Freshwater fish are those that spend some or all of their lives in freshwater such as rivers and lakes with less than 1.05% salinity. These environments differ from marine conditions in different ways, the difference in salinity levels being the most obvious. To survive in freshwater, fish need a range of physiological adaptations.
41.24% of known species of fish are found in fresh water. This is largely due to the rapid speculation that makes scattered habitat possible. When working with ponds and lakes, one can use the same basic models when studying island biology. Fish can react [
Freshwater fish are physically different from saltwater fish in many ways. Their gills must be able to disperse the dissolved gases while keeping the body fluids inside the salt. Their fibers reduce the spread of water through the skin: freshwater fish that have lost too much scale will die. The kidneys have developed very well to recover salt from body fluids before their excretion.
Many species of fish reproduce in freshwater, but most of their adult lives are spent at sea. These are known as andromas fish and include, for example, salmon, trout, sea lamprey  and three spined stalkbacks. In contrast, there are other species of fish born in salt water but most of them or some of their adults live in fresh water; For example, these are known as catadromas fish.
Migratory species in both sea and freshwater require adaptation to the environment; In salt water when their body salt concentration has to be kept lower than in the vicinity and vice versa. Many species solve this problem by associating different habitats with different levels of life. Both els law, Andromas salmoniform fish, and marine lampreys have different tolerances to different types of salt in the United States.
Among fishermen in the United States, freshwater fish species are generally classified in the water temperature where they survive. Water temperature affects the amount of oxygen available as cold water contains more oxygen than hot water. 
Cold-water fish species survive in the coldest temperatures, preferring water temperatures of 50 to 60 degrees Fahrenheit (10–16 সেল C). In North America, air temperatures resulting in adequate cooling water temperatures are found at higher altitudes in the United States of America, Canada, and South America. Common cold water fish include brook trout, rainbow trout and brown trout.
Cold-water fish species prefer temperatures of about 60 to 80 ° F (16-25 সেন C) between cold water and warm water species. These are found throughout the Americas except the southern part of the United States. Common cool water species include Maskelange, Northern Pike, Wally and Yellow Perch.
Warm-water fish species can survive at different water levels, preferring water temperatures of about 80 degrees Fahrenheit (27 degrees Celsius). Warm-water fish can survive winter temperatures in northern climates but can thrive in warm water. Common warm-water fish include catfish, largemouth bush, nilgil, crepe, and many other species from the family Centarchidae. Intestinal anthropogenic restructuring and regeneration of waterways affect the flow of water, water temperature, and more that affect the functioning of the general habitat. Dams not only disrupt the flow of linear water and divert major geological channels, but also limit the amount of water available for fish in lakes, streams and rivers, and these habitat changes have the potential to alter trophic structures and limit movement and connectivity. [] ৮]
Dams can create problems for freshwater habitats.
The unnatural flow of water beneath the reservoir causes a lot of habitat degradation by reducing effective alternatives for aquatic organisms. Upstream mountain migration to the dam structure is hampered and the population may decline because the fish do not have access to common food and / or spawning areas. Dams affect the richness of flowing species, i.e. the number of fish species in the ecological community [Furthermore, dams can cause isolation of fish populations and lack of connectivity creates potential problems for infection and low genetic diversity. Connection damage affects the structure of population assemblies and increases the fragmentation of habitats, which can further complicate existing problems for vulnerable species. 
Temperature changes are another unintended consequence of the dam and land use project. Temperature is a very important part of the stability of the aquatic ecosystem, and thus changes in flow and river water temperature can have a major impact on the biotic community. Many aquatic larvae use thermal signals to control their life cycle, most notably insects here. Insects are a big part of most fish diets, so it can create a great dietary problem. The temperature varies with the behavior of the fish
And distribution habits as well as increasing their metabolic rates and thus driving them into Spain and the feed []
Linear systems break down more easily and the connection to the aquatic ecosystem is essential. Freshwater fish are particularly at risk of habitat destruction because they live in small bodies of water that are often very close to human activity and are thus easily contaminated by waste, chemicals, waste and other agents harmful to freshwater habitats.
Changes in land use can lead to major changes in the aquatic ecosystem. Forests can change the structure and sediment composition of streams which can change the habitat function of many fish species and reduce the ness ecosystem, parity and diversity of the species.  Agriculture, mining and basic infrastructural buildings can reduce freshwater habitat. Fertilizer ranfus can produce excess nitrogen and phosphorus which feeds on abundant algae flowers that block sunlight, limit water oxygen and make the habitat virtually unstable for aquatic species. Chemicals from mines and factories enter the soil and flow through the stream. Wide roads, cement and other basic infrastructure do not absorb materials and more harmful pollutants move directly into rivers and streams leading to more floodplains [fish are highly sensitive to changes in water pH, salinity, hardness and temperature, which are runoff pollutants and land use May be affected by indirect changes from x to exotic species
An exotic (or non-native) species is defined as a species that does not occur naturally in a particular region or ecosystem. These include eggs and other biological components associated with the species. Non-native species are considered invasive if they cause environmental or economic harm. 
Blue perch is important for popular sport fish and commercial fishermen on the African Great Lakes
The introduction of exotic fish species into ecosystems is a threat to many indigenous populations. Native species fight for survival alongside exotic species that surpass prey populations or indigenous fish. The high concentration of exotic fish is negatively related to the ness of the native species.  Since exotic species were suddenly thrown into a community instead of evolving alongside other organisms, it is not established by other species as predators, prey, parasites, etc., and exotic species have more fitness benefits than such native organisms.