Community structure of mesopelagic fishes constituting sound scattering layers in the eastern tropical North Atlantic

• Published in: Journal of Marine Systems
• Main Authors: Czudaj, Stephanie, Koppelmann, Rolf, Möllmann, Christian, Schaber, Matthias, Fock, Heino Ove
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2021
• Subjects: Text; article; ddc:570; Mesopelagic zone > Vertical migrations > Scattering layers > Oxygen minimum layer > Tropical fish > Aquatic communities > Eastern Tropical Atlantic > 0–12°N and 20–26°W; North Atlantic
• Online Access: https://doi.org/10.1016/j.jmarsys.2021.103635; https://www.openagrar.de/receive/openagrar_mods_00075854

Mesopelagic sound scattering layers (SSLs) are predominantly associated with mesopelagic fish taxa with gas-inflated swimbladders that perform active nocturnal diel vertical migration (DVM), like species of the families Myctophidae, Gonostomatidae, Sternoptychidae and Phosichthyidae. Larger-sized species with limited, partial, asynchronous and non-migratory migration pattern (non-DVM) and without gas-bearing swimbladders are presumably invisible by hydroacoustic methods operating at lower frequencies (18–38 kHz). Their vertical migration behaviour and functional role in the mesopelagic community remain largely disregarded. The present study investigated (i) the taxonomic and functional mesopelagic fish composition (migration behaviour and feeding guild as traits) of the main SSLs and (ii) regional variation in the vertical community structure related to the presence of the oxygen minimum zone (OMZ) in the eastern tropical North Atlantic. We analysed hydroacoustic and biological trawl data collected concurrently with a pelagic midwater trawl in distinct SSLs between 45 and 680 m depth by means of ordination, cluster and compositional analyses. Whereas daytime hydroacoustic backscatter profiles demonstrated higher mean Sv (38 kHz) backscatter at shallower depths at low-oxygen (LO) station 309 compared to the equatorial (EQ) station 330, night-time hydroacoustic profiles at LO stations could not be directly related to OMZ conditions. Catch abundances and biomass showed no clear regional pattern between EQ and LO stations. The analysis of trawl samples emphasised the importance of non-DVM species to the composition of tropical SSLs and further indicated regional and vertical variation in the mesopelagic fish community structure. At all stations, non-DVM species made up the largest proportions of total tow community composition (abundance and biomass) in the principal and secondary mesopelagic SSL during night-time (375–680 m) and during most daytime tows (LO: 390–555 m, EQ: 325–500 m). Our analysis suggests a more ...