Pelagic High Seas Ocean Ecoregionalisation of the Indian Subantarctic (PHOCIS)

In 2019 and 2023, the Expert Workshops on Pelagic Ecoregionalisation for the Subantarctic Region aimed to establish a comprehensive work programme addressing abiotic and biotic pelagic regionalisation (Makhado et al., 2019 and 2023). The area that is concerned spans from 20°W to 160°E and from 30°S to 60°S, excluding Exclusive Economic Zones (EEZs). The integrated ocean management process will encompass the high seas within the CCAMLR planning domains 5 and 6, as well as the non-CCAMLR subantarctic and subtropical areas north of CCAMLR planning domains 5, 6, and 7.

One of the main objectives is to delineate ecoregions for understanding the distribution patterns of plankton, mesopelagic fish, seabirds and marine mammals to advance conservation efforts. In the northern sectors of the Southern Ocean and Southern Indian Ocean, major frontal zones (such as the Subtropical Front, Subantarctic Front, Polar Front, and Agulhas Return Current Front) are highlighted as being the significant drivers of regionalisation. However, these frontal zones vary both geographically and temporally. They may be juxtaposed in some locations while being far apart in others, rendering the study of ecoregions quite complex.

In this area, the Subantarctic islands, crucial habitats for seabirds and marine mammals, are experiencing localized population declines, attributed largely to diminished prey availability and the impacts of climate change. Addressing these challenges necessitates the adoption of a holistic biogeographic approach to ecoregionalisation, which accounts for the diverse array of environments, species communities, and productive pelagic zones. The overarching aim is to delineate priority conservation areas within the high seas, thereby bolstering protection measures for species and ecosystems in regions significantly impacted by climate change.