STUDIES ON THE ANTARCTIC SHAG Phalacrocorax atriceps bransfieldensis AT THE SOUTH SHETLAND ISLANDS, ANTARCTICA.

Ricardo Casaux

Instituto Antártico Argentino, Cerrito 1248, 1010 Buenos Aires, Argentina. e-mail: pipocasaux@overnet.com.ar

As part of a programme aimed at understanding the relationships between fish and their predators, from 1990 the Instituto Antártico Argentino is developing studies on the diet of the Antarctic Shag Phalacrocorax atriceps bransfieldensis at the South Shetland Islands, Antarctica. These studies were mainly focused on the colony at Duthoit Point, Nelson Island (Fig. 1), but from 1991 the colonies at Half-moon Island and Harmony Point, Nelson Island, were also considered.

Both, the analysis of stomach contents and regurgitated pellets indicated that the diet of this bird at these localities is diverse and that benthic-demersal fish are largely the main prey, followed by octopods, gammarid amphipods and polychaetes. Among fish, the Antarctic cod Notothenia coriiceps is the most important prey, whereas Harpagifer antarcticus and/or Lepidonotothen nudifrons, depending on the study area, followed in importance (Casaux & Barrera-Oro 1993, Coria et al. 1995, Barrera-Oro & Casaux 1996, Casaux 1997, among others).

The variation in the composition of the diet throughout the breeding season was also analysed. It was found that in order to respond to chicks’ increasing energetic demands, during the main rearing period the parents forage on larger fish such as N. coriiceps, than in periods of chicks’ lower requirements (Casaux & Barrera-Oro 1995, Favero et al. 1995). This strategy at Duthoit Point occurred with an increase in the number and a decrease in the duration of the foraging trips along the breeding season, although the extension of the daily foraging activity and the mass of the loads carried to the nest did not vary significantly (Favero et al., 1995). Differently, it was observed that at Harmony Point the shags also increased the extension of the daily foraging activity as well as the mass of the loads (Casaux, unpublished data).

In the 1995/96 summer season we started studies on the foraging strategy of this bird at Harmony Point. For such purpose we are analysing sexual differences in diving depths, organisation of the bouts, selection of foraging areas, etc.; aspects that are correlated with the composition of the diet as reflected by the analysis of pellets and stomach contents, breeding parameters, activity patterns and prey availability. Simultaneously, a tagging program is developed at Duthoit Point and Harmony Point.

This information is being processed and is considered of particular importance to understand on the biology of the Antarctic Shag at the South Shetland Islands, a species whose breeding population size at the three colonies under study was steady decreasing during the last decade (Table 1).

Table 1. Variation in the size of the breeding populations of the Antarctic Shag Phalacrocorax atriceps bransfieldensis at three localities of the South Shetland Islands.

⁽¹⁾ Araya & Aravena 1965.
⁽²⁾ Favero, Bellagamba & Farenga 1991.
⁽³⁾ Favero, personal communication.
⁽⁴⁾ Casaux et al., unpublished data.
⁽⁵⁾ Casaux & Barrera-Oro, 1993.
⁽⁶⁾ Coria et al. 1995.
⁽⁷⁾ McDermott & Vázquez, personal communication.
⁽⁸⁾ Esponda, personal communication.

Figure 1. Study area of Blue-eyed Shags Phalacrocorax atriceps bransfieldensis at the South Shetland Islands, Antarctica.

References

Araya, R. & W. Aravena 1965. Las aves de Punta Armonía, Isla Nelson, Antarctica Chilena: Censo y distribución. Publicación del Instituto Antártico Chileno, 7: 1-18.

Barrera-Oro, E. & R. Casaux 1996. Fish as diet of the blue-eyed shag Phalacrocorax atriceps bransfieldensis at Half-moon Island, South Shetland Islands. Cybium, 20 (1):37-45, 1996.

Casaux, R. 1997. On the accuracy of the pellet analysis method to estimate the food intake in the Antarctic shag Phalacrocorax bransfieldensis. Commission for the Conservation of Antarctic Marine Living Resources, Ecosystem Monitoring and Management Working Group, Document WG-EMM-97/61, 14 pp.

Casaux, R. & E. Barrera-Oro 1993. The diet of the Blue-eyed Shag Phalacrocorax atriceps bransfieldensis feeding in the Bransfield Strait. Antarctic Science, 5 (4): 335-338.

Casaux, R. & E. Barrera-Oro 1995. Variation in the diet of the Blue-eyed Shag Phalacrocorax atriceps throughout the breeding season at Half-moon Island, South Shetland Islands. Commission for the Conservation of Antarctic Marine Living Resources, Ecosystem Monitoring and Management Working Group, Document WG-EMM-95/78, 12 pp.

Coria, N., R. Casaux, M. Favero & P. Silva 1995. Analysis of the stomach content of the Blue-eyed Shag Phalacrocorax atriceps bransfieldensis at Nelson Island, South Shetland Islands. Polar Biology, 15:349-352.

Favero, M., P. Bellagamba & M. Farenga 1991. Abundancia y distribución espacial de las poblaciones de aves de Punta Armonía y Punta Dedo, Isla Nelson, Shetland del Sur. Revista Italiana de Ornitología, 61 (3-4): 85-96.

Favero, M., R. Casaux, P.Silva, E. Barrera-Oro & N. Coria 1995. The diet of the blue-eyed shag Phalacrocorax atriceps during summer at Nelson Island, Antarctica: Temporal variations and consumption rates. Commission for the Conservation of Antarctic Marine Living Resources, Ecosystem Monitoring and Management Working Group, Document WG-EMM-95/82, 17 pp.

Ricardo Casaux, Instituto Antártico Argentino, Cerrito 1248, 1010 Buenos Aires, Argentina. e-mail: pipocasaux@overnet.com.ar