Temperature differences and slow-moving water at the confluence of the Clearwater and Snake rivers in Idaho might delay the migration of threatened juvenile salmon and allow them to grow larger before eventually heading to the Pacific Ocean.
Scientists used to think all juvenile fall Chinook salmon in the Clearwater River migrated to the ocean during the summer and fall after hatching in the spring. But federal researchers and the Nez Perce Tribe began learning in the early 1990s that some stick around until the next spring.
Similar delays have also been found in a select number of other rivers, but this is still the exception rather than the rule. The Clearwater is unique because a high number - as much as 80 percent in some years - of its fall Chinook salmon don't enter the ocean before they're a year old.
The delayed migration could also mean Clearwater salmon are more robust and survive better when they finish their ocean-bound trek, said Billy Connor, a fish biologist with the U.S. Fish & Wildlife Service.
"It may seem counterintuitive, but the stalled migration of some salmon could actually help them survive better," Connor said in a news release. "Juvenile salmon may gamble on being able to dodge predators in reservoirs so they can feast on the reservoirs' rich food, which allows them to grow fast.
"By the time they swim toward the ocean the next spring, they're bigger and more likely to survive predator attacks and dam passage."
Scientists from the U.S. Geological Survey, the U.S. Fish & Wildlife Service, the Department of Energy's Pacific Northwest National Laboratory and the University of Washington are wrapping up field studies this fall to determine if water temperature or speed encourage salmon to overwinter in the confluence and in other reservoirs downstream. The Bonneville Power Administration is funding the research to help understand how Snake and Columbia River dams may affect fish.
USGS and USFWS are tracking fish movement by implanting juveniles with radio tags, which are more effective in shallow water. PNNL is complementing that effort with acoustic tags, which work better in deeper water.
The lab also is contributing its hydrology expertise to measure the Clearwater and Snake rivers' physical conditions. The University of Washington is providing the statistical analysis of the tagging.
"Fall Chinook salmon on the Clearwater River have a fascinating early life history that may contribute to their successful return as adults," said PNNL fish biologist Brian Bellgraph. "If we can support the viability of such migration patterns in this salmon subpopulation, we will be one step closer to recovering the larger fall Chinook salmon population in the Snake River Basin."
--Eric Mortenson; firstname.lastname@example.org