Coldwater fish in a warm water world: Implications for predation of salmon smolts during estuary transit

Integrating otolith and genetic tools to reveal intraspecific biodiversity in a highly impacted salmon population

Intraspecific biodiversity is vital for species persistence in an increasingly volatile world. By embracing methods that integrate information at different spatiotemporal scales, we can directly monitor and reconstruct changes in intraspecific biodiversity. Here we combined genetics and otolith biochronologies to describe the genotypic and phenotypic diversity of Chinook salmon (Oncorhynchus tshawytscha) in the Yuba River, California, comparing cohorts that experienced a range of hydroclimatic conditions. Yuba River salmon have been heavily impacted by habitat loss and degradation, and large influxes of unmarked hatchery fish each year have led to concern about introgression and uncertainty around the viability of its wild populations, particularly the rarer spring-run salmon. Otolith strontium isotopes showed that Yuba River origin fish represented, on average, 42% (range 7%-73%) of spawners across six return years (2009-2011, 2018-2020), with large interannual variability. The remainder of adult Chinook salmon in the river were primarily strays from the nearby Feather River hatchery, and since 2018 from the Mokelumne River hatchery. Among the Yuba-origin spawners, on average, 30% (range 14%-50%) exhibited the spring-run genotype. The Yuba-origin fish also displayed a variety of outmigration phenotypes that differed in the timing and size at which they left the Yuba river. Early-migrating fry dominated the returns (mean 59%, range 33%-89%), and their contribution rates were negatively correlated with freshwater flows. It is unlikely that fry survival rates are elevated during droughts, suggesting that this trend reflects disproportionately low survival of larger later migrating parr, smolts, and yearlings along the migratory corridor in drier years. Otolith daily increments indicated generally faster growth rates in non-natal habitats, emphasizing the importance of continuing upstream restoration efforts to improve in-river growing conditions. Together, these findings show that, despite a long history of habitat degradation and hatchery introgression, the Yuba River maintains intraspecific biodiversity that should be taken into account in future management, restoration, and reintroduction plans. The finding that genotypic spring-run are reproducing, surviving, and returning to the Yuba River every year suggests that re-establishment of an independent population is possible, although hatchery-wild interactions would need to be carefully considered. Integrating methods is critical to monitor changes in key genetic, physiological, and behavioral traits to assess population viability and resilience.

Storing and managing water for the environment is more efficient than mimicking natural flows

Dams and reservoirs are often needed to provide environmental water and maintain suitable water temperatures for downstream ecosystems. Here, we evaluate if water allocated to the environment, with storage to manage it, might allow environmental water to more reliably meet ecosystem objectives than a proportion of natural flow. We use a priority-based water balance operations model and a reservoir temperature model to evaluate 1) pass-through of a portion of reservoir inflow versus 2) allocating a portion of storage capacity and inflow for downstream flow and stream temperature objectives. We compare trade-offs to other senior and junior priority water demands. In many months, pass-through flows exceed the volumes needed to meet environmental demands. Storage provides the ability to manage release timing to use water efficiently for environmental benefit, with a co-benefit of increasing reservoir storage to protect cold-water at depth in the reservoir.

Quantification of thermal impacts across freshwater life stages to improve temperature management for anadromous salmonids

Water temperature is the major controlling factor that shapes the physiology, behaviour and, ultimately, survival of aquatic ectotherms. Here we examine temperature effects on the survival of Chinook salmon (Oncorhynchus tshawytscha), a species of high economic and conservation importance. We implement a framework to assess how incremental changes in temperature impact survival across populations that is based on thermal performance models for three freshwater life stages of Chinook salmon. These temperature-dependent models were combined with local spatial distribution and phenology data to translate spatial-temporal stream temperature data into maps of life stage-specific physiological performance in space and time. Specifically, we converted temperature-dependent performance (i.e. energy used by pre-spawned adults, mortality of incubating embryos and juvenile growth rate) into a common currency that measures survival in order to compare thermal effects across life stages. Based on temperature data from two abnormally warm and dry years for three managed rivers in the Central Valley, California, temperature-dependent mortality during pre-spawning holding was higher than embryonic mortality or juvenile mortality prior to smolting. However, we found that local phenology and spatial distribution helped to mitigate negative thermal impacts. In a theoretical application, we showed that high temperatures may inhibit successful reintroduction of threatened Central Valley spring-run Chinook salmon to two rivers where they have been extirpated. To increase Chinook salmon population sizes, especially for the threatened and declining spring-run, our results indicate that adults may need more cold-water holding habitat than currently available in order to reduce pre-spawning mortality stemming from high temperatures. To conclude, our framework is an effective way to calculate thermal impacts on multiple salmonid populations and life stages within a river over time, providing local managers the information to minimize negative thermal impacts on salmonid populations, particularly important during years when cold-water resources are scarce.