Freshwater fish and marine fish live in two different water environments, which also leads to the fact that freshwater fish and marine fish cannot live in interchangeable environments.
If you rashly throw freshwater fish into the seawater, the freshwater fish will not stay long before they die.
But both the Yellow River and the Yangtze River will eventually flow into the sea, especially during the annual flood season, when the water at the mouth of the sea is choppy and will wash a large number of freshwater fish into the sea.
Will these freshwater fish, who are unable to live in the ocean, die en masse when they are washed into the sea?
Before answering this question, let's discuss why freshwater fish can't live in seawater.
In fact, the biggest difference between freshwater fish and marine fish is that they have different adaptability to salt in the water. 、
As the name suggests, freshwater fish are fish that live in freshwater, and fish in these rivers, lakes, and reservoirs in inland areas are all freshwater fish.
And those fish that live in the ocean, as the name suggests, are marine fish.
For example, Dalian in Liaoning, Yantai in Shandong, Ningbo and Zhoushan in Zhejiang, Zhanjiang in Guangdong, etc., most of the fish that people living in these coastal areas usually eat are marine fish.
Freshwater fish live in a normal environment that allows them to only move in water bodies below 0.05% salinity, while the salinity of the ocean makes it impossible for them to adapt.
Seawater contains a large number of metallic and non-metallic ions, the most common being sodium ions and chloride ions, which are similar to the edible salt we usually eat.
Seawater contains not only salt, but also calcium, magnesium, sulfur, carbon and other ions, because the ratio of these chemical ions is different, resulting in different tastes and properties of seawater.
However, in general, the salt concentration of seawater is about 3.5%, which is about 7 times that of freshwater.
Although the sea in which the fish live is salty, these fish are not salted fish.
This is because there is a reasonable osmotic pressure difference between the cell fluid in the body of the fish living in the sea and the seawater outside.
Fish living in the ocean have been actively drinking water, not only to obtain oxygen, but also to expel excess salt from the water.
The gills are the organs that fish use to breathe, but what many people don't know is that there are actually many blood vessels and capillaries in the gills, which allow blood and seawater to exchange gases, which is also the secret that fish can breathe in water.
Marine fish have a special type of cell on their gills called chlorine cells.
The chloride and sodium ions that are drunk into the stomach and enter the blood will be transferred to the seawater by the chlorine cells of the marine fish, so that the salt concentration in the marine fish will also be reduced.
In addition to the gills, the kidneys are also good helpers for marine fish to remove salt.
As the excretory organ of fish, the kidneys can excrete excess water and waste products from the blood of marine fish in the form of urine.
The kidneys of marine fish are also a bit special, that is, they are not large, so the amount of urine that can be stored at a time is not much. In order to make proper use, the kidneys of marine fish concentrate urine.
The kidneys concentrate the sodium and aluminum ions in the blood into the kidneys for processing, and then excrete them with the concentrated urine, so the urine of marine fish contains a high proportion of salt.
Marine fish drink a large amount of seawater and then excrete salt through their bodies to maintain a balance of osmotic pressure.
Adaptation to osmotic pressure is a life-threatening event for any living thing on Earth.
Like freshwater fish, it can't adapt to the osmotic pressure of seawater, because once freshwater fish enter the sea, due to the difference in osmotic pressure, the water in the body of freshwater fish will continue to seep out of the skin, so it will die when it enters seawater.
On the contrary, when a marine fish enters fresh water, it cannot learn to regulate and increase the salt content in the body like fresh water, so as to avoid the outside fresh water from entering the cell fluid, which will eventually lead to the rupture of the cell due to over-filling of water.
Therefore, once the freshwater fish in the Yellow River are washed into the sea, there is really no way to survive.
However, when these rivers flow into the ocean, they do not change in the form of salt cliffs, but have a slow change process.
The inertia of the discharge, coastal currents, coastal drifting formed by the monsoon, etc., all cause the runoff to flow into the sea, backward and outward, or to one side, and the shape of this area is like a tongue when you look at the analysis map.
The area of the isosalinity line tongue extending in the sea area off the estuary is officially called "freshwater tongue" in academic terms, and is also called "low-salt water tongue".
Like the freshwater tongue at the mouth of the Yellow Mouth, the range is almost to the middle of the sea area.
In the freshwater tongue at the mouth of the Yangtze River, the salinity of the estuary is about 5‰, and the maximum salinity difference along the tongue axis is 27‰, which has been affected until around 30°30'~34°N.
Therefore, even if the waves of the Yellow River and the Yangtze River are strong during the flood season, these freshwater fish will only be washed into the freshwater tongue and will not directly enter the ocean, so there will be no eventual death due to the inability to adapt to the change in salinity.
In addition to the presence of freshwater tongues, there will not be a large number of dead fish in the estuary, which is actually related to the special living habits of some fish.
There is a species of fish that has an inherently excellent ability to adapt to salinity, and they are the migratory fish of the sea and rivers, which belongs to the "wide haline fish".
This fish is also divided into two main groups.
One is to lay fish eggs in seawater, but they usually live in fresh water. Eels, like eels, live in freshwater for most of their lives, but when it comes to spawning season, eels swim into the sea. This is called a migratory fish.
The other is to lay eggs in fresh water and live in the sea. Salmon is a fish that usually lives in the sea, but when it comes to spawning, it will try to swim upstream and find a suitable place in fresh water to give birth to offspring.
The cell structure and physiological structure of migratory fish are relatively special, which is specifically manifested in the gills, and its gill filaments are spiral-shaped, which can adapt to waters with different salinity.
When they swim from freshwater to seawater, they can excrete excess salt from their bodies through their skin, gills, urine, etc., like marine fish, to achieve a balance of osmotic pressure, and the cell fluid will not be dehydrated.
As they swim from seawater to freshwater, their bodies adjust to a salinity increase and live like freshwater fish.
In fact, these special features are the expression of animals adapting to nature, and they are also the way they actively respond to environmental changes.
Resources:
[1] Dr. Gorai, "The sea is salty, but the "fish" living in the sea are not salty?", 2023-08-22