Mortality Estimates of Juvenile Spring–Summer Chinook Salmon in the Lower Columbia River and Estuary, 1992–1998: Evidence for Delayed Mortality?

Abstract

Recovery of Endangered Species Act—listed salmonids in the Columbia River basin has relied upon the efficacy of the U.S. Army Corps of Engineer's juvenile salmon transportation program to move fish past Snake and Columbia River hydropower dams. The effectiveness of this program has been assessed by the indirect method of comparing smolt‐to‐adult returns. We present some of the first data and mortality estimates of barged and run‐of‐river (ROR) radio‐tagged juvenile spring–summer Chinook salmon Oncorhynchus tshawytscha after release in the lower Columbia River, representing years of study. Our data suggest that smolt mortality (1) is very low for ROR and barged fish between Bonneville Dam and the estuary proper, a migratory distance of 180 river kilometers (rkm); (2) occurs in the lower estuary (rkm 0–46); (3) varies more across dates within a year than between years or between passage types (barged or ROR); (4) increases with time within a season and increasing numbers of avian predators, including Caspian terns Sterna caspia and double‐crested cormorants Phalacrocorax auritus; and (5) is estimated to be 11–17% of all smolts annually. Preliminary evidence suggests that at least some smolt mortality is influenced by differential predation by avian predators on Chinook salmon infected with Renibacterium salmoninarum and possessing low smoltification levels (relatively low gill Na+,K+‐ATPase activity). Fish type (barged or ROR) did not appear to influence mortality because of avian predation. This project was also the first to identify avian predators as a major source of mortality for out‐migrant Columbia River basin salmonids.

Testing for delayed mortality effects in the early marine life history of Columbia River Basin yearling Chinook salmon

“…Despite tracking smolts as far as northern Vancouver Island, 750 km beyond the last dam and for approximately 1 mo after ocean entry, we did not observe lower survival of SIR smolts. Consistent with several studies (Schreck et al 2006, Clemens et al 2009, McMichael et al 2010, Harnish et al 2012, survival in the lower river and estuary was high, and although subsequent marine survival was low, smolts originating from the Snake River apparently did not suffer deleterious effects (i.e. extra mortality) from additional dam passage.…”

“…Modelling exercises have demonstrated that even if hydrosystem survival was 100%, population growth rates would continue to decline unless reductions in first-year mortality, particularly early ocean and estuarine mortality, occurred (Kareiva et al 2000). Several studies have shown that survival of spring Chinook is high in the Columbia River estuary (Schreck et al 2006, Clemens et al 2009, McMichael et al 2010, Harnish et al 2012, leaving little room for improvement. Given the significant correlation between ocean conditions that juvenile spring Chinook salmon encounter following ocean entry and the number of adults subsequently returning to the Columbia River (Burke et al 2013, NOAA Fisheries Service 2013 and that depressed Chinook salmon populations are not unique to the Columbia River Basin (Chinook salmon in the highly altered Sacramento River suffered a recent collapse, Lindley et al 2009, as did several populations originating from pristine Alaskan rivers, ADF&G Chinook Salmon Research Team 2013), increasing scrutiny of marine survival is warranted.…”

Migratory Behavior and Survival of Juvenile Salmonids in the Lower Columbia River and Estuary in 2009

“…In 2009, 17% of acoustic-tagged steelhead smolts detected at the Bonneville Dam forebay array (CR236.0) were detected on the East Sand Island nesting colonies. These percentages compare closely to previous studies that have found 9-16% of steelhead that pass Bonneville Dam are eaten by birds in the estuary (Collis et al 2001;Roby et al 2003;Ryan et al 2003;Schreck et al 2006). Based on last detections of acoustic-tagged fish, particularly steelhead, it appears that the foraging distance of Caspian terns may have been greater in 2009 than reported for radio-tagged terns by Anderson et al (2007).…”

“…Questions remain regarding whether the passage experience of a juvenile salmonid through the FCRPS may influence its probability of survival as it migrates through the lower 235 km of the Columbia River and its estuary. Some researchers have suggested that the previous passage experience through the FCRPS may influence the survival of juvenile salmonids as they transit the lower Columbia River and estuary (Budy et al 2002;Ferguson et al 2006;Schreck et al 2006;Schaller and Petrosky 2007). To better understand where fish losses were occurring in the lower Columbia River and its estuary, PNNL deployed additional autonomous receiver arrays in strategic locations to separate the lower 235 km of the Columbia River and estuary into specific reaches in 2007 and 2008.…”

Mitigating the effects of high-head dams on the Columbia River, USA: experience from the trenches

“…Based on results from earlier transportation studies, transportation was hypothesized to decrease overall adult returns because it caused stress which increased susceptibility to disease or predation (Congleton et al, 2000;Mesa et al, 2000;Budy et al, 2002;Schreck et al, 2006). Now, with recent information on temporal, seasonal changes in adult return rates, the past physiological hypotheses of mortality do not explain return rates as well as hypotheses on differential rates of predation in the estuary and ocean based on size of fish predators and juvenile salmonids (Muir et al, 2006).…”