why do salt water fish need salt water

In the same way that humans require oxygen to sustain our lives, fish require water to sustain theirs. There are also reasons why some fish require saltwater and some require freshwater to exist. There are two different classifications that address species of fish. Stenohaline species are groups of fish that are able to tolerate only narrow ranges of salinity in the water. This can be true for freshwater or saltwater fish. For example, freshwater fish like goldfish can only survive in freshwater, while tuna can only survive in saltwater. Other species of fish, such as salmon or striped bass, can live or survive in wide ranges of salinity. The salt content in the blood of fish is very high (approximately 9 grams per liter of blood), which are primarily sodium and chloride. This can create a problem for fish that exist in the very salty oceans. Fortunately, their systems are suited to deal with, and survive in, this type of environment. Gills help fish excrete excess salts in their system. The kidneys of fish are also designed to produce small volumes of fluid that contain salt. This is part of the reason why saltwater fish must live in saltwater - that was the way their systems function. They must continue to drink saltwater to replace lost fluids and then eliminate the excess slats in their systems. Freshwater fish have systems that function somewhat differently. They must retain salt through what they eat and through absorbing the smaller salt concentration that exists in freshwater.

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