Nonlinear survival of imperiled fish informs managed flows in a highly modified river

Abstract

Water is a fundamental resource in freshwater ecosystems, and streamflow plays a pivotal role in driving riverine ecology and biodiversity. Ecologically functional flows, managed hydrographs that are meant to reproduce the primary components of the natural hydrograph, are touted as a potential way forward to restore ecological functions of highly modified rivers, while also balancing human water needs. A major challenge in implementing functional flows will be establishing the shape of the managed hydrograph so as to optimize improvements to the ecosystem given the limited resources. Identifying the shape of the flow-biology relationship is thus critical for determining the environmental consequences of flow regulation. In California's Central Valley, studies have found that increased streamflow can improve survival of imperiled juvenile salmon populations during their oceanward migration. These studies have not explored the potential nonlinearities between flow and survival, giving resource managers the difficult task of designing flows intended to help salmon without clear guidance on flow targets. We used an information theoretic approach to analyze migration survival data from 2436 acoustic-tagged juvenile Chinook salmon from studies spanning differing water years (2013)(2014)(2015)(2016)(2017)(2018)(2019) to extract actionable information on the flow-survival relationship. This relationship was best described by a step function, with three flow thresholds that we defined as minimum (4259 cfs), historic mean (10,712 cfs), and high (22,872 cfs). Survival varied by flow threshold: 3.0% below minimum, 18.9% between minimum and historic mean, 50.8% between historic mean and high, and 35.3% above high. We used these thresholds to design alternative hydrographs over the same years that included an important component of functional flows: spring pulse flows. We compared predicted cohort migration survival between actual and alternative hydrographs. Managed hydrographs with pulse flows that targeted high survival thresholds were predicted to increase annual cohort migration survival by 55-132% without any additions to the water budget and by 79-330% with a modest addition to the water budget. These quantitative estimates of the biological consequences of different flow thresholds provide resource managers with critical information for designing functional flow regimes that benefit salmon in California's highly constrained water management arena.

Factors Affecting Spatiotemporal Variation in Survival of Endangered Winter‐Run Chinook Salmon Out‐migrating from the Sacramento River

“…Flow management is often used as a primary tool for mitigating impacts to fish. When high flows are not available, maintaining functional flows through flow pulses offers managers another way to improve survival under low‐flow conditions (Michel et al 2021). Figure 8B describes how the slope of the intra‐annual reach flow–survival relationship changes with mean annual flow.…”

One hundred‐seventy years of stressors erode salmon fishery climate resilience in California’s warming landscape

“…Historically, precipitation may have determined habitat capacity via the inundation of massive floodplains and wetlands. At present, precipitation may be important in overriding the effects of water regulation and extraction on the flow regime to determine whether flow within the simplified, channelized watershed does not drop below harmful thresholds as juveniles migrate to sea during the wet season (Michel et al, 2021; Munsch et al, 2020; Figure 2).…”

“…We inferred from these patterns that constraints of spring-summer temperature on fishery performance increased over time and that the impact of dams on the salmon and habitat portfolio was a major contributor to this transition, among other contributing stressors (e.g., wetland destruction, nonnative predators). Considering that contemporary flow requirements for salmon are evident from many previous studies (e.g., Michel, 2019;Michel et al, 2021;Munsch et al, 2020), the generally positive (albeit weaker) relationships between fishery performance and precipitation in the contemporary era (Figure 8), and recognizing that we employed a correlative analysis that evaluates model performance and parsimony, we interpreted the rise of temperature constraints as reducing the fishery's climate resilience by adding to --rather than replacing --the constraints of precipitation.…”

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

“…The concept of implementing functional flows (Yarnell et al, 2020) aims to create more seasonally inundated shallow-water habitats and allow for pulse flows to mimic the recession of the historical spring hydrograph. This could provide salmon with a last chance to transit the river and Delta before waters become exceedingly warm (Michel et al, 2021). Reduction of SAV either through removal or hydraulic modifications could reduce the predation risk posed by Largemouth Bass along the salmon outmigration corridor (Conrad et al, 2016).…”