Shearwater Foraging Figure 5. Simulated wind costs of long foraging trips by sooty and short-tailed shearwaters. A. Simulated wind costs (background colours) for sooty shearwaters. B. For South Australian short-tailed shearwaters. C. For Tasmanian short-tailed shearwaters. Costs are shown as percentage residuals from smooth regression of cost against distance. For example, a value of 25% indicates that the cost to visit the area in question is 25% higher than the average cost for potential foraging locations at the same geographical distance from the colony. Insets show regressions of cost against distance. The thin green lines show outward flights from the colony; the thick green lines show the simulated minimum-cost paths from the colony to representative points on the birds’ foraging grounds. The orange lines show the same information for the return trips. Grey lines show the foraging components of flights (not shown in panel A for clarity). The purple lines show the direct (geodesic) routes. The northern and southern branches of the Polar Front are also shown (black). doi:10.1371/journal.pone.0010960.g005 the surface [66,67], and rise to the surface waters at night [68,69]. Some myctophid species are also found north of the Polar Front zone (e.g. E. carlsbergi, G. microlampas, G. piabilis), but inhabit deeper waters (250–600m) in these areas [67,70], which may limit their availability to surface-diving seabirds. These general distribution patterns were observed in the trawl data, with increased myctophid densities south of 50uS. The observed timing of sooty shearwater dive activity in Antarctic waters is also consistent with the proposition that the sooty shearwaters were foraging for myctophids, or other prey that migrate to near-surface waters during night-time hours. Sooty shearwaters use visual prey detection methods, scanning for underwater prey by immersing their heads before diving [71,72]. The observed distribution of dive activity in Polar Front waters (peaking just after sunrise and just before sunset, with no night- time diving) might represent a compromise between increased prey availability (proximity to surface) and decreased visual prey detectability during the hours of darkness. Sooty shearwaters have been observed to feed on myctophids during the day in the presence of other marine predators that drive the fish towards the surface waters [65], and shearwaters are commonly observed in association with cetaceans in Antarctic waters [e.g. 73]. The dive depths reported here for sooty shearwaters are underestimates of the true maximum dive depths, due to the low sampling rate (one sample per 24 or 32s) of the tags relative to the time spent by the birds at the deepest part of their dives. Capillary tube measurements show that sooty shearwaters are capable of diving to depths exceeding 60m [74,75]. The relatively low sampling rate of the tags also means that the probability of failing to detect a dive is greater for dives of shorter duration. It is possible that our finding of dawn and dusk diving in high latitudes has been confounded by this bias. If, for example, dive activity was in reality constant throughout the day, but the dive durations were shorter around midday and longer around dawn and dusk, then the distribution of detected dives would peak around dawn and dusk, as we found here. However, we compared the diel distribution of dive activity from fast sampling tags (sampling rate 24s or 32s) to the distribution obtained from slow sampling tags (432s; n = 3301 dives total by 18 individual birds; results not shown), and found the distributions to be very similar. Missed detections of short duration dives should be more pronounced with the slow sampling tags, and since the distributions match, we do not believe that the diel distribution of dive activity is an artefact of sampling bias in this case. Cherel et al. [57] reported that Euphausia vallentini were the main crustacean prey from long-trip samples in short-tailed shearwaters from Tasmanian (Bruny Island) colonies. E. vallentini is distributed to the north of the Polar Front [14,76,77]. The short-tailed shearwaters in this study, particularly the South Australian birds, were observed to forage in this region. Although E. vallentini have been reported in sooty shearwater diet samples [63], it seems unlikely that sooty shearwaters principally target this species on long foraging trips, since their primary foraging zones lie around and to the south of the Polar Front zone (Figure 1). The South Australian short-tailed shearwaters also foraged well south of the Polar Front, in waters where E. superba (Antarctic krill) is the most abundant euphausiid. Birds tracked from Montague Island, NSW [53] were observed to travel to Antarctic waters where krill is common, as did a post-breeding bird from French Island, Victoria [78]. Connan et al. [56,79] reported that lipid concentrations from 14 short-tailed shearwaters after long foraging trips did not match those of E. superba, and concluded that this euphausiid might not be a major component of their diet. This result appeared to contradict the above tracking studies as well as earlier stomach samples and observed predation of shearwaters on E. superba [80,81]. However, the results of Connan et al. [56,79] were obtained from birds breeding on Bruny Island, Tasmania. The small sample of Tasmanian (Wedge Island) birds tracked in this study did not forage in waters where E. superba might be expected. Thus, this apparent contradiction might be a result of site-specific differences in foraging area preferences of birds from different regions and colonies. E. superba might still be an important dietary component for some populations of short-tailed shearwaters, such as those from South Australian islands. Figure 6. Vertical water velocity at 1095m in the Southern Ocean. Twenty-year mean vertical velocity from the CSIRO Mk3.5 climate system model. The northern and southern branches of the Polar Front are shown. doi:10.1371/journal.pone.0010960.g006 PLoS ONE | www.plosone.org 9 June 2010 | Volume 5 | Issue 6 | e10960