If you really think about it, all of the answers point out the fact that fish are opportunistic feeders. Even that very "selective" trout is taking advantage of that particular food item that is present in greatest abundance at the time. (Taking advantafge of the opportunity!) This minimizes the amount of energy expended. Their energy expense must be less than the return for them to survive. Some years ago I had the privelege to fish the salmon fly hatch on the Deschutes in Oregon. There was also a caddis fly hatch at teh same time. You could catch the "red sides" on a 2 1/2 inch Sofa Pillow or a #16 Elk Hair Caddis. It was a "name your poison" day.

Also, a fish is a fish is a fish, regardless of where it lives. 25 years ago I was responsible for a redfish (red drum) tagging project where we used conventional tackle to catch the fish. We used a lead-head jig with plasctic shrimptails of various colors. Some days they would hit anything you threw at them, including the bare hook and lead head only (no tail), other days they could be very color specific in their "taste". One never knew what to expect from them. (We caught, tagged and released some 4,000 redfish in a three year study period.)

I have observed bluegills being very selective on occassion, especially when the large sulfur mayfly is emerging. The bluegill schooled and hung at a very specific depth and fed voraciously on the nymphs as they were migrating to the surface.

Classic examples of saltwater fish feeding selectively is when stripers or bluefish concemtrate on a school of bait fish. However, their frenzy is so great that you can toss about anything in among them and get a hookup. The school as a whole was feeding selectively on the baitfish, but that individual you caught on a squid patter was feeding opportunistically.

(FIW, I am a retired marine fisheries biologist.)

Cheers,
Frank