I once watched a TV program here in Oz by a leading Sydney Fish Resteurantuer Expert.

He had studied in Japan. He had the fish caught specially for him by a local fisherman. The technique was to catch them individually on a handline, then as soon as they were on deck a spike or knife was thrust into the brain before they could trash about, killing them instantly.

This stops the muscles generating a lot of lactic acid, which turns the flesh (for normal 'white flesh fish') a pale white colour.

If done correctly, Sashimi Grade fish will keep on ice for about 10 days, and the flesh will stay totally translucent. (If kept on ice, it will not 'go off' in that time.)

Fellow Mudcat Fishermen might like to try that 'instant death' technique too - even if you do plan to cook the fish.