The place to start is in killing your fish humanely. Too many times people will throw their catch into a bucket of water or tub of some sorts until the fish slowly but surely drowns.
This is common practice in so many places, but apart from being inhumane, this will leave your fish in a stressed state which causes stiffness and makes the flesh deteriorate much quicker. In simple terms, by the time you are dining, the quality of the flesh won't be all that it could be.
How do we lengthen the shelf life of our recently caught fish?
If you go to any decent fishmonger you'll notice that their fish stocks are flexible and have a certain limpness to them. This is because their boat people have treated the fish correctly at the point of capture.
You can do this too. You need to brain spike the fish which can sound disturbing to begin with, but it is much more humane and beneficial in the long run. Hold the fish whilst taking a sharp implement and pushing it quickly through the fishes head to pierce its brain.
It can take practice, but the signs that you've done it correctly will be shown by the fish flaring its gills and flapping its tail one or two times before it rests for good. Scientists tell us that certain enzymes are produced when fish are killed quickly like this which delays rigormortis kicking in and this is what lengthens the shelf life.
Now storing the recently deceased fish in a bucket of its natural water might seem like the logical thing to do, but this is the worst thing you can do. The fish need to be immediately stored on ice or in an ice slurry of sorts.
An ice slurry is simply achieved by placing a bag of ice into a cooler mixed with some saltwater. This creates a brine and will store your fish at 0C (32F) bringing the core temperature of the fish down very quickly.
We are essentially slowing down the bacteria growth rate and hence making our fish shelf life much longer.
Fish can be stored on ice now for up to 8 high quality days whilst we don't break the ice cycle we have started.