Cooperative Fish and Wildlife Research Units Program: Washington
Education, Research and Technical Assistance for Managing Our Natural Resources

Differences in the life history pathways of juvenile animals are often associated with differences in demographic rates in later life stages. For migratory animals, different life-history pathways often result in animals from the same population occupying distinct habitats subjected to different environmental drivers. Understanding how demographic rates differ among animals expressing different life history pathways may reveal fitness tradeoffs that drive the expression of alternative life history pathways and enable better prediction of population dynamics in a changing environment. To understand how demographic outcomes and their relationships with environmental variables differ among animals with different life history pathways, we analyzed a long-term mark-recapture dataset for Chinook salmon (Oncorhynchus tshawytscha) from the Wenatchee River, Washington, USA. Distinct life history pathways represented in this population include either remaining in the natal stream until emigrating to the ocean as a one-year-old (natal-reach rearing) or emigrating from the natal stream and rearing in downstream habitats for several months before completing the emigration to the ocean as a one-year-old (downstream rearing). We found that downstream-rearing fish emigrated to the ocean 19 days earlier on average and returned as adults from the ocean at higher rates. We detected a positive correlation between rate of return from the ocean by downstream-rearing fish and coastal upwelling in their spring of outmigration, whereas for natal-reach-rearing fish we detected a positive correlation with sea surface temperature during their first marine summer. Different responses to environmental variability should lead to asynchrony in adult abundance among juvenile life history pathways. A higher proportion of downstream-rearing fish returned at younger ages compared to natal-reach-rearing fish, which contributed to more variability in age at reproduction and greater mixing across generations. Our results provide an example of how diversity in juvenile life history pathways is associated with heterogeneity in demographic rates during subsequent life stages. This demographic heterogeneity can in turn affect variance in the aggregate population abundance and population response to environmental change. Our findings underscore the importance of considering life history diversity in demographic analyses and lay the foundation for the development of population models that account for different life history pathways.