The easiest way to ensure your catch is fresh is to keep your catch alive. There are a whole host of reasons for doing this with possibly the most important being the prevention of contamination through spoiling via bacteria and other nasties. It simple, dead flesh requires the appropriate storage, particularly when it is destined for human consumption. This is something that just about all of us knows but rarely does. You see and angler on the wharf, rocks or beach with an ice laden esky in which to store captured and killed fish. Boat anglers are the exception here with even the smallest tinnies having appropriate ice filled storage or refrigeration with which they store their catch.
The solution really is quite simple. Purchase a Keeper Bag. They are very easy to store, transport and clean. When a fish is caught you can add it to your keeper bag and hang the bag back into the oxygen rich, and replenished, water in which it came from.