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3. Stomach Contents, Eclectic Feeding, and "Selectivity"

Killed a small trout yesterday for the table -this morning's breakfast in fact. Below is what was dissected from the stomach. It came from a high gradient stretch of mountain stream at about 6500ft elevation.

Stomach contents tell me more than anything else about how trout are actually spending their time. I try to pump at least one or two fish every outing and carry tubes of different sizes for different size trout. But nothing beats actual dissection for the full story. Since I frequent small unsung streams mostly there is little danger in taking some fish for the table.

What always strikes me upon viewing trout stomachs is how good trout are at discerning food from flotsam. Beyond the occasional oddball tidbits, trout stomachs tend to contain almost entirely food items many of which are truly minute and seemingly well camouflaged. Trout are good at identifying “food” from “not food”.

This 8" female brown was thin following winter as snow had recently left its stretch. The water is low due to lack of precipitation but the high gradient still offers good current speed for drift feeding. The stomach contents show that she had been feeding eclectically. It is likely that the low water has consolidated living space and competition amongst inverts which may have put some into the drift. It is also likely that this fish was gleaning larvae from the substrate -possibly evidenced by both cased caddis and esp, several Heptageniids. The fish took a #20 Baetis nymph fished off a #16 Baetis dry.


This little brown had a rather large bait hook fully in its stomach. The hook was well in process of dissolving, which can happen, if the fish doesn’t die from hook wound trauma first.


The Brachycentrus cases all had larvae within, meaning the trout chose live critters versus the empty cases one can often find on rock tops along with occupied ones. Either the trout took them from the drift, or gleaned them after seeing the movement of the feeding larvae. These may be occidentalis (spring emergence) or americanus (summer emergence). They had not begun pupating yet, which I’d expect is underway or imminent in occidentalis by this date. Other suggestions?


The interesting thing about these midge pupae is that they were the only critters that floated in the dish of water I dissected into. I wonder if this buoyancy is involved in getting those pupae to the surface as has been suggested for caddis. I can’t imagine how such tiny pupae could get to the surface in such cold turbulent water otherwise.

I hear my tying bench calling ...

Paul -


Another possibility is that they were taken while repelling to a different location on their silken threads. On smaller streams I've had good luck letting a cased imitation simply swing slowly into and dangle in shallow riffles.

I like your fly! I think I posted a similar style on another thread. Dead drifting head down is what the naturals do and your creation mimics that perfectly. For the dangling technique though, I prefer the head at the other end as that is where the silk thread it dangles from emanates.

Thanks, Paul. Excellent job.

Very true! This is the subject of one of my dissertation chapters, although I'm studying it in juvenile Chinook salmon instead of adult trout. The fish I'm studying have similarly clean stomach samples, but and what's even more surprising is that they catch inedible debris in their mouths all the time -- roughly 5-10 times more often than real food. They just spit it all out.

They're using taste/touch as a critical part of that excellent filter they have to decide what is and isn't food. It does make one feel a little less self-satisfied after catching a nice trout. You might not have fooled it into thinking your fly was insect, but instead into thinking your fly was maybe an insect, just similar enough to be worth a taste (like much of the other random debris in the river).

Really good point Jason, and neat that you can put some numbers to it. My tactic of switching flies before leaving a good run, sometimes several times, takes advantage of trout's efforts at sampling the drift. There are usually a lot more fish to be caught in a given pool than one fly will show.

Jason & Paul,

You're right. That's why the old time pro bait fishermen (most of them market fisherman that supplied restaurants) were real vacuum cleaners. They could read the water and understood drifting like modern day nymphers - only the fish didn't let go! I often muse over the damage we could do with Spring night crawlers and Summer crickets!:) My dad used to love fishing big stoneflies impaled on a hook, especially the adults in big pools below a riffle...:)

Orn - looking forward to the forensics. :)

Great posts and pictures Paul, some really interesting stuff.

