Shade affects magnitude and tactics of juvenile Chinook salmon antipredator behavior in the migration corridor

Megan C Sabal,Joseph E Merz,Michelle L Workman,Eric P Palkovacs

doi:10.1007/s00442-021-05008-4

Abstract

Environmental conditions strongly affect antipredator behaviors; however, it is less known how migrating prey adjust antipredator behavior in migration corridors, in part, because active migrants are difficult to observe and study. Migrants are vulnerable and encounter many predators in the corridor, and their propensity to travel towards their destination ties antipredator behavior with movement. We evaluated how environmental risk cues in the migration corridor including in-water habitat structure (present, absent) and overhead shade (sun, shade), and salmon origin (hatchery, wild) affected how juvenile Chinook salmon (Oncorhynchus tshawytscha) reacted to a live predator. We measured how salmon react to predation risk as the difference in time to swim downstream through a 9.1-m long field enclosure with or without a live predatory largemouth bass (Micropterus salmoides). Shade significantly modified the reaction to the predator, and it did so in two ways. First, the magnitude of antipredator behavior was larger in shade compared to direct sun, which suggests salmon perceived shade to be a riskier environment than sun. Second, the escape tactic also varied; salmon slowed down to be cautious in shade and sped up in sun. Structure did not significantly affect behavior and hatchery and wild salmon behaved similarly. Our study suggests that environmental risk cues can shape the magnitude and tactics of how migrants react to predation risk and illustrates how these responses relate to movement with potential to scale up and affect migration patterns.

Highlights

Prey use environmental cues to assess predation risk and adjust their antipredator behavior (Lima and Dill 1990)
Antipredator behavior in the migration corridor affects finescale movement creating a potential pathway to affecting migration duration or arrival time (Hope et al 2014; Sabal et al 2020)
Because shade was the only significant variable that interacted with predator presence, we subsequently examined the effects of structure and salmon origin within shade and sun categories (Table 1)

Summary

Introduction

Prey use environmental cues to assess predation risk and adjust their antipredator behavior (Lima and Dill 1990). Habitat-dependent antipredator behavior has been heavily studied in resident. Prey and can scale up to affect populations and communities (Preisser et al 2005; Wirsing et al 2021). These antipredator decisions are much less studied in migrating animals, despite that migrants often experience high predation risk in migration corridors where they are concentrated and conspicuous (Furey et al 2018). Migratory prey are economically and ecologically valuable and, it is important to examine their context-dependent antipredator behavior in the risky migration corridor

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Results

Conclusion