Do fish sweat when they enter warmer waters?
Fish lack sweat glands because it wouldn’t do them any good. You need air to dry the sweat for removal of excess heat. Fish are cold-blooded, which means they will be the same temperature as their environment. Their physiology does nothing to regulate their body temperature. If they get too hot, they can simply move to deeper water where it is cooler or migrate someplace else. One anomaly is the tuna, in which some species have a special network of blood vessels that trap heat to protect their eyes, brain, and other internal organs when they dive to deeper colder water to feed. This physiology helps them to cool off slower so the temperature differential does not shock their systems.
