Get co-operation from some boat owners and marina people and use one of their fish baskets and mount an appropriate sized coni-bear at the entry door. To avoid owls landing on the trigger or people close the door the otter is already opening it. If you think closing the door will stop him build a wire U over the entry for the same reasons.
Take a dead fish wire it to the bottom of a five gallon bucket, put a rock in it also to help stabilize it, put a 220 in the top and hang it with parachute chord or wire off the marina either by one at the top of the bucket causing it to hang at 45 degees or both ends causing it ti hang horizontal.
Make a wire cage 12 to 18 inches deep and the square of a 220 thru 280. You can use 1/2 by 1/2 rabbit wire or 1/2 by 1. It doesn't have to be real strong the otter will take the route of least resistance. Cut some slots in the top to mount the 220-280, zip tie a noogle around the top of the square and cut a slit for the springs to fit in. Just a slit because the springs will work, small fish can't slip out, and the noogle will make the coni very stable. Get the marina owner to give you some minnows or shiners put a dozen or so in the cage and install the trap. Float it off the marina. Attach the trap to the cage with a line long enough for the otter to sink out of site for a lot of reasons.
I gotta go I'll post some more marina options later. Live fish are best but dead fish might work if necessary.
