I wonder if you could use some type of large net? Maybe something a 100' long or so that could be pulled by a 4WD pickup or tractor. This is probably a hair-brained idea, but it sounds like something that might work.
If the pond is shallow (<6ft deep), roughly circular or rectangular, and has no brush, stumps, logs, cover etc to hang the net on then a big seine net can effectively empty a pond if you keep the lead line on the bottom and don't loose the catch when you start pulling the net out of the water. You'll need trucks/tractors for big nets. Probably a good YouTube video on harvesting aquaculture ponds that can show you how to do it. Usually have to drag the net a few times to get everything.