Face down in frigid water, Stillaguamish biologist Jody Brown arranged plastic buckets of gravel in the North Fork of the Stillaguamish River.
The 2-quart buckets are artificial salmon nests, or redds, which the tribe is using to study the fine sediment that accumulates in the gravel where salmon spawn.
Fine sediment can smother chinook redds. When small granules fill in the space that allows water to flow through gravel, sediment cuts off oxygen to the eggs. With larger particles up to a quarter of an inch, oxygenated water may get to the eggs, but newly hatched fry may not be able to emerge from the gravel.
By measuring fine sediment in the buckets during an "incubation cycle," the tribe can estimate survival-to-emergence rates for naturally spawning North Fork Stillaguamish chinook.
"We incubate them from when chinook spawn in September and October to when you'd normally expect to see fry emerge from gravel in January," Brown said.
This fall, Stillaguamish Natural Resources staffers dug three artificial redds each at seven locations on the North Fork. They buried six buckets in each redd and covered them with the clean gravel that chinook use. Buckets will be retrieved at intervals throughout the incubation period to observe changes in sediment concentration.
To get a sense of which sites are likely to have high sediment deposits, natural resources staff members are measuring suspended solid particles in the water at each location.
During the first year of the study in 2006, Brown found there was a 15 to 66 percent chance of survival; in 2007, the estimate ranged from 6 to 40 percent.
"The estimates are based solely on fine sediment infiltration," he said. A number of other factors affect the survival of eggs to emergence, such as flooding, redd disturbance by other spawning fish and natural survivability.
This is the final year of a three-year project to provide insight on the survival-to-emergence of naturally spawning North Fork chinook. The impact of fine sediment on chinook redds is listed as a data gap in the Stillaguamish Salmon Recovery Plan.
While all Puget Sound chinook are listed as "threatened" under the Endangered Species Act, North Fork Stillaguamish chinook have been supplemented for 20 years through the tribe's hatchery program. Because of the success on the North Fork, the tribe plans to replicate the program in the South Fork where the population is nearing extinction.
The U.S. Environmental Protection Agency provided funding through a Clean Water Act tribal grant. A similar project on the South Fork Stillaguamish is being conducted by Snohomish County Surface Water Management and funded by the state Salmon Recovery Funding Board.