Medetomidine or a salt thereof for use in controlling parasitic crustaceans, such as sea lice, on fish, e.g. salmon. A method of improving water flow into and out of a cage or net for fish farming, by providing said cage or net with a surface coating containing medetomidine or a salt thereof in an amount effective to reduce biofouling of said cage or net. The coating is capable of releasing medetomidine or the salt thereof into the water in the cage or net in an amount effective to reduce or prevent parasitic infestation of the fish in the cage or net.