A krill-dominated micronekton and macrozooplankton community in Croker Passage, Antarctica with an estimate of fish predation

Abstract

Abstract A micronektonic and macrozooplanktonic community was sampled with midwater trawls in the Croker Passage, vic. Antarctic Peninsula. Species composition suggested the area was a faunal transition zone between oceanic and nearshore communities. In a comparison between the study area and two oceanic areas, four of the top five numerically dominant species were identical ( Euphausia superba, Salpa thompsoni, Thysanoessa macrura and Sagitta gazellae ). In the nearshore region a mysid ( Antarctomysis ohlinii ) replaced the oceanic chaetognath Euhrohnia hamata in numerical ranking . E. superba was primarily responsible for the much higher nearshore biomass values when compared with those of oceanic regions (52.6 vs 2.4–3.1 g DM/m 2 ). The ctenophore Callianira antarctica , the mysid A. ohlinii , and the pelagic nototheniid fish Pleuragramma antarcticum were important neritic biomass species. Some oceanic species, notably S. thompsoni and the mesopelagic fish Electrona antarctica , were also important contributors to total micronektonic biomass. Fish predation by P. antarcticum and E. antarctica on E. superba (assuming constant fish and krill populations through the season that was sampled, the austral fall (90 d), and using our daily consumption values) is estimated at 2.5% and 2.9% of the total krill numbers, respectively, mostly on the juvenile segment of the population. Krill densities vary considerably in time and space, while available data suggest that pelagic fishes show less seasonal and spatial change. The most likely explanation for those observations is that the fishes are reliably more vulnerable to sampling gear than krill, but it also implies a consistent low level of fish predation on krill throughout the year, particularly on juveniles.