I can say, success rates of RES preying on FHM in an aquarium setting would be substantially higher than in a wild environment such as a pond.
I agree CJ, there won't be near as many FHM available in a pond setting to go after. Hopefully mine will convert to eating YOY GSH and other types of YOY when I turn them loose in a few months.
Over the last few months my aquairum RES have become quite good at eating FHMs in the first 10 minutes after they are released in tank. The RES don't chase FHM around the tank when the light is on but simply wait until one swims close enough to grab. The bulk of the FHM get eaten after the lights go out for the night. I have seen a single RES eat two FHM 30 seconds apart just sitting in the same spot right after the FHM are released. I suspect that that RES mostly graze on small things during the day and become more predatory after dark.
They do seem to express more agressive behavior towards each other when when I feed them nightcrawlers as opposed to FHM or bloodworms.