I've watched trout feed a fair amount, and have shot some video too. One clip I liked to show in my classes showed a drift-feeding brown eye-balling and rejecting items –mostly pieces of grass leaves, a pine needle, and a catkin. So they don't need to taste test everything. That fish was feeding eclectically (very common) and it took food items from the bottom to the surface film from its 2.5ft deep foraging site. At one point it spied something in the substrate a good 3 feet ahead, shot forward (those are the takes that really pop your indicator), watched, then dropped back to its hold without a capture. Then it shot forward again, gleaned from a rock crevice, and turned back toward the camera with its white mouth flashing as it handled the item –a larva of some sort. That fish had ID’d the item as food from a distance –probably saw movement –and needed two attempts to make the capture. On a more productive stream, or time period, the fish may have been more myopic.

Eclectic feeding, the most common scope most of the time on most waters I think, goes a long way in describing why there are so many methods, techniques and variations of each, as well as fly patterns that can potentially work on a given day. At the same time, the act of fishing to these fish, (esp with sloppy technique), goes a good part of the way in defining “selectivity”. It’s not necessarily what a given trout will accept as “food”, but what it will reject as “not food”. Certain techniques are hindered not by the trout’s willingness but in the fact that certain conditions (esp sky, water, and angling pressure) limit what you can get away with.

Fun game; A multi-tiered chess match you can walk right into. My tactics are layered with presentation first, then fly pattern. I spend time observing, watching for an “in” –which can be an approach, a presentation method, technique, a correct read on, or change in, conditions, or a specific food item trout recognize as falling into the “FOOD!” category. Collecting stomach contents is an important observational tool, a direct look at what’s potentially important to the trout.

Jason, is their much of a difference in handling time for food versus non-food items?

My research shows the same thing. For my fish, less than 10% of capture attempts lead to ingestion. The rest are split roughly 50/50 between things they pursued and rejected, and things they captured and spit out.



I haven't quantified that yet (I will) but my best guess is that there's no significant difference.

Thanks, Jason. Doesn't sound terribly "energetically efficient". I wonder how more mature fish would compare?

Myopia:
I ran into something yesterday that LOOKED like “selectivity”. But it wasn’t really, just “myopia”. A flat pool on the same small stream as the fish above had trout rising and leaping all over it. Midging they were, taking pupae. The activity looks like classic caddis pupa activity, with boils, surges, and leapers. But if you look close you’ll see noses, wakes, and bulges too. You might think that it’s actual surface feeding and pretty eclectic at that. But just throw a small dry fly to them and you’ll be snubbed. The old “angler’s curse”, or in my case the “old angler’s” curse, as I was forced to re-rig some in fading light with these old failing eyes of mine. Adding a midge pupa pattern off the tail of the dry and I was in business.

I'm thinking that this is not selectivity (fish making “choices” exactly), but “myopia”, the fish focused below the surface film and ignoring what’s above it. It is likely that pupae are easier to catch than adults and the fish focus there. Sometimes I run into fish that focus on adults –egg layers and spent adults– and then too big a dry can still catch some fish. I suppose the true test would have been to run a larger generic nymph off the dropper to see how willing they were to take that. Wish I'd done it.

Yesterday I started with the cased Brachycentrus in the image above in pockets and the trout were happy with it, then switched to the midges in the flat pool at dusk.

I think the debris effect grows less important as fish get larger, because their typical food is larger, and there's less debris of the same size as their prey. I haven't tested this but it makes clear sense.

It doesn't become completely unimportant, though. In one of the clearest streams in the world in New Zealand, with almost no debris, researchers found discrepancies between adult brown trout foraging attempt rate and a foraging model's predictions, which could be explained by a "debris effect" strictly from the exuviae of their main insect prey. And you mentioned another case with pine needles and such.



I think it's likely that everything we call "selective feeding" in fly fishing could actually be better termed "myopic feeding" for the reasons you described. Lloyd makes this point in his book, too. I don't think trout ever decide that they want to ignore things they know to be food. Instead, when one type of food is really abundant, they feed most efficiently by developing that myopia and ignoring the possibility that anything else might be food. That way, they don't waste time with debris.

I was thinking myopia of the eyes only, and not brain processing -"decisions". I was thinking they weren't "rejecting" my dries, but not seeing them bc they were focused on a different environment -subsurface and not above the film. But debris filtering certainly is a matter of decision making. Interesting.

