The streamlined chub is an important food fish in the free-flowing portion of the Allegheny River. Jeff Knapp/Special to the Eagle

I’m always intrigued by what fish eat in their natural world.

Fly fishermen have been known to use special stomach pumps to extract and examine from trout what’s been recently ingested in the quest of duplicating the same.

Though I spend most of my time pursuing warmwater species, I’ve found observing the gullets of fish, as well as the deck of the boat, can be revealing. At times the forage fish can be so concentrated that you intentionally snag them.

Take, for instance, an early May trip on the Allegheny River.

From an anchored position near a mid-river island, my guests Ed and Thomas caught one smallmouth bass after another casting into a plume of fast water at the head of the riffle. The action lasted for about 30 minutes before dying down.

When it was finished I noticed regurgitated baitfish I later identified as darters on the deck of the boat. Historically, the spot produced a fish or two, a half dozen at most. But on this day we happened upon a feeding spree of bass corralling darters in the fast water.

The streamline chub is a popular forage fish on the free-flowing portion of the Allegheny. Locally, they are often called riffle runners or blue dots as they frequent faster flows and also feature a series of blue dots down their sides. Intolerant of pollution, the riffle runner is an indicator of good water quality. It’s also an excellent natural bait.

So when my client Al snagged one on his minnow-shaped jerkbait, he decided to add it to the rear treble hook of the lure. The next cast produced a nice smallmouth. Who is to say he wouldn’t have caught the fish without the added baitfish? But it was interesting.

It pays to take a peek down the mouth of a fish while you’re unhooking it.

It’s not uncommon to see the pinchers or antenna of a crayfish protruding from the gullet. Bass must turn the crustaceans around in their mouths and swallow them tail first as it’s always, in my experience, the head end that sticks out.

The same goes with larger baitfish such as gizzard shad and alewife. The forked tail of such can be seen poking out of the throat of bass, even crappies. It’s fascinating crappies that refuse a 2-inch bait but will bite a smaller one, are foraging on thick-bodied shad that stretch at least 4 inches in length.

Walleyes must process their food faster. Though I’ve been in many scenarios when they were feeding on shad or alewifes, I don’t recall seeing evidence other than what’s been spit up in a livewell.

Of the more interesting observed in the gullet was the tail of a mouse or similar rodent-like critter in a Clarion River smallmouth bass.

I’m certain subsurface bugs are at times included in the diets of warmwater species, though not to the extent of their coldwater (trout) brethren.

During an early spring trip last year on the lower Allegheny, one of the caught bass spewed out a giant stonefly nymph. Perhaps that had something to do with why the fish were responding so well to rabbit fur jigs that mimicked the bug in size, color and profile. Or maybe just a happy coincidence.

A lake’s surface can also be an indicator of the food below. During the late fall and early spring (just before and after ice out) baitfish die-offs can be revealed by floating food fish. This is quite common with shad and alewife. For many years I’d seen the occasional rainbow smelt on the surface of Keystone Lake, but not so the last couple, leading me to think the smelt numbers might have tanked.

Unlike trout, warmwater species tend to not be overly selective when feeding, especially so for smallmouth bass. I’ve caught many a smallie on a minnow-imitating lure that was tossing out crayfish parts on the way in.

So, rather than seeing all this as an attempt to match lures to the day’s food du jour, I find it absorbing the way fish make use of the bounty in their natural world.

Jeff Knapp is an outdoors columnist for the Butler Eagle