In the beginning, the earth was full of fresh water, but now there is a distinction between seawater and freshwater because the salts and minerals from the land are washed into the sea by rivers, and slowly accumulate to form a high concentration of seawater.
Freshwater fish account for 41% of the world's approximately 33,000 fish species, and they can tolerate salinity of less than 0.05%, while seawater has a salinity of up to 3.5%, what happens to freshwater fish once they enter the ocean?
Here we must understand the principle of osmotic pressure, easy to understand, nature pays attention to balance, and the same is true of the salinity in the organism, assuming that the salinity in the cell is "5", and the salinity in the seawater is "10", when the organism ingests seawater, the salinity inside and outside the cell will produce obvious differences, in order to maintain balance, the water in the cell will be precipitated to desalinate the high concentration of seawater.
In other words, under the desalination of a large amount of water, the salinity outside the cell decreases to "7.5", and the salinity in the cell rises to "7.5" due to the decrease of water, and at this time, the salinity of both inside and outside reaches "7.5", which maintains balance.
But what is the consequence? The consequence is that a large amount of water in the cells will "escape" out, resulting in dehydration of the body, which is why human beings drink seawater, the more they drink, the more thirsty they become, because the water in our body runs out to reduce the salinity of seawater, and if the dehydration exceeds a certain level, it will die.
The same is true for fish, when freshwater fish enter the seawater, they will also dehydrate and die, and the reverse is also applicable, when the marine fish enter the freshwater, the cells will be filled with a large amount of fresh water, causing the cells to continue to absorb water, and the sea fish will also "explode and die" after an excess.
In the past three years, the average annual inflow of the Yellow River has exceeded 30 billion cubic meters, and the value is more than 10 times that of the 90s of the 20th century.
Is the salt concentration in the mouth of the Yellow River too low?
The average salinity of the Yellow River estuary is less than 15 during the flood season and 29 in the non-flood season, with an annual fluctuation rate of 2.1%, but regardless of whether the Yellow River is in the flood season or not, the salinity of the estuary exceeds the salinity range of freshwater.
There are about 8,600 species of freshwater fish in China, which will die quickly as soon as they enter the seawater, but there are also a small number of freshwater fish, they have strong adaptability to salt, usually live in freshwater, and occasionally enter the seawater, such as the perch, which has not only appeared in the lower reaches of the Yellow River, but also has their presence in the estuary, and the perch belongs to the broad salt fish.
In addition to freshwater fish entering the seawater, marine fish also enter freshwater, so is the principle of salinity balance mentioned above invalid? Why can freshwater fish live in the estuary? The reasons are as follows:
The estuary is actually a very wide area, for example, we have the impression that the estuary is as big as an airport, but in fact, the real estuary may be as big as a small town.
If the salinity of the estuary is divided into 10 stages, 1~3 is the freshwater salinity value, 8~10 is the seawater salinity value, and the middle 4~7 is the buffer zone.
Freshwater fish will not suddenly enter the "8~10" area, but start from "1", slowly transition to the seawater, once the salinity exceeds the limit, freshwater fish will immediately swim back, and vice versa, the same is true for marine fish.
Fish live in the water all year round, extremely sensitive to changes in the surrounding environment, they are not as stupid as imagined, and as mentioned above, a small number of freshwater fish can enter the premise of living in the sea water is that "time can not be too long", once the stay time is exceeded, whether marine fish or freshwater fish, in the "other place" will die, so aside from the time discussion is also a little not rigorous.
This is like Zhang San wearing a T-shirt, from the 36 °C area, to the -10 °C area, during which the temperature is not a sudden drop, but slowly decreasing layer by layer, if Zhang San stays in the -10 °C place for too long, he will undoubtedly freeze to death, but it is instinct to seek advantages and avoid disadvantages, when Zhang San feels cold, he will turn back to the high temperature place.
Humans are human beings, fish are fish, human beings in an unsuitable environment, will take the initiative to find solutions, and fish live in waters, greatly affected by the current, when the boiling Yellow River water quickly rushed into the ocean, freshwater fish have not even reacted, they are already in the high concentration of seawater, is this possible?
The possibility is very small, the Yellow River Basin originates in the Bayankala Mountains of Qinghai Province, China, and spans the Qinghai-Tibet Plateau, the Inner Mongolia Plateau, the Loess Plateau and the Huang-Huai-Hai Plain from west to east.
However, in the eastern plains, the flow rate of the Yellow River water will be significantly reduced, and the S-shaped bend is more conducive to "buffering and unloading", and the silt that the Yellow River has impacted into the east over the years will also reduce the flow rate of the water.
And the estuary is rich in nutrients, many fish adapt to thrive here, in general, there are very few freshwater fish washed into the sea water by the Yellow River to die, after all, after so many years, the fish living nearby have two "brushes", and most of them can be safe and sound.