By Bob Hoffmann, College of Agricultural, Human and Natural
PULLMAN, Wash. - A slender, dark-haired woman in her
forties shoulders a backpack loaded with dead fish ash she
hikes a long, rocky trail to a mountain stream in southern
Idaho. Arriving on the bank, she drops the pack and starts
winging fish carcasses into the water.
This is science. And this is Laura Felicetti, a research
scientist in the lab of Charles Robbins, professor in the
School of the Environment and School of Biological Sciences
at Washington State University. Felicetti is a member of a
team that's trying to quantify the success of nutrient
replacement in an area where dams have stopped salmon and
steelhead from migrating.
Soils in Idaho's Boise-Payette-Weiser sub-basin are
nutrient poor, according to Katy Kavanagh, a University of
Idaho forest ecology professor and a collaborator on the
project. One reason is that natural processes in this
ecosystem poorly incorporate atmospheric nitrogen into the
soil in a form that is usable to plants. Also, the region's
dry summers and cold winters are not favorable to
decomposition, so dead trees are slow to decay and make
their nutrients available to other plants.
Bears - a fertilizer factory
For thousands of years, nutrient-rich salmon and steelhead
spawned in the area's streams, and their offspring swam to
the ocean to feed and grow. Several years later, these same
fish returned to the streams to spawn and die. In nature,
everything eventually gets consumed by something else.
Bears were among the great fertilizer factories of the
watershed, eating fish and processing them, through
digestion, into a form that the plants would readily use.
As the bears wandered, they distributed nutrients into the
forest. Insects fed on some of the remaining fish
carcasses, and in turn were eaten by other animals, such as
frogs, birds and bats, which also would distribute the
nutrients. The fish in the river eventually would fertilize
trees from the riverbanks to the ridge tops.
But with the construction of dams such as
Black Canyon in the early 1900's, salmon and
steelhead could no longer make their spawning migration.
Many suspect that forest health and productivity are
gradually declining, like a garden that is never
fertilized. Dam operators have been trying to mitigate
the loss of this nutrient flow, sometimes by distributing
dead fish into the streams. But scientists needed a way
to measure the effects of these efforts, and therefore
evaluate the benefits of mitigation efforts.
Throwing dead salmon
Felicetti stood streamside, chucking dead salmon, which she
obtained from a fish hatchery. "Marine environments,
where the salmon spend most of their lives, have different
carbon and nitrogen isotopes than terrestrial
environments," Felicetti said. "We can trace these
marine isotopes through the food web."
The stream was deprived of marine nutrients and the
hitchhiking isotopes for a century. By pitching the salmon
carcasses into the stream, Felicetti reintroduced marine
isotopes and tried to follow them, like Hansel and Gretel's
bread crumbs, through the food chain.
Felicetti is a bear biologist by trade, so she knew how to
determine if bears were eating the fish. She set bait
stations and recovered hair from the scene. She also
targeted bats with a mist net, which is like a volleyball
net with ultra-fine webbing. She could take hair samples
from the captured bats and release them unharmed. This way,
she could test the hair for isotopes, and determine whether
the fish nutrients were working their way through the
environment. The presence of marine isotopes in bear and
bat hair would provide evidence that this type of
mitigation was successful.
Preliminary data shows that "mitigation doesn't work
beyond the stream bank when conducted on a small-scale
basis," Felicetti said. Aquatic vertebrates and
invertebrates feed on the fish carcasses within the small
stretches where carcasses were provided, but when only a
few hundred yards of streams can be fertilized economically
in this manner the larger terrestrial system does not
benefit. The isotopes were not detected in the
terrestrial food chain, meaning that nutrients from the
dead fish were not making their way into the forest.
"Bears are very tied to fish when abundant," Felicetti
explained. For example, a large, adult, male Alaskan brown
bear can consume as much as 6,600 lbs. of salmon per bear
per year, while females can consume 3,300 lbs. This
amount of food provides from 50 to 70 percent of the bears'
annual nourishment. "But bears don't key in on dead
fish like they key in on live fish," said Felicetti.
"They will respond to the motion and sound of a fish
in shallow spawning streams, but they don't get that with
the fish carcasses and we could never distribute the amount
of hatchery fish that healthy, natural salmon runs would
This project was funded by the Bonneville Power
Administration. Colden Baxter, associate professor of
Ecology at Idaho State University and his Ph.D. student
Scott Collins also collaborated on the project, focusing
on the aquatic aspects. This study was depicted on the
PBS program Nature, on their "Running the Gauntlet"
episode, which can be viewed at http://to.pbs.org/LkJhxL.
Laura Felicetti, research scientist, WSU College of
Agricultural, Human and Natural Resource Sciences,
Bob Hoffmann, WSU College of Agricultural, Human and
Natural Resource Sciences, 509-335-7744, email@example.com
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