Shearwater Foraging Figure 8. Latitudinal distributions of pelagic trawl abundances and sooty shearwater diving activity. A. Pelagic prey abundances (individuals/trawl) in the 60uS to 45uS latitude band, 0–500m depth, at approximately 140uE. B. Sooty shearwater relative dive distribution in the 135uE–145uE sector for the same latitude range. Dotted lines show the latitudes of the southern and northern branches of the Polar Front. doi:10.1371/journal.pone.0010960.g008 Relationship with physical oceanography in the Polar Front zone The foraging effort of the shearwaters within the Polar Front zone was not uniform in longitude, but was focused on areas of persistent upwelling. The association of shearwater foraging activity with these areas reflects elevated prey availability (as measured by near-surface copepod abundances), resulting either from elevated production in these areas, and/or the aggregation of prey in near-surface waters by upwelling and convergence [e.g. 87]. The persistent upwellings in the Polar Front zone are largely coincident with the surface productivity, as measured by satellite estimates of surface chlorophyll (Figure 7; see also Figure 8 in [54]). In the open ocean away from shallow regions, blooms along Southern Ocean fronts are generally initiated by upwellings due to the interactions of the Antarctic Circumpolar Current with large- scale bottom topography [54]. A large and intense topography- induced upwelling is found where the Polar Front interacts with Pacific-Antarctic Ridge, from ,170uE–200uE, and this zone corresponds to the strongest chlorophyll bloom found in the Pacific sector of the Southern Ocean [54], and also to the sooty shearwaters’ foraging zone south-east of New Zealand. The upwellings in the vicinity of the South-East Indian and Macquarie Ridges (140uE and 165uE; also foraging zones) have relatively weak signatures in surface chlorophyll distribution (Figure 7a). However, in-situ observations along the World Ocean Circulation Experiment SR3 section (140uE line) show a persistent subsurface chlorophyll maximum at depths of 50–100m [88,89]. This subsurface maximum does not extend northward into the subantarctic zone. Parslow et al. [88] concluded that vertical processes (rather than horizontal advection) were the dominant controllers of this feature, consistent with the vertical velocity distributions shown here (Figure 6). Consistency and overlap of foraging areas The overlap of foraging zones between the two shearwater species, and with commercial fisheries, is an issue of ongoing interest [2]. The Polar Front region around 140uE was utilised by shearwaters from all three tracking studies incorporated here. The range of years covered by those studies, combined with the consistency of high at-sea densities in this region, strongly suggests that this region is used repeatedly across years. This is also consistent with previously published tracks of short-tailed shear- waters from south-eastern Australia [53,78], and with land-based observations of southward-, but not northward-flying shearwaters at Macquarie Island [90]. The south-eastern sooty shearwater foraging zone matches that recorded in 2000 (Figure 1 in [2]), suggestive of inter-annual consistency in the use of other foraging locations. The sooty shearwaters also showed foraging zone fidelity at an individual level, with only one bird foraging in both the south-eastern and south-western zones. Fidelity to mesoscale features such as fronts and upwelling zones is extremely common in pelagic seabirds [91]. Central-place foragers, such as breeding seabirds, tend to repeatedly utilise foraging zones where prey are consistent [92,93]. The strategy of returning to learned foraging zones provides a means of counteracting the natural patchiness of marine prey [94]. The lack of foraging activity by the sooty shearwaters in the Polar Front zone sector from ,145uE–180uE does not appear to be a result of wind-cost factors, but likely prey availability and other factors. This sector encompasses a sector of persistent PLoS ONE | www.plosone.org 11 June 2010 | Volume 5 | Issue 6 | e10960