We use the following points system in our competitions, it mite work for you guys too but I dont know how many species of fish you have in your lakes, so use it dont use it, its just an outsiders 2cents.
Your fish has to weigh in at or above a certain weight, normally the legal take limit for each specie. So its different weight category for each specie allthough some have the same take limit.
Now the points system:
You are only allowed to weigh in one fish from each specie, and in areas with limited species a amount of weighers agreed upon before the start of the comp say 2 or 3. So you get one point for a specie and a point for say each 100gr your fish weighs more than the legal limit with a limit of 4 points per fish for reef fish and 6 points for game fish and a biggest fish of each specie bonus point.
Eg, you weigh in 3 different species one of wich is a game fish, your legal take limit on the 2 reef fish is 800gr and 900gr and the game fish is 1.2kg, so your 800gr reef fish weighs in at 980gr youll get 3points total for the fish, your 900gr fish weighs in at 1.2kg, you get 2points in total and your game fish weighs in at 3.6kg so you get full 6points, you now have 11points for the comp and as a extra your game fish was the biggest of em all so you have 12points with a bonus point for the biggest take of game fish specie.
Our club runs a comp once a month for a year with small prizes like shafts weights, floats and float lines. And have a total tally of all points at the end of the year where we use part of the entry fees for each comp to buy prizes, last year we had a speargun, some knives a spearfishing dvd and a few books on hunting. When we have the prize giving the guy in first gets his choice of the prizes and so it goes down the line till the last of the decided amount of prizes are taken, the others get something like a roll of fishing line and a hook or a pair of cheap lobster gloves or a bottle of energade or something arb like that