A recent study shows that beaver dams not only
create more complex habitat for juvenile steelhead, but that the steelhead will
actually move to different parts of that habitat during the day and night to
feed and rest, as well as to find thermal refuge.
The study found that individual steelhead
established more distinctive patterns of their use of the space within beaver
impoundments on Bridge Creek, a tributary of Oregon’s John Day River, than they
did in a separate run without the benefit of the beaver-created complex
The study published in the Canadian Journal of
Fisheries and Aquatic Sciences is a sequel to another study on the impacts of beaver
impoundments on water temperature published last year.
“In the temperature paper we demonstrated that
beaver impoundments can also create thermal habitat complexity,” said Gus
Wathen, fisheries scientist with Ecological Research in Bend, Ore. “This study
demonstrates that fish can set up distinct and disparate movement patterns in
highly complex habitat, allowing then to satisfy daily needs (food consumption,
rest, digestion, predator avoidance, seeking thermal refuge, etc.) without
having to search far, and expend a lot of energy to find microhabitat suitable
for their specific need.”
The previous study (a ten-year study) was
about the influence of beaver dams – some artificially added to the stream
during the study – on stream water temperature. It found that the dams increase
surface water storage and encourage cool groundwater flow. The results are a
cooler stream at a time when typically high summer temperatures stress
threatened juvenile steelhead (see CBB, June 2, 2017, “Partnering With Beavers:
10-Year Study Shows How Beaver Dams In Arid Streams Moderate Water Temps,” http://www.cbbulletin.com/439009.aspx).
That study was published in the journal PLOS one May 17, 2017.
This latest study, “Beaver activity increases
habitat complexity and spatial partitioning by steelhead trout,” was published
online August 29, 2018 (http://www.nrcresearchpress.com/doi/abs/10.1139/cjfas-2018-0171#.W6qEr2hKjIU).
Wathen’s co-authors are Jacob Algeier,
assistant professor in the Department of Ecology and Evolutionary Biology at
the University of Michigan; Nicholaas Bouwes, fish ecologist, Ecological
Research; Michael Pollock, research fish biologist, NOAA Fisheries’ Northwest
Fisheries Science Center; Daniel Schindler, professor, School of Aquatic and
Fisheries Sciences, University of Washington; and Chris Jordan, research fish
biologist, Northwest Fisheries Science Center.
“Our study showed that within the beaver
complex, individual fish established more distinctive patterns of spatial use
of microhabitats than in the run,” the study says. “This pattern is reflected
in increased specialization in activity and boldness behaviors, and variation
in diel movement patterns of individuals in the beaver complex relative to
those in the run where individuals showed less variation in their habitat
In the run without beaver activity, the
steelhead either established use patterns in deeper areas or they used other
habitat “in seemingly random ways, presumably as if they are searching for
quality microhabitat,” the study continues.
On the other hand, steelhead in beaver
impoundments “display more specialized habitat use and movement patterns that
changed through the day, presumably allowing individuals to fulfil daily needs
(i.e., foraging, digestion, predator avoidance) in different physical space at
different times,” the study says.
This additional movement within beaver
impoundments can also result in higher densities of steelhead.
“More juvenile steelhead used the beaver
complex for longer durations, compared to the run,” the study says. “From our
fish capture surveys, we know that the density of juvenile steelhead was almost
three times higher in the site impacted by beaver activity and, on average,
these fish spent longer amounts of time within this habitat than in the more
“Therefore, we can conclude that availability
of quality habitat constrains the degree to which fish can vary their daily
habitat preference and that habitat heterogeneity allows variation in habitat
selection over the diel cycle.”
Wathen said that “almost all stream dwelling
fish in North America evolved with beaver and were accustomed, and maybe even
dependent, on having them on the landscape. It wasn't until the fur trade came
through and wiped out almost all beaver, that we developed this idealized
notion of an unencumbered free flowing trout stream. Especially in environments
where water is at a premium, like the arid ecosystems in the west, beaver are
essential to ecosystem health.”
The “take home” for those charged with
managing fish habitat, he said, is that complexity is a good thing.
“Stream restoration is big business across
North America and I think a premium should be placed on creating habitat
complexity in depth, water velocity, fish cover, and even thermally if it can