Aquatic animals are adapted to living within certain salinity ranges. Sunfish, for example, common in ponds and freshwater streams, could not survive in salt water.
Animals living in salty conditions tend to be more tolerant of a wider range of salinities.
Many species of fish migrate from salt to fresh water to spawn. Salinity is affected by weather, especially at the surface of the water. During dry seasons, water evaporates, making salty water. When it rains, salty water is diluted by the added fresh water. After a heavy rain, look for freshwater fish in normally saltier water.