Rocking motions and rubbing of lips which are signs of pain in most species
Fish have social intelligence, the ability to manipulate, punishment, reconciliation, cooperation, tradition, recognise individuals, monitor social prestige, track relationships, use tools, build nests, and have long term memories
Fish can definitely feel pain. Examples are heart rate, breathing, adrenalin, gasping, struggling, writhing, endeavouring to escape which shows intelligence and will to survive. Brain chemicals analogous to ours are released (Endorphins and Enkephalines) when caught.