Search for the Smoking Gun: Identifying and Addressing the Causes of Postrelease Morbidity and Mortality of Hatchery‐Reared Snake River Sockeye Salmon Smolts

Abstract

As part of the Snake River Sockeye Salmon Oncorhynchus nerka recovery effort, a dedicated smolt rearing facility was constructed in 2013 near Springfield, Idaho. In‐hatchery performance and survival were typical for the species, but unexpectedly high mortality rates were observed in the first cohorts of Springfield‐reared smolts upon release into Redfish Lake Creek (RFLC) and during out‐migration. In response, a series of iterative experiments was conducted to identify the cause of the morbidity and mortality observed and to test a range of strategies to mitigate effects on postrelease survival. In the search for possible contributing factors, a difference in water chemistry was noted: whereas Springfield Hatchery's water source is “hard” and has high calcium concentrations, water at the RFLC release site is “soft” and has very low calcium concentrations. In both manipulative experiments and field evaluations, we demonstrated that juvenile Snake River Sockeye Salmon are profoundly affected by instantaneous transitions from high‐ to low‐hardness water. Furthermore, we established a causal link between differences in water chemistry, the associated physiological stress, and morbidity/mortality observed during smolt releases and subsequent out‐migration. A variety of mitigation strategies, including water mixing and water softening, was tested, but stepwise acclimation from high‐ to medium‐hardness water and then from medium‐ to low‐hardness water proved to be the most biologically and logistically effective means of addressing the identified water chemistry differences. Estimates of postrelease survival to Lower Granite Dam (~430 river kilometers downstream) indicated significantly higher survival for acclimated groups (68.7–75.5%) compared to smolts directly released into RFLC (18.1%). Although Snake River Sockeye Salmon smolt survival rates will undoubtedly fluctuate annually with environmental conditions, it is clear that the elevated morbidity and mortality observed in previous years can be addressed through proper acclimation of smolts prior to release.

Population Dynamics and Temporal Trends of Bull Trout in the East Fork Salmon River, Idaho

“…However, the fact that the Smolts × Avgflow interaction term was not significant (i.e., the 95% CIs around the parameter estimate overlapped zero) suggests that lower streamflow was not altering Bull Trout foraging success, at least not over the range of flows that we observed. Nevertheless, foraging conditions for adult Bull Trout in central Idaho may become even more favorable as climate change continues to reduce the magnitude and duration of high streamflows during late spring to early summer (Mote et al 2005) when the majority of smolt emigration occurs (e.g., Copeland and Venditti 2009; Trushenski et al 2019). However, given that the effects of climate change on coldwater ecosystems are somewhat unpredictable (Isaak et al 2012), how climate change will affect adult Bull Trout survival and growth in central Idaho is a matter of conjecture (Lynch et al 2016).…”

Fisheries Volume 45 Number 3 March 2020

“…By contrast, water quality professionals customarily focus on the quantifi cation of chemical constituents or physical habitat and subsequent comparison against thresholds that are indicators of biotic health. When aquatic communities are explicitly considered, thresholds are often based on protecting 95% of individuals exposed to a contaminant (Grist et al 2002 ;Cormier and Suter 2013 ;van Dam et al 2014 ). However, the most sensitive or commercially, recreationally, or environmentally important species (e.g., Endangered Species Act listed species) can drive the selection of the protective thresholds.…”

“…An estimated 200,000-300,000 fall Chinook Salmon Oncorhynchus tshawytscha smolts were killed in an incident at a California hatchery because debris from a spillway failure caused oxygen levels to drop (Cahill 2017 ). Recently, the Idaho Department of Fish and Game in collaboration with National Marine Fisheries Service researchers discovered that low survival of endangered, hatchery-reared Sockeye Salmon O. nerka smolts was the result of differences in water hardness between the hatchery and the release site (Trushenski et al 2019 ). In each of these situations, desired fi sheries goals cannot be achieved without addressing underlying water quality issues.…”

A Call for Collaboration among Water Quality and Fisheries Professionals

“…Recently, the Idaho Department of Fish and Game in collaboration with National Marine Fisheries Service researchers discovered that low survival of endangered, hatchery‐reared Sockeye Salmon O. nerka smolts was the result of differences in water hardness between the hatchery and the release site (Trushenski et al. ). In each of these situations, desired fisheries goals cannot be achieved without addressing underlying water quality issues.…”

“…An estimated 200,000-300,000 fall Chinook Salmon Oncorhynchus tshawytscha smolts were killed in an incident at a California hatchery because debris from a spillway failure caused oxygen levels to drop (Cahill 2017). Recently, the Idaho Department of Fish and Game in collaboration with National Marine Fisheries Service researchers discovered that low survival of endangered, hatchery-reared Sockeye Salmon O. nerka smolts was the result of differences in water hardness between the hatchery and the release site (Trushenski et al 2019). In each of these situations, desired fisheries goals cannot be achieved without addressing underlying water quality issues.…”