Mesopelagic predator-prey interactions revealed by joint passive and active acoustic observations

Abstract

The ocean’s mesopelagic zone is one of the Earth’s largest habitats and contains large numbers of animals, playing important roles in aquatic food webs—notably as food for many marine mammal species. While these communities have been observed for decades on echosounders as sound scattering layers (SSLs), their ecological dynamics remain poorly understood. Using a broadband (0-128 kHz) hydrophone and an upward-looking echosounder (38 kHz) at a cabled observatory in Monterey Bay, CA in early 2019, we observed numerous occasions where SSLs abruptly increased their depth during bouts of echolocation clicks from odontocete whales. Clicking bouts occurred 6.1 times per day on average, mostly at night, lasting minutes to several hours. Pacific white-sided dolphins and Risso’s dolphins produced most of these clicks. Increases in clicking were significantly cross-correlated with increases in SSL depth. The deepest SSL, centered at 400–500 m depth, dove 15 m during an average clicking bout, with some excursions up to 100 m. Video surveys from a remotely operated vehicle identified juvenile Pacific hake, myctophids, and sergestid shrimp as likely constituents of these SSLs. Our results suggest these animals actively dive to avoid odontocetes, and that fear of predation can restructure mesopelagic ecosystems on short time scales.