Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 8556
Title: Post-hydropower system delayed mortality of transported Snake River stream-type Chinook salmon: unraveling the mystery
Author: William D. Muir, D. M. Marsh, Benjamin P. Sandford, Steven G. Smith, John G. Williams
Publication Year: 2006
Journal: Transactions of the American Fisheries Society
Volume: 135
Issue: 6
Pages: 1523-1534
Keywords: Chinook salmon, smolt-to-adult return rates, transportation,

Past research indicates that on an annual basis, smolts of stream–type Chinook salmon Oncorhynchus tshawytscha collected at Snake River dams and transported by barge to below Bonneville Dam have greater post–hydropower system mortality than smolts that migrate inriver.  To date, this difference has most commonly been attributed to stress from collection and transportation, leading to decreased disease resistance or predator avoidance ability. 

Using both hatchery and wild passive integrated transponder (PIT) tagged Chinook salmon, we explored two mechanisms that either separately or jointly contributed to an alternative explanation:  Altered timing of ocean entry and lost growth opportunity leading to size–selective predation.  Based on weekly estimates of inriver survival and adult return rates of smolts that were transported or that migrated inriver between Lower Granite and Bonneville Dam, we found greater post–hydropower system mortality for smolts transported early in the season but greater mortality for inriver migrating smolts later in the season. 

Migrants took 24 weeks to travel between the two dams, while transported fish took less than 2 d.  Thus, fish leaving Lower Granite Dam under the two transit modes encountered different conditions downstream from Bonneville Dam.  Further, wild and hatchery migrants grew 68 and 56 mm, respectively, while transported fish had no apparent growth in the less than 2–d barge ride.  Using length data and regression equations of size selectivity, we found that transported smolts were more vulnerable to predation by northern pikeminnow Ptychocheilus oregonensis (freshwater) and Pacific hake Merluccius productus (marine) than were migrants; this was particularly true for the smaller wild smolts transported early in the season.  We concluded that the most parsimonious explanation for differential post–hydropower system mortality of transported Chinook salmon smolts related not to effects of stress but to differential size and timing of ocean entry.

After completion of the federal hydropower system on the Snake and Columbia rivers in 1975, the number of returning adult, stream–type, springsummer Chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss declined, in large part as a result of reduced survival and delay of juvenile fish (smolts) during their downstream migration (Raymond 1979, 1988).  For several decades, smolts have been collected at Snake River dams and trucked or barged downstream to below the last Columbia River dam for release to avoid mortality caused by dams (Ebel et al. 1973; Ebel 1980). 

During this period, dam mortality of inriver migrants has been reduced through structural and operational improvements at the dams, including improvements to or installation of juvenile bypass systems to keep smolts from entering turbines; predator removal and exclusion programs; flow augmentation; and the use of spill to pass smolts through nonturbine routes and reduce migrational delay.  As a result of these efforts, the conditions encountered by inriver migrants at dams improved substantially (Williams and Matthews 1995), as did their survival (Muir et al. 2001; Williams et al. 2001, 2005).

Studies to evaluate the efficacy of smolt transportation (barging and trucking smolts) have generally shown a benefit, especially for steelhead.  Yet, adult returns of transported smolts have generally fallen short of expectations, particularly for wild springsummer Chinook salmon (Ward et al. 1997; Williams et al. 2005).  Inriver survival of springsummer Chinook salmon (hereafter referred to as Chinook salmon) from the uppermost dam on the Snake River to below Bonneville Dam averaged about 4050% in non–drought years (Williams et al. 2005), while survival of smolts transpo

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Theme: Recovery and rebuilding of marine and coastal species
Foci: Describe the relationships between human activities and species recovery, rebuilding and sustainability.