I rarely keep a trout these days, but when I was a kid my grandmother always asked me to bring her the trout that I caught.
I did a lot of fish cleaning, but I never thought to look inside a trout’s stomach to see what it had ate.
Early on, I assumed they were eating worms since that is what I caught them on, and later, when I switched to flies, I guess I assumed they liked flies too.
Fishing was so important to me as a kid that by high school I had decided I would find a job that involved working with trout. I earned a master’s degree in fisheries biology and my thesis work included determining population estimates, the age and size distribution of the trout in the rivers. It also included the examination of stomach contents from 500 trout from the Manistee and Au Sable rivers in Michigan.
As a result, I got good at being able to tell what the fish were eating; sometimes by finding pieces of the bugs in the samples.
I loved that work, but decided that I preferred teaching and working with kids so I left fisheries biology for a 30-year career in education as an elementary and middle school teacher and an elementary school principal.
After retiring and moving to Payson, I have found a great way to get back to my fisheries roots and combine it with working with kids. I meet with several Julia Randall Elementary fourth- and fifth-graders and Rim Country Middle School students twice a week to teach fly-fishing. I teach about learning the aquatic insects that we imitate with our artificial flies.
Recently, my middle school students got to practice fisheries science and see firsthand how important aquatic insects are to trout. They had earlier surveyed the aquatic insect populations collected from three sections of Tonto Creek — a riffle, stream edge, and deep pool.
They identified the “bugs” by stream section, and from their data were able to determine what types were most common to Tonto Creek and which parts of the stream each type seemed to prefer. They found that the most common insects in the creek were mayfly nymphs (82 percent) and caddisfly larvae (9 percent) while other bug groups made up the rest of the sample. The mayfly nymphs were in the 16-20 hook size range, while the caddisfly larvae were in the size 8-12 range. One crane fly larva was also in the sample.
It was about a size 4. Crane fly larvae are often found in streams, but are much less common than mayflies and caddisflies in Tonto Creek.
The students were preparing for a presentation on what they had learned and needed a trout to show external and internal anatomy features, so I HAD to go fishing and catch a trout.
It is amazing how much kids learn with a slimy fish in their hands rather than looking at a diagram of a fish!
When we examined the stomach contents of the Tonto Creek trout, we expected to see a lot of mayfly nymphs and caddisfly larvae based on the insect samples from a week prior. About 10 percent of the bugs in the stomach were mayflies, and about 30 percent were caddisfly larvae. Surprisingly, about 60 percent of the stomach contents were crane fly larvae.
During normal flows in the summer, stomach contents of trout closely match the distribution of bugs found in stream sample surveys. The shift to a much higher percentage of bigger bugs in the trout’s stomach got the students hypothesizing about why the trout had targeted these bigger insects.
I explained to the students that I caught this trout in quiet water next to a very fast current that was much faster, deeper, and muddier than the normal stream flow.
They offered a couple promising ideas about why the bigger insects were in the trout’s stomach at a higher percentage than expected. One idea offered was that the trout could see the bigger bugs easier in the faster, muddier water than it could see smaller insects. Another suggestion was that the trout was reluctant to leave the quieter water that was much easier to swim in to go grab a bug in the fast, cold current unless it was worth the effort. Both ideas made a lot of sense to me.
The elementary group of students will be doing their bug study in about a month when the water should return to a more normal flow. We will have the middle school data to compare and I will do my best to catch another trout to see how the stomach content data compares as well.
Collecting data and doing science with kids. What a great way to spend my retirement.