Since the Ban on Fishing in the Yangtze River, more and more rare species have begun to emerge, as evidenced by the recent discovery of helio. In the "Red List of Chinese Vertebrates" released by China in 2016, helix has been listed as one of the 22 critically endangered fish species in the Yangtze River, along with the Yangtze River anchovy, Chinese sturgeon and other valuable fish species.
Since 2017, the frequency of wild hemorring in the Yangtze River has begun to increase, and so far there have been 6 sightings. The scientific name of the helio is helios, commonly known as wheat straw, the historical catch is quite rich, and the "Three Fresh Yangtze River" has formed a considerable fishing flood. According to the current signs, after the end of the fishing ban on the Yangtze River, this species is expected to prosper again.
The number of knifefish in the Yangtze River is increasing
On the other hand, exotic species in the Yangtze River are also taking advantage of the "east wind" of the ten-year ban on fishing to multiply, posing a great threat to native fish. These "outsiders" have been artificially selected, grow fast, adaptable, and mostly omnivorous fish, which are more competitive than native fish. Among them, tilapia is one of the most potentially harmful species, and the aquaculture production has ranked among the top ten in the country, and if it can adapt to the low temperature, the consequences will be unimaginable.
The Yangtze River is the first river in China and the third largest river in the world, and has flowed through a variety of complex landforms, forming a variety of fish resources, which can be called a natural "fish reservoir".
At the source of the Jinsha River, schizophreniae fish and plateau loach that are adapted to low temperatures breed here; in the upper reaches of the Yangtze River, the canyon is deep, the water flow is rapid, and the fish are mainly endemic species such as the subfamily catfish, the subfamily catfish, and the subfamily wild catfish; and in the middle and lower reaches of the Yangtze River, the water surface is wide, there are many tributaries, the lakes are dotted, the climate is mild and humid, and the composition of fish is complex, represented by four large fish, henfish, eels, copper fish, catfish, etc.
Considering the topography and water temperature distribution of the Yangtze River, the high-risk area of tilapia invasion is the Yangtze River estuary area, followed by the middle and lower reaches of the Yangtze River and the Three Gorges Reservoir Area. Actual investigations also show that tilapia has indeed invaded the middle and lower reaches of the Yangtze River, mostly from aquaculture escape, and when floods occur, ponds overflow, and farmed tilapia sneak into the Yangtze River.
From 2005 to 2007, when the Institute of Fisheries of the Chinese Academy of Sciences conducted a survey of alien species in the lower reaches of the Yangtze River, it was found that a total of 45 alien species in the lower reaches of the Yangtze River were artificially farmed, and tilapia ranked first and the production was far ahead. In fact, tilapia has become a popular choice for farmers in the Yangtze River Basin, and no one can guarantee that the situation of abandonment and release will not occur again in the future, and the hidden dangers behind it should not be underestimated.
Tilapia is native to eastern Africa, also known as "African crucian carp", adult fish body length of up to 20 cm or more, back with hard spines, eating miscellaneous, not picky eaters. China first introduced different varieties of tilapia in 1957, such as Nile tilapia, Mozambique tilapia, Orea tilapia and so on. Today, tilapia has "blossomed everywhere" in the country, and it is also farmed in the Yellow River Basin in the north, with a total production of up to 1.6 million tons.
Worryingly, tilapia are capable of breeding. In a still-water pond, tilapia can reproduce naturally in just 6 months, and in high-temperature zones such as Hainan, the shortest gonadal development takes only 3 to 4 months. A female tilapia weighing 200g can lay up to 1000 to 2000 eggs, and the special oral incubation method improves the survival rate of offspring.
When the natural water temperature is 20 to 30 ° C, the male fish begins to build a nest in the silt of the pond, and then the female fish enters the nest and pairs up, excretes mature eggs and contains them in the mouth; at the same time, the male fish excretes mature sperm, which flows into the mouth of the female fish with the water flow, so that the eggs are fertilized. During the entire incubation period, the fertilized eggs are held in the mouth of the female until the yolk sac disappears, and the juveniles do not leave the mother until the yolk sac disappears, a process that takes only 4 to 5 days.
