Having not only a minimum size, but also a maximum size on fish can make it very difficult for spearfisherman to stick within the rules. Judging fish lengths underwater gets easier with experience, but having to select snapper between 50-70cm makes life very difficult.
Diving in shark bay recently we came across a mega school of snapper in a wide range of sizes. Diving down onto the sand and waiting, built up the schools confidence and before long a range of fish would be circling above.
Swimming ahead of the school I came across two big fish sleeping on the weed. I dropped down silently above them, and lined up, getting closer and closer until I was within 50cm of the fish. While drifting down, I was trying to get a sense of the size of the fish, and the closer I got the bigger they looked. I would estimate the two fish were well over 80cm, around 7kg and well over the max size.
Swimming back to the school I picked a fish that I thought would be in the range and it measured just over the 50cm minimum size. Having to be so picky certainly adds a new dimension to diving and the pleasure of swimming with so many snapper is never dull!