When I say rules, I mean finding out in advance what fishing areas are allowed, what marine reserves are allowed, what the mesh size of the fish is (the minimum size allowed) and when you can catch them.
When all these conditions are met, spearfishing allows you to take only the fish you need, you can select a single, good-sized specimen and leave the others to swim around quietly, preserving the resource. There is also a form of beauty in this approach which requires preparation, an understanding of the environment in which you dive and which also simply allows the fish to escape.
And it is precisely on this last point that I think the fishing industry is mistaken at the moment...