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Community structure and feeding ecology of mesopelagic fishes in the slope waters of King-George Island (South Shetland Islands, Antarctica)

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Pusch, C. , Hulley, P. A. and Kock, K. H. (2004): Community structure and feeding ecology of mesopelagic fishes in the slope waters of King-George Island (South Shetland Islands, Antarctica) , Deep-sea research i, 51 (11), pp. 1685-1708 . doi: 10.1016/j.dsr.2004.06.008
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The role of mesopelagic fishes in the Southern Ocean ecosystem and more particular their trophic effect on the standing stock of mesozooplankton is at present poorly understood. To get a deeper insight in the Antarctic mid-water ecosystem the mesopelagic fish community of the King George Island slope (South Shetland Islands) was sampled with a pelagic trawl in 1996. The community structure was analysed and the feeding ecology was studied of the five most abundant species. A total of 18 mesopelagic fish species in 10 families was identified. Of these, the Myctophidae was the most important family by species number (9 species), individual number (98.5% of all individuals)and fish wet weight (87.3% of the total weight). The assemblage was numerically dominated by four myctophids (Electrona antarctica, Gymnoscopelus braueri, Gymnoscopelus nicholsi,Protomyctophum bolini) and one gempilyd (Paradiplospinus gracilis). Multivariate statisticalanalysis of the mesopelagic fish data reveals two major groups of stations according to the sampleddepth: a shallow group of stations (295â450 m depth) and a deeper group of stations (440â825 mdepth). The change in relative abundance of mesopelagic fish species at 440â450 m coincides withthe presence of warmer and denser Circumpolar Deep Water at and below these depths. Deeperstations were characterized by a higher density and increased diversity of mesopelagic fish species.The community patterns identified correlated well with the vertical depth distribution of the mostabundant species. Dietary analysis reveals that myctophids are mostly zooplanktivorous, while thegempilyd P. gracilis is classified as a piscivorous predator. The small P. bolini feed mainly oncopepods of the species Metridia gerlachei, while the most important prey item of the largermyctophids E. antarctica, G. braueri, and G. nicholsi were various species of euphausiids.Investigation of feeding chronology showed that G. nicholsi and P. bolini were feeding day andnight. Daily ration estimates for myctophid species ranged from 0.28% to 3.3% of dry body weight(0.5â5.94% of wet body weight). Krill (Euphausia superba) were the most important food of E.antarctica and G. nicholsi, accounting for 53.1% and 58.3% of the total food weight, respectively.The annual removal from the krill stock by both species was estimated to amount to 11.1â26.7% inthe South Shetland Islands region. This estimate emphasizes the important role of mesopelagic fish inthe Antarctic ecosystem as a prevalent consumer of krill.

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