Study measures nutrients in river

Fewer dead fish may mean less food for young fish

By Marcy Stamper

A dramatic drop in the number of salmon returning to the Methow basin to spawn clearly reduces the number of eggs deposited in local rivers. But that next generation may also be at a disadvantage because of the decrease in decaying fish carcasses, which constitute an important food source for growing fish.

This is one of the hypotheses being explored in a new study of nutrients available in the Twisp River. “Most traditional fisheries restoration projects look at habitat and hatchery operations. We’re looking at what the food web looks like,” said John Jorgensen, fish biologist with Yakama Nation Fisheries, who is directing the multi-year research project.

The premise of the study is that food available for growing fish is considerably depleted when compared with historic levels, since a high percentage of nutrients are derived from saltwater fish that migrate back to freshwater streams, where they decompose after spawning, said Jorgensen.

“The Methow was probably pretty smelly with rotting salmon carcasses this time of year,” he said. While fish are not typically thought of as cannibals, juvenile fish benefit from “a direct uptake of flesh from their parents,” said Jorgensen. Crystal-clear streams may seem pristine, but they actually may not provide adequate nutrients for fish, he said.

Studying available food sources – minerals like phosphorous and nitrogen, micronutrients including algae and chlorophyll, and insects and aquatic invertebrates – is expected to yield valuable statistics for assessing the availability of food needed by growing fish in the river. Biologists are also collecting water samples.

Jorgensen acknowledged that the information they obtain will be hard to evaluate, because there are no data from 150 to 200 years ago, when an estimated 50,000 salmon and steelhead typically returned to the Methow basin to spawn. Still, it is important to gain a more holistic understanding of the underlying conditions that may limit productivity, which can be a critical step in salmon restoration, he wrote in a description of the project.

Standard hatchery operations are more quantifiable – biologists can tally how many fish are released, how many return, and where they die – but hatcheries keep fish for their eggs and do not typically release the fish into the river after spawning because of the possibility of disease. When they do, they remove the guts and head to prevent disease transmission, further reducing the available nutrients, said Jorgensen.

As a result, nutrient studies like this one are still fairly rare, said Jorgensen. There are a few being conducted in the Lower Columbia, and several have been done in Canadian watersheds. The Twisp River assessment is patterned after a 10-year study in Idaho.

Researchers will compare data obtained in this assessment with nutrient levels in rivers in British Columbia, where fish populations are closer to historic levels, and in coastal systems, where numbers of fish tend to be higher because they do not have to traverse nine dams to get to those waterways, said Jorgensen.

Although they ultimately hope to survey the entire Methow basin, Jorgensen and his team are starting with the Twisp River because it is a small system that is home to spring Chinook and summer steelhead, both federally endangered species, and to bull trout, a federally threatened species. They also hope to draw on data from other ongoing sampling on the Twisp River for overall fish conditions such as length and weight.

Jorgensen and his team have placed three 12-inch-square Styrofoam grids at each of six bridges on the Twisp River, from the town of Twisp at the mouth to South Creek, 22 miles west. Jorgensen and fish biologist Kraig Mott visit each site every two weeks to collect random samples from the grids. The Styrofoam pieces are sent to a lab, where all accumulated algae is evaluated for levels of chlorophyll, one of the basic building blocks for nutrient development, said Jorgensen. Signs posted at the six bridges explain the project so that the Styrofoam squares will not be mistaken for trash, said Mott.

The researchers also collect bugs and other invertebrates from each location, carefully scraping each creature off the rocks and curing it in denatured alcohol for evaluation at their in-house lab.

The nutrient study is funded by the Bonneville Power Administration, as part of its overall commitment for projects to benefit fish affected by the Federal Columbia River Power System, according to a BPA fact sheet. Because it is part of a series of mitigation projects, it is difficult to isolate the amount being spent on the Twisp River Nutrient Assessment alone, said Jorgensen.

The Twisp River assessment began last May, with samples being gathered from April or May through October or November. The researchers expect to have results after three years.

If they find that nutrient levels are not adequate to support larger fish populations, scientists will propose a prescription for replenishing the nutrients, which could include the addition of salmon carcasses, fish meal or minerals, said Jorgensen.

Photo by Marcy Stamper

Fish biologists John Jorgensen, left, and Kraig Mott collect algae and invertebrate samples from the Twisp River near the bridge in town.