Without intervention, tilapia breeding continues, multiplying several generations in a year with endless children and grandchildren. If the tilapia is premature and over-multiplying, more energy will be used for gonadal development rather than weight gain. For farmers, this will not only lead to feed waste, reduced meat production rate, so that commodity specifications are uneven, different sizes, and economic benefits are greatly reduced.
In order to limit the reproduction of tilapia, scientists have come up with another way to "birth control" - hybrid breeding to breed all-male tilapia. Full male tilapia grow faster than ordinary tilapia, the individual is larger, and more importantly, this tilapia has three pairs of chromosomes and loses the ability to reproduce.
In recent years, breeding experts have made great breakthroughs and cultivated super male tilapia, which has greatly improved its commerciality and has been strongly welcomed by the industry. However, those common tilapia that are "exiled" into natural waters are not subject to "birth control" restrictions, and can always reproduce as long as the water temperature is suitable, and the potential threat cannot be ignored.
The "progenitor" of tilapia is the Mozambican tilapia, an indigenous fish from Lake Tanganyika in Africa, typical of small tropical fish. When it entered China at the beginning of the last century, tilapia was not welcomed, and there were not many people who raised it, and the people even called it "garbage fish" and "cheap fish".
Overall, tilapia has both merit and experience: on the one hand, tilapia has created considerable foreign exchange for China and has become a "hard ticket" to seize the European and American markets, and this contribution cannot be ignored; on the other hand, wild tilapia flows into the Yangtze River Basin, and ecological harm cannot be brushed over, and the two should be treated differently.
The problem now is that there are already traces of wild tilapia in the Three Gorges reservoir area. As early as the 2005 fishery resource survey, the catch data of many tilapia on the river section of more than 600 kilometers upstream of the Three Gorges Dam had been generated, but it was not taken seriously at that time. In the spring and autumn of 2006, scientific expeditions surveyed foreign species in the upper reaches of the Yangtze River in Panzhihua, Yibin, Wanzhou and other sections, and tilapia was once again on the list.
Tilapia in the Three Gorges Reservoir area should be paid attention to, and the hydrological conditions of this section of the river are special and may become a high-risk area for the large reproduction of tilapia, mainly for the following four reasons:
First, the water temperature in the Three Gorges reservoir area is relatively suitable. A survey published in June 2017 in the Water Conservancy and Hydropower Express showed that after the Three Gorges Reservoir area was stored, the water temperature at Zhutuo Station, Cintan Station, Yichang Station and other monitoring stations was relatively suitable, with the lowest temperature being 11 °C in January and the highest temperature being 25 °C in August. It should be known that as long as the water temperature is above 10 degrees Celsius, most tilapia will not be frozen to death, and the possibility of survival and reproduction in the reservoir area is very high.
Second, after the water was stored in 2003, the ecological niche of the Three Gorges Reservoir area was seriously vacant, which provided convenient conditions for the infiltration of tilapia. According to the survey, there are 108 species of fish in the Three Gorges reservoir area, of which 17 species are endemic fish. After the water is stored, the reservoir area changes from flowing water to slow water, and the natural river section becomes a semi-open artificial reservoir, and migratory fish are intercepted under the dam and cannot reproduce. Therefore, a large number of idle ecological niches have been freed up in the reservoir area, which objectively provides a favorable factor for the "upper position" of tilapia.
Third, tilapia itself is also adaptable to low temperatures. Although 10 °C is the "lifeline" of tilapia, many tilapia species have lethal low temperatures below 10 °C. For example, Orea tilapia can maintain half of the survival rate at a low temperature of 7.82 °C; He Jinzhao and other scholars have found that the red tilapia in Florida can achieve half the survival rate at 6.77 °C.
Fourth, artificial breeding accelerates adaptation. At present, tilapia cannot overwinter normally in the north, but in order to increase production, there is a demand in the industry to improve the hardiness of tilapia in order to promote it in a large area in the north. With the progress of artificial breeding, the ability of tilapia to adapt to low temperatures will be greatly accelerated, and the rate of replacement will be unimaginable. For the northern aquaculture industry, this is a great thing to increase production and income, but for the ecology of the Yangtze River and even the Yellow River, hardy tilapia will also become a greater hidden danger.
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