Seasonal and mesoscale variability in the distribution of antarctic krill, Euphausia superba, west of the Antarctic Peninsula

Observations collected between November 1991 and September 1993 during four multidisciplinary cruises were analyzed to provide a description of seasonal and mesoscale variability in the distribution and abundance of Antarctic krill, Euphausia superba, within continental shelf waters west of the Antarctic Peninsula and to investigate possible relationships between variability in krill distributions and variability in selected environmental parameters. Acoustic measurements of krill were made each season at designated locations to estimate the abundance of krill, in terms of vertically-integrated krill biomass and the number of aggregations, characterize the depth distribution of krill, and provide a quantitative description of the size, shape, and biomass of individual krill aggregations. The acoustic observations were coupled in time and space to environmental conditions defined by measurements of weather and sea ice parameters, hydrographic structure, concentrations of algal pigments, and net-based density estimates of krill and other zooplankton.; Spatially-averaged estimates of krill biomass increased three-fold from spring to summer (34 to 110 g m{dollar}sp{lcub}-2{rcub}{dollar}) and then decreased by an order of magnitude to the low values ({dollar}{dollar}150 g m{dollar}sp{lcub}-3{rcub}{dollar}) aggregations dominant during summer and large ({dollar}{lcub}>{rcub}10sp4{dollar} m{dollar}sp2{dollar}), less dense ({dollar}{dollar}50 g m{dollar}sp{lcub}-2{rcub}{dollar}), in all seasons, at locations on the inner shelf within 100 km of the Antarctic Peninsula. Across-shelf gradients were also observed in the environmental conditions measured during this study and these gradients were strongest during the summer and fall. A spatial separation in krill size classes was observed in all seasons except winter, with small adults ({dollar}{dollar}45 mm), reproducing adults during the summer was spatially correlated with surface salinities exceeding 33.8 and the presence of Circumpolar Deep Water at depth. The seasonal and mesoscale variability in the distribution of krill along the west coast of the Antarctic Peninsula observed in this study was consistent with descriptions of krill distributional patterns provided for other regions of the Southern Ocean. Moreover, the temporal change in the mesoscale distribution of krill over an annual cycle is consistent with a shift in the primary habitat of krill between seasons.