I go on the principal of what a feller from the MCA told me when he came to do a seasearch course. He was asked what he thought about taking crabs and lobsters, and replied that he never took more than he was going to eat that day.
I think this is a sensible policy and is one I stick to. I also try not to take the ones that are too small, and indeed too big, as the huge ones are the ones that breed lots.
Given that I eat fish, I think that diver caught food is perhaps the most environmentally friendly way to go about it.
Obviously I don't take stuff from marine reserves, and if i'm diving with a buddy who's doesn't share my views, then I won't take stuff with them as I don't want to upset them.
But there's nothing worse then someone who takes stuff and doesn't eat it (or collects spidge and then it sits there rusting before being binned)
Janos