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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 statistical analysis of the mesopelagic fish data reveals two major groups of stations according to the sampled depth: a shallow group of stations (295-450 m depth) and a deeper group of stations (440-825 m depth). The change in relative abundance of mesopelagic fish species at 440-450 m coincides with the presence of warmer and denser Circumpolar Deep Water at and below these depths. Deeper stations 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 most abundant species. Dietary analysis reveals that myctophids are mostly zooplanktivorous, while the gempilyd P. gracilis is classified as a piscivorous predator. The small P. bolini feed mainly on copepods of the species Metridia gerlachei, while the most important prey item of the larger myctophids 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 and night. Daily ration estimates for myctophid species ranged from 0.28% to 3.3% of dry body weight (0.5-5.94% 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 11.1-26.7% in the South Shetland Islands region. This estimate emphasizes the important role of mesopelagic fish in the Antarctic ecosystem as a prevalent consumer of krill. r

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Lanternfishes (Myctophidae) constitute the most important component of the daily vertically migrating mesopelagic fish community. This research addresses the estimation of the trophic position and diet of myctophids using stable isotope analyses. Fishes were collected across the central Atlantic, from a very productive zone influenced by the Mauritanian upwelling to the western oligotrophic equatorial waters. The survey also encompassed a zone of low oxygen concentration in the mesopelagic layers. Determinations of δ13C and δ15N values were made on the 20 most frequent and abundant myctophids, from small-sized species (e.g. Notolychnus valdivae) to larger ones (e.g. Myctophum punctatum). Isotope analyses on the seston and several plankton groups were also performed to assess the influence of zonal differences in trophic position (TP) calculations, and to use as food sources in diet estimations. Myctophids displayed a narrow range of trophic positions, being greater than 2 and less than 4, except for N. valdiviae (TP = 1.7). Comparisons of diets estimated through an isotopic mixing model differentiated the smallest species, with a strong seston signature (Diogenichthys atlanticus and N. valdiviae), from the Diaphus species of medium sizes, (D. brachycephalus, D. holti, and D. rafinesquii), which feed on prey of higher TP values.