TOXICITY
Water and dissolved solutes, including various salts, calcium chloride and sulphates, pass through a fish’s body (cells, tissues, and organs) in a process known as osmosis. Larger molecules inside the blood and tissue fluids of fishes and sharks, including proteins and trimethylamine oxide (TMAO*), also exert a contributory osmotic effect but they are too large to pass through the channels that govern salt balance.
* By-product of the metabolic breakdown of proteins and amino acids.
If the salt concentration in a fish’s tissues is lower than that of the surrounding water, its body will absorb salt from the environment until both levels become equal. If a marine fish swims up a freshwater river, the reverse phenomenon will occur and it will diffuse salt into the environment via specialised sodium secreting cells located primarily in gill tissue. In both cases, too much or too little salt is detrimental to most fish species because they can only survive within a specific range of salt concentrations. Fish that are thus restricted to either freshwater or seawater are known as stenohaline. Some species, such as salmon, are able to osmoregulate in varying levels of salinity. These fish are called euryhaline.