1. The trouble of Dianchi Lake. Dianchi Lake, also known as Kunming Lake, Yunnan Nanze, is located in the southwest of Kunshan, the water surface area of 330 square kilometers, is the largest freshwater lake in Yunnan, since ancient times has been known as the pearl of the plateau, the importance of Yunnan, Kunming is self-evident. Unlike the current Dianchi Lake, which has suffered from deterioration in water quality and a sharp decline in the number of species, the Dianchi Lake in the early days of the founding of the People's Republic of China can be described as beautiful: aquatic plants are abundant, once accounting for 90% of the water surface; under sunny weather, the visibility of the water body is as much as 2 meters, and some places are even "clear to the bottom", and the surrounding residents can directly pan rice and wash vegetables; carp is fat and rich in production, even valuable goldfish can catch a lot.
The first turning point of Dianchi Lake was the 1969-1972 lake reclamation campaign, in which 100,000 people gathered around Dianchi Lake every day, relying only on cattle, flattened burdens, dustpans, hands and feet, and in just a few years, 30,000 mu of water surface of Dianchi Lake was buried and filled in by stones and mud, and 407 kilograms of grain from 1971 to 1972 were returned. The implementation of the campaign of "asking for fields from Dianchi Lake and grain from Dianchi Lake" is undoubtedly a product of the specific environment at that time, but the ecological impact on the next few decades is difficult to estimate.
The time for reclamation around the lake is short, and it is a very small fragment in the development process of Dianchi Lake, and the real dominance is still fishing. As early as the early 1950s, Kunming consciously released fry into Dianchi Lake, which is different from the purpose of purifying water quality now, when it was more to improve catch production, and the stocking fry was mainly carp and grass carp, which are fast-growing fish species. Subsequently, in 1958, the fishery management department was formally established, and since then Dianchi Lake has begun to enter systematic and large-scale aquaculture.
In order to meet the needs of the market, the farmed fish species were constantly adjusted and replaced from carp and grass carp in 1950-1960 to blue carp, carp, Wuchang fish and silver carp between 1960 and 1970. The popularity of the market has further stimulated the fishing and fishing in Dianchi Lake. In the mid-to-late 1960s, annual production stabilized at 3,000-4,000 tonnes, and because of this successful attempt, the stockings and numbers of fish in Dianchi Lake were also more radical, laying the groundwork for the later local "indigenous" fish species.
2. Invasion of alien species. As mentioned above, from 1960 to 1970, Dianchi Lake was market-oriented and constantly changed the cultured fish species, and achieved good results, so when the economic fingerlings were reintroduced, they were more decisive and radical. In 1973, Dianchi Lake first introduced beautiful white shrimp and giant river prawns from Japan, and it was not expected that Dianchi Lake would be rich in food, suitable temperature, and lack of natural enemies for the two new species introduced, so the number increased sharply. According to the data, the aquatic products in 1975 were 8363 tons, while the difference between shrimp and shrimp reached 8027 segments, occupying a dominant position.
Another species introduced and having a significant impact is the Taihu lake silverfish, which has a high economic value. White shrimp, white fish and Taihu lake silver fish are collectively known as "Taihu Three Treasures", of which Taihu lake silver fish is rich in nutrients, containing 8.2% protein, 0.3% fat and vitamins B1, b2 and other human essential trace positions, slender body about 7-10 cm body, and ginseng size is similar, so it is called "fish ginseng". Taihu silverfish is not limited to the country, has long been famous, like South Korea, Japan and other East Asian gourmets through various ways to buy high prices. Therefore, Taihu silverfish is often exported to foreign countries as high-grade ingredients in exchange for foreign exchange, which is the story of one ton of silver fish in exchange for 20 tons of grain in the 70s.
In the early 1980s, 700,000 Taihu silverfish were officially introduced into Dianchi Lake, similar to Japan's giant shrimp, Taihu silverfish were very successful in Dianchi Lake, and only by 1985, the production of silver fish easily exceeded 1,000 tons, and in 1989 it reached 3,500 tons.
The side effects of the introduction of alien species and overcultured fishing soon became apparent. In the 1990s, the abundant aquatic plants and low aquatic plants in Dianchi Lake quickly faded, accounting for only 20% of the water surface in the 90s, and some fish species attached to the aquatic weeds with fish eggs, due to the lack of spawning places, such as some carp and redfin protosthetes had to lay their eggs on abandoned fish nets, and the number of them shrank repeatedly; a number of native fish species in Dianchi Lake had a high degree of overlap with taihu silverfish in active aquatic weeds and food intake, and silverfish occupied the advantage of size and quantity, and the living space of indigenous fish was repeatedly compressed until it disappeared.
