I see that "Fish become the property of WP and will not be returned" and hadn't understood why. After pondering it a bit I assume it's so people won't try the same fish in future tournaments.

My suggestion is to allow players to keep the fish (if they so desire) after the tournament in the form of a trophy, made with a taxidermy kit. Maybe even have staff engrave something on it mentioning the tournament. That way it's fair for all, it doesn't jeopardize future tourneys, and it's cool deco.
P.S. Looking back I would have really liked to have kept that whopper I caught last year. I suppose that's how fish stories go though. hehe