Never pull your line up immediately after losing a fish. Wait to see if it will take the second bait. A gaff is useful to land larger fish. Gaff the fish through the lower jaw and gently pull it up through the hole. Releasing fish - keep the fish out of water for as little time as possible and handle the fish with moist hands.
Fish produce a protective mucous covering which acts as a line of defence against infection.
Handling the fish with moist hands will decrease the damage to the protective covering. Fish which are caught in deep water must be brought up slowly if they are to survive. This will allow the fish to adjust its physiology to drastic pressure changes which occur with changing water depth. Hooks which can't be removed without doing structural damage to the fish are better left in the fish. The fish has a better chance of survival this way.