Marine and freshwater fish. The similarities and differences.
The Caspian sea should be considered a sea or a lake? Because it is not connected directly with the ocean, according to the strict requirements of geography, therefore, is the lake. But the water in this lake is salty, marine. Therefore, we call this the closed reservoir of the sea. Lake Baikal is a lake or the sea? Baikal is also considered the lake as it is fresh water.
Fish who spend their lives performing in high salty waters of the sea; fish, living from birth to death in fresh water – freshwater.
And the fish that live in the coastal parts of seas, where flow of fresh water rivers? They are usually considered as brackish.
In the Caspian sea is the real sea herring, which live and breed in the sea and in fresh water will never enter. All three groups of Caspian herrings that peacefully exist in one place, related to each other, but they differ in their appearance and biological features: first marine, second is saltwater, third freshwater.
There are species of herring, living in the sea for spawning going far in the river. These fish have a characteristic title: “diadromous fishes”.
Atlantic salmon – salmon reproduces only in freshwater rivers. Here spend the first years of his life, the juveniles and in the adult salmon, except in the spawning period, lives in the sea. Salmon can be attributed to both marine and freshwater, as salmon reproduction occurs in the river. River eel, living many years in fresh water, breed in sea. Therefore, it can be attributed to the group of marine fish, but also to the group of freshwater anadromous.
Back to the history of the development of the fish. Animals of the same class, the class of fishes, and behave completely differently! On fossils of fishes ichthyologists come to the conclusion that some fish, having lived a long time in the sea, moved in bodies of fresh water, others of freshwater moved into the sea and became marine.
Geology of our earth is constantly changing: some part joined, some were disconnected. Sometimes between internal aqueous and external education was not formed and even the isthmus. The water in this closed reservoir, a lake, some time remains salty, sea, but gradually due to rainfall, snow, due to the runoff of the rivers it becomes fresh. Marine fish, resposables first brackish, and then to fresh water change, and of course sooner or later become freshwater.
Grave lake on the island of Kildin in the Barents sea were formed by separating a small part of the Barents sea; now the living water link between the lake and the sea there. In the Grave the lake is home to sea cod, although the conditions in this pond are becoming more and more severe: in the upper layers of water strongly desalt, and deep layers of the its are poisoned by hydrogen sulfide, so that cod can live only in the 6-7 meter layer of water: to rise above interfere with fresh water, to descend below the hydrogen sulphide.
This experiment produced by nature itself, and I wonder what will happen next: not addressed whether sea cod in freshwater.
When dividing fish on saltwater and freshwater is taken into account the salinity of the water.
But what kind of water is considered salty, some brackish and some freshwater? Sea water recognizes that water in which salt (sodium chloride) contains more than 25 grams per liter of water. Brackish water containing from 25 to 3 grams of salt per liter. Even less if salt – water is fresh. In the water, besides sodium chloride and other dissolved salts, but in this case these salts are not taken into account.
However, the share of fish in marine and freshwater only on the basis of salinity of the reservoir is not always correct, because there are fish that tolerate different salinity of water. For example, trout can live in the sea, in the lake and in the river. In the sea – the sea trout (in the West it is called taimen, Northern – trout), lake trout, or brown trout, and river trout.
Sea trout, large fish with a length of 1 meter, lives in the sea off the coast of Europe, but the spawning is in the river. Brown trout inhabits the lakes, and lays eggs in rivers. Brown trout less than sea, but sometimes there are very large specimens. Brown trout, smaller lake.
The trout here which is great. It is proved that the sea trout, for some reason, remaining in the lake, may appeal to the lake, and brown trout remaining in the river – strand. After several generations change the size and color of the fish. More interestingly, the reverse is observed: brook trout refers to lacustrine, and lacustrine – marine. So to turn all the fish, but, of course, and some trout.
Consequently, the division of fishes in marine and freshwater is rather arbitrary, but nevertheless it was made.