The 2011 spring season’s high, cool flows down through the Columbia-Snake river hydro system may have been both a blessing and a curse, with overall survival of juvenile steelhead and yearling spring chinook above and near, respectively, long-term averages but lagging behind the past two years’ rates.
“Estimated survival for Snake River yearling chinook salmon and steelhead through the hydropower system (Snake River trap to Bonneville tailrace) in 2011 was lower than in the previous two years,” according to a recent report, “Preliminary survival estimates for passage during the spring migration of juvenile salmonids through Snake and Columbia River reservoirs and dams, 2011.”
The trap is located just below Lower Granite Dam on the lower Snake River in southeast Washington. Lower Granite is the first dam the juvenile fish must pass on their way from hatcheries and spawning grounds in the lower Snake and its tributaries, many of which are in Idaho. Bonneville is the eighth and final dam the salmon and steelhead negotiate on their migration to the Pacific Ocean.
“For yearling Chinook, the 2011 estimated hydropower system survival was 48 percent, which is nearly equal to the long-term average of 49 percent, but lower than (though not significantly different than; P = 0.31) the 2010 estimate of 55.1 percent.
“For steelhead, the 2011 estimated hydropower system survival was 56.9 percent, which is higher than the long-term average of 41.7 percent, but lower than (though not significantly different than; P = 0.40) the 2010 estimate of 61.8 percent.”
Study results compiled through the monitoring of PIT-tagged wild and hatchery produced fish are summarized in a Sept. 13 memo from the Northwest Fisheries Science Center’s John Ferguson to Bruce Suzumoto, NOAA Fisheries assistant regional administrator, Hydopower Division. The preliminary data was compiled this year as part of a long-running study dating back to 1993 headed by the NOAA Fisheries’ NWFSC.
The summary notes river conditions and survival estimates, as well as preliminary estimates of the proportion of Snake River smolts that were collected from at juvenile bypass systems at three lower Snake dams and transported downstream through the hydro system aboard barges.
The “summer memo” cautions that the results are preliminary and could be changed following the more detailed analysis to be undertaken for a final report due by year’s end. In the past differences have ranged up to 3 or 4 percent in estimated survival values.
The memo notes that “Snake River flow volume in 2011 was higher than in recent years for most of the migration period.” In some cases those high flows, resulting from the meltdown of near-record snowpack, created obstacles for the migrating fish.
“High flows resulted in increased debris, which caused problems for dam operations and juvenile bypass systems at several projects. Debris clogged orifices in the gatewells at Little Goose Dam on 19 May, resulting in smolt mortalities. Debris at Bonneville Dam necessitated removal of the bypass screens from several turbine units starting on 19 May, with screens not reinstalled until 12 July. High flow and debris also caused problems for the towed PIT-tag detection array in the estuary, resulting in less sampling time and restricted sampling areas compared to previous years.”
“The removal of the bypass screens at Bonneville Dam resulted in greater numbers of fish going through turbines, which likely resulted in lower survival through the dam than would have occurred otherwise.
“At the same time, screen removal also resulted in an extreme reduction in PIT-tag detections, which meant that survival could not be estimated during that period. Therefore, any annual average survival estimate for chinook or steelhead that includes the John Day to Bonneville reach in 2011 is most likely an overestimate of the actual survival of the population,” the memo said. “Smolt indices at John Day Dam indicated that when the screens were first removed at Bonneville Dam on 19 May, approximately 40 percent of chinook population and 45 percent of steelhead population originating from above John Day Dam had not yet passed John Day Dam.”
A powerhouse shut down May 24-June 2 at Little Goose Dam on the lower Snake, forced by a damaged transformer, was also troublesome.
A “consequence of the powerhouse shutdown at Little Goose was an increase in the amount of discharge forced to go through the spillways. This occurred during a period of high flow and resulted in high levels of dissolved gas in Lower Monumental reservoir. These increased gas levels may have resulted in increased smolt mortality for both yearling chinook salmon and steelhead,” the memo said.
“Average Snake River spill percentages in 2011 hovered around 30 percent until the flow increase in early May, when average spill percentages increased to levels higher than any seen in recent years,” the memo says of water volumes that forced involuntary spill at dams beyond what is prescribed in NOAA Fisheries’ biological opinion on federal hydro system operations. The prescribed release of water through spill gates, as opposed to sending the water through power-generating turbines, is designed to provide what is considered a more benign route of passage for salmon and steelhead stocks that are listed under the Endangered Species Act. Generally, higher cooler water, and spill, are believed beneficial to fish.
“Water temperatures in the Snake River in 2011 were on the cool side of average for most of the season.”
The memo notes that the percentage of chinook salmon and steelhead collected and transported from Snake River dams down through the Columbia-Snake system in 2011 were similar to 2010, which were among the lowest seen from 1995-2009. Transportation is intended to help the young fish avoid the rigors, and predators, of swimming down through the dams and reservoirs.
“Throughout the migration season, high spill percentages, in combination with surface bypass collection at each of the collector dams on the Snake River, resulted in low proportions of fish entering juvenile bypass systems,” the memo says. “These relatively low collection rates, a late start in collection relative to run timing, and temporary suspensions of transportation resulted in relatively low proportions transported in 2011.”
For PIT-tagged hatchery yearling chinook salmon originating from the upper Columbia River in 2011, estimated survival from the Columbia’s McNary Dam tailrace down past John Day, The Dalles and Bonneville dams was 70.1 percent, down slightly from the 1999-2011 average of 75.8 percent. Columbia River origin steelhead survival down through stretch was 65.1 percent this year, according to the preliminary. That is also lower than the average, 75.2 percent.
Estimated 2011 survival of Snake River sockeye salmon from Lower Granite to McNary Dam was estimated at 65.9 percent, which is up from the 1998-2011 average of 60.2 percent. Poor detection in the lower Columbia make it impossible to estimate survival to Bonneville Dam tailrace for Snake River sockeye, the memo says.