The vast majority of salmon are getting up, over, around and through the four lower Snake River dams even as legal challenges and political battles swirl around them, according to the federal agency in charge of monitoring fish health.

For every 100 young Chinook and steelhead that head downstream and past the four dams every spring, about 75 survive.

“That’s pretty good,” said Ritchie Graves, Columbia hydropower branch chief for the National Oceanic and Atmospheric Administration. “In a lot of river systems, that would be something they would shoot for.”

For each of the four dams, NOAA maintains a separate survival standard for juvenile salmon heading downstream. The agency wants 96% survival for yearling Chinook and steelhead, and 93% for subyearling Chinook less than a year old.

The dams are achieving those performance standards, Graves said.

For adult fish swimming upstream, the survival rate is above 90%.

How many fish survive the trek past Ice Harbor, Lower Monumental, Little Goose and Lower Granite dams is at the core of the decadeslong conflict over them.

Environmental groups and Native American tribes want the dams removed as a way to increase salmon populations.

Farmers and other stakeholders support salmon recovery, but say dam breaching isn’t a silver bullet that will automatically lead to more fish.

In the middle are agencies such as NOAA’s National Marine Fisheries Service, referred to as NOAA Fisheries, which monitors the dams’ impact on fish, and the U.S. Army Corps of Engineers, which operates the dams.

Of 12 distinct populations of fish in the interior Columbia basin, seven are protected under the Endangered Species Act and five are unlisted.

“Clearly, the fish aren’t recovered, they’re still listed,” Graves said. “The good news is, we haven’t lost any populations in 25 to 30 years of listing, either.”

How fish move

In the last five years, the number of fish passing upstream past Lower Granite Dam each year averaged:

• More than 55,000 adult Chinook.

• Nearly 16,000 Chinook jacks, male salmon that return to their fresh water stream a year or two earlier than normal.

• More than 55,000 hatchery steelhead.

• More than 14,000 wild steelhead.

Numbers fluctuate depending on the year, weather conditions and how many predators are feeding on them.

Adults swim upstream, spawn in the river systems above the dams and their offspring migrate downstream at varying times to the Pacific Ocean. They then live in the open ocean for several years.

To get downstream, the juvenile fish either swim or ride the river flow near the surface of the water, said Michael Milstein, a NOAA spokesman.

They get past the dams several ways. The dams are built so that spill gates open from the bottom up, so the fish can dive deep to find their way through.

They can also get past the dams through spillway weirs, which allow them to ride the water as it passes over the spillway.

The spillways greatly reduce the chances the juvenile fish will pass through the dams’ turbines, Milstein said. Turbines operate depending on the river flow and are screened at some dams to prevent fish from entering.

For adult fish returning from the ocean, the passage is less stressful.

“Upstream is actually easier,” Graves said. At the dams, the fish swim up the gradual incline of the ladder that spirals from the downstream side of the dam over to the upstream side.

Elsewhere in the U.S., fish ladders “often perform much worse,” he said.

“The fish ladders in the Columbia River basin are as good as it gets,” he said.

“Salmon are so driven to get to where they want to go, if you give them an opportunity, they will take it,” he said.

The salmon die after spawning. Some steelhead attempt to return to the ocean and then make another run to spawn again, Graves said.

Migration timing

Fish populations are typically named for the time of year they swim upstream as adults, Graves said.

Fall Chinook spawn in the autumn and the subyearlings emerge in March and April. They migrate downstream to the ocean in late May through July.

Spring Chinook and steelhead can spend up to three years in fresh water before migrating out to the ocean as juveniles.

NOAA biologists believe adult upstream migration hasn’t changed much because of the dams. They might be slightly delayed, but they’re also not fighting as strong a current as before the dams were constructed.

“It kind of washes out,” Graves said.

On the other hand, the dams create reservoirs, which slows the water flow and increases exposure to predators.

Swimming downstream, juveniles face a bit of a headwind the whole way, Graves said.

“It makes it a little harder, it costs some more energy than it did historically,” he said of the juveniles. “They’re probably delayed pretty substantially compared to historical conditions.”

Latent or delayed mortality is the biggest unanswered question surrounding the salmon and the dams, Milstein said.

That’s when juvenile fish die later because of the stress they experience getting past the dams. If that happens, it’s not reflected in NOAA’s fish passage numbers.

“The problem is that delayed mortality is difficult to measure because it occurs in the ocean, and different ways of estimating it lead to different results,” he said.

NOAA is looking for better ways to measure delayed mortality, Graves said.

“It’s devilishly hard,” he said. “The science is challenging on this.”

Zero mortality, or 100% survival, is not possible, he said. “High” or “significant” juvenile mortality would still occur naturally.

Predators such as sea lions and orcas are primarily interested in adult salmon, Milstein said. The biggest predators for juvenile fish include birds such as gulls, terns and cormorants and nonnative game fish such as bass and walleye.

“It’s a long and dangerous trip for a small fish, and there are many hungry mouths along the way,” Milstein said.

Ocean warming due to climate change may be the biggest factor of all, Milstein said.

“Salmon survival in the ocean is closely related to ocean temperatures, and as the temperatures increase, survival declines,” he said.