproduction was delayed by 37 days. They estimate that this has further reduced the effective breeding population to less than 7% of the original population.
Chinese sturgeon do not take heat
To make matters worse, three other large dams were added upstream of Gezhouba: the Three Gorges Dam in 2003, the Xiangjiaba Dam in 2012 and the Xiluodu Dam in 2013.
The possible effects of these last dams on the sturgeon have not been taken into account, the pbadage of the sturgeon in these areas upstream having already been blocked by the Gezhouba. But the new report shows that the new dams have had a combined effect on the river's water temperature, which could lead to a final disaster for the sturgeon.
Chinese sturgeons have a short window in which, biologically, they can reproduce. They must be reproductively mature, but not yet out of the fat stores they feed on to go to the spawning grounds.
Normally, this window appears when conditions in the river are conducive to spawning. The fish will only spawn in waters around 16 to 20 degrees Celsius. But because the dams are warming the river slightly, the weather when conditions are favorable has shifted later in the season. At present, the water temperature window is not synchronized with the sturgeon's biological reproduction window. If the weather gets warmer, for example due to climate change, they will be completely out of sync.
The researchers estimate that this is the last straw for Chinese sturgeon, which reduces its effective nesting population between 4 and 0%.
Zero percent of a population means extinction.
Researchers recognize the enormous challenges of sturgeon conservation. The four dams of the Yangtze, which accounted for more than 3% of China's total electricity production from 2015, are certainly not going to be there soon.
The only hope for Chinese sturgeon could be that improvements be made to the dams, which would help maintain the natural and cooler water temperature in the Yangtze.
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