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

CASAUX R.; BARRERA-ORO E.

Workshop; The CCAMLR Fish Stock Assessment Working Group; 2005

Shags in Antarctica: their feeding behaviour and ecological role in the marine food web.   Ricardo Casaux1,* and Esteban Barrera-Oro1,2     1 Instituto Antártico Argentino, Cerrito 1248, (1010) Buenos Aires, Argentina. e-mail: rcasaux@dna.gov.ar and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Av. Rivadavia 1917, (1033) Buenos Aires, Argentina.   2 Museo Argentino de Ciencias Naturales ?Bernardino Rivadavia?, División Ictiología, Ángel Gallardo 470, C1405DJR Buenos Aires, Argentina   * Present address: Laboratorio de Investigaciones en Ecología y Sistemática Animal (LIESA), Universidad Nacional de la Patagonia, Ruta 259 km. 5, 9200 Esquel, Chubut, Argentina                 CCAMLR FISH STOCK ASSESSMENT WORKING GROUP   Agenda item Nº                         Re: Dr. Ricardo Casaux        Instituto Antártico Argentino        Cerrito 1248        1010 Buenos Aires       Argentina       e-mail rcasaux@dna.gov.ar                                                                                                                     Shags in Antarctica: their feeding behaviour and ecological role in the marine food web.   Ricardo Casaux1,* and Esteban Barrera-Oro1,2     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 increment 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.