Question

why do freshwater fishes excrete dilute urine and marine fish excrete salt concentrated urine
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Answer 1

Answer:

In Fresh water fishes:

Conc. of Environmental water < Conc. of the water in fish body fluids

So water moves into fish bodies by osmosis. Because of the large water intake, urine is dilute.

In Marine fishes:

Conc. of Environmental water > Conc. of the water in fish body fluids

So marine fishes tend to lose water to their surroundings because their bodies are less salty than the surrounding water.