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

Thomas M. Lancraft, Kim R. Reisenbichler, Bruce H. Robison, Thomas L. Hopkins, and Joseph J. Torres

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 nearshore mysid (Antarctomysis ohlinii) replaced the oceanic chaetognath Euhrohnia hamata in numerical ranking. Euphausia 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 DW/m²). The ctenophore Callianira antarctica, the mysid Antarctomysis ohlinii, and the pelagic nototheniid fish Pleuragramma antarcticum were important neritic biomass species. Some oceanic species, notably Salpa thompsoni and the mesopelagic fish Electrona antarctica, were also important contributors to total micronektonic biomass. Estimates of fish predation (P. antarcticum and E. antarctica) on E. superba ranged from 10.9% to 21.9% of the total annual krill population (particularly the juveniles), depending on whether individuals were considered to feed for half the year or all year long. Those values were considered to be conservative, as fish populations appear to remain seasonally constant whereas krill populations may diminish during the winter.

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