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I am not an expert or DIVE instructor so please only take this comment for what it is; a guy on a message board sharing his experiences...
I have been spearing on scuba for about 15 years now and find it a lot of fun, and challenging. While spearing on scuba; yes, you do have the advantage that the air gives you, however as you found out quickly, that advantage also becomes a disadvantage in that the noise can scare off your quarry or make it very skittish; resulting in you moving up and down in the water column hunting the fish. While this does not lend it self to a normal dive plan, it does make it essential to having a great dive computer to keep up with the dive plan and following it to the letter. Should your dive computer fail during the dive, then the thing I do is always end my dives for the day. There is no way for a backup computer to know what your previous dives have been during the day unless it has been with you on every dive.
The hardest thing when spearing on scuba is to maintain depth while fighting any fish, especially an 80# AJ. If possible find a reference point and keep a close eye on it. Also I have found it helpful that when fighting a big AJ is to rip out the gills. This will dispatch the fish much faster than most methods.
Good luck in your spearing!
Regards,
HD_0027