Climate change threatens Chinook salmon throughout their life cycle

Widespread declines in wild salmon populations have significant consequences for fisheries, cultural heritage, and other marine species. Overfishing, migration barriers, habitat loss, and regime shifts in marine ecosystems have contributed to these declines. However, anthropogenic climate change is emerging as an overriding threat. Previous predictive models have focused on freshwater life stages or used climate indices that cannot be readily linked to future climate projections. These models are inadequate for informing management actions because they fail to account for "carryover" effects from freshwater to marine stages, rely on climate indices not directly linked to GCM projections, and use independent downscaling methods for freshwater and marine environments. This study aimed to address these limitations by developing a modeling framework that integrates information on environmental structure and climate drivers, conducts model evaluation, and allows for the variation of juvenile and adult migration timing based on environmental conditions.

The literature review highlights the existing gaps in predicting the biological responses of climate change on salmon conservation. Existing models predominantly focus on freshwater conditions, neglecting the crucial "carryover" effects from freshwater to marine stages, where climate change impacts survival. Furthermore, these models rely on climate indices that lack a direct link to global climate model projections, hindering formal analyses of how alternative scenarios might affect marine survival. The inconsistent downscaling methods used for freshwater and marine environments further complicate the assessment of climate impacts, making it difficult to account for carryover effects between stages. The authors emphasize the limitations in current approaches and their inadequacy in informing potential management actions for salmon conservation, particularly for migratory species with complex life histories.

The researchers used a stochastic, age-structured life-cycle model to simulate the population dynamics of eight wild Chinook salmon populations within the Snake River spring/summer Chinook salmon Evolutionarily Significant Unit (ESU). The model incorporated density-dependent and density-independent climate effects on survival, forced by environmental drivers and future climate trends based on ensemble projections from global climate models (GCMs). A simulation framework was used to explore model assumptions and quantify uncertainty. The model included five annual time steps corresponding to life-stage transitions (spawner to parr, parr rearing, ocean entry and first winter/spring, second year in the ocean, and return migration). Individual life-stage survival estimates were obtained from various sources, including PIT tagging data. A multivariate state-space model was used to estimate an unstructured covariance matrix for environmental covariates, accounting for their correlations. Climate change scenarios were simulated by adding climate trends to the historical environmental data, retaining the autocorrelation and variance of the historical environment. Sensitivity analyses were conducted by comparing model outcomes with alternative freshwater and marine covariates and by applying climate trends to individual life stages.

The key findings indicate that Chinook salmon populations declined dramatically in response to warming climate, particularly due to rising sea surface temperatures (SST). This decline in marine survival occurred despite shifts in salmon migration timing and changes in hydrosystem management. Small populations showed minimal resilience and quickly fell below the quasi-extinction threshold in most simulations. Larger populations also faced high extinction risk under the climate change scenarios, raising serious concerns for the long-term viability of the ESU. Sensitivity analyses revealed that SST was the dominant driver of extinction risk, with alternative models and covariates producing similar overall patterns. While freshwater climate impacts varied depending on population and model specifics, the negative effects of rising SST on marine survival overwhelmed any potential benefits in freshwater stages. The analysis showed strong density dependence at the parr-to-smolt stage, suggesting this life stage as a potential target for conservation efforts.

The study's findings highlight the significant threat posed by climate change to Chinook salmon populations, emphasizing the dominance of rising SST in driving extinction risk. While efforts to improve freshwater survival have been made, these are insufficient to offset the drastic decline in marine survival. The strong density dependence at the parr-to-smolt stage suggests potential avenues for conservation intervention. The results suggest a need for a dramatic increase in smolt survival to counter the negative impacts of climate change. The study's conclusions underscore the urgent need for effective conservation strategies targeting marine survival, given the inadequacy of current freshwater-focused efforts. Further research is needed to understand the complex trophic interactions and potential ecological surprises that may affect salmon survival in a warming ocean.

This study demonstrates the critical threat posed by climate change, particularly rising sea surface temperatures, to the survival of Chinook salmon. While management efforts focusing on freshwater stages have shown some improvement, these are insufficient to compensate for the drastic declines in marine survival. The strong density dependence in early life stages highlights a potential target for conservation interventions. Future research should prioritize understanding the complex interactions in the marine environment and developing strategies to enhance smolt survival and overall resilience to climate change.

The study acknowledges that the Northeast Pacific might not warm at the modeled rate, and that ecological surprises could alter the relationship between SST and salmon survival. The model also does not account for the negative effects of ocean acidification, which could further exacerbate the threats to salmon populations. The study's reliance on existing data and model assumptions introduces uncertainty, and future work could incorporate more detailed information on trophic interactions and a wider range of climate projections.