To shed some light on this topic, have you tried to drop snails into the aquarium with the RES to see how they respond when presented a choice of food items?
With other predators (juvenile bass etc) present with RES I wonder if small "dumb" fish will even wander into areas where the RES spend their forage (hunt) time. Are not RES more inclined to be found in the deeper water vs the shallow water 'haunts' of small fish?
Just a couple of comments from someone that knows little about fish.
When I dropped snails into a 10 gallon aquarium with 4 3" RES they would immediately attack them on the way down. If the snail was small enough, it would be consumed immediately. You could see the RES crunching up the shell. This was fairly early on when the RES were still becoming accustomed to pellets (at the time they would attack the pellets and spit them back out). The RES seem to be attracted to movement. They would strike anything that moved, but once it was stationary on the bottom, loose interest. For the snails that made it to the bottom (I would dump in quite a few at once) the snail movement would get the RES attention, then the RES would "stare" at the snail with the body of the fish being completely motionless except for fin movement to keep it in place. It would stare at its prey till it saw the snail move, then strike. No movement, no strike. I even saw one once before being pellet trained strike at a pellet sitting on the bottom because another fish fin movement had caused the pellet to move.
As far as FHM going into deeper water (5-6' at least) they do. As I was scuba diving I would lay motionless on the bottom above the thermocline. Not wearing a wet suit in the summer I could tell when a FHM school found me. Usually took about 30 seconds to one minute. No matter where I laid on the pond bottom, as long as it was above the thermocline, the FHM's would find me in short order. I could tell because I would have 25-50 fish nibbling all over whatever body parts were exposed (arms and legs mostly, it was summer) including my head (I keep short hair). They would be pecking all over me. Don't know if they thought they could get a chunk off of me or if salt on my skin attracted them or what, but they would be all over me, from shallow depth to the thermocline anywhere in the pond. This was with only BG, a few RES, and FHM in the pond. So I would have to assume FHM do range deeper in the pond than some might expect. I rarely see adult FHM's in the shallow in my pond unless I am feeding. Then the adults will come up for the feed. Mostly along the shore line I see the Juvenal FHM's in the very shallow water.
When I put crushed limestone rock around the bank down to a couple feet deep, it almost immediately grew a layer of algae. This attracted snails by the tens of thousands. In some areas, snails on the order of being an inch apart in the very shallow where the fresh growth is (one reason I got some more RES and stocked them last fall). These snails are in very shallow water. If the RES are going to eat them, they are going to have to come up from the depth. Just today I pulled a little fresh filamentous algae out around the edge of the pond where the ice had receded. Snails in the FA.
So I have to wonder, is there maybe a time of day or night that RES frequent the shallows to feed on snails?
My observations. Don't know that much about fish.
