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Fish carcass tossing helps track food chain nutrients

black bear
Grizzly bears and black bears feed on naturally spawning salmon and
distribute nutrients into the forest.Photo by U.S. Fish & Wildlife Service
 
 
 

PULLMAN, Wash. – A slender, dark-haired woman in her forties shoulders a backpack loaded with dead fish as 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.

 
Nutrient-poor soil

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 Arrowrock and 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
salmon
Salmon, such as these Alaskan sockeye, are a significant source of nutrients in free-flowing rivers. Photo by Charles Robbins

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

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 provide.”

As this research project draws to a close, Felicetti said the preliminary data shows that “mitigation doesn’t work beyond the stream bank when conducted on a small-scale basis.”

 
The 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.

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