A handy trick I learned from saltwater fishing: Dry the filets with paper towels -- bacteria prefer a wet medium for growth. Place dry filets in plastic bags, then put in cooler with ice, some water, and road salt. This will keep filets super cold & dry. You can even put them (still in sealed bag) in fridge in a bowl with salt, ice when you get home.

Meat stays good for a LONG time without being frozen. I've eaten fish stored like this nine days after catching and they tasted just about as fresh as the day they were caught.

If freezing, Alaska Dept of Wildlife found that best was to put filet in a plastic bag with a small amount of water. This will freeze rapidly & hopefully coat fish overnight. Next morning, put in more water to form second layer of ice. A few drops of lemon juice can be added to the water if desired, but don't overdo it.