Community structure of mesopelagic fauna and the length-weight relationships of three common fishes in the Cosmonaut Sea,Southern Ocean

This study used specimens of marine organisms caught by rectangular midwater trawl in the Cosmonaut Sea,Southern Ocean,in austral summer 2019/2020,to determine species composition and spatial distributions of mesopelagic fauna in this sea.The data were also used to calculate the length-weight relationships of three common fishes captured during the survey(Electrona antarctica,Bathylagus antarcticus,and Cyclothone microdon).A total of 385 individual organisms with a total weight of 15462.2 g and representing 17 species were collected across the 11 stations visited.The small-sized crown jellyfish Atolla wyvillei(order Coronatae)was the most-dominant species by number(96 individuals),and the zooplankton Cyllopus lucasii(Amphipoda)also showed high abundance(54 individuals).Among fishes,81 individuals of Notolepis coatsorum(Paralepididae)were caught,followed by 71 individuals of E.antarctica(Myctophidae),25 individuals of B.antarcticus(Bathylagidae),and 16 individuals of C.microdon(Gonostomatidae).Clustering analysis divided the mesopelagic community into“west”and“east”groups,and a greater number of individuals were collected in the western part of the sea.The length-weight relationships of the three common fishes revealed positive allometric growth for B.antarcticus(b=3.16),and negative allometric growth for E.antarctica and C.microdon(both b=2.53).Our descriptions of the mesopelagic-zone community structure and biological features of three common fishes provide basic information on the ecology of the Cosmonaut Sea,Southern Ocean,and will be useful for ecosystem-based fisheries management in this region.