First, I agree with drbill that spearfishing is the most sustainable form for fishing. However, I disagree with the assertion that the bigger fish should be left to reproduce. I know it sounds shocking but let me explain.
The slot limits in my area (Florida) are intentionally sized so that fish outside of the slot (smaller fish) are of sexual age to reproduce. Therefore, there are hundreds of fish swimming around me that I cannot target that are going to breed.
Secondly, if I target 1 large fish, I have enough meat to take home and will leave the other 4 within my daily limit to live. That takes only one fish out of the breeding pool. So if I target smaller fish, I will kill all five within my limit and get less meat. I've effectively taken five breeders out of the breeding pool instead of one. There are also studies done showing that taking the alpha males helps make the fishery stronger by replacing with younger alpha males.
In fact, our recreational fisheries laws have become so restrictive for some species "Gag grouper" that all of the one's we see are well over the slot and 15lbs plus.
Please understand that I make these comments respectfully and I'm sure that we will agree to disagree.
lemke: I don't know why you are saying that shooting a big fish means that you are going to loose your gun? Remember the guy is scuba diving. If you get a proper shot you will paralyze or disable the fish no matter what the size. Unless it's a cobia they never die ha ha ha.
I believe that you "rig" dive. We dive ledges and wrecks in my area. If you shoot a fish on a lineshaft or reel you have nothing to get "wrapped" on, you just let the fish tire itself out. I've been on boats with 85lb AJ's, 55Lb Cobias brought onboard using linshafts, reels and even freeshafting. Also had friendly bullsharks taste test my fish before I could get him to my stringer. No guns were ever lost. Had some guns lost for other reasons but that's another story. ha ha
Here is some video of a buddy of mine getting owned by a cobia. This proves my original point about the safety of getting pulled up and down the water column. Likely a reason why Scuba guys are starting to use reels.
Epic Cobia Spearfishing Pandemonium - YouTube
(disclaimer - the music in the video is terrible)
Respectfully - Waterboy