Fear dynamically structures the ocean's pelagic zone.

Fear of predation can have wide-ranging ecological effects.1-4 This is especially true in the ocean's pelagic zone, the Earth's largest habitat, where vertical gradients in light and primary productivity force numerous taxa to migrate vertically each night to feed at the surface while minimizing risk from visual predators.5-7 Despite its importance and the fact that it is driven by spatial differences in perceived risk,8 diel vertical migration (DVM) is rarely considered within the "landscape of fear"3,8,9 framework.10 It is also far from the only such process in the pelagic zone. We used continuous, year-long records from an upward-looking echosounder and broadband hydrophone at a cabled observatory off Central California, USA, to observe avoidance reactions by several groups of pelagic animals to the presence of their predators. As expected, vertical migration was ubiquitous, but we also observed behaviors at shorter and longer timescales that were best explained by fear of predation. The presence of foraging odontocetes induced immediate diving behavior in mesopelagic sound-scattering layers, and schools of epipelagic fishes induced similar reaction in layers of zooplankton and mesopelagic micronekton. At longer timescales, the presence of fish schools significantly deepened vertical migration, rearranging life throughout the water column. We argue that behavioral reactions to predation risk are common in the pelagic zone at a range of spatiotemporal scales and that our understanding of food webs and biogeochemical cycling in this immense biome will be incomplete unless we account for fear.