Challenges in the Identification and Interpretation of Phenological Shifts: Anthropogenic Influences on Adult Migration Timing in Salmonids

Abstract

Migration timing has evolved in many animals, allowing them to maximize breeding and feeding success by matching seasonal changes in abiotic conditions and resource pulses. These seasonal changes can shift with the climate, resulting in mismatches between migrations and resource availability unless the populations respond through phenotypic plasticity or evolutionary adaptation. It is common, however, for factors unrelated to climate to affect phenology. Salmon are an exceptionally well-studied group of fishes whose breeding migrations can serve as a template to consider the complex factors affecting migration phenology. In this paper, hypotheses for explaining changes in adult salmon migration phenology are reviewed. Pathways through which climate change may influence migration timing are first summarized, including shifting migration cues, limiting freshwater conditions, changes in distribution and conditions at sea, and alterations in embryo development. Alternative causes of phenological change in salmon are then explored including anthropogenic modifications of river habitat, demographic effects, hatcheries, and fisheries. The effects of these factors on phenology can mimic and mask climate effects, making it challenging to disentangle the causal basis of observed patterns. Instead of inferring shifts from trends in timing data (as is often done), it is suggested that specific mechanistic hypotheses be proposed and tested rigorously, and alternative causes systematically ruled out. Overall, it is challenging to attribute causation to phenological change, but salmon exemplify the many ways in which migration timing can change, including shifts due to climate and other processes.

Keywords:

• Salmon
• phenology
• climate change
• migration breeding

Acknowledgments

We thank Garrett McKinney for providing helpful input on the current state of research into the genetics underlying migration timing of adult salmon.

Additional information

Funding

MT was supported by the University of Washington’s IGERT Program on Ocean Change and School of Aquatic and Fishery Sciences and by the Achievement Rewards for College Scientists (ARCS) Foundation Seattle Chapter10.13039/100008227. MCA acknowledges past support from the Richard T. Whiteleather Fisheries B.S. 1935 Endowed Scholarship, Floyd E. Ellis Memorial Scholarship, and ARCS Foundation Seattle Chapter via the Barton family. CSA received support from the Richard and Lois Worthington Endowment and the H. Mason Keeler Endowment. AL was supported by the H. Mason Keeler Endowment and the Dayton Alverson Endowment, and WM was supported by the Sonoma County Water Agency. KNM received support from the National Science Foundation10.13039/100000001 Graduate Research Fellowship Program under Grant No. DGE1339067, and TQ gratefully acknowledges the support of the Richard and Lois Worthington Endowment.