A dynamic vision of trout

A technician uses an antenna and radio receiver to locate fish in Milk Creek.

By Brian Hodge

As every TU member knows, salmon and trout depend on a steady supply of cool or cold water to survive. Accordingly, steam temperature serves as a relatively good index or predictor of salmonid habitat.

Although we don’t expect to find salmon and trout in waters with bath-like temperatures, this usually-reasonable generalization has the risk of lulling us into an overly conservative vision of salmonid habitat. As my colleagues and I were reminded, what’s unfavorable (if not lethal) habitat for trout in one season, might be important (if not critical) habitat for trout in another.

In the summer of 2013, and in conjunction with an ongoing cutthroat restoration project, biologists from TU and Colorado Parks and Wildlife collected stream temperature data from Milk Creek, Colorado. Temperatures in the downstream reaches of the project area were, by all accounts, too warm for native cutthroat trout to persist. Nevertheless, when we returned to the same reaches in October, we found cutthroat throughout. That sparked our curiosity and launched us into a study of trout distribution and movement.

In 2014, my colleagues and I tagged and tracked the movement of cutthroat trout in Milk Creek (Hodge et al. 2017). All but one fish migrated upstream to spawn, and the majority spawned in tributaries not much larger than a street gutter. Then, as fast as the cutthroat had crammed into the tributaries, they slipped back out to the main stem, not to return again that year. When summer arrived, we observed another interesting pattern: fish that started in cool, upstream locations stayed put and fish that started in warm, downstream locations moved upstream.

Very small radio transmitters were implanted into fish so that their movements could be tracked through time.

After a while, it started to make sense. The small tributaries provided what appeared to be the best spawning habitat, but they were, for one reason or another, unfavorable during other times of the year. The downstream reaches of the main stem were too warm for cutthroat during summer, but evidently offered favorable habitat during winter and spring. In short, cutthroat trout moved across time and space, presumably to find and occupy the most suitable habitat of the period.

The majority of cutthroat spawned in small, headwater tributaries of Milk Creek.

In the case of Milk Creek, it would have been too easy to discount the importance of the warm, downstream reaches. Likewise, the small, headwater tributaries looked relatively insignificant in the grand scheme of things. But, as we learned, all of the pieces are at least seasonally important to the existing cutthroat population, and removing or degrading any one of those pieces could have deleterious, population-level effects. Protecting, and maintaining the connectivity of, habitat in Milk Creek will thus be critical for maximizing the likelihood of population persistence.

Our findings at Milk Creek serve as a reminder that, to effectively conserve salmon and trout, our thinking regarding the species needs to be as dynamic as the species themselves.

Brian Hodge is a fisheries biologist and project coordinator for TU in Colorado. Read the full article by Hodge, Battige, and Rogers here

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