Abstract
In 2011, unusually high flows caused total dissolved gas (TDG) levels in the Columbia River, USA, to escalate well above the 120% regulatory limit that was imposed to prevent harmful impacts to aquatic organisms. After observing gas bubble trauma (GBT) in dead yearling Chinook Salmon Oncorhynchus tshawytscha (smolts) held in tanks, we compared estimated survival rates of acoustic-tagged in-river-migrating (IR) and transported (TR) smolts that were released below Bonneville Dam prior to and during the period of elevated TDG (>120%). The log odds of estimated daily survival in the lower river and plume was significantly lower for IR smolts that were released during elevated TDG (maximum possible exposure = 134%) than for IR smolts released when TDG was less than 120%. The TR smolts that were released 10-13 km below Bonneville Dam during elevated TDG had lower maximum possible exposure levels (126% TDG), and the log odds of estimated daily survival in the lower river and plume did not differ from that of TR smolts released when TDG was less than 120%. Direct mortality due to GBT is probably reduced in natural settings relative to laboratory experiments because smolts can move to deeper water, where pressure keeps gasses in solution, and can migrate downstream of the spillway, where TDG levels decrease as the river returns to equilibrium with the atmosphere. However, initially nonlethal GBT may reduce survival rates by increasing smolt susceptibility to predation and infection. Although our findings are limited by the observational nature of the study, our analysis is the first direct assessment of gas supersaturation's potential influence on survival of free-ranging smolts in the river and coastal ocean below a large dam. Experiments using simultaneous releases of control and gas-exposed groups are warranted and should consider the possibility that the chronic effects of TDG exposure on survival are important and persist into the early marine period. Received October 5, 2015; accepted August 18, 2016 Published online October 24, 2016.
Highlights
IntroductionT on salmon smolts (reviews by Weitkamp and Katz 1980; McGrath et al 2006; Maynard 2008)
In 2011, high flows in the Columbia River resulted in total dissolved gas (TDG) reaching 134% below Bonneville Dam, which was fortuitously coincident with the release of both barge-transported and in-river-migrating, acoustic-tagged juvenile yearling Chinook Salmon Oncorhynchus tshawytscha during a long-term study (Rechisky et al 2012, 2013, 2014; Brosnan et al 2014)
Seventy-two in-river low exposure (IR-LOW) fish were detected at the Columbia River mouth and remained somewhere upriver of the Astoria and Sand Island subarrays after May 14, when TDG reached 120% in the Bonneville Dam tailrace
Summary
T on salmon smolts (reviews by Weitkamp and Katz 1980; McGrath et al 2006; Maynard 2008). Because the timing of tagged smolt releases covered the period before and after TDG levels climbed above the state limits, we were given a unique opportunity to compare the potential effect of exposure to gas supersaturation on the subsequent survival of free-ranging Chinook Salmon smolts in the lower Columbia River and estuary (Bonneville Dam to Astoria; lower river) and the Columbia River plume (Astoria to Willapa Bay; Figure 1) Our objective with this retrospective cohort study was to compare the survival of transported (TR) and in-river-migrating (IR) smolts that were released into gas-supersaturated waters below the 120% state water quality limits (i.e., classified as “low exposure”) with that of their counterparts, which were released when TDG levels were above the limits (classified as “high exposure”; Figure 2). Other factors, such as temperature, turbidity, emigration timing, and diseases other than GBT, did not appear to play an important role in survival over the study period (see Discussion)
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