LEWISTON, Idaho — Another set of scientists, this one more than five dozen deep, is sounding the alarm over Snake River salmon and steelhead, saying if the imperiled fish are to be saved, the four lower Snake River dams must go.

On Feb. 22, 68 fisheries researchers from the Pacific Northwest released a letter penned to the region’s congressional delegation, governors and fisheries policymakers methodically making the case for breaching the dams.

“This scientific recommendation wasn’t taken lightly. This is relying on a review of a large preponderance of information that a bunch of us analyzed over and over again over the years,” said Howard Schaller, a retired fisheries research biologist who worked for the U.S. Fish and Wildlife Service.

They compared the lifecycle survival, known as smolt-to-adult survival rates, of Snake River salmon and steelhead, and note the runs, which must pass eight dams as they migrate to and from the ocean, have lower survival rates than similar stocks in the Columbia Basin that only have to make it past four or fewer dams.

For example, wild steelhead from the John Day River in Oregon have an average smolt-to-adult return rate of 5% and wild chinook from the same river have a survival rate of 3.6%. The Northwest Power and Conservation Council has set a survival goal of 2% to 6% for anadromous fish runs from the Snake and Columbia rivers.

At 2%, the runs replace themselves. At an average of 4%, they grow.

But the smolt-to-adult return rate for wild Snake River steelhead is 1.4%, below replacement level, and for wild spring and summer chinook, it is just 0.7%.

The difference, they say, is caused by the number of dams and reservoirs each run encounters during juvenile migration to the ocean. For the fish from the John Day River, it’s three dams. At each of the eight dams on the Snake River, fish face hardships, including delays caused by slowed water velocity, predation, injury and stress. The scientists point to research that indicates many of the young fish that make it past each of the eight dams succumb from delayed mortality, the result of accumulated stress and injuries incurred along the way.

“When all of the existing credible scientific evidence is taken into account, it is clear that removing the four lower Snake River dams, with adequate spill at the remaining lower Columbia River dams, is necessary to restore Snake River salmon populations,” they write.

The work they cite was looked at during last year’s Columbia River Systems Operation Environmental Impact Statement, authored by the Army Corps of Engineers, Bureau of Reclamation and Bonneville Power Administration. The agencies chose a plan that calls for water to be spilled at each of the dams during the juvenile migration period.