I think Pirtech did a comp like this a while ago. It went something like this, Entry was online, a date was set, photo's of fish were sent in after the days fishing including a pic of the fish being weighed or measured, then there were mystery weights/lengths for each species and prizes to suit.
I thought the best thing is the mystery size, it makes it more achievable for the average punter. eg if it was say biggest fish, then the chances of catching a 50kg plus fish in Aus are a lot lower than say New Cal and would probably reduce entries.
Great idea I recon.