Shags in Antarctica: their feeding behaviour and ecological role in the marine food web

Abstract

Feeding behaviour, ecological role in the marine food web and population trends of the Antarctic shag Phalacrocorax bransfieldensis and the South Georgia shag P. georgianus in Antarctica are analysed. The diving depths and duration recorded for these shags are the deepest and longest among all flying birds in Antarctica and match deep dives performed by small Antarctic penguins. Individual shags of both sexes partition foraging depths and food resources, which might diminish intra-specific competition. Like other sub-Antarctic shags, P. bransfieldensis and P. georgianus are bottom feeders that prey predominantly on demersal fish. In the southern Scotia Arc and west Antarctic Peninsula, nototheniids, mainly Notothenia coriiceps, constitute their main prey. Shag partners alternate the time at sea and, as the energy requirements at the nest increase, they increase the number but reduce the duration of the feeding trips. A steady declining trend in the number of breeding pairs of both species has been observed in the last decade at several Antarctic localities; this phenomenon at the South Shetland Islands might be at least partially explained by the effect of the commercial fishery on their prey. In inshore shallow waters shags occupy the trophic niche of main predators of demersal fish and play an important ecological role as regulators of populations of particular fish prey that have marked site fidelity. The potential use of shags as biomonitors in Antarctica is discussed.

Combined bio-logging and stable isotopes reveal individual specialisations in a benthic coastal seabird, the Kerguelen shag

“…Male and female Kerguelen shags in the present study did not differ in mean maximum dive depths, which contradicts other studies performed on species of the blue-eyed shag complex [20, 29, 30, 56, 57, 60]. The small sample size for birds equipped with dive recorders and the large inter- and intra-individual variation in depths in males might have been responsible for the lack of statistical significance in regards to the influence of sex on dive depths.…”

Slow recovery of previously depleted demersal fish at the South Shetland Islands, 1983–2010

“…The absence of otoliths from the two exploited species contrasts with the high occurrence of those from N. coriiceps. These results are consistent with the high incidence of G. gibberifrons in the diet of shags and in trammel-net catches both at the Danco Coast and the tip of the Antarctic Peninsula (Joinville-D'Urville Islands; Casaux and Barrera-Oro, 2006), reflecting higher availability of this fish in an area remote from the main historical fishing grounds.…”

The Roles of Sex, Mass and Individual Specialisation in Partitioning Foraging-Depth Niches of a Pursuit-Diving Predator

“…All members of the taxon are colonial seabirds with short foraging ranges that dive to prey on a variety of benthic fish and invertebrates [25]. Blue eyed shags are sexually size dimorphic, with males being larger than females and also diving to greater depths [26-29].…”