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NW Fishletter #387, Nov. 5, 2018

[7] Some Juvenile Salmon Survived Well, Others Didn't Under New Spill Regime

A preliminary report estimating this spring's juvenile salmonid survival rates in the Columbia River Basin shows that the high runoff conditions supplemented by a court-ordered spill at eight dams on the lower Snake and Columbia rivers doesn't always translate into better survival for young fish.

For Snake River juveniles--many of which travel through all eight dams affected by the spill order--sockeye experienced exceptional survival rates, steelhead came in above average, but spring Chinook survival rates were significantly below the long-term average, especially those originating at a hatchery. That's according to NOAA Fisheries' preliminary survival estimates for the passage of spring-migrating juvenile salmonids through Snake and Columbia river dams and reservoirs.

"One of the key observations is that there was substantially higher spill this year, but there was not a corresponding increase in the survival of yearling Chinook," said NOAA Fisheries spokesman Michael Milstein said, adding "It's unclear why that is."

The estimated survival of both hatchery and wild Chinook migrating through all eight dams under this year's court-ordered spill was 43.2 percent--substantially below the 20-year average of 52.1 percent. The report notes, "Yearling Chinook survival through the hydropower system has been consistently below the mean for the past four years, despite a range of different environmental conditions within these years."

Of these four years, this year's survival rate was second lowest, following the 42.8 percent survival in 2015--a drought year with very low flows.

The poor survival rates seem to be driven mostly by poor survival in the McNary-to-Bonneville reach, Milstein notes. The survival rate through the Snake River dams was on par with the long-term average.

Data for the Bonneville Power Administration-funded juvenile survival study--which has been ongoing since 1993--comes from tracking fish with passive integrated transponder (PIT) tags through the system. The rates of survival are monitored for each hatchery, and from fish captured and tagged at a trapping site on the Snake River near Lewiston, Idaho.

Total survival declines as the fish pass through each project on the river. The study breaks out total survival rates for reaches above the eight dams, for the four lower Snake River dams, and for the four lower Columbia River dams. It also provides larger scale pictures of survival rates for all eight dams from Lower Granite Dam to Bonneville Dam, and from the trap to Bonneville Dam.

Overall, the survival rate for both hatchery and wild spring Chinook traveling all the way from the trap and hatcheries to Bonneville Dam--through all eight projects under a spill order--was 38.1 percent, significantly lower than the long-term average of 48.9 percent.

By reach, spring Chinook survival from the trap to Lower Granite Dam was 88 percent; lower than the long-term average of 93 percent. Their survival rate through the four lower Snake River dams was 73.3 percent, similar to the 20-year average survival rate of 73.8 percent. Survival from McNary's tailrace to Bonneville's tailrace was just 59 percent, much lower than the 20-year average of 69.5 percent.

Wild Chinook fared better than their hatchery cousins, with a total estimated survival from the Snake River trap to the Bonneville tailrace of 50.4 percent, above the long-term average of 44.8 percent.

Milstein said there's evidence that some spring Chinook suffered from gas bubble trauma, but whether that was a big enough factor is still unclear. In general, he said, NOAA Fisheries scientists are more concerned about the impacts of high spill and gas bubble trauma on returning adults because they're exposed to the higher gas levels for a longer period of time.

As for the juveniles, he said, "There are a number of different factors that could be potentially contributing." Bird and aquatic predators can have significant impacts on juvenile survival, depending on where they are and how the fish are exposed to them as they pass through the dams. "Typically with the birds, you see more impact on steelhead than Chinook, so that's a bit of a puzzle as well," he said.

NOAA Fisheries scientists will be compiling more PIT-tag data and conducting more detailed analysis before completing a final report, expected to be released in February. The final report will include a more complete database, which could result in differences of up to 3 or 4 percent in the estimated survival rates, the preliminary memo noted.

The report characterizes this spring's environmental conditions as "a year with average water temperatures, but high flow and very high spill for most of the migration season." The average spill discharge at the Snake River dams during this year's downstream migration was 41,300 cfs, substantially higher than the average of 27,700 cfs over the past 25 years.

Spill as a percentage of flow was also much higher this spring, averaging 37.2 percent compared with the long-term average of 27.2 percent.

Juvenile Snake River steelhead--both hatchery and wild--saw much better survival rates this spring. From the trap to the Bonneville tailrace, about 52.4 percent of the young steelhead survived, compared with a long-term average of 45.6 percent. The improved survival rates held true for all stretches of the Snake and lower Columbia rivers.

And, if estimates are accurate, juvenile Snake River sockeye experienced their third-highest survival rate since 1998, with 64.3 percent survival from Lower Granite Dam to the Bonneville Dam tailrace. The high survival rate comes on the heels of three consecutive years with very low survival. "The Idaho Department of Fish and Game has adjusted their acclimation methods this year in order to address the causes of the low Snake River Sockeye survival from the past three years; their efforts almost certainly contributed to the higher survival estimate this year," the report said.

The travel time for juvenile Chinook and steelhead through the eight dams this spring was short--ranging from about two weeks in early April to a few days in mid-May--continuing the trends from recent years, the report said. The travel times are substantially shorter than the long-term average dating back to 1997.

"Since the institution of court-ordered spill in 2006, and the concurrent installation of surface collectors at four additional federal dams during that period, travel times have decreased on average between Lower Granite and Bonneville dams for steelhead, but the effect is less apparent for Chinook," the report notes. "Differences in travel times for low-flow years versus other years are not so well pronounced for either species. Day in season is a stronger predictor of travel time for Chinook than either flow or spill," the report noted.

This year's spill operations also impacted the transporting of juvenile salmonids in the Snake River, with higher-than-average proportions of smolts collected for transportation due to the earlier start date and higher collection rates, the report said. "This was due to the combination of spill operations and river conditions experienced by the fish as the passed the collector dams," the report said.

About 45 percent of the Snake River steelhead and spring Chinook salmon were transported this year--transportation rates that haven't been seen since 2008. -K.C. Mehaffey

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Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: K.C. Mehaffey
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