Slime coat

The coat is an active immune barrier, not just lubrication. Strip it and you leave open ground for bacteria and fungus. A fish that swam away strongly can still die days later from an infection that started as a dry handprint.

Everything abrasive removes it: dry hands, dry gloves, knotted nylon mesh, a boat deck, grass on a bank. Wet hands, wet gloves and rubber netting leave it largely intact.

This is the single most useful thing to know about handling fish you intend to release, and it costs nothing to act on.

You can see where it has been removed. A fish that has been handled with dry hands shows dull patches where the sheen has gone, and those patches are where fungus takes hold. Wet everything that will touch the fish, including the mat or the deck you lay it on.