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Now I See You: How Trout Appetite Shifts from Hatch to Hatch

How a fisherman-scientist's puzzle on the Missouri River led to a laboratory study revealing the secrets of trout prey selection.

Now I See You: How Trout Appetite Shifts from Hatch to Hatch
Fly fishermen seem to enjoy the sport because it is a nice mix of mystery and science. (Jason Jagger photo)

It was at the crack of dawn one morning in late July and I (JS) was standing waist-deep in a cove within the famed Missouri River below Holter Dam. Why? I was anticipating an aerial blizzard of tiny female Trico (Tricorythodes spp.) spinners briefly followed by the spent insects, wings outstretched, littering the water’s surface and drifting towards me and the rainbow and brown trout sharing this cove with me. While waiting for the drama to unfold several PMDs (Pale Morning Duns, Ephemerella excrucians), some of which were attacked by trout, floated by. I chose to ignore this minor distraction. My 9-foot, 5-weight rod, rigged with floating line and a 12-foot 6X leader ending in a #24 poly-wing black Trico spinner was at the ready for the massive flotilla of naturals to drift by….and it did. The trout keeping me company were now eagerly gulping at the surface and inhaling these tiny morsels. Those of you who have had this or a similar experience know that spotting your tiny imitation amongst a dense cluster of naturals at 20 or more feet is a challenge requiring intense concentration, and more than a smattering of faith. I was aided somewhat by the glint of the early-morning sun off the reflective wing of my fly, and I managed to hook and land five 16- to 19-inch fish. For me catching big fish on tiny flies floating amongst a crowd of naturals is the pinnacle of trout fishing.

On a high, I knew that I had to return to the Big Mo and I did, the following year. This time it was mid-July, a week earlier than the previous year. July is a very popular month for dry-fly fishing and I was hoping for the same experience as the prior year. So I woke up very early, put on my gear and high-tailed it to “my” cove before another angler could usurp it. I eased off the bank into the cold water and shuffled along the silty bottom to the exact same location as before…and waited. There were clearly more PMDs on the water than I recalled from last year, many of which were eaten by trout. I was rigged and at the ready for an anticipated Trico spinner fall. True to form, it happened, although Trico abundance was lower than what I had previously experienced. To my great surprise these spinners drifted by the resident fish eliciting little response and, to my even greater dismay, my Trico imitation was ignored.

Artwork of the nymph, dun, and spinner of the pale morning dun mayfly.
The three stages of the Pale Morning Dun mayfly. (Dun & Dun art)

The spinner fall ended by late morning and I left the river defeated, but more-so puzzled. Why were these Trico spinners inhaled by trout last year, but snubbed this year? On the walk to my car I first stopped at a local fly shop in the trout town of Craig to pick up a few items. While checking out I shared my experience on the river across the two years with the proprietor, a guide. Why the great interest in Tricos by trout in late-July of last year and practically no interest in this same abundant food this year (mid-July)? He responded matter-of-factly but with great certainty, “The trout just haven’t yet seen enough Trico spinners to turn onto them.” He shared this observation as being common knowledge, applying not just to Big Mo trout but to trout everywhere. I thought about it and on the surface it made sense, but wondered–was there solid scientific support for this anecdotal evidence.

Artwork of the nymph, dun, and spinner stages of the trico mayfly.
The three stages of the Trico mayfly. (Dun & Dun art)

Turning Experience into Science

In my many years of experience, fly fishermen enjoy the sport because it is a nice mix of mystery and science. If all is explained by science where fish, bugs, and environment are predictable, the sport becomes boring. All mystery or luck and the experience is fruitless and frustrating. We stay engaged by learning through our own experiences, from talking to other anglers, and by experimenting while on the water. But how much of what we learn is largely anecdotal, not backed by scientific rigor?

Being formally trained in animal behavioral ecology, my conversation with the guide raised a question that tickled at me–both as a fisherman and as a scientist–and I enjoy tackling just this sort of question. In other words, developing a hypothesis based on field observations and rigorously testing to determine if it can be supported. In this case does greater experience by trout with a novel food source (e.g., Tricos) result in its acceptance? In other words, is it true that the more a trout sees it, the more likely it is to go for it?

The next day I returned to the Missouri River, not to fish, but to observe mayflies and trout while considering how such a study might be conducted. Ideally one might perform this study on this same river with trout and mayflies, but there are many confounding factors, such as interacting trout, inconsistently appearing trout and mayflies, and changing environmental conditions, that make a solid replicated study of this nature just not possible.

Upon returning to North Carolina, I discussed this research challenge with my colleague Brad (BT). Brad, an aquatic ecologist who at that time taught a graduate Aquatic Entomology class with me, agreed that such an on-stream study was not possible. So we centered our attention on developing a laboratory study using carefully delivered prey to individual trout thus avoiding the on-stream “noise." (EDITOR’S NOTE: If you are interested in the full scientific publication, visit https://doi.org/10.1098/rsos.241042)

A fly angler wading knee deep in a broad river with her dog; desert cliffs in the background.
Dry-fly fishing the Missouri River around Craig, Montana is one of fly fishing's greatest joys. (Josh Bergan photo)

Experimental Model–How to Get Answers

First, individual 13- to 16-inch brown trout were caught by barbless fly from the South Holston River in eastern Tennessee and transported to our lab in aerated coolers with a block of frozen river water. The fish were then housed in 7-foot-long circulating refrigerated stream tanks–private albeit unfamiliar accommodations. It was critical that they eat to help overcome the relocation stress of stream to aquarium. Trout naturally encounter aquatic annelids as well as terrestrial earthworms washed into the river. Fortunately, our lab trout accepted earthworms within a couple of days. But how to do our planned experiments with smaller floating prey? We would ideally mimic the Missouri River smorgasbord with PMD duns and Trico spinners. However, this was not possible. These tiny fragile insects could neither be collected, nor reared, in the numbers required (greater than 500,000) over many months. It would appear that we had encountered a formidable roadblock in testing our hypothesis of trout prey switching. We needed large numbers of an insect prey that both floats and that our trout would eat. Enter Blattella germanica. You know them as German cockroaches (see video below).

My main research focus is behavioral resistance and nutritional ecology of cockroaches, specifically German cockroaches: those pesky critters that thrive in homes, apartments and many of your favorite restaurants, sometimes reaching alarming numbers. We rear and maintain tens of thousands of German cockroaches for our studies. Why not use these same insects in different sizes and stages of development for our trout prey-switching experiments? The simple answer is that, unlike earthworms, to our knowledge wild brown trout from the South Holston River have never seen a German cockroach. How do we even convince the trout that these are prey?


Three graphs showing the rates at which trout ate German cockroaches in a laboratory setting.
It took an intermediate step for trout in the aquarium to be able to recognize small prey after keying on large prey. (Jules Silverman graphic)

Each isolated trout was at the rear end of the tank facing forward into the current. We stopped feeding them earthworms. We then placed several live adult male cockroaches on a fixed floating platform at the upstream end of the tank. For several days the trout appeared to show no interest in these clearly visible prey…and they were getting hungry. Then an occasional cockroach would fall off the platform and slowly drift on the surface towards a fish…and voila, a fish would eat it! Once they enjoyed the taste of cockroach there was no going back. These same fish would now frequently bump the platform to dislodge, then eat, a cockroach. Accepting cockroaches was a big deal as we now had the resources needed for our experiments. We now have an easily-sourced “focal” prey – ¾-inch (~#12 dry fly) adult male cockroaches against which other size cockroaches could be compared.

However, before starting experiments it was important for all trout to be at about the same level of stomach fullness. We first determined how many cockroaches a trout would eat in a meal before refusing any more. A few years ago when I presented this study to our local fly-fishing club, I had asked the audience, “How many cockroaches will a brown trout eat?”. A hand shot up, “4”, another “10” and another “25”. I paused, then displayed a photo for effect, “Way off guys–around 200!” To this day, when this talk comes up in conversation, this fact is what most folks remember. This level of gluttony should not surprise anglers. Fish are opportunistic foragers–they need to eat to fullness when food is available, as they never know when their next meal will appear. How many of you have caught a largemouth bass or northern pike with a 5-inch shad or shiner (or small trout) sticking out of its mouth?

Recommended


We needed equally hungry, but not starving, trout for the experiments. We settled on 25-percent satiation, or feeding trout 50 adult male cockroaches, prior to each experimental replicate. We had an accepted prey item of a specific size to compare smaller size prey to. As stated, this adult male cockroach was our “focal” prey to compare with “novel” prey. Recall my observations on the Missouri River. Would trout ultimately accept previously unseen smaller novel prey, in this case 1/8-inch (hook size #24) cockroaches? We drifted individual focal and novel prey sequentially past the trout and recorded the fish’s response. We performed this exercise daily, increasing the ratio of smaller to larger cockroaches each day. We expected (like my river observations) that the trout would consume all the smaller cockroaches once they reached a high enough frequency; yet, to our surprise virtually none of the over 200 small cockroaches offered were attacked and consumed (see graphs). We were disappointed, but not deterred. We considered continuing this approach, exposing trout to more and more tiny prey, but we decided a different tactic was a better use of our resources.

Try, Try Again

Perhaps these novel prey were so small that they went undetected, that there was just too much of a size difference from the “focal” cockroaches which trout did see as prey? We thought that if this was true then perhaps increasing novel prey size to “medium” (1/4-inch, #18) and increasing the ratio of medium to large focal prey, the medium-size prey would be detected. This was indeed the case. All medium-size novel prey at all ratios were attacked and consumed (see graphs).

A graphic illustrating how trout appetite changes over time.
A stepwise adjustment in trouts' field of vision to detect prey. (Mayfly art by Dun & Dun)

Brad and I now wondered, what if these once-novel medium size prey were now the focal prey and, if drifted with the once-rejected small novel prey, would these small prey now be detected and attacked by the trout? We did not know what to expect. If so, this would be an exciting result.

We first floated a medium size prey–accepted; then another and another and another–accepted, accepted, and accepted. No surprise. Then the moment…dropping a small prey upstream it drifted for about 30 seconds, then slurp; it was sucked in by a trout. This pattern continued across all fish and medium-to-small prey ratios (see graphs). We had our answer: The once-rejected small prey (when compared to the large prey) were now accepted when compared with the most recent focal medium-sized prey. So through a two-step process, our trout’s visual template had expanded to encompass large to small prey (see conceptual diagram). Furthermore, the trout now remembered these once-undetected small prey, even when withheld for at least 14 days.

What about prey considerably larger than our focal adult male cockroaches? We discovered they were initially ignored by trout but gradually accepted with continued exposure.

In conclusion, trout develop a search image for a certain (energetically favorable) prey type and potential prey occurring outside this search image are not detected. However, this search image can be expanded over time to include the initially rejected prey by exposure to intermediate-sized prey. In the case of Missouri River trout and their PMD and Trico prey, greatly enhanced continuous exposure to Tricos (more than the small prey we could deliver in our experiments), as well as possible exposure to fewer, smaller, later emerging PMDs may have triggered the feeding shift to Tricos.

What Does This Mean For You?

An illustration of the cross section of a river showing a brown trout looking up mayflies floating by on the surface.
What does this mean for you? When multiple sizes of trout prey are hatching, step down your fly sizes rather than jumping right to the exact size of the new bug. (Adobe Firefly illustration)

A practical outcome of our findings for anglers on streams with multiple insect hatches is that near the tail end of a major hatch, an imitation matching the size of subsequent hatching and initially less abundant, and substantially smaller or larger, insects may not trigger a response, but one intermediate in size might. Step down your fly sizes rather than jumping right to the exact size of the new bug. Matching the hatch also requires some knowledge of both the current ratio of insect species and previous ratio of exposure of insect species to trout. This period of search image adjustment allows individuals of some insect species to escape being eaten and many anglers frustrated.

Our science suggests that fishing an intermediate-sized fly (#18 to #20), even when not exactly matching a specific insect but that is within fish’s established search-image range, could have been effective that fateful day on the Missouri when the trout took my #14 to #16 PMDs but ignored my #24 Tricos. We are excited to continue testing.


Jules Silverman, Ph.D. Is Distinguished Professor Emeritus at North Carolina State University. Formally trained in animal behavioral ecology, his research program focused on behavioral, chemical and nutritional ecology of cockroaches and ants. He is a lifelong fly fisher and tier, pursuing trout, bass, bonefish, redfish, and other fish species throughout the U.S.A…and beyond.

Brad W. Taylor, Ph.D. is Associate Professor at North Carolina State University and The Rocky Mountain Biological Laboratory. His research focus is freshwater ecology, especially the ecology of stream insects and fish-insect interactions. His teaching responsibilities include aquatic entomology (with Silverman for several years, now taught solo), limnology, and community ecology.

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