The secret to preserving fish is to keep it alive or cold.  If the surface water is cool, a stringer or wire basket can keep some species alive.  Bring the fish aboard when moving the boat to a new spot.  Return them to the water as soon as possible.

Check your catch often, whether on a stringer or in a live well.  Transfer dead ones to an ice filled cooler immediately.  Dead fish left in the water spoil rapidly.

Large game fish should be killed immediately.  Use a stout stick and rap them across the back of the head.  Their flesh bruises easily if they flop around in the boat.  Field dress as soon as possible and place fish on ice.

Tips:  Wire baskets or net bags hung over the side of the boat help keep fish alive.  Aerated live wells keep bass, northern and other hardy fish alive.  Avoid placing fish in the sunshine or a non-porous wrapping material such as a plastic bag.  Fish spoil rapidly without air circulation.  Crushed ice chills faster than a solid block.  Drain the cooler often so the catch does not soak in water.  Burlap bags, newspaper, moss or materials that "breathe" help preserve fish when ice is not available.  Keep the covering moist as evaporation helps cool fish.