Two years ago today, the lifeline of the Yangtze River Dragon was cut off and it was declared extinct.

Two years ago today, researcher Wei Qiwei, who had spent most of his life studying the rare aquatic life in the Yangtze River, announced a regrettable and helpless news to the public: the world’s largest freshwater fish, the white sturgeon Psephurus gladius, had become extinct!

Although the news came very suddenly, this tragedy did not happen overnight. In fact, according to model estimates, the Chinese sturgeon was functionally extinct as early as 1993, and then completely disappeared between 2005 and 2010. In other words, the Chinese sturgeon may have been completely extinct at the latest ten years ago.

Commemorative stamp of white sturgeon

The fish has passed away. In addition to the discussion on the cause of the extinction of the white sturgeon (analyzed in previous articles), people are more concerned about whether the white sturgeon is really extinct. After all, the Yangtze River is thousands of miles long, and no matter how precise the data model is, there is no guarantee that there will not be one or two white sturgeons lingering on by chance. Can the current environmental conditions of the Yangtze River still support the "Dragon of the Yangtze River" of the past?

Can't have a pure land

Although in situ conservation is recognized as the most effective means of protecting endangered species, for large animals this usually requires a suitable environment, a large habitat, sufficient food and a population large enough to reproduce.

In situ protection of aquatic organisms usually involves designating river sections where they grow fat or reproduce as protected areas, and restricting or prohibiting human activities such as fishing. Such protection measures are quite effective for sedentary fish, but for migratory fish, the effects of such protected areas are often unsatisfactory. Unlike mountain forests on land, rivers have flowing characteristics. Once fish migrate outside the protected area with the water flow, they will be threatened by the loss of protection.

Until the extinction, the white sturgeon had never had a "white sturgeon protection area". Even if there were individual Yangtze River rare fish protection areas involving white sturgeons, the earliest one was not established until 2000, when the white sturgeon had long been nowhere to be seen.

White sturgeon specimen | Shaoxia little yellow chicken

In fact, for the Chinese sturgeon, the significance of in-situ protection in the form of protected areas is too limited. As the largest aquatic organism in the Yangtze River, the Chinese sturgeon has the characteristics of large individuals, late sexual maturity, and large living space. A complete life cycle often requires a long time and a huge space. Therefore, for the Chinese sturgeon, which can grow up to 7 meters long, the approximately 3,000-kilometer section of the river from the lower reaches of the Jinsha River to the mouth of the Yangtze River is its world. Like other sturgeons, the Chinese sturgeon is a typical migratory (semi-migratory) fish. Every spring, adult Chinese sturgeons in the breeding season will leave the deep riverbed where they usually live, follow the memory engraved in their blood, and go upstream from the downstream to the spawning grounds of the Jinsha River thousands of miles upstream to complete the most important mission of their life-reproduction.

Acipenser ruthenus drawing | rawpixel

Afterwards, some white sturgeons will return to the downstream to fatten, while others will continue to live in the main and tributary streams upstream; the hatched young fish will go down the river and feed and grow in waters rich in bait such as estuaries. However, since the 1980s, the river-blocking dams across the main stream of the Yangtze River have completely blocked the migration channel that the white sturgeons have traveled through for generations, and the white sturgeon populations upstream and downstream have also been isolated. For the white sturgeons living on the dam, they can still reproduce normally, but the young fish cannot return to the downstream to fatten. For the white sturgeons under the dam, migration, which was once the most basic need, has become a luxury. Although they can fatten and grow normally, they can no longer return to the spawning grounds in their blood memory to reproduce.

Unable to return home

As a representative of ancient fish, sturgeons not only inherit the original morphological characteristics of their ancestors, but also solidify the life habits passed down from generation to generation into their nature, so their ecological niche is often special. As early as the Soviet era, people tried to eliminate the impact of dams on the migration of Russian sturgeons (Acipenser gueldenstaedtii) and shining sturgeons (A. stellatus) by means of fishways and fish lifts. However, these methods, which were effective for salmon and trout, which are also migratory fish, ended in complete failure for sturgeons.

A 'fish ladder' is a device that helps migrating fish pass through a dam | Qurren / Wikimedia

After learning from the experience of the past, people can only hope that the white sturgeon can use their "ancient wisdom" to overcome the current difficulties, and optimistically put forward the beautiful idea that the white sturgeon can adapt to the new environment and achieve "fattening in the reservoir area and breeding under the dam". However, the reality is cruel, adaptation is not omnipotent, not to mention that the upper limit of the white sturgeon's environmental adaptability is extremely low.

In fact, although the three types of sturgeons in the Yangtze River have different shapes and life histories, they all prefer the rapid gravel environment upstream for breeding, which shows their extremely limited ecological plasticity. For the young white sturgeons trapped on the dam, although they do not have to return to the sea for fattening like the Chinese sturgeons, they are still fish that migrate in both salt and fresh water, so the reservoir area cannot meet the environmental conditions necessary for their growth and development. And for the white sturgeons gathered under the dam, it is even more difficult to find a suitable spawning ground in a short period of time!

A white sturgeon caught in Gezhouba in 1993 | Hui Zhang, et al. / Science of The Total Environment (2020)

Not to mention that there are few river sections below the dam that can meet the necessary rapid gravel environment, the hydrological environment of the Yangtze River, such as water temperature and flow, has also changed to a certain extent after the dam was filled with water. Any of these factors may become the breeding start signal for these ancient sturgeons, but the spawning grounds that have the consistency of these environmental signals are not easy to reassemble. The spawning grounds are the product of the white sturgeon's adaptation to the climate, geological processes and the continuous evolution of river landforms in the long history of natural evolution. Therefore, the white sturgeon has formed a special perception of the natural process of the river, which is so deeply rooted that it is irreplaceable.

Escaped from death but suffered a heavy blow

Although the irreplaceable living environment has become the fatal weakness of the white sturgeon, this ancient family that has survived in China for 120 million years has not given up hope. Although no breeding activities of white sturgeon broodstock have been observed below the dam, people can still catch white sturgeon fry hatched that year at the mouth of the Yangtze River in Chongming. It is speculated that some of the white sturgeon fry bred upstream may have drifted to the dam through the sluice gate water.

But what awaits these tenacious white sturgeons is a worsening future for the Yangtze River. Although the scale of reproduction is shrinking, the scale of fishing for white sturgeons has reached its peak. In that era of material shortage, the white sturgeons trapped under the dam and refusing to leave received "special care" from the fishermen, and the replenishment group of white sturgeons suffered a heavy blow, with the population showing a cliff-like decline, and followed by the coercion of negative factors such as shipping, pollution, and food shortages. Although isolated on both sides of the dam, the upper and lower white sturgeon populations, lacking effective replenishment, almost simultaneously ushered in the fate of extinction in different ways but the same destination.

Hongze Lake Fishing Port before the 10-year fishing ban | Tuchong Creative

Since 1993, no young white sturgeon has been detected in the Yangtze River estuary. It can be concluded that neither the Yangtze River nor the white sturgeon meets the basic conditions for in situ protection. A dragon in a shallow pond has great ambitions but is hard to achieve. The largest aquatic animal in the Yangtze River needs such a vast living environment. When humans cut off the Yangtze River, they also cut off its lifeline.

This article comes from the Species Calendar, welcome to forward

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