For the record, I'm talking about spearing adult fish intended for the dinnertable, not "shorts" or killing for sport. How is spearing different than line fishing from a boat, btw? The advantage if spearing is that you know the type and size of the fish *before* you hook it and haul it in.
And bywarren, I think you're right that commercial fishing, especially without regulation or enforcement, is the the real hazard to the marine ecosystem. Spearfishers don't kill 10 fish to get one, nor do they hunt undersized fish or create "bycatch". The good news is not "everyone" can or will come down with a spear because very few folks can freedive at all. The long and short of it is that recreational fishing, above and below water, is considerably lower-pressure and more sustainable than any commercial fishing.