Diel Activity of Five Anadromous Species Along Gradients of Migration Complexity

In many species, diel activity during reproductive migrations diverges from patterns established in non-breeding and rearing habitats.  Migrants must balance risk-avoidant behaviors, such as nocturnal movement, against fitness consequences imposed by reproductive timetables.  Diel activity along migration routes is further constrained by animal sensory and orientation mechanisms and is mediated by proximate stimuli.  We used telemetry data to test the hypothesis that diel behavior of five anadromous species (Chinook salmon, Oncorhynchus tshwaytscha; sockeye salmon O. nerka,; steelhead, O. mykiss; Pacific lamprey, Lampetra tridentata; American shad, Alosa sapidissima) would differ among migration environments varying in biotic and abiotic complexity.  Fifteen freshwater study sites ranged from relatively undisturbed to evolutionarily-novel (i.e., concrete fishways).  Diel activity data from ~13,000 radio- and PIT-tagged adult fish strongly supported the migration complexity hypothesis.  Salmonids were active day and night in riverine habitats, for example, but were predominantly diurnal at fishways where hydraulic complexity and predator gauntlets likely elevated the importance of visual cues.  Similarly, habitually cryptic Pacific lamprey were more nocturnal at fishway sites than in other environments.  Results suggest that context-dependent behavioral plasticity is likely common in long-distance migrants, and that animals may increasingly rely on their dominant sensory mechanisms in challenging environments.