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NW Fishletter #375, November 6, 2017

[7] Dam Passage Survival Estimates for 2017 Juvenile Salmon a Mixed Bag

This year's survival of juvenile spring salmonids migrating through the Columbia-Snake river system was below average for some stocks and above average for others.

That's the finding of the Preliminary Survival Estimation Memorandum from NOAA's Northwest Fisheries Science Center, which was presented at the Oct. 4 meeting of the interagency Technical Management Team.

The science center annually estimates juvenile survival of Snake River hatchery and wild Chinook, steelhead and sockeye migrating through the dams and reservoirs in the lower Snake and Columbia rivers from April 1 to June 15.

It also estimates the juvenile survival of upper Columbia hatchery Chinook and steelhead from McNary Dam to Bonneville Dam, and upper Columbia sockeye from Rock Island Dam to Bonneville during that April-to-June period.

The science center has estimated survival for spring migrating PIT-tagged salmonids since 1993. The report's principal author is Richard Zabel, the center's director of fish ecology.

For yearling Snake River Chinook--hatchery and wild combined--the 2017 survival was above average or nearly average through the individual reservoirs and dams on the lower Snake and Columbia rivers, except for the reach between McNary and John Day dams.

The report said the estimated survival of these juveniles in the McNary-to-John Day reach was the lowest on record.

This resulted in overall below-average survival from Lower Granite Dam to Bonneville Dam of 47.8 percent for juvenile Snake River Chinook, compared to 52.4-percent average survival since 1993.

For juvenile Snake River steelhead--hatchery and wild combined--2017 was above average in all of the individual reaches, except for the area between John Day and Bonneville dams, where survival was lower.

The poor survival between John Day and Bonneville brought the overall survival rate from Lower Granite to Bonneville down to 45.9 percent this year, compared to 46.3 percent since 1993, making it the third consecutive year of below-average survival for the Snake River juvenile steelhead outmigration, the report said.

For juvenile Snake River sockeye, 2017 was also the third year in a row of below-average survival. From Lower Granite to Bonneville, survival was estimated at 17.7 percent; the average since 1996 is 39.2 percent. Low survival from McNary to Bonneville dams accounted for 22 percent, or the largest portion of the drop.

For juvenile Chinook and steelhead originating from the upper Columbia, survival was above average, while upper Columbia sockeye survival was just average.

The survival of upper Columbia hatchery yearling Chinook from McNary to Bonneville was 94.4 percent, while the average is 81.8 percent.

For upper Columbia hatchery steelhead, the McNary-to-Bonneville survival was 96.4 percent, while the average is 74.7 percent. The science center memo said both these Chinook and steelhead estimates have "high uncertainty."

For fish released from upper Columbia River hatcheries, "we cannot estimate survival in reaches upstream from McNary Dam (other than the overall reach from release to McNary Dam tailrace) because of limited PIT-tag detection capabilities at Mid-Columbia River PUD dams," Zabel noted in the report.

Survival for upper Columbia juvenile sockeye from Rock Island Dam to Bonneville was 50 percent, while the average since 1996 is 50.3 percent.

The science center said environmental conditions this year produced average water temperatures and very high flows and spills. Daily flow values were above long-term daily means for most of the April 1-June 15 period, the report said. Only 1997 had higher flow.

Columbia and lower Snake River dams. Credit: Courtesy of CRITFC

At Snake River dams, spill as a percentage of flow averaged 44.5 percent in 2017, the highest spill percentage since 1993. The long-term mean percentage is 26.8.

The memo said that juvenile fish showed up at lower Snake River dams earlier than normal this year. When transportation began May 2 at Lower Granite, Little Goose and Lower Monumental dams, more than half of the Chinook and steelhead had already passed.

As a result, only some 20 percent of the Snake River Chinook and steelhead juveniles were transported during the spring migration period. The report said the estimates for 2015, 2016 and 2017 were the three lowest transportation levels since the scientists started tracking transport levels in 1993.

The report said the reason for the lower-than-average proportions of juveniles collected during the spring of 2017 was "presumably" higher spill levels.

"The combination of early migration timing and of relatively low collection proportions late in the season resulted in the low percentages of smolts [juveniles] transported in 2017," the memo said.

Regarding fish travel time, the science center reported that juvenile Chinook and steelhead migrating from Lower Granite to Bonneville last spring traveled through the rivers' reservoirs and dams faster than in any other year in the time series.

Shorter travel time is associated with higher juvenile-to-adult survival rates.

While 2017 travel times coincided with high flows and high involuntary spill at dams, high spill was not necessarily the reason for the shorter travel time, the memo said.

"Day in season is a stronger predictor of travel time for Chinook than either flow or spill. Some of the lowest flow years were also low-spill years that occurred before the new spill regime [which started in 2006], so the effect of average flow on travel time is difficult to separate from that of spill by simply inspecting the figures without the assistance of a statistical model," Zabel wrote.

The estimates are preliminary and have in the past resulted in differences of up to 3 or 4 percent in estimated survival values. A final report will be available in the next few months.

The juvenile survival estimates for migration through the Columbia-Snake dams do not include summer-migrating juvenile salmonids. -Laura Berg

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Publisher/Editor-in-Chief: Mark Ohrenschall, Editor: Laura Berg
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