Blog

May 15

Naturalist Notes: Life at the Waterline

It’s finally warm enough to sit by the river. I’ve found a perfect rock perch to listen to the tree swallows, smell the cottonwoods, and watch the water flow by. The rolling wave in front of me sweeps around a rock, forming a back eddy that collects foam. What’s it like at the bottom of that flow? Is it moving fast, churning the water and sediment? Or perhaps there’s a still spot, a perfect hiding hole for a Yellowstone Cutthroat trout?

I am fortunate to experience this scene on a regular basis here in Gardiner, our town that sits on the edge of Yellowstone, right at the confluence of the Yellowstone and the Gardner rivers (The river and the town are named after the same person, but a southern drawl led to the additional ‘i’ in the town’s name). My significant other and I come to the river often. He is an excellent angler, but I have not quite learned to fly fish myself. I’d much rather find an aforementioned sit spot and observe the world around me. Spring is an exciting time when so many familiar sights return to Yellowstone. Among returning migratory birds and first blooms of long dormant plants, I especially appreciate the insect life around me. There can be so much biodiversity in a single spot if you look close enough!

I notice a few things right away. The first, unfortunately, is a mosquito, probably freshly emerged from its underwater nymph stage in a beaver-dammed section of the river a bit upstream. As pesky as these insects are, they play a vital role in the ecosystem. That, however, does not stop me from slapping away! I feel something on my leg and go to smack it, but this time I’m pleasantly surprised to find a caddisfly instead of a mosquito. I tell my partner about this observation, he stops to change his fly, and lands a trout on the next cast! The art of fly fishing has to include the science of entomology, and a good angler will pay attention to what’s emerging and flying around. That’s fish food!

There are several types of insects that begin life in a freshwater habitat. Some examples include mayflies (Ephemeroptera), dragonflies and damselflies (Odonata), stoneflies (Plecotera), and of course caddisflies (Trichoptera). All of the above start as an egg, then hatch into their immature stage, known as ‘nymphs.’ These insects, plus several other aquatic species, are good indicators of a healthy waterway.

Mayflies are the only insect to have a winged immature stage. Entomologists call that the ‘subimago’ phase, and anglers know them as ‘duns.’ The final molt brings them to the sexually mature ‘imago’ phase, or ‘spinners’ to anglers. These adults only live a few hours to a few days, hence the ephemeral nature of their order name (Ephemeroptera).

The nymphs of the Odonata order are also worth mentioning. These prehistoric creatures breathe through their butts and are predatory from the day they hatch. Their mouth part (known as the labium) is retractable, and they punch it out to snatch other insect nymphs and even small fish and tadpoles. Imagine a miniature and aquatic version of the terrifying xenomorph from Alien.

Stoneflies are another angler favorite, specifically the famous Salmon fly. These gentle giants (relatively speaking) can grow up to 3 inches long and have only vestigial mouthparts as adults. They do all their eating in the nymph stage, and so once they emerge from the water, they mainly focus on adult activities (mating). I look forward to seeing these monsters in a couple weeks.

Last but certainly not least, are the caddisflies. These are remarkable insects, and their underwater life is beautiful. They create protective cases to live in, and use whatever materials are available to them, including small rocks, twigs, grass, and even an occasional snail shell! From my sit spot, I can see a few larger caddisfly nymphs scooting along the bottom of the eddy, and I hear a big splash downstream. Another fish on the line! Not only are these aquatic insects a favorite food for trout, but they also provide an important food source for many other riparian species like bats, dragonflies, birds, spiders and more. I was happy to see some caddis this past weekend, for the famous ‘Mother’s Day Hatch,’ even though we didn’t have the clearest water for fishing. Some years there are so many caddisflies in the air that it almost seems as if it is raining insects!

As the days warm up, I anticipate more river time in my future appreciating the life teeming above and below the waterline. Even if most people don’t appreciate the ‘bugs’ like I do, they are still an important part of our ecosystem, not only as a food source but also for the role they play in decomposition and nutrient cycling. The presence of these insects indicates a healthy body of water, as many are sensitive to pollution, and don’t necessarily have the means to migrate to cleaner waters. Thankfully here in Yellowstone we have pristine waterways and lots of critters to find. The Yellowstone Forever Institute educators especially enjoy taking students to participate in stream ecology activities. In addition to collecting samples from a stream and recording the number of species present, we also measure the water’s temperature, pH, and flow rate. After all critters are safely deposited back into the water, this data is saved to Yellowstone Forever’s phenology log. By understanding and protecting these small yet significant creatures, we can ensure the sustainability and resilience of Yellowstone’s wetlands for future generations.

by Katy Fast, Yellowstone Forever Institute 

Photos Top to Bottom:

Yellowstone River at sunset, NPS / Jacob W. Frank
Caddisfly, NPS / Jacob W. Frank
Damselfly larvae, NPS / Jacob W. Frank