Residence Time of AcousticTagged Juvenile Chinook Salmon in off-Channel Areas During Winter and Early Spring

In 2007, 2008, and 2010, we studied migration pathways and residence times for acoustic-tagged juvenile salmonids in off-channel, tidal freshwater habitats of the Columbia River in the vicinity of the Sandy River delta (SRD, rkm 188-202) using acoustic telemetry.  The study had two phases, one that was conducted in 2007 and 2008 and the second during 2010. We deployed autonomous acoustic receivers to detect fish, which included yearling and subyearling Chinook salmon (Oncorhynchus tshawytscha) smolts every year and juvenile steelhead (O. mykiss) in 2008.  Juvenile salmonids were tagged with acoustic transmitters and released at or upstream of Bonneville Dam (rkm 234). Lengths of tagged fish during Phase I averaged 144 mm for yearling Chinook salmon, 110 mm for subyearling Chinook salmon, and 215 mm for steelhead. In 2007, there were 575 Chinook salmon with unique detections in off-channel areas and 981 Chinook salmon and steelhead detections in 2008. Coupled with data on main channel detection rates from other studies, we determined 10.9% of yearling and 3.7% of subyearling Chinook salmon migrated through the SRD and vicinity in off-channel habitats during 2007. During the 2008 study period, 8.4% of the tagged yearling Chinook salmon, 6.9% of subyearling Chinook salmon, and 3.1% of steelhead used off-channel migration routes. Mean residence times in off-channel areas were short (<4 hrs) for all species; steelhead mean residence time was the shortest with an average of <1hr and subyearling Chinook salmon residence time was the longest with a mean of 3.69 hrs. In Phase 2 during winter 2010, we used a beach seine to capture juvenile Chinook salmon in the SRD. Fifty-one fish (mean FL 103 mm) were tagged with transmitters, released, and detected on receivers in off-channel areas of the Columbia River near the Sandy River delta from January through April 2010. Genetic stock identification analysis indicated most of the tagged fish (84.3%) belonged to the West Cascades and Willamette stock groups. We detected 48 of 51 tagged fish from January 28 to April 15. Mean winter residence time was 34 days (median 26 d). These findings support restoration of off-channel habitats in tidal freshwater areas of the Columbia River to aid Columbia River and Willamette River stocks.