Salmon release their nitrogenous wastes mostly as ammonia. Ammonia is a byproduct of protein catabolism; when proteins are converted to carbohydrates to provide energy, the amino group is removed. This amino group is then oxidized to form ammonia, which is highly toxic and also highly soluble in water. So in freshwater, the ammonia can simply diffuse out of the salmon. But in saltwater, the salmon are not as likely to waste water excreting nitrogen. They will often invest some energy to convert the ammonia into urea, which is less toxic and less dilute.Some of the nitrogenous wastes diffuse through the gills into the surrounding water. Others are removed by the kidneys, excretory organs that filter wastes from the blood. In freshwater, the salmon get rid of excess water as urine. So the kidneys function to prevent water loading as a result of osmosis. Freshwater fish absorb salts through their gills, since their bodies are low in salt.
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