In their first months at sea, salmon that have migrated through the eight dams of the Snake River are no more likely to perish than juveniles that traversed fewer dams, according to a new PNAS Early Edition paper. The finding builds a case that there’s something other than dams driving down the population of Snake River Chinook salmon and could have policy implications on river management if the results hold up in future studies.
Salmon that hatch in freshwater streams of the Pacific Northwest migrate to the ocean when they reach a juvenile stage of their life cycle, where they spend multiple years before returning to their upstream place of birth to spawn. In the late 1970s, the construction of the Snake River dams was wrapping up, ocean conditions were shifting due to climate change, and the population of salmon born in the Snake River was on the decline—at least, fewer adult salmon were returning to the Snake River. Because of the timing, many scientists assumed that the dams were to blame for the low rates of salmon survival. After crossing so many dams, they hypothesized, the young salmon were physically beat up and couldn’t survive the conditions in the ocean.
“What’s been done in the past is that the adult return rates, after the fish have been out in the sea for two to three years, have been used to make inferences about the early marine survival of the fish,” says first author of the new paper Erin Rechisky of Kintama Research in British Columbia Canada.
But Rechisky and her colleagues wanted a more direct measurement of salmon survival during this crucial juvenile time period. So they turned to new acoustic tags which can track fish for many miles. Arrays of receivers for the tags were arranged in the known migratory paths of the salmon.
“We’re able to get an answer before the fish spend two years out at sea,” she says. “We look at their survival from the time they migrate down the river, through the estuaries, and up along 500 kilometers of the Pacific coast.”
The survival rates during the journey—which spans about a month—were statistically no different between Snake River Chinook and salmon that were born in the mid-Columbia and traversed fewer dams to reach the ocean.
Rechisky et. al. hypothesize that factors other than the dams could set the populations—which are genetically distinct—apart. “They might have different distributions out in the ocean that cause this difference in adult return,” says Rechisky. “We know very little about where the fish go after about six months.”
Ray Hilborn of the University of Washington, Seattle, whose commentary on the work appears in a separate PNAS Early Edition piece, says the altitude and climate of the fish’s spawning grounds could also affect growth rates and the ability of the fish to thrive when they reach the ocean. But the new findings offer a starting place, he says, toward more fully understanding the various influences of different environmental factors on the salmon’s survival.
“Why these upriver fish have such poor survival has been a major scientific and management question for thirty years, ever since we started to measure survival,” says Hilborn. “This paper is really an example of how we do have the ability to study the survival of fish in the ocean.”
The work is not without caveats though. Only the largest fish can be tagged, so the survival rates of the smallest fish could still be influenced by the dam crossings. And the fish’s mortality rates later in their ocean journey—during the first winter or in deeper waters—could still be affected by delayed influences of the dams. Future studies tagging smaller fish and following the populations for even longer periods of time could help clear up exactly what makes the Snake River Chinook so vulnerable in the ocean.