Saving native fish: When does a barrier help?

An upstream view of the impassible, but soon to be navigable, culvert on Poose Creek.

By Brian Hodge

In the world of fisheries conservation, we often emphasize the importance of large, interconnected habitats and populations.  Generally speaking, a trout population that occupies 25 miles of stream is more likely to persist than a trout population that occupies only 5 miles of stream.  Also, because some streams are more suitable than others for spawning and rearing, we spend a lot of time removing passage barriers from streams so that fish have options.

Now that the importance of stream connectivity is clear, I’ll the muddy the waters.  In Colorado and elsewhere, TU partners on both barrier removal and barrier construction projects—in some instances our goal is to facilitate movement of fishes and in other instances our goal is to prevent movement of fishes.

Unfortunately, there’s no one-size-fits-all formula for determining when and where stream connectivity should be expanded vs. restricted.   Rather, a number of factors, such as downstream threats and extent of habitat, must be considered.  These factors are often “offsets” where barrier construction is concerned:  construct a barrier too far downstream and you increase the risk of locking a fox in the chicken coop; construct a barrier too far upstream and you risk isolating a fatally small and unsustainable population.  In short, scientific principles must be weighed and evaluated on a case-by-case basis.

A Colorado River Cutthroat Trout from Poose Creek. This fish has been marked so that it can be recognized as originating from below the culvert. (Source: Kevin Rogers, Colorado Parks and Wildlife). 

 

Cases and points:

Poose Creek, a tributary to the East Fork Williams Fork, is occupied by a population of native Colorado River Cutthroat Trout.  Unfortunately, an impassible concrete culvert on Poose Creek has long prevented cutthroat from accessing the headwaters.   To reconnect Poose Creek and expand the distribution of cutthroat, TU and the U.S. Forest Service are constructing a fish ladder within the culvert.

 

An upstream view of the fish barrier on Trout Creek.

 

Meanwhile, and not far from Poose Creek, a population of native Colorado River Cutthroat Trout occupies the headwaters of Trout Creek.  This population was at imminent risk to invasion of whirling disease-infected nonnative trout.  Whirling disease can result in mortality of native cutthroat.  Nonnative trout compete with and sometimes hybridize with native cutthroat.  To secure cutthroat in Trout Creek from these threats, TU, Colorado Parks and Wildlife, and the U.S. Forest Service constructed a fish passage barrier.

One of my colleagues jokingly refers to this concurrent practice of removal and construction as hypocrisy.  I call it strategy.

Brian Hodge is northwest Colorado project manager for TU's Colorado Water Project.

 

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