The more we study other animals the more we learn about their fascinating cognitive, emotional, and moral lives. Many animals display moral behavior (wild justice), mice, chickens, and other animals display empathy, ravens punish other ravens who steal food, pigs can be optimists or pessimists, and now we know that fish punish others who steal their food. It's been well-demonstrated that fish are sentient beings who feel pain (see also) and don't like winding up with a hook in their mouth, and now we've learned they show complex behavior that indicates they're well aware of when they're being hooked by another fish and don't like it.
Redouan Bshary and his colleagues studied punishment in a fish known as the bluestreak cleaner wrasse. These fish live in coral reefs and establish feeding stations where larger fish come to have their skin cleaned. The wrassses prefer the carbohydrate rich mucus that coats the skin of the fish who come in to be cleaned. It turns out that males don't like it when females come in and steal their food in the wild or in laboratory experiments. The males go berserk and females learn to stop stealing food. In the wild it's suggested that male cleaners punish females because after they bite the fish who come to be cleaned the fish swim away and the male loses food. The males make sure stealing will be less likely to occur in the future. The researchers suggest that the behavior of the wrasse may be a precursor to our more complex systems of punishment. If this seems like a flight of fancy consider that we also know that fish are able to make choices based on numbers.
Fish are amazing beings and the more we study them the more we learn about their well-developed cognitive and emotional capacities, and now it looks like they, like many other animals, don't like being harmed and also don't like being treated unfairly and will do something about it.