According to the data, the Kunming fishery department tracked and recorded as many as 26 species of indigenous fish in Dianchi Lake in the 1960s, and at least 15 species were on the verge of extinction in the early 1990s due to food shortages, deterioration of water quality and other factors, including mid-rumped crucian carp, middle carp, pond silver whitefish, and mid-rumped anchovies. Today, the number of indigenous fish species that can be seen has dropped to 4.
3. Deterioration of the water environment and countermeasures. If the number of fish species in Dianchi Lake is blindly reduced, and the blame is all on over-breeding and fishing, I am afraid that the crux of the problem will never be found. The deterioration of water quality and ecosystem destruction in Dianchi Lake are the main reasons for the decline of species in Dianchi Lake, and there are many factors that cause this situation.
Dianchi Lake is located in the lower reaches of Kunming City, is the lowest terrain of the entire Kunming Basin, due to objective conditions, in the absence of any measures, Kunming City discharged domestic sewage, wastewater, including a large amount of phosphorus, light agricultural water spontaneously flow to Dianchi Lake and its tributaries. In addition, the production enterprises near Dianchi Lake have risen from the ground, chemical and fixture production flowing out of the industrial wastewater, before the 90s or even without any treatment directly discharged to Dianchi Lake, the water around the discharge port is either reddish brown, or light brown, a strong, pungent smell of nearby insects smoked far away. According to statistics, 200 million tons of sewage are discharged to Dianchi Lake every year, which is undoubtedly a terrifying figure.
In addition, Dianchi Lake is a typical semi-enclosed lake, with an average water depth of only 5 meters, and there is no injection of large-flow water purification, and the ability of Dianchi Lake to purify and recover itself is limited. The climate near Dianchi Lake is special, and the annual rainfall season is concentrated in May-October, accounting for up to 90% of the annual precipitation; from November to April, the rainfall decreases and the evaporation increases significantly. Relying entirely on several sewage plants around Dianchi Lake, it is unable to play an effective purification effect.
A large number of phosphorus and nitrogen elements can not be eliminated in time, and the most direct result is the eutrophication of water. Once the threshold range is exceeded, cyanobacteria, or blooms, are immediately infested. In the mid-to-late 1990s, there were frequent blooms in Dianchi Lake, especially in The summer of July, the waters with high odor and a large number of cyanobacteria gathered, the lake water was as green as paint, the air was filled with a strong fishy smell, and the surface of the water was floating with dead fish with white bellies.
In order to improve this situation, Dianchi Has already taken action. After the 1990s, the fingerlings stocked in Dianchi Lake gradually changed from blue carp, grass carp and Wuchang fish to silver carp, and it is expected to use silver carp to purify the water quality of Dianchi Lake. There is a reason for this change: in the 1970s, a large number of rivers and lakes in the United States were polluted by sewage discharge, and a large number of grass carp and carp, especially silver carp, were introduced from Asia, and the water quality was greatly improved after several years. Silver carp are filter-feeding fish that gain 1 kg of body weight for every 40 kg of cyanobacteria swallowed. From 2013 to 2015, 3,600 tons of silver carp, 2 million native "indigenous" golden-lined catfish and 100,000 Yunnan light-lipped fish were stocked into Dianchi Lake to achieve dynamic ecological balance.
The release of silver carp to control cyanobacteria is only a "superficial work" and does not fundamentally change the problem of poor water quality. In addition, too many silver carp will definitely squeeze the activity space of other species of organisms, which is the so-called "killing one thousand enemies and losing eight hundred." Therefore, banning sewage discharge and removing silt from the bottom of the water are the focus of restoring the water quality ecology of Dianchi Lake.
Dianchi Lake began a 10-year ban on fishing in the whole water, which is not a new thing in Dianchi Lake, as early as the 80s Dianchi Lake adopted a phased ban on fishing, but it was more to facilitate the mating and spawning of fish, this time more attention to ecological benefits, rather than economic benefits. The reason why the fishing ban cycle has been greatly extended, no longer like the previous six months or months, is because there is enough time to ensure that the juvenile fish grow into big fish, rather than being caught again in childhood. According to the breeding cycle of carp and grass carp in 3-4 years, there will be 3 generations in 10 years, and the recovery effect of fish resources is remarkable.
Original: Old High