Ask and ye shall recieve... these are just a quick search on Spearboard...
http://www.spearboard.com/showpost.php?p=27770&postcount=4
http://www.spearboard.com/showpost.php?p=27904&postcount=3
http://www.spearboard.com/showpost.php?p=16538&postcount=1
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There are sharks, and then there are Bull sharks.
I characterize Bull sharks as "untrustworthy," "erratic," "unpredictable."
They aren't particularly scared of bubbles - or anything else, for that matter, except maybe porpoises/dolphins.
They are as liable to come check you out as they are to ignore you. If you move towards one he may flee... or he may make a brief charge, or he may not respond at all.
While they tend to hug the bottom, they will come up to "see" you - and there's no timidity in 'em when they do.
The "typical" Bull shark encounter, if there is one, is a sudden unexpected dash straight at you, veering off at an uncomfortably close range, followed by a brief period of hanging out at the edge of visibility, appearing and disappearing in such a way you may swear there are three or four sharks when there's only one. Probably not dangerous but it can be very scary.
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In answer to PF's hypothetical... first, in over 30 years diving in the Bull's back yard (the northern Gulf), the most I've ever seen together is
four, and they were actively hunting a big school of redfish & jack, so I think the chance of encountering a "school" of Bulls is pretty remote.
If I did, however, I would commence a slow non-threatening ascent and a below the surface swim back to the boat.
Rick