Recent research takes a detailed look at one of the most misunderstood aspects of salmon migration -- homing movements -- and why some Columbia River basin adult salmonids fail to pass dams, as well as why some fish fall back over dams.

"Some failed passage attempts and fallback events can be linked to dam operations, but others are probably the result of orientation and homing behaviors unrelated to dams, including overshoot of natal tributaries," according to recent work by University of Idaho and NOAA Fisheries researchers.

The study, "Non-Direct Homing By Adult Spring-Summer Chinook Salmon: Tributary Overshoot, Overshoot Fallback, and Temporary Non-Natal Tributary Use in the Columbia River Basin ," was funded by the Army Corps of Engineers and is available at http://www.cnr.uidaho.edu/uiferl/

The report presents summary information on overshoot behavior of radio-tagged adult spring-summer chinook salmon from 1996-2003 and examines relationships between overshoot, temporary straying, and a suite of fish characteristics (e.g. sex, origin, migration timing).

In the study, three migration behaviors -- overshoot of natal tributaries, fallback at dams upstream from natal tributaries, and temporary non-natal tributary use -- were evaluated for more than 5,200 radio-tagged spring-summer chinook salmon.

"Over seven years, from 1 percent to almost 80 percent of eleven Columbia River study stocks initially overshot natal tributaries and were recorded at upstream Columbia or Snake River dams (mean = 29 percent)," says the study abstract.

"Smaller proportions of each stock were recorded falling back at upstream dams (range = 0–49 percent, mean = 17 percent).

"The contribution of overshoot fallback to total fallback by salmon that returned to tributaries was approximately 20 percent at The Dalles, John Day, and Ice Harbor dams and was about 40 percent at McNary Dam," says the abstract.

"Proportions that temporarily entered non-natal tributaries ranged from 8 percent to 42 percent (mean = 22%) of each stock."

Overshoot, overshoot fallback, and temporary tributary use behaviors were greatest when spawning tributaries and/or dams where in close geographic proximity, says the report.

"For example, 39 percent of Klickitat River salmon fell back at The Dalles Dam, 49 percent of Umatilla River salmon fell back at McNary Dam, and salmon from Bonneville pool tributaries were most likely to temporarily use nearby tributaries."

Overshoot and temporary tributary use events, says the report "were complex and interrelated, likely reflecting a combination of active searching for olfactory cues from natal tributaries, behavioral thermoregulation, and geographic proximity among sites."

The researchers say the findings suggest " that direct, point-to-point homing may be less common than is typically reported for adult salmonids. Instead, initial passage of natal tributaries and temporary non-natal tributary use by the study stocks demonstrate that the homing process in the Columbia River system can be quite complex.

"The results suggest that extensive searching behaviors may be necessary for some populations to detect and respond to appropriate olfactory and environmental cues from natal sites. Understanding these behaviors is important for differentiating temporary tributary use and permanent straying, managing fisheries that target specific stocks, and interpreting salmon movements near dams and river confluences.

"The results also provide some insights regarding fish that fail to pass dams and those that fall back downstream over dams, two important management concerns in the Columbia River . Geographic proximity of tributary confluences and dams, salmon orientation tactics, river environment, and impoundment all appeared to affect the observed salmon migration behaviors and these mechanisms are discussed below."

Given that many chinook salmon do overshoot natal sites, researchers say "it may be important for dams in the Columbia River to offer benign downstream passage routes for adult fish. Such routes may be especially important for those stocks whose natal rivers are relatively close to upstream dams, such as those from the Klickitat, Deschutes and Umatilla rivers."

The researchers were M.L. Keefer, C.T. Boggs, C.C. Caudill, and C.A. Peery of the Idaho Cooperative Fish and Wildlife Research Unit at the University of Idaho and M.L. Moser of NOAA Fisheries.