Biological characteristics of Uphausia superba dan in acoustically detected aggregations near Elephant Island, Antarctica

Abstract

Euphausia superba aggregations were sampled with a Multiple Opening/Closing Net and Environmental Sensor System (MOCNESS) during the austral summer 1991-92 Antarctic Marine Living Resources (AMLR) survey in the vicinity of Elephant Island, Antarctica. Four areas of high krill concentrations were identified with a towed l2OkHz single frequency spift-beam acoustic transponder. Discrete samples were collected at user-designated depths to examine length and developmental stage variability vertically and horizontally within aggregations and to determine the influence of physical and biological factors on Euphausia superba distributions. The MOCNESS study area included a major hydrographic front marking the mixing of Weddell Scotia Confluence (WSC) and Antarctic Surface Water (AASW). On the spatial mesoscale, Euphausia superba distributions showed a distinct separation of length and developmental stages by water mass, with juveniles <30mm associated with WSC in shallower shelf waters and adult krill >45 mm associated with Transition Type 11 water and the presence of Circumpolar Deep Water (CDW) on the west shelf and slope. There were significant differences in Euphausia superba length distribution and the proportion of developmental stages between the thermocline and mixed layer that varied with the diel period of the samples and the degree of water stratification. These data suggest vertical diel migration by Euphausia superba which may have been affected by the degree of water stratification, which in turn may influence phytoplankton population growth. On a spatial fine-scale, vertical stratification of krill with significantly different mean lengths and proportions of developmental stages were observed in the mixed layer of weakly and strongly stratified water. As many as four stratified layers of Euphausia superba were observed from the surface to 20 meters depth. These data suggest behavioral responses may be affecting krill distribution on this finer scale. For example, Euphausia superba may be responding to patchy phytoplankton distributions or schooling with other individuals with the same developmental swimming skills. No significant differences in Euphausia superba mean length were observed in nets which collected at one depth over horizontal distances of 127-225 meters. However, some nets which fished within a restricted stratum grouped separately in Duncan's Multiple Range Analysis which implies that horizontal variability exists on spatial scales <125 meters.