Fish and zooplankton from RMT-8 net hauls on the BROKE voyage

Brief description

Distribution patterns of pelagic fish, larvae and juveniles collected by RMT trawls during BROKE survey to CCAMLR Division 58.4.1 were investigated. Nearly 2000 individuals, weighing 1210 g, were collected from approximately 1.5 million cubic metres of the upper 200 m of ocean, supporting the theory that Antarctic ichthyoplankton has low biomass. The collection consisted mainly of P. antarcticum larvae and juveniles and E. antarctica sub-adults, with a range of other notothenioid fish and myctophids. Three distinct biogeographic zones, with characteristic ichthyo- and zooplankton assemblages, were identified. The Oceanic Zone was dominated by myctophids and, in the western reaches, the paralepidid N. coasti. The shelf break zone comprised of myctophids, and the juveniles of notothenioid fish. The shelf zone consisted of notothenioid juveniles and sub-adults. Characteristic water masses and associated zooplankton assemblages were found throughout these three zones. Analysis of fish stomach contents indicated feeding on locally abundant zooplankton taxa. There was niche-partitioning of prey taxa and size classes, between both sympatric species and between different ontogenetic stages. Fish distributions corresponded to known patterns, and extended the geographic range of several species.

Zooplankton data from routine 0-200 m oblique trawls were analysed using cluster analysis and non-metric multidimensional scaling to define the communities in Eastern Antarctica (80-150 E), their distribution patterns, indicator species, and species affinities. Three communities were defined based on routine trawls. The Main Oceanic Community comprising herbivorous copepods, chaetognaths, and the euphausiid Thysanoessa macrura dominated the area west of 120 E. The area east of 120 E was dominated by Salpa thompsoni. The third community located in the neritic zone was dominated by Euphausia crystallorophias. Antarctic krill Euphausia superba did not form a distinct community in its own right, unlike previous observations in Prydz Bay. Krill were distributed throughout most of the survey area but generally in higher abundances towards the shelf break. Overall, krill abundance was low compared with previous net surveys in Prydz Bay. Three main types of assemblages were identified based on target trawls. The first group was dominated by krill (mean 1149 individuals per 1000 cubic metres) which represented greater than 99% of Group 1 catches in terms of numbers and biomass. Group 2 comprised the bulk of target trawls and comprised a wide diversity of species typical of the main oceanic community, with a mean abundance approximately half of that observed in the routine trawls. The third group comprised trawls in the neritic zone dominated by E. crystallorophias. No salp-dominated aggregation was found. While E. superba did not dominate a distinct community geographically as seen in previous Prydz Bay surveys, it did dominate discrete layers or aggregations, showing that both horizontal and vertical separation of communities exist.

Notes

Includes: point occurrence data