Moldy Chum talks about the rather horrific practice of spear fishing for Tarpon and links over to someone spearing a large tarpon using scuba...
You should go read it and check out the video but I warn you it is ugly and very depressing.
Personally, I feel the only sporting way to spearfish is free diving and that killing for the simple act of killing something is just plain wrong... there is simply no reason whatsoever to spear a tarpon.
In the old bad days of spearfising where slaughter was the order of the day, scuba-equipped divers using spearguns were even then considered to be less than sportsmen, losers, and as my dad used to say "Bottom feeders might as well fish with a hand grenade". He also said a lot of other things about guys like this but as it involved cutting certain body parts off, we'll refrain from going into detail.
I spearfish because it gives much more choice in harvesting, involves no by-catch, no catch & release mortality, and all things considered, it is the most sustainable method of catching a fish to eat.
What these Helldiver folks are doing is simply not right and about as sporting as shooting a tethered deer from six feet away.