How do fish wee?
Urine
is liquid comprising water and soluble nitrogenous waste products. It is produced in the kidneys of vertebrate animals, and fish are no exception.
Unlike vertebrates on land, fish can also excrete unwanted compounds via the ultra-fine membranes of their gills – salts and nitrogen-containing ammonia are expelled this way, by simple diffusion into the passing water. For freshwater fish, it’s easy to flush their kidneys with large quantities of water and they wee copiously, but marine fish are limited to water acquired from their food and a limited amount gained at considerable metabolic cost by drinking sea water and excreting the excess salt. Body water is thus a precious resource for fish in the sea, and their urine is highly concentrated. This means that a far greater proportion of healthy lake or river water is likely to be fish wee, compared to sea water!
Amy-Jane Beer