Predation, fear, and dynamic vertical structure in the mesopelagic

The ocean’s pelagic zone is the Earth’s largest habitat and contains large numbers of animals, playing important roles in aquatic food webs. Similar to some terrestrial and benthic ecosystems, such as mountains or the intertidal, the pelagic is characterized by strong physical gradients, which lead to distinct vertical zonation in biology. Unlike these environments, it lacks hard structures, and so the zones can rearrange themselves on short time scales, with diel vertical migration (DVM) in response to food, light, and predation being the classical example. It is not, however, the only such process. Since February 2019, continuous observations from an upward-looking echosounder (38 kHz) and broadband (0-128 kHz) hydrophone at a cabled observatory in Monterey Bay, CA have revealed several novel responses of epi- and mesopelagic animals to the presence of predators. On hundreds of occasions, bouts of odontocete echolocation (mostly by Pacific white-sided and Risso’s dolphins) provoked mesopelagic sound-scattering layers (SSLs) to dive 5-100 m over tens of minutes. Video surveys from a remotely-operated vehicle identified juvenile Pacific hake, myctophids, and sergestid shrimp as likely constituents of these SSLs. On other occasions, the appearance of epipelagic fish schools was associated with increases in depth of vertical migration by SSLs. On the most extreme occasion, DVM range increased from 50 m on 7 April 2019 to 300 m on 8 April. Finally, the frequent presence of multiple thin SSLs, separated vertically on scales of meters and persisting for hours, suggests the existence of ecological zonation emerging dynamically from the movements and interactions of multiple groups of mesopelagic animals. While the processes remain poorly resolved, these examples suggest ways that predation and fear can restructure mesopelagic ecosystems at short time scales, with potentially significant effects on food webs and biogeochemical fluxes.