news

my country is a vast country with abundant resources. There are countless rivers, lakes and reservoirs on its 9.6 million square kilometers of land. According to incomplete statistics, if all the rivers in my country were connected end to end, they could circle the earth's equator more than 10 times. There are more than 50,000 natural rivers in my country with a basin area of ​​100 square kilometers.

The spatial distribution of rivers in my country is very uneven, with most of them distributed in the outflow area, namely the Greater Khingan Range-Yinshan-Helan Mountain-Qilian Mountain and the southeast side of the Bayankala Mountain-Gangdise Mountain. The total water resources of rivers in the outflow area account for more than 95% of the total water resources of rivers in my country. In other words, most of the water resources of rivers in my country will flow into the ocean.

Take the Yangtze River and the Yellow River, the two largest rivers in my country, for example. They not only account for the vast majority of the total freshwater river water resources in my country, but also breed very rich biological resources in their waters, the most important of which are various fish resources. For example, there are almost 400 kinds of fish in the Yangtze River, among which the white sturgeon, the Dabryan sturgeon, the Chinese redfin bream, the giant salamander, etc. are all national protected animals;

There are more than 100 species of fish in the Yellow River, among which species such as the Yellow River knife fish, the Yellow River big carp, the Yellow River naked splittail fish, and the walleye fish are currently relatively rare.

As we all know, the final destination of outflowing rivers is the sea. These rivers start from their source and continuously move towards the ocean along the terrain difference from high to low. The river water carries with it different levels of silt and biological resources, and finally flows into the sea through the estuary, completing a "semi-cycle" process of water resources, silt and nutrients.

So, as freshwater fish, their daily living environment is very different from those of marine fish living in the ocean. Freshwater fish are definitely unable to adapt to marine life. Where do these freshwater fish washed into the sea eventually go? Did they die because they could not adapt to the marine environment?

In the natural environment, there is no absolutely fresh water on the earth, because in the process of water flow, it will inevitably interact with the surrounding soil, rocks, etc., so that some minerals will be dissolved in the water, and the salt content of the water will inevitably be greater than 0. In order to distinguish the salt content of the earth's water, scientists divide the water on the earth into fresh water and salt water. The dividing line between the two is the salt content of 0.5 g/L (0.05%). Water above this value is salt water, and water below this value is fresh water.

Among my country's coastal waters, the South China Sea has the highest average salt content, about 3.5%, followed by the East China Sea and the Yellow Sea, which are close, at about 3.2%, while the Bohai Sea has the lowest average salt content, about 3%. In coastal waters, generally speaking, the closer to the land, the lower the salt content of the seawater. This is directly related to the fact that the closer to the coast, the more fresh water flows from the land to the ocean, which is equivalent to diluting the salinity of the seawater.

Fish that have lived in freshwater rivers such as the Yangtze River and the Yellow River for a long time naturally cannot adapt to the pure marine environment. There are two important factors at work. One is the pressure problem. The depth of the ocean (even in coastal waters) is often incomparable to that of rivers, lakes and land. As the depth increases, the pressure of the water body continues to increase. The body structure of freshwater fish cannot withstand the impact of this ocean pressure.

Another is the salinity of seawater. The average salinity of my country's coastal waters is at least ten times higher than that of the Yellow River and the Yangtze River. Freshwater fish that have lived in freshwater for a long time cannot adapt to such high salinity changes. Once the salt content of the surrounding water increases sharply, the osmotic pressure of the fish's cells will be unbalanced, and the water in the cells will continue to lose water, causing the serious problem of "death by thirst". This is also the fundamental reason why freshwater fish cannot survive in seawater.

If this is the case, then every year there is so much water flowing from the Yangtze River and the Yellow River to the sea, which must contain so many fish. Do these fish just "accept their fate"? In fact, the vast majority of these fish will not sit and wait to die.

On the one hand, after the water from the Yangtze River and the Yellow River flows into the sea, there will be a relatively large buffer zone. In the buffer zone, fresh water and sea water slowly mix, and the salinity value slowly increases as the distance from the coast increases. Therefore, the salinity value of the water from the Yangtze River and the Yellow River flowing into the ocean will not suddenly increase in a short period of time. This also provides the necessary buffer for freshwater fish to adapt to this gradually mixing environment.

Secondly, when the water from the Yangtze River and the Yellow River flows into the ocean, it suddenly becomes "clear" and the flow rate decreases a lot. As fresh water continuously rushes in from upstream, silt will slowly settle in the buffer zone, and sea water and fresh water will fight against each other in the boundary area. With the continuous injection of fresh water from upstream, the closer the area is to the estuary, the greater its tendency to be freshwater in nature. Therefore, in an environment with low water speed and low salinity, these freshwater fish essentially still live in a freshwater environment.

Thirdly, after being washed into the ocean, many freshwater fish can still successfully return to the "embrace" of freshwater by relying on the skill of "migration". In the long process of evolution, these freshwater fish gradually formed the habit of reproductive migration and feeding migration. Even after being washed into the sea, they will swim upstream in the direction of coming, fighting against the turbulent river water and steep terrain, and finally migrate to a certain area upstream, either to mate and lay eggs, or to find more abundant food, so as to continue the fate of the species.

Therefore, we can see that the fish that are washed into the sea from the Yangtze River, the Yellow River and other rivers have developed the ability and survival skills to adapt to the huge changes in space, climate and other factors in the long process of evolution. What awaits them is not death, but a new beginning.