Long term study analyses sea bird communities

Since observations of seabirds at sea within Prydz Bay began in 1980–81 with the start of the BIOMASS (Biological Investigations of Marine Antarctic Systems and Stocks) program, almost 32,000 observations of seabirds and associated environmental variables (such as sea ice, sea surface temperature and wind speed) have been made. Forty species of seabirds have been reported from the Prydz Bay region.

Of the 26 most frequently observed species, nine breed within Prydz Bay (emperor and Adélie penguins, southern giant petrels, southern fulmars, cape, Antarctic and snow petrels, Wilson’s storm petrels and south polar skuas) and 17 visit Prydz Bay from their breeding grounds farther north, such as albatrosses and shearwaters. There have been no winter observations (June to August, inclusive) of seabirds within Prydz Bay over the 22 years, so the analyses were confined to spring, summer and autumn observations.

A recently completed study has used this extensive data set to identify and examine three species assemblages, or communities, of seabirds within Prydz Bay. Observations of assemblage 1, comprising all nine breeding species, were consistent with the influx, presence, and departure of breeding birds to their colonies around Prydz Bay – a major breeding area for all nine breeding species, with large breeding populations during summer months. In midsummer this assemblage was observed in the southeast of Prydz Bay, adjacent to known Vestfold and Larsemann Hills breeding areas.

Assemblage 1 is similar to pack ice associated seabird assemblages identified elsewhere around Antarctica, with minor differences in species composition due to different seabird breeding distributions. For example, the sea ice assemblage in the South Atlantic sector of the Antarctic includes chinstrap penguins that do not breed in the Australian sector.

Assemblage 2 comprises all 26 commonly observed seabird species from Prydz Bay and represents an overlap (in time and space) assemblage, being observed in late summer in central Prydz Bay.

Assemblage 3 consists principally of the non-breeding species that visit Prydz Bay each summer from subantarctic and temperate breeding localities, including some species from as far afield as southeast Australia, such as shearwaters. This assemblage was observed most frequently in mid- to late summer and well offshore.

Two species (cape petrels and Wilson’s storm petrels) were found to be members of assemblages 1 and 3. This is believed to be a result of their widespread breeding distributions, with colonies in the Antarctic and subantarctic. Individual cape and Wilson’s storm petrels forage within Prydz Bay before returning to their colonies. This behaviour produces broad ranges in their ‘at -sea’ distributions in the Southern Ocean.

The study provides the foundation for further investigations such as predicting seabird distributions from physical environmental parameters at localities elsewhere in East Antarctica.

Eric J Woehler, Ben Raymond & David J Watts