The irony in this discussion is that in my experience very few anglers show much interest in, let alone knowledge of , what fish actually eat - other than our baits. In my area , I can think of perhaps 4 or 5 miles of river where enough free offerings are thrown in perhaps for fish to treat them as part of their diet , at least in season. But that leaves hundreds of miles of lightly fished , and purely fly fished water.
As part of the Riverfly Partnership I've been monitoring invertebrates for about 15 years and that has offered some insight into what food sources there are. The lessons I've learned are that in any one spot (we monitor exactly the same location ) there are huge variations in numbers of bugs (upwinged flies like mayflies and olives, caddises, shrimps , stoneflies, chironomids, leeches etc ) according to flow and oxygenation , and season. I've had 1000+ in good conditions, and zero in bad ones, from a 3 minute kick sample. So do the fish move, go hungry, eat something else ? All three I suspect. It's harder to monitor bugs on deep coarse fishing rivers but it's easy enough to spot what's hatching if you pay attention . And sadly , very few coarse anglers do . That isn't a patronising comment from a posh dry fly angler as I am an all rounder , but it is rare that much interest is shown.
I remember John Bailey doing very well with tench on caddises, bloodworm and jokers were once essential for winter canal matches ,I've done well with chub on slugs and crayfish (when legal) , and even had some trout on live mayfly but like most of us, most bait comes from the tackleshop and Tescos