Most fish cannot endure an extreme change in salinity, but there are some fish that are able to regulate their bodies' intake of salt and water so that they can survive in salt and fresh water. This ability to maintain the balance in their bodies allows for their species and life cycles to remain productive.
Why Is This Regulation Necessary?
Osmosis and diffusion is the passing of fluid and molecules between an organism and its environment. Salinity directly affects fish and sea creatures much like humidity can affect land creatures. It is that diffusion and loss (or gain) of water that can cause these animals to perish or survive.
How Does Osmosis/Diffusion Affect Fish?
When introduced to fresh water, saltwater fish will begin to lose the salt in their bodies and take on large amounts of water. Just the opposite happens to freshwater fish when they are introduced to saltwater. In either case, most fish placed in these circumstances will die.
Osmoregulation: How They Survive
Some fish, such as Atlantic salmon and brook trout, are forced by their life cycle to adapt to wide ranges of salinity. They do so through osmoregulation. This is a process through which they are able to regulate the intake and release of water and salt.
How Do Freshwater Fish Osmoregulate?
To release the excess water that enters their bodies through their gills, freshwater fish release a constant stream of very dilute urine. This allows them to maintain a perfect body composition. A fish might urinate up to one-third of its body weight in a day.
How Do Saltwater Fish Osmoregulate?
Saltwater fish are losing water constantly because of the high salinity of their environment. They are drinking sea water throughout the day to replace the water they lose. This allows them to make sure they do not become dehydrated and die.
Fish That Can Survive Extreme Changes
The fish that can survive at both extremes of the salinity spectrum are able to regulate the function of their kidneys and gills in a manner that allows them to exist in both extreme environments. Salinity begins to change as these fish migrate toward their freshwater spawning areas, and they adapt gradually with the changes in the environment.
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