Role of carryover effects in conservation of wild Pacific salmon migrating regulated rivers

Abstract

Determining which factors are most effective for mitigative strategies in conservation management can be difficult for species with complex life cycles. Salmon (Oncorhynchus spp.) migrating through a hydroelectric power system experience conditions that can affect their survival directly within a life stage and indirectly in subsequent life stages through carryover effects. We quantified the association of covariates with survival across life stages of Chinook salmon (O. tshawytscha): juveniles migrating downstream, juveniles in the estuary, subadults in the ocean, and adults migrating upstream. We applied a hierarchical Bayesian mark-recapture model to~400,000 wild Snake River Chinook salmon (Pacific Northwest, USA). Modeled covariates of survival included migration timing, river temperature, flow, percent of water spilled over dams, snow water equivalent, juvenile fish transportation, juvenile fish length, dam powerhouse passage, sea surface temperature, and the North Pacific Gyre Oscillation. By analyzing these covariates in a unified model, we evaluated their relationships to survival within and across life stages in a common currency. Among direct effects, the negative relationship of sea surface temperature to ocean survival had the largest marginal effect size. Carryover effects also had substantial marginal effect sizes. Ocean survival was associated positively with juvenile length and snow water equivalent and negatively with river temperature. In comparison, direct effects on juvenile and adult survival generally showed smaller marginal effect sizes. Both juvenile and adult survival were negatively associated with river temperature. Juvenile survival increased with fish length, while adult survival was positively related to flow and negatively to percent water spilled at dams. The greatest variability in survival, however, was in the ocean stage. Thus, as the ocean continues to warm and as the human population exerts more pressure on the ecosystem, carryover effects will be increasingly important for recovering salmon and other migratory species.

Targeting river operations to the critical thermal window of fish incubation: Model and case study on Sacramento River winter‐run Chinook salmon

“…Surprisingly, the models suggest that hypoxia persisting greater than a week would be largely independent of temperature and this insensitivity may account for the low variability in T crit with CW type. Additionally, the CW framework arises in physiological and ecological studies (Mueller et al, 2015; Uchida, Uesaka, Yamamoto, Takeda, & Irie, 2018) and is implicit in studies that consider cross life‐stage effects (Del Rio et al, 2021; Gosselin et al, 2021; Stoks & Córdoba‐Aguilar, 2012). We suggest that further studies in a CW framework will provide new insights into embryogenesis and population regulation in a warming climate.…”

An information theory-based approach to characterize drivers of upstream salmon migration

“…This shows that water temperature is a major factor in Chinook dynamics for both instantaneous interactions, as it influences migrating adult Chinook within a season and over multiple years. Within a year, water temperature can lead to higher disease, parasites, higher predation rates, and poorer ability to evade predators, and across years it may influence salmon over different life stages due to carryover effects [ 18 ].…”

“…For example, considering nonlinear environmental drivers has been found to improve forecast accuracy in fisheries models [ 16 ] and the extent to which a variable is predictive can depend on nonlinear thresholds [ 13 ]. It has also been found that there are multiple relevant indicators of salmon returns [ 17 ], carryover effects within salmon lifecyles in which conditions experienced at one life stage affect a subsequent stage [ 18 ] and marine survival [ 19 ], but no single indicator or subset of indicators is highly explanatory. Moreover, it has been found that the relationships between indicator variables themselves can be non-stationary [ 20 , 21 ], which has implications for predictive skills of models based on certain input combinations, and indicates the difficulty in distinguishing causal drivers from temporary correlations.…”

Life in the Fast Lane: Feeding and Growth of Juvenile Steelhead and Chinook Salmon in Main‐Stem Habitats of the Columbia River Estuary

“…These studies suggest that estuarine growth can be an important “carryover” effect of estuary rearing (Gosselin et al. 2021) that relates to subsequent ocean survival. Together, these findings reinforce the importance of understanding the link between wetland restoration, chironomid production and export from wetland areas, and consumption of chironomids by migrating salmon smolts.…”

“…Several recent studies have demonstrated that the ocean survival of Columbia River steelhead and Chinook Salmon is positively related to juvenile length or growth rates at ocean entry (Claiborne et al 2011;Wilson et al 2021;Gosselin et al 2022;Norrie et al 2022). These studies suggest that estuarine growth can be an important "carryover" effect of estuary rearing (Gosselin et al 2021) that relates to subsequent ocean survival. Together, these findings reinforce the importance of understanding the link between wetland restoration, chironomid production and export from wetland areas, and consumption of chironomids by migrating salmon smolts.…”