您眼中的黄河：
Source: Self-drawn. The actual Yellow River:
Source: Self-drawn using a mouse， a bit abstract but the meaning is conveyed. Some people online wonder what happens to freshwater fish that are washed into the sea by the Yellow River daily， given their inability to survive in saltwater. The reason for this question might be the misconception that the Yellow River is a straight channel， always leading fish directly to the ocean.
However， after viewing the two images above， it is believed that everyone now understands that this is not the case. Today， we will discuss this topic in detail.
The Yellow River is not a straight channel.
Due to variations in water flow speed and gradient， as well as differences in geological hardness， rivers carry different amounts of sediment in various regions， leading to the formation of numerous natural underwater deep pools.
Source: 500 Cases of River Management
Source: Network. In areas with open terrain， large lakes can also form.
As vertebrates， fish also need to sleep and rest. If the Yellow River were like the first image， with a continuous downward flow throughout its course， it would be very difficult for fish to survive. Even then， they would only be found in areas with abundant rocks and aquatic plants. Those with fishing experience in rural streams know that areas with established fish populations are typically shallow pools and depressions.
Deep pools and shallow areas with aquatic plants are more likely to harbor fish. Source: 500 Cases of River Management. A winding river， due to the random distribution of riverbanks and floodplains， experiences significant variations in water flow speed. Areas with slower currents are often richer in aquatic vegetation， have complex ecological transitions between land and water， and consequently， more established fish populations.
Source: upraveno podle HUGGET， 2007
Source: FISRWG， 1998， Chinese online translated version. Streams with slightly faster currents are less likely to have fish concentrated in fixed locations. If fish do appear， they are either quickly swept downstream to depressions or they strive upstream to depressions further upriver. It is precisely because the habitat of freshwater fish consists of a series of depressions connected by rapid currents that freshwater fish have evolved the instinct to swim upstream.
Every heavy rain creates larger or new depressions upstream. Generally， small fish are easily swept away by the current， but with slower and smaller currents upstream， not only are small fish less likely to be swept away， but they also encounter numerous depressions if they are， providing more survival opportunities. Essentially， most medium and small fish exhibit an upstream swimming behavior during heavy rains and floods.
However， in reality， most fish are swept back upstream. Anyone who used to catch fish as a child knows that during heavy rains， even with a bamboo basket placed in a small stream， you could catch fish swimming up from downstream ponds， only to be exhausted and swept back down. The fewer fish that reach the upper reaches， the greater their survival advantage there. Not only is food abundant with plentiful insects and plant fruits， but predators are often scarce.
Beyond basic survival， spawning and reproduction in the upper reaches also offer advantages. In summary， the complex geographical environment of rivers ensures that a large number of fish are not directly swept into the sea.
Winding and meandering rivers. Copyrighted image， reuse may lead to copyright disputes. Of course， many fish also live in the lower reaches of rivers， and it is not uncommon for some to be washed into the sea. So， what happens to the freshwater fish that end up in the sea?
What happens to freshwater fish washed into the sea?
1. They swim back.
We know that river mouths are where the outflow is greatest. However， due to the wide expanse of water here， the current is actually slower.
Yellow River Estuary (Source: Yellow River Estuary Satellite Image， Dongying， Shandong， 8.14)
The vast majority of fish that mistakenly enter estuaries have the ability to swim back. Furthermore， river water and seawater do not form distinct layers but rather a special salinity buffer zone [1]. The salinity gradually changes from approximately 0.1% near the shore to 3% in the open sea， forming a natural transition zone.
Typical salinity gradient change at an estuary.
Salinity gradient changes for different estuary types (slope differences). Source: libretexts.org
Therefore， it often forms a tongue-like extension， also known as a “freshwater tongue” or “low-salinity tongue.” The surface area of the freshwater tongue is positively correlated with the river’s daily flow rate. When the flow rate exceeds 200 m³/s， the area of the freshwater tongue can exceed 70 km². The Yellow River’s maximum flow rate reaches 4700 m³/s， with an average flow fluctuating around 2000 m³/s.
The Yangtze River’s maximum flow rate is even higher， reaching 85700 m³/s， with an average flow of about 34000 m³/s. The Yellow River possesses a very rộng freshwater tongue， and the Yangtze River’s is about 10 times larger than the Yellow River’s. Complex water environments often attract various marine/freshwater fish species， forming complex ecosystems [2].
This area not only hosts mistaken marine/freshwater fish but also occasional opportunistic feeders， resident fish， and anadromous and catadromous species. For typical freshwater fish， such as carp， that mistakenly enter the freshwater tongue， they can keenly sense the changes in salinity as they enter the sea. This is because their taste receptors are distributed not only in the mouth and pharynx but also on the lips， fins， and barbels. Beyond taste， their sense of smell can also detect salinity， and they possess high sensitivity.
Source: Living Ocean， CRDG， University of Hawaii at Manoa
While the lateral line system does not directly sense salinity， it can indirectly detect changes in water density and other properties. Crucially， fish directly regulate osmotic pressure through the water and salt in their environment [3].
Osmoregulation in freshwater fish (A) and marine fish (B).
As environmental salinity changes， fish experience osmotic imbalance. While this is not a direct perception of salt， the discomfort， combined with high salinity information from taste and smell， helps fish detect danger and thus return to low-salinity waters promptly. Generally， for freshwater fish like crucian carp and common carp， if they can adjust their condition in time within the buffer zone， they can swim back upstream to the Yangtze or Yellow Rivers.
For fish that cannot return to freshwater areas for an extended period， the vast majority will die. Besides succumbing to osmotic imbalance， they may also be preyed upon by opportunistic marine predators. Of course， the above analysis focuses on individual fish. But what if a certain fish species， over long periods， is consistently washed into estuaries in large numbers， and some survive?
2. Adaptation to the environment can trigger an evolutionary revolution.
There are living examples in reality， the most typical being salmon. It was once believed that salmon ancestors originated from the sea， but it is now generally thought they originated from freshwater [4][5]， with origins dating back 65 to 95 million years ago [6][7][8].
Source: Reference [8]. Constantly changing geographical environments have shaped the evolution of salmon ancestors. Between 37 and 57 million years ago， the Columbia River basin was a vast coastal plain， and salmon ancestors lived prosperously.
The Columbia River Basin in North America. Source:wikimedia. Between 6 and 17 million years ago， volcanic eruptions and massive basalt flows covered the center of the Columbia River basin， eventually flowing into the Pacific. Surrounded by lava or seawater， salmon ancestors lived as if in purgatory. Between 8 and 15 million years ago， the gradual uplift of the Rocky Mountains， Olympic Mountains， and Coast Ranges transformed the world of salmon ancestors.
Source: Reference [5]. These dramatic environmental changes led to differentiation in the adaptation of salmon ancestors to different environments. The last common ancestor of Pacific salmon (Oncorhynchus) diverged during this period.
Dark areas indicate salmonids. The earliest salmonid species to enter the sea may have originated over 10 million years ago [9]. They may have undergone multiple independent “freshwater to marine” evolutionary cycles tens of millions to millions of years ago [10].
Source: Reference [9]. However， 2.58 million years ago， Earth entered the Quaternary glaciation. The Quaternary glaciation was not consistently cold but experienced numerous interglacial periods lasting 1-40，000 years. The repeated alternation of cold and warm periods subjected salmon ancestors to a “hellish” existence. During the Quaternary glaciation， up to 19-20 cycles of glaciation， climate fluctuations， and drought-flood cycles resulted in salmon evolution through continuous selection and extinction.
Many early freshwater salmonid species went extinct， while Pacific salmon， through constant elimination， adapted to the cyclical glacial periods. Twenty thousand years ago， a group of Pacific salmon survived the Quaternary glaciation on the North American continent， entering a peak of radiation and development， finally seeing the dawn of survival. But they did not realize that with the end of the ice age， they would face an even more “hellish” existence. With global warming， ice dams broke periodically， causing recurring floods.
The Ice Flood Era in North America began over 10，000 years ago. Source: hugefloods.com. The intermittent action of glaciers and seawater continued for thousands of years， with the Glacial Lake Missoula ice floods scouring the Columbia Plateau over a hundred times. These ice floods， covering a vast area， had a flow rate equivalent to 60 Amazon Rivers today， carrying car-sized boulders embedded in the ice for as far as 800 kilometers.
Area covered by Glacial Lake Missoula ice floods (left). Source: Montana Natural History Center.
Salmon that survived， do they choose glaciers or the sea? Clearly， from an evolutionary perspective， surviving in the sea came at a much lower cost than in glaciers. We can imagine the evolutionary process of migratory salmon. From the beginning of the Quaternary glaciation to the present long period， the ancestors of these salmon encountered icy floods， and the vast majority died. Then， some entered the sea， and the vast majority of the remaining ones died. Ultimately， individuals with slightly higher body fluid osmotic pressure， after enduring for a period， returned to freshwater and hence reproduced.
(As their ancestors may have already undergone a “freshwater to marine” adaptation process， re-adapting to marine environments would have been easier.) Once they developed to a sufficient scale， encountering cold floods again， through continuous repeated selection， and after N iterations， the surviving salmon developed increasingly higher body fluid osmotic pressure， falling between that of freshwater and marine fish， and even developing the ability to excrete salt through their gills. They not only survived longer in the sea but also evolved strong reproductive capabilities due to the high survival elimination rate.
A high number of offspring is needed to balance the high elimination rate. Furthermore， to ensure the survival of enough offspring， it means needing to migrate upstream， choosing areas with abundant food and fewer predators. They would even sacrifice themselves for the sake of their offspring. Thus， through millions of years of ancestral natural selection， enduring the “hellish” pressure of the Quaternary glaciation， and experiencing tens of thousands of years of intense ice flood culling， Pacific salmon and other salmon developed their migratory habits.
It is generally believed that Pacific salmon reached their peak radiation development 20，000 years ago， and migratory habits existed at least 11，500 years ago [11]. Perhaps it is precisely because of the extremely harsh evolutionary process that salmon reproduction has reached an unusually high level of “involution” in nature.
Salmon life cycle. Source: Elizabeth Morales.
Salmon parents travel 800 kilometers， sacrificing themselves to spawn once. They lay an average of 4000 eggs， of which about 1/5 hatch， with 200 entering the sea， 10 successfully migrating back， but only 2 reaching their birthplace， thus completing the lifelong cycle. Salmon spend 1-5 years in rivers before entering the sea. Their growth is guaranteed by their ability to inhabit depressions， riverbeds， bays， aquatic vegetation， and lakes.
Fish like salmon that enter the sea and survive are relatively rare， requiring complex and specific evolutionary pressures. The vast majority of those washed into the sea， apart from those that return to freshwater， naturally die and re-enter the natural cycle.
The magnificent natural scenery in your eyes may be a cruel survival selection in their world.
