Another possibility I considered was to build a buoyant "cage" of large diameter PVC pipe.

Make the bottom third enclosed in netting. Use the largest opening size possible that will still contain the smallest fish you wish to include in your sample. (Large mesh to reduce water resistance as the trap is rising.)

Put a concrete anchor on the bottom with a stainless steel pulley. Run a rope to the dock. Release the rope while the fish are actively feeding, and capture the entire feeding ring plus all of the slow/surprised fish.