Fish are helping scientists identify different ecological regions in the Dumont d'Urville Sea.

The Dumont d'Urville Sea, ranging from Terre Adélie to the Mertz Glacier tongue, in George V Land was investigated during the Collaborative East Antarctic Marine Census (CEAMARC) to understand the composition of the marine biota in relation to their environments, and to establish baseline information that could be used to track changes over time.

One aim of the sampling network was to conduct an ecological regionalisation of this area. 'Ecoregionalisation' is a combination of regional oceanographic features (such as banks, depressions, coastal zones and open ocean), and the spatial distribution of species. To do this, scientists from the Tokyo University of Marine Science and Technology, the Muséum National d'Histoire Naturelle of Paris, the Laboratoire d'Océanographie of Villefranche-sur-mer on the French Riviera, and the Australian Antarctic Division, studied the fish of the Dumont d'Urville Sea. Antarctic fish are good indicators for delineating ecoregions because they are long-lived, exhibit a range of functional roles in the ecosystem, and are adapted to different ecosystems.

Historical surveys listed about 20 species of fish, but we have now identified 91 species belonging to 21 families. This shows the importance of having integrated international surveys. Fifty-one per cent of the species belonged to the Notothenioids, a group highly endemic (or specific) to the Southern Ocean shelves (such as icefish, toothfish and Antarctic silverfish). These are followed by Myctophids (lantern fish), a deep sea family very abundant in the Southern Ocean that migrates from the deep layers (greater than 200m) to the surface layers.

We also collected all the life stages of the fish; from larvae to the adults of many species. Early life stages of Channichthyidae (bloodless icefish) are known to be associated with swarms of Antarctic krill, but Pleuragramma antarcticum (Antarctic silverfish) is the dominant pelagic fish (living in the water column) over the shelf. All have major roles in the food web between plankton and top predators (such as penguins, seals and whales). However, our intensive surveys were not able to collect all potential species, such as Dissostichus mawsoni (toothfish) or skates – although these species were collected by others or viewed on video, respectively.

Different ecoregions were found in the pelagic and continental shelf zones. In the pelagic zone, we found a clear distinction in species composition between the surface water layer (0–200m), the intermediate layer (200–600m) and the deep layer.

The continental shelf regionalisation, based on fish living near the sea floor, showed a clear difference between continental margins, inner-shelf depressions, banks and coastal zones. Some species were caught specifically in inner-shelf depressions and especially in the George V Basin. This shelf regionalisation is probably related to both recent environmental features and past environment. Further investigation is needed to determine if colonisation of the shelf occurred from the continental margin itself, or from shelter sites over the shelf that were not covered by ice, after Last Glacial Maximum.

What have we learned from CEAMARC? The list of species known for this area has increased for all the taxa, especially for fish but also for gelatinous plankton and benthos. The Mertz Glacier tongue broke off at the beginning of the year, releasing a very large iceberg. This will change the currents and the biological productivity of the area, it will impact benthic communities (organism living on the sea floor) by scouring, and it will then modify fish habitats. Other surveys in the Dumont d'Urville Sea will try to follow the consequences of this major change in the area. Some will be led by the Australian Antarctic Division, others like the French ICO²TA project (Integrated Coastal Ocean Observations in Terre Adélie) supported by Institut Polaire Paul Emile Victor, will continue to study the changes in the pelagic environment (plankton and pelagic fish) each year. 

PHILIPPE KOUBBI

Observatoire Océanologique de Villefranche sur Mer, France