Saline water has more salts than fresh water. The amount of salt influences many things, but most of all it influences the salt and water balance of living organisms.

Oceanic fish have adapted to living in the high salt concentrations of the oceans. If an oceanic fish moves into fresh water, its cells begin accumulating too much water. However, some oceanic species have adapted to both highly saline and less saline environments. Herring and cod are examples of such species. 

Fresh inland water contains less salts than saline water. Because of this, freshwater fish must actively receive salts from the water and remove excess water from their systems. 

Most freshwater species have adapted to environments with low salt concentrations, but require fresh water to reproduce. For example salmon must move into rivers to spawn. However, some freshwater species such as pike have adapted to reproduce in brackish water.

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​The pike is an example of a freshwater fish that has adapted to reproduce in brackish water.