Why do fish from the sea die when they are put in a river?

The water at sea and water in a river have different salt concentrations and therefore, are habitats to completely different organisms. These organisms must keep an internal environment similar to the ionic composition of their habitat. This is why marine fish excrete excess salts in the hypertonic sea water, while Freshwater Fish still have the ability to concentrate salts inside their bodies in an environment where there is none. If you were to put one of these types of fish in the other's environment, it would die due to the imbalance that occurs inside the organism's body.

However, there are some species that can tolerate both fresh and salt water, like the Delta smelt that lives in between freshwater and the sea. There are also some species that live in one type of water, and then migrate to the other in a later part of their life, like salmon whom are born in freshwater, then move to saltwater later on.