Informed water management alternatives for an over‐allocated river: Incorporating salmon life stage effects into a decision tree process during drought

Abstract

Water competition in overallocated rivers is often extreme, and climate change exacerbates the challenge of balancing ecosystem and societal water needs. During a severe California drought in 2013–2014, storage in a strategic reservoir dropped to critically low levels, necessitating reduced downstream discharge during Chinook salmon, Oncorhynchus tshawytscha (Walbaum), incubation and rearing. In response, stakeholders developed an adaptive management process to balance competing water needs, including reservoir storage and salmon survival. This approach incorporated decision tree models, integrating salmon life stage transitions to define potential impacts. Life stage‐specific thresholds were identified and monitored to determine management triggers and actions. Flow reduction stranded up to 12% of incubating embryos and thus was used to trigger a flow pulse. Frequency of stranded fry in redds was reduced post‐pulse. Water quality did not change but remained within the species’ tolerance. High densities, poor body condition and aggressive behaviour of stranded fry triggered a second pulse three weeks later. This pulse reduced stranding and initiated downstream migration. Prescribed flow pulses supported outmigration of a keystone species while minimising water use during a critical period. This study evaluated ecological responses to management actions intended to ameliorate stressful low flow conditions and provided a decision‐making framework that can be used when resource use conflicts arise again.

Applying parentage methods to detect gravel augmentation effects on juvenile Chinook Salmon recruitment rates

“…Given that females were assumed to have spawned shortly before collection, the spawning day was calculated by converting the sample date to Julian days using the yday function in the lubridate package, version 1.7.9.2, (Grolemund & Wickham, 2011) and adjusted so that Julian Day 301 (October 28), the first day on which spawning was observed across sample years, became spawning day 1. The predicted hatch date was then calculated using a temperature‐based embryo development model (see Sellheim et al, 2020 and references therein) and mean daily water temperature (USGS #11446500). The duration between spawning day and predicted hatch day defined the assumed incubation period over which environmental covariates (e.g., mean daily temperature and variance daily flow) were calculated.…”

“…These include numerous barrier dams that now block Chinook Salmon from over 90% of their historical spawning range (Yoshiyama et al, 2001). Reservoir operation also disrupts the natural hydrograph, decoupling high‐flow events from the timing of salmonid freshwater life stages (Sellheim et al, 2020; Zeug et al, 2011). Further, a legacy of gold and gravel mining, leveed riverbanks, and severed sediment transport from upstream watersheds, reduces remaining spawning and rearing habitat quantity and quality below these structures (Kondolf, 1997).…”

A Comprehensive Review of the Impacts of Climate Change on Salmon: Strengths and Weaknesses of the Literature by Life Stage

“…Management. Management actions that affected the egg stage included improving flow and thermal regimes by altering dam operations [100,[356][357][358][359][360]. Habitat restoration options [361,362] included removing barriers to historic habitat, augmenting gravel for spawning, and reintroductions to newly accessible or restored habitats.…”