Freshwater fish have a reputation for urinating profusely a dilute urine, whether handled or not. The high urine volume is needed to compensate the continuous leaking of external freshwater into fish, especially across membranes of gills.

Do fish pee when caught?

Like you, fish have kidneys. Kidneys help the body make urine. The shape and size of kidneys can be different depending on the species. ... A lot of fish get rid of the pee through an tiny opening, called a pore, that's near their rear ends—and in some fish, waste also goes out through the skin or the gills.

Why do fish urinate?

Freshwater fish will passively intake water from their environment and then, as their insides are saltier than their surroundings, will excrete a diluted urine. Saltwater fish have to drink water more actively and, as their surroundings are saltier than their insides, will expel a more concentrated urine.