Gene,
Over what period of time do you think the bass have been accumulating under the dock? Are you close the a river or large stream?
Alligator snapping turtle would be my guess. See the link below for the Missouri info. They eat fish and have been known to lure them using their tongue as bait. Less likely a standard snapping turtle since they tend to be more of a scavenger as opposed to predator.
If it is an Alligator snapper remember they are protected in MO.
I would suggest you try and contact the State Herpetologist Tom Johnson The only number was for the headquarters of the Conservation Department at 573/751-4115.
http://mdc.mo.gov/nathis/herpetol/turtles/ Here is an example of a middle weight at 70lbs I would not want to be in the water with it
http://www.bigwoods.net/images/sidalturtle.jpg Good luck with your problem.
JW.
