Vertical distribution, composition and migratory patterns of acoustic scattering layers in the Canary Islands

Abstract

Diel vertical migration (DVM) facilitates biogeochemical exchanges between shallow waters and the deep ocean. An effective way of monitoring the migrant biota is by acoustic observations although the interpretation of the scattering layers poses challenges. Here we combine results from acoustic observations at 18 and 38kHz with limited net sampling in order to unveil the origin of acoustic phenomena around the Canary Islands, subtropical northeast Atlantic Ocean. Trawling data revealed a high diversity of fishes, decapods and cephalopods (152 species), although few dominant species likely were responsible for most of the sound scattering in the region. We identified four different acoustic scattering layers in the mesopelagic realm: (1) at 400–500m depth, a swimbladder resonance phenomenon at 18kHz produced by gas-bearing migrant fish such as Vinciguerria spp. and Lobianchia dofleini, (2) at 500–600m depth, a dense 38kHz layer resulting primarily from the gas-bearing and non-migrant fish Cyclothone braueri, and to a lesser extent, from fluid-like migrant fauna also inhabiting these depths, (3) between 600 and 800m depth, a weak signal at both 18 and 38kHz ascribed either to migrant fish or decapods, and (4) below 800m depth, a weak non-migrant layer at 18kHz which was not sampled. All the dielly migrating layers reached the epipelagic zone at night, with the shorter-range migrations moving at 4.6±2.6cms−1 and the long-range ones at 11.5±3.8cms−1. This work reduces uncertainties interpreting standard frequencies in mesopelagic studies, while enhances the potential of acoustics for future research and monitoring of the deep pelagic fauna in the Canary Islands.