Krill on a tray !

by Clara Péron, LOCEAN (Laboratoire d’Océanographie et du Climat) and CEBC (Centre d’Études Biologiques de Chizé)

        Birds and marine mammals represent a considerable carbon biomass in the Southern Ocean, and especially around oceanic archipelagos such as Kerguelen. The MOBYDICK cruise took place in February-March, while the breeding season was in full swing on the archipelago’s bird and marine mammal colonies. At this season, the constraints are strong to find food at sea because the parents must both feed and bring food to their young remained on the colony. Adults go to sea for periods of 5 to 15 days and can travel very long distances (500-1000 km) to find favorable feeding areas. Even penguins move several hundred kilometers (300 km) offshore during their feeding journeys. They are therefore able to explore the entire plateau of Kerguelen and the deeper waters beyond the continental slope.

       Birds and marine mammals play a role in the carbon cycle through their consumption, excretion and decomposition. A blue whale (Balaenoptera musculus) weighs up to 190 tons and consumes up to 4 tons of krill a day! With the exception of large whales, penguins represent the highest biomass of marine predators around Kerguelen, home to more than 1.5 million pairs of macaroni penguins (Eudyptes chrysolophus) and more than 300,000 pairs of king penguins (Aptenodytes patagonicus). Albatrosses and petrels are much more diverse than penguins in terms of species (> 40 species against 4 species of penguins) but their biomass and consumption are lower.

       The MOBYDICK project aims to include, for the first time, this biomass of marine megafauna in ecosystem carbon balances. It is for this purpose that we spent long hours scanning the horizon in search of these animals. The weather conditions were not always favorable (fog, rain, strong swell) but our observer was well equipped for the wind and the cold! On average 4°C (39°F) and 26 knots of wind …

Left – Clara equipped with a wind mask, hearing protection to withstand the howling wind and engines of the boat, a pair of binoculars and a notebook. Right – Time series of wind speed over the boat during the MOBYDICK cruise (and associated Beaufort state).

        The analysis of the first results shows that all the species that feed on krill (euphausiids) or amphipods (Themisto gaudichaudii) at the surface and / or at depth were more abundant on the plateau (depth <500 m) than outside the Plateau (depth > 4000 m)

        The vast majority of large whale sightings (28 individuals) were made on the plateau, particularly at the M2 station. The sei whale (or Rudolphi, Balaenoptera borealis), which is 12 to 21 m long, was the most commonly encountered and repeatedly approached the research vessel at M2. This species was abundantly fished in the region at the beginning of the 20th century and it is reassuring to see it coming back. Many blows were observed at great distances without being able to identify the species of whale. A blue whale was stealthily observed on a stormy day near the M2 station and a group of 30 pilot whales remained 30 minutes around the ship at M4.

        Sea lions and elephant seals are very abundant in the region at this season but no individual has been observed because they are very difficult to sight at sea.

Map of marine mammal sightings during the MOBYDICK campaign. The gray dots are the stations where the observer was at the observation post (one hour counting) but did not observe any marine mammal.

        Macaroni penguins and small petrel species (prions and storm petrels <300 g), which mainly feed on crustaceans, were also more abundant on the plateau. On the other hand, king penguins, which feed mainly on lantern (myctophid) fish, were not observed on the plateau but rather at the deeper stations (M1 and M4).

Map of penguin sightings during the MOBYDICK campaign. The gray dots are the stations where the observer was at the observation post (one hour counting) but did not observe any marine mammal.

        The acoustic and pelagic trawl data reveal the presence of crustaceans at all the depths sampled on the plateau with higher concentrations and biomasses at shallow depths at night.

Example of “patches” of euphausiids observed by acoustic on the plateau (near M2) at the time of the blue whale sighting and photo of the crustacean content (Themisto gaudichaudii, Euphausia triacantha and Euphausia vallentini) of a pelagic trawl over the plateau (M2).

        Although the same crustacean species are present in trawls at off-shelf stations (M4), we assume that they are more accessible (more concentrated and at shallower depths) for predators on the shelf.

King penguin observed near the ship at M4 and M1 and its favorite prey,s lantern fish (Krefftichthys anderssoni) caught by pelagic trawl. (photo credits: O. Crispi, C. Péron)

        Among the albatross and petrel species (> 300 g), four species were present at almost all sighting stations, the most abundant being the wandering albatross