In China, blue carp, grass carp, silver carp, bighead carp are known as the four major fish, in the freshwater fishery is an uncompromising "carry handle". In the 1960s, the sum of the production of four major fish accounted for 75% of the total freshwater aquaculture in the country, and the title of "family fish" can be said to be well deserved.
Four big fish
By 2019, the aquaculture production of the four major fish has exceeded 13 million tons, and economic fish such as carp and crucian carp have not been called "family fish", but they have also reached the level of "family fish" in terms of production. More embarrassingly, the annual production of bluefish is only 680,000 tons, which is not only at the end of the domestic fish ranks, but also completely lost to the imported tilapia (more than 1.6 million tons).
In this regard, some people even say that the bluefish does not deserve the title of "family fish". Purely from the perspective of production, the status of bluefish has indeed been challenged, but the title of the family fish will not change, and this "id card" will be permanently valid.
Giant bluefish in Poyang Lake
Speaking of the origin of the title of "Four Big Fish", we must also start from the Yangtze River. The Yangtze River is a natural aquatic and vivid species gene bank, and it is also the origin of the four major fish. In the middle reaches of the Yangtze River, which stretches 400 kilometers from Yichang to Chenglingji, there are 11 spawning grounds for domestic fish, accounting for 42.7% of the entire main stream of the Yangtze River.
Interestingly, the eggs produced by the four species of fish of "grass carp" are drifting eggs, the density of eggs is similar to that of water, the surface is non-sticky, and it will float on the upper layer of the water body under the action of water flow, and after a period of drifting, it can hatch on its own. Carp and crucian carp produce sticky eggs with a sticky surface that adheres to the stems and leaves of aquatic plants, with a specific gravity larger than water.
Spawning habits are similar to grass carp, silver carp and bighead carp, and the natural production in the Yangtze River Basin is relatively high, which is an important reason why bluefish is classified as one of the "four big fish". It is important to note that drifting eggs must hatch in a running water environment, while sticky eggs can hatch normally in both still water and running water. This is why in still water ponds, domestic fish cannot reproduce naturally, while carp and crucian carp can reproduce on their own.
Don't look at such a simple phenomenon, but it has plagued domestic aquatic experts for more than 30 years. In order to break through the artificial breeding technology of the four major fish, the domestic research team has carried out long-term tracking and monitoring of the spawning conditions of the four large fish, and recorded a large number of environmental data such as water temperature, water level and weather, and the whole process is full of hardships and challenges.
For the artificial reproduction of fish, the conventional idea is to play oxytocin, but this operation does not work at all in the four major fish. Later, it was noted that when the family fish spawned, it was often accompanied by the soaring water level of the Yangtze River, and the water flow and water temperature stimulation at this time were likely to be necessary factors to promote the spawning and reproduction of the domestic fish.
Domestic fish in ponds and enclosed lakes do not reproduce smoothly
This is indeed the case. When the water level rises, the yangtze river flow rate is significantly accelerated, and the lateral line system in the skin of the fish senses this change and will go upstream. The external water flow stimulation is transmitted to the hypothalamus and pituitary gland of the domestic fish through the nervous system, and the gonadotropin released by the pituitary gland further acts on the gonads, at which time the male and female broodstock begin to chase the tail and spawn.
In June 1958, the older generation of aquatic experts conducted a breeding experiment of domestic fish under the effect of water flow stimulation, and successfully conquered the artificial breeding of silver carp and bighead carp, and later solved the artificial breeding of blue carp and grass carp with the same idea. Since then, the wild grass carp in the Yangtze River has successfully "landed", and the output of land aquaculture has also increased by leaps and bounds, truly becoming a veritable "family fish".
Silver carp and carp in the wild
Although the production of domestic fish farming is booming, the four major fish in the wild in the Yangtze River later encountered a serious crisis - the construction of hydropower dams. In fact, in the 1950s, when people first proposed the construction plan of the Three Gorges Project, they took into account the impact on the resources of the four major fish, and in the subsequent demonstration, construction and operation stages, the four major fish were also the focus and discussion object.
After the Completion of the Three Gorges Project and the storage of water, worrying things still happened - the survey showed that in 1965, the number of seedlings naturally bred by the four major fish in the Yangtze River was as high as 129 billion, and the natural fry of domestic fish dropped sharply after the completion of the Three Gorges, and the number of fry in recent years was less than 2 billion, and resources could only be restored through artificial breeding and stocking. Although overfishing and water pollution are also partly responsible, the main reason is the construction of hydroelectric dams.
Three Gorges Hydropower Station
The impact of the Three Gorges Dam on the spawning of domestic fish is mainly manifested in the following three aspects:
First of all, the interception of the dam will cause the four large fish to be unable to migrate smoothly, even if there are many high-quality spawning grounds in the middle and upper reaches, which is the direct reason.
Second, dam storage has changed the hydrological conditions downstream (Yichang River section), resulting in a delay in the breeding period of domestic fish. In natural circumstances, the most suitable spawning water temperature for domestic fish is 20 to 24 ° C, and when the water temperature is lower than 18 ° C, they will not spawn. After the dam is filled, the water level in the reservoir area rises, and the phenomenon of water temperature stratification is particularly obvious, and the low temperature water discharged by the gate directly affects the downstream water temperature.
In addition, the water body when the high dam is discharged will violently impact, resulting in a phenomenon of gas oversaturation, which can have a fatal effect on fish eggs and juveniles. Under normal circumstances, fish breathe through the gills, dissolved oxygen is in a dissolved state in the blood, but if the oxygen is oversaturated, the dissolved oxygen will return from the dissolved state to the gas state, and the precipitated bubbles will block the blood vessels and embolize.
Dam discharge causes oxygen saturation of downstream water bodies
It is precisely because of these adverse effects that the natural spawning volume of the four major fish has dropped sharply. Although it is far from extinction, if the problem of natural reproduction is not solved, wild domestic fish will get closer and closer to the "red line", and the depletion of resources is only a matter of time.
In order to promote the restoration of the four major fish resources in the middle reaches of the Yangtze River, the Three Gorges Reservoir began an ecological dispatch experiment in 2011, taking a big step in the "home fish defense war". In the past, reservoir scheduling was mainly for agricultural irrigation and flood control, but in recent years, it has begun to shift to the direction of ecological dispatch, that is, to incorporate various ecological factors into the traditional reservoir dispatch, and then gradually restore the ecological environment of the Yangtze River.
May to June of each year is the breeding period of the four major fish, and the experiment of ecological dispatch in the Three Gorges is also carried out at this stage. From 2012 to 2018, the Three Gorges Reservoir carried out a total of 11 ecological dispatch tests, the main goal is to regulate the water level and water temperature downstream.
11 ecological dispatch tests carried out by the Three Gorges Reservoir
During the ecological dispatch period, the continuous water rise time of the Yichang River section reached a maximum of 9 days, the water level rose by an average of 0.43 to 1.83 meters per day, and the downstream water flow increased by 1080 to 5800 cubic meters per second. In addition, ecological dispatch also has a significant impact on the downstream water temperature, during the dispatch period, the water temperature of the Yichang River section was basically maintained at 17.5~23.5 °C, except for 2013, which did not reach 18 °C, the rest of the years were in the moderate temperature breeding range (20~24 °C) of the four large fish.
Schematic diagram of the sampling river section
Judging from the reproduction of the four major fish in the Yangtze River, the ecological perspective has indeed produced positive effects. In 2012 and 2017, the maximum fish egg density monitored in a single day appeared in the artificial peak process of ecological dispatch, and as long as there is an obvious water rise process, the spawning activities of the four large fish can basically be monitored, which shows that artificial ecological scheduling can indeed promote the large-scale reproduction of the four large fish.
Domestic fish seedlings found in the middle and lower reaches of the Yangtze River
Historically, the natural reproduction survey of domestic fish can be divided into four stages: the 1960s were the bottom-up stage, the 1980s were mainly to assess the impact of GezhouBa, from 1997 to 2010, the impact of the Three Gorges Project was mainly assessed, and from 2011 to the present, the ecological scheduling of the Three Gorges was assessed.
Whether it is the Yichang river section or the Jianli river section, a large number of research reports show that from the 1960s until the Three Gorges flood storage period, the breeding scale of the four major fish has been declining continuously; after the ecological dispatch in 2011, the breeding scale of the four large fish has increased. Of course, this is not only the credit of ecological scheduling, but also has a lot to do with factors such as strengthening the ban on fishing and artificial release implemented in recent years.
Breeding and stocking activities of four large fish
In short, the ecological dispatch of the Three Gorges, the ten-year ban on fishing in the Yangtze River and the large-scale artificial breeding and stocking have opened up a "life channel" for the survival of the four major fish. Although it is still not comparable to the glory days of the 1960s, this has won more living space for the four major fish.
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