Freshwater fish
Freshwater fish inhabit the inland waters, lakes and rivers of the continents and islands. Almost all freshwater fish are among the bony fishes ( Osteichthyes ). Of the cartilaginous fishes ( Chondrichthyes ) some ray species live permanently in fresh water. There are also a few species of sharks that occasionally the estuaries hike up in freshwaters.
One differentiates the primary freshwater fish that have originally developed in fresh water, the secondary freshwater fish derived from marine ancestors, and the peripheral freshwater fish that spend part of their life cycle in freshwater.
Most freshwater fish are safe to eat, such as Pike, trout, catfish, carp and eel (see also: food fish ). Small Consistent, colorful styles are often popular aquarium fish.
Primary freshwater fish
Primary freshwater fish have undergone a great deal of their evolutionary history in fresh water, completely lost their salt tolerance and can not swim in the sea or in highly brackish water. Consequently Salty waters provide propagation barriers this group; they represent about 33 % of the total fish fauna. There are mainly fish from the orders of the Characins ( Characiformes ), carp -like ( Cypriniformes ) and catfish -like ( Siluriformes ), which are combined to form the superorder Ostariophysi the. The Ostariophysi have developed special adaptations to life in fresh water, such as a particularly good hearing. Provide you with more than 6,000 species two-thirds of all freshwater fish.
Secondary freshwater fish
The secondary freshwater fish are descended from marine ancestors that have migrated into fresh water. You do not have a weak salt tolerance. They make up about 8 % of the total fish fauna. Cichlids ( Cichlidae ), Zahnkärpflinge ( Cyprinodontiformes ) and ears Fishy ( Siluriformes ) belong to this group. Their representatives often dominate waters with very hard water or water whose pH is very high, as the rivers of Central America or the great East African lakes. The fish fauna of the rivers and lakes of Australia consists of the Australian lung fish with exceptions ( Neoceratodus forsteri ) and less of other types exclusively from secondary freshwater fish. Also, Madagascar, New Guinea, Sulawesi, the small Sunda Islands and the Antilles were inhabited exclusively by secondary freshwater fish. Have Oceanic, never standing in connection with the mainland islands, if any, also a fauna of secondary freshwater fish that here mainly of the family of gobies ( Gobiidae ) will be provided.
Peripheral freshwater fish
The peripheral freshwater fish do not have a pronounced salt tolerance and can still spread across the seas. They often spend a stage of their life in the sea. Among the peripheral freshwater fish include sturgeons (Acipenser ), the eels ( Anguillidae ), many salmonid fish ( Salmoniformes ) and smelt behaved ( Osmeriformes ).
Living conditions
The living conditions in freshwater are often subject to strong fluctuations. Depending on the season flows can cause different amounts of water. Lakes can dry out, the temperature can change. Previously associated water can be separated by geological events. All these factors affect the fish fauna. The separation of a population in geographically isolated regions results in the formation of new species. This often results in so-called species flocks, such as in the salmon -like of the genus Coregonus in Northern and Central Europe and in the Central American viviparous tooth-carps of the genus Xiphophorus.
In contrast to the sea fresh waters are usually relatively flat. Lake Tanganyika and Lake Malawi in East Africa with 1,470 meters and 704 meters, although very deep. Higher life is no longer possible below 200 meters. Because of the lack of water circulation, the lakes are at these depths almost without oxygen ( anaerobic). Only in the Siberian Lake Baikal there with the Baikal oil fish ( Comephoridae ) which is adapted to life at great depths fish fauna.
Endangering
According to the IUCN report are from the 522 European species of freshwater fish are already classified 200 species as threatened.
Freshwater fish with its various species and subspecies have usually a very small area of distribution as opposed to marine life. They often live in small, isolated lakes or endemic to islands and their small river systems. One extreme is the habitat of Teufelskärpflings ( Cyprinodon diabolis ), the 3.5 m × inhabited only a small limestone basin of 5 m × 3 m size in Nevada (USA). The limitation of their range makes these fish populations vulnerable to changes in their habitat, which they can not escape.
A major reason for the extinction of species in freshwater fish has been the regulation of rivers and the drainage of wetlands. In the past, they have also to climate change and water withdrawal. In Europe, more and more water is withdrawn for irrigation of fruit and vegetable plantations in formerly dry areas, especially in the Mediterranean countries. This results in the summer to partially dried-up riverbeds. Also, weirs, grit chambers in the main circuit and dams of power plants can endanger especially the migratory fish species.
Overfishing was in the course of population growth in many parts of the world to the problem. The intensive fishing industry with the onset of alien fish that displace native species mix with regional subspecies or bring pathogens, hazards or other cause of danger to biodiversity. In tropical regions Wild caught endemic species for rearing in aquariums contributed to the decline in populations.
In Eastern Europe were and are lack of environmental protection measures, a main reason for the decline of species and individuals. Agriculture and industry are growing in many states along the rivers without appropriate measures for the protection of waters and of the living beings.