John Hale, of Dunedin, asks :-
I've heard that whereas salmon leave the sea and return to their fresh-water birth-place to spawn, some fresh-water fish go to the sea for the same purpose. This suggests that some fish can live and feed in both kinds of water. What differentiates fresh-water fish from sea-fish?
Mark Lokman, a zoologist at the University of Otago, responded.
Fish are a diverse group of animals, numbering around 20,000 species. Most of these either only ever live in seawater or only ever in fresh water. However, some species spend part of their live in seawater and part of it in fresh water. The best-known example is salmon, whose eggs hatch in fresh water and whose offspring move to the ocean to grow, only to return as adults some 2-5 years later.
Eels do the opposite, hatching in the ocean, making their way into freshwater streams and returning to the ocean to breed. For some fish, movement from freshwater into estuaries or the sea is aimed at finding the most food-abundant place - a 'wandering' approach, which may give the it the best opportunities for growth. So why do not all fish move between fresh and seawater environments?
Seawater, as we all know, contains salt, the composition of which is roughly similar to table or rock salt. Generally, the amount of salt in seawater is around 25-35 gram / litre. This salt can 'sting' - one only needs to have a small cut on hands or feet and then put this in seawater to experience this. The same happens to fish that move from fresh to seawater: the salty water stings and literally "sucks" the water out of the fish. Seawater fish therefore drink lots of (sea!)water to make up for the water loss and they use special cells in their gills to excrete excess salts.
Freshwater fish do not have this machinery. Conversely, fresh water contains almost no salt at all, and the fish will absorb water from its environment. It gets rid of excess water through the kidneys, which are well-developed in freshwater fish, but not in marine fish. Salmon, eels and some other fishes have both the special cells in the gills AND a well-functioning kidney, enabling them (sometimes only during specific times of the year) to move between these different environments.