Shearwater Foraging Figure 1. Sooty and short-tailed shearwater foraging in the Southern Ocean. A. Sooty shearwater tracks (grey lines) and dive locations (black dots), with the short-tailed shearwater habitat utilisation from panel B included for reference. Note the overlapping use of the Polar Front zone around 140uE. The northern and southern branches of the Polar Front (black) and the trawl transect (dotted orange) are shown. B. Short-tailed shearwater tracks from two South Australian islands (red lines) and from Wedge Island, Tasmania (green lines), and their corresponding combined habitat utilisation (background colours). doi:10.1371/journal.pone.0010960.g001 depth of ,10m, at ,100m behind the vessel. A 270 mm mesh is used to filter the water flowing through the instrument and the samples preserved in formalin. In subsequent laboratory analysis, the silks are cut into segments representing 5 nautical miles (referred to as ‘‘tow segments’’) and all zooplankton on the silk identified [34,35]. The SO-CPR survey has been operating since 1991, and due to the relative ease of deployment of the instrument, the survey has broad coverage of the waters south of Australia, and to a lesser extent, New Zealand. We used data from 48 voyages that surveyed areas of the Polar Front zone between 130uE and 200uE during the months of February–April. These data comprised a total of 1288 tow segments within the Polar Front zone — that is, within the northern and southern branches of the Polar Front, as defined by Sokolov & Rintoul [36]. The continuous plankton recorder survey effort in the Polar Front zone was mostly concentrated between 130uE and 155uE, with particularly heavy coverage at 140uE and 150uE resulting from repeat sampling along the World Ocean Circulation Experiment SR3 transect (,140uE) and by the Japanese vessel Shirase along 150uE. We examined the pooled abundances of all copepod taxa, since copepods are a common prey of myctophids [37,38,39]. We note that the patterns of total zooplankton abundance were very similar to those of copepods. Physical oceanography Mean positions of Southern Ocean fronts were obtained using the technique of Sokolov & Rintoul [36]. The Polar Front zone is defined here as the zone between the northern and southern branches of the Polar Front. Estimates of vertical water velocity at 1095m depth were obtained from the CSIRO Mk 3.5 model [40]. Surface chlorophyll concentrations derived from satellite ocean color measurements were based on eight day mean level 3 standard mapped images of chlorophyll on a global 9 km equidistant cylindrical grid from SeaWiFS and were obtained from the Goddard Space Flight Center [41]. Except where otherwise specified in the text, we use surface chlorophyll to refer to chlorophyll-a detected by satellite and use this quantity as a proxy for phytoplankton biomass. Analyses All analyses were conducted in Matlab (Mathworks, Natick MA, 2009) and R (R Foundation for Statistical Computing, Vienna 2009). At-sea observations from all years were pooled. The continuous observations were binned into records representing 10-minute surveys. There were a total of 13097 10-minute at-sea surveys PLoS ONE | www.plosone.org 3 June 2010 | Volume 5 | Issue 6 | e10960