I am in need to discuss about how the fish and other aquatic animals survive or managing to adopt them while migrating from freshwater to sea and vice versa.
Dear Noorjahan,
According to my opinion, we should determine our main topics and basic concepts.
In this process many aspects would be considered such as Habitat, Nutrition, Physiology, Genetic, Biodiversity and diseases and risk factors.
We can discuss on main topics more and more.
I think trout is a good example of a fish with a perfect adaptation. An interesting fact is that migration will depend not only on environmental factors (temperature variation or water level) and habitat, but also on biotic factors : high densities will increase the probability that a fish will migrate. Which is "good" because mortality increases with density, so if fishes migrate, then it's in a way "better" for the population.
The largest challenge is the change over in osmoregulatory response from a freshwater fish that must retain salts and get rid of excess water to a saltwater fish that must drink seawater, retain the water and get rid of the excess salts. Salts are regulated through the chloride cells in the gills. The references in the attached paper may help.
Brazilian freswater fishes that do long distance migration upstream rivers for reproduction need accumulate energy for it. Generally this energy is accumulated as fat. If the fish have no enough fat accumulation it doesn't migrate upstream, which happens in floodplain fishes in the Pantanal, one of the largest freshwater floodplain in the world.
Whether the fish is anadromous or catadromous, it can manage the migration with the help of its physiology especially balancing its kidney and gills.
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