Handling the catch in the correct way is one of the important parts of Sport fishing. First you have to dispatch your catch with a hit on the back of the head using a tool called a "Priest". This kind of tool should be in every angler's tacklebox.
Why bleed a fish?
The answer is simple; fish blood deteriorates very quickly & will taint the flesh.
A simple, clean and neat way to bleed a fish is: make a cut on the right side of the fish from the outermost arc of gills and continue the cut to the end near the mouth as shown in the picture. Hang the fish so that the head is lower than its tail which will promote blood flow. Compared this method with the classic way of cutting the throat and I think that you would agree that this method of "Bleeding" has two advantages; First - The fish will stay whole and keeping the fish in one complete piece makes it also a prospect to turn it into a trophy. Freeze the fish immediately and take it to the taxidermist. The second advantage is photographic: Rinse the fish and you have a stylish mannequin, whose gills are not hanging like a bloody whisk.