I'm guessing that the fuel prices in Tahiti are not yet as high as here if this is the most efficient way for hunting catching MahiMahi. This is an awesome diving video to check out.
Either way it takes mad skills to throw a spear from a moving boat at a moving target.
I've heard about this in my travels and what they do is ride around on the front of these purpose built boats and chase the Mahi down. The more tired they get as they follow the boat the better they can position themselves and then make a throw securing the fish.
Pretty awesome. Might have to try this sometime in my Salt Life spearfishing shirt.