Shearwater Foraging Figure 4. Diel distributions of sooty shearwater diving activity. A. Distributions of sooty shearwater dives with respect to time of day. B. Dive depths with respect to time of day. The median (cross) and interquartile range (bars) are shown. Grey bars indicate dives made from 30uS–50uS, and white bars indicate dives made south of 50uS. doi:10.1371/journal.pone.0010960.g004 shearwaters extend into this zone, and the observed at-sea shearwater densities (Figure 3) tend to coincide with the changes in the upwelling rates and primary production (e.g. the higher densities of shearwaters observed west of approximately 85uE correspond to the region of strong upwelling and intensive chlorophyll bloom near the southern Kerguelen Plateau [54]). Potential prey abundances The trawl transect was made in the longitude band 140uE– 146uE, and gives an indication of latitudinal variation in pelagic prey availability. North of 50uS, the trawls from the top 500m of the water column were dominated by pyrosomes (65% by number) and salps (11%), and the fish Photichthys argenteus (8%; Figure 8). From 50uS–54uS, myctophids overwhelmingly dominated (90%), but these numbers were strongly skewed by one particularly large haul of Electrona carlsbergi, which contributed 61% of the total myctophids caught in this latitude band. Temporarily excluding this haul of E. carlsbergi (Figure 8 is presented with this haul of E. carlsbergi excluded), the total myctophid component in the 54uS–50uS band was 54% (mainly Gymnoscopelus microlampas, 15%; G. piabilis, 8%; and unidentifiable Protomyctophum, 8%). Other fish (41%; mainly unidentifiable fish, 17%; and Persparsia kopua, 13%) and small numbers of cnidarians and salps (3%) made up the remainder. South of 54uS the hauls were again dominated by myctophids (67%; mainly G. braueri, 22%; Krefftichthys anderssoni, 15%; and E. antarctica, 9%), with smaller numbers of other fish (14%; mainly Bathylagus species, 6%; and unidentifiable fish, 4%), and the coronate scyphomedusae Atolla wyvillei (11%) and Periphylla periphylla (6%). Figure 7c shows the longitudinal distribution of near-surface copepod abundances in the Polar Front zone. Abundances were highest around 145uE and to the east of 170uE, although survey effort in the latter sector was sparse. The regions of higher copepod abundance generally coincided with the regions of persistent upwelling (Figure 7a), high surface chlorophyll concentrations (Figure 7b), and with shearwater foraging effort (Figure 7d). Discussion Likely prey of shearwaters on long foraging trips The diet of short-tailed shearwaters has been relatively well described [14,56,57], and on long foraging trips they predomi- nantly consume myctophids (lantern fish), with a small euphausiid component (likely Euphausia vallentini) [56,57]. Some myctophids, including the relatively common E. antarctica, G. braueri and K. anderssoni are rich in wax esters [58,59], which act as long-term energy reserve molecules rather than immediate energy supply [60] and they form an important component of stomach oil that is fed to chicks [61]. A number of studies show that sooty shearwaters consume a wide range of crustaceans, fish, cephalo- pods, and salps [e.g. 62,63,64,65]. The specific prey of sooty shearwaters on long foraging trips has not to our knowledge been studied, although it has been suggested that in Antarctic waters sooty shearwaters also principally forage for myctophids [12]. The results from our study support this suggestion. The observed foraging zones of the sooty shearwaters in this study, like those of the short-tailed shearwaters, are consistent with the general patterns of myctophid distribution in the Southern Ocean. Myctophids are widely distributed and abundant, particularly from the Polar Front zone south to the Antarctic continental shelf [37,38,66]. Around and to the south of the Polar Front zone, myctophids are commonly recorded within 200m of PLoS ONE | www.plosone.org 7 June 2010 | Volume 5 | Issue 6 | e10960