I'm not surprised, the RES I had in my aquarium a few years ago loved eating FHM although 2-1/2" was about the upper limit of what they would eat. They would try to eat longer ones but would spit them out if they couldn't get their mouth closed. The shorter ones they would suck in head first, close thier mouth, then pop thier jaw two or three times crushing the FHM as they swallowed it. I'd see cloulds of small fish scales coming out of thier gill plates every time they popped thier jaw, it was fun to watch. I have a suspicion that small fish are a regular part of thier diet especially after the sun goes down as most of the FHM I fed them disappeared overnight after the lights went out.
I had found a post on this forum once where a guy was trying to control common carp reproduction. At the time he stated he did so by stocking RES. I tried sending him PMs but never heard back to hear how it went. I thought it was an interesting concept.
I always wondered if my RE were somewhat abnormal or just hungry. Glad to hear that you have seen this behavior. I frequently catch RE on minnow or wooly bugger patterns as well as the more expected nymph patterns. Maybe some of the contributors that are trying to catch the RE and not succeeding would have better luck with flies or even spinning tackle using this info.
RES do like eating fish fry. This last winter I passively feed trained 30 small RES in my aquarium and they ate every single albino bristlenose pleco that hatched in the tank. I have one male pleco and four females, the poor male spent 5 solid months tending eggs and fry only to see them disappear once they left his spawning cavity. About every 5 or 6 weeks there would be a new batch of fry and I could always tell when they had hatched as there would be one or two RES intensely focused on the spawning cavity waiting for fry to swim out. The RES would aggressively chase other RES away from the cavity while waiting for fry to emerge. This aggressive defending of a "feeding spot" would go on for about 5 days until all of the fry were gone. Normally I see very little aggressive behavior with the RES in my aquarium, but when I do see it, it's very short lived, the only exception I saw to this was when fry were present.
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