I did get some interest on a #18 Usual but only when it sunk deep into the film. But those 4 responses (I saw) were two half-hearted dimples (sample and rejections), and two that I pricked -felt a little weight, but failed to hook.

Sounds like there was some decision making going on, and not simple "myopia", even in these "unsophisticated" small-stream fish. Despite having seen many situations where fish were clearly onto certain "search images", I seem to only begrudgingly give fish credit where its due. Gee, maybe its offensive in some way to have been "out-thunk" by a fish lol. I sure am quick to credit myself when I've shown some bit of erudtiion.

Water type has a lot to do with this topic. Eclectic foraging from the benthic drift is largely a freestone phenomenon that becomes less prevalent as the water gets richer. Spring Creek fish get extremely discriminating and usually waste little effort on sampling. During non-hatch periods, their sampling would be more accurately described as grazing. On many I fish, the trout will either be stuffed to the gills with what's hatching (usually with a ratio of 5 to 1 or more nymphs to adults) or mostly plant matter mixed with sails, scuds and the odd midge or mayfly larvae thrown in. Sometimes a "salad" fly is very effective.

BTW - Ever observe trout "sample" the same object multiple times? It's funny to watch.:) Perhaps attempting to "taste" for a larvae inside?

BTW, Paul that is so cool. Thanks for posting. I haven't killed a trout in a few years but I use to be amazed to see what they have been eating.

I have sat on our dock with a glass of adult beverage and enjoyed watching the bluegills sample everything you throw in the water. They always seem much bolder when the small object is sinking then when they have to go to the surface to sample it. Even bluegills can take in an object, sample it and spit it out very quickly.

Often when my fly is drifting and I feel a little nick I surmise it's a trout that just sampled my fly and I was too slow. I often wonder in the dark, iron colored freestone streams how often my fly is being sampled and I have no clue it was happening.

Trout ARE sampling our offerings, and our detection rate is probably lower than we'd like. I'm reading George Daniels "Dynamic Nymphing" now and all those techniques are in large part about detection.

From what I've seen, once a trout has sampled our fly (or the way it's presented) it can get placed into the "not food" category for a time.

Speaking of bluegills, here's a cool thing: Fish (and other critters) have visual receptors that fire upon having certain (important) objects or motions come into the visual field. Bass (and possibly bluegills) have receptors that fire on objects falling through the visual field, which probably indicates to the fish vulnerable prey. Trout are known to have visual receptors tuned to fire at objects enlarging, or moving towards them, as in drift feeding.


Yes, good point worth clarifying... eclectic feeding is likely most prominent in relatively unproductive freestoners.

I guess putting it all together trout discriminate ("select") both by filtering out "non-food" and recognizing "search images" (along with taste) of "food".

Great pictures Orn!

Neat pictures Orn.

I wonder if the trout full of sticklebacks didn't capture them at a different site and then moved into that area. Only bc the stickles and capelin are in a state of somewhat advanced digestion. Whereas the amphipods look fresher. I wonder if that piscivorous trout would switch to amphipods upon entering the new site? Or does it simply live differently than the amphipod-eater. I've read that coastal (semi-anadromous Cutthroat trout) are known to have different feeding habits depending, in part, on how far out to sea they venture.

You are probably right, but the pool I caught them in is full of sticklebacks so I think it's probably the opposite with the first trout just entering the pool after eating amphipods in the saltwater.

The pool though is in a tidal area so you have saltwater and amphipods coming in with the tide.

Ah. Interesting. Fun to know what's going on.

Paul...Wonderful thoughts from you as usual. Had an odd "modern" encounter earlier this afternoon...I have been out of the loop (fly-fishing & hockey) since I was down near Tampa with my wife closing out her mother's estate...I was sitting at the Tampa airport waiting for my flight to Detroit...I logged on with my new iphone and got to read this interesting thread...You started it from Colorado...Jason added his two cents from Alaska. Kurt from California and Orn from I don't know where...All being read by someone sitting in the Tampa airport...I truely enjoyed the read but I couldn't stop shaking my head about the technology that lets us join in together and share some ideas! :)

Thanks!

Spence