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A historic picture of three men fishing on a boat.
© NRCM |
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Passing between freshwater and marine habitats requires fish to osmoregulate, that is, to maintain their concentration of body fluids independent of the concentration of the outside environment. Because of osmosis—the diffusion of a solvent (water) through a semipermeable membrane (the gills), tending to equalize the concentrations on either side of the membrane— fish living in salt water are vulnerable to water loss. They compensate by drinking sea water, excreting salt through their gills, and passing concentrated urine. Freshwater fish, on the other hand, are susceptible to water retention, so they never drink, they produce highly diluted urine, and they have special cells lining their gills that extract salt from the water. As they migrate between fresh and salt waters, diadromous species have to undergo fundamental physiological changes in order to adapt to the new habitat.
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