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Trout Aren’t Lazy. They're Smart. How Smart?


If you’re just starting out in fly fishing, you might run across articles suggesting the laziness of trout. "Look for the slowest water," they claim. Or, "Look for areas where trout can indulge in their 'lazy' tendencies." This might make sense at first glance, after years of studying trout and their riverine habitat, these statements have one glaring issue — trout just aren’t lazy! A lazy trout would be unable to survive the perpetual treadmill of living in a river. A lazy trout would struggle to find food... a lazy trout would be skinny and malnourished, because even though rivers bring food to the fish, it’s not as easy as just waiting for it to show up. The trout need to know where to position themselves. 


Trout are optimizers; they are immensely adept at gauging the energy requirements of their environment and responding in a way that maximizes the amount of energy that can be extracted with the tiniest amount of effort. Trout don’t work harder, they work smarter. When temperatures climb into a trout’s preferred feeding range, and when bug activity is abundant (common during the summer months), a trout can be found feeding in surprisingly fast water. They move out of deep pools into the fast water, riffles as many anglers know them, places where bugs thrive. These riffles are the food factories of the river, and when the factory is running, why not get as close to the source as you can? Sure, trout might have to fight swifter current as stream velocity is greater in riffles, but the food is so abundant that the energy gain outweighs the energy expenditure (Naman et al., 2017). Macroinvertebrates (the bugs trout mainly eat) thrive between river rocks in the velocity breaks provided by that interstitial space between river rocks. Whether intentional or accidental, eventually some insects lose their grip and drift downstream, and unsurprisingly, trout know how to position themselves accordingly, waiting for wayward insects. They live it day-in and day-out and quickly capitalize on dislodged prey.



Caption: This figure shows the cross-section of how water moves within a stream channel (left) and the associated velocity at different depths (right). Notice how velocity decreases as depth increases, this is where trout tend to lay to avoid current, even if the water looks like it's moving quickly at the surface.  Wetzel & Likens, 1979


Stream dynamics can make things confusing to anglers approaching a fast-moving stretch of water. Without an understanding of how rivers work, their surface appearance can be deceiving. Even though surface currents may appear to produce rapidly moving water, the velocity on the stream bottom is much slower. This phenomenon allows fish to steadily hold in seemingly quick water while the energy cost near the stream-bed is surprisingly manageable. The above figure shows how velocity changes in a 3-dimensional cross-section of a river. Take note of how low the velocity is at the streambed. Even though trout may be holding in fast riffled water, they might not be facing the full extent of stream flow as it appears from the angler’s perspective. 


Interestingly, different species have varying behavioral tendencies when foraging. For instance, rainbow trout are more focused on energy acquisition and are typically more likely to brave faster water for an ideal spot on the fast-food conveyor belt. Brown trout, on the other hand, are much more focused on energy conservation. They tend to feed in slower water, along banks, and places where the energy cost is significantly less (Gatz et al., 1987). But don't be mistaken, they still strike on high-calorie food items when presented. Risking faster current can be worth it for a meaty reward, that’s true for any trout.




Author Bio:


Andy Witt, scientist and angler obsessed with chasing and understanding all gamefish, writes on the intersection of science, conservation, and fly fishing for Due West Anglers, based out of Denver, CO. Paxis is proud to share his insight.




Sources:

1. Wetzel & Likens. 1979. 3 Idealized Stream Current Velocity. A. Channel cross-section. B. Profile at stream midpoint.

2. Naman, S.M., et al. 2017. Habitat-Specific Production of Aquatic and Terrestrial Invertebrate Drift In Small Forest Streams: Implications for Drift-Feeding Fish. Canadian Journal of Fisheries and Aquatic Sciences, vol. 74, no.8. 1208-17.

3. Gatz, A.J., et al. 1987. Habitat Shifts in Rainbow Trout: Competitive Influences of Brown Trout. Oecologia, vol. 74, no.1 7-19.

4. Olsen, D. 2019. Tactical Fly Fishing: Lessons Learned from Competition for All Anglers. Stackpole Books.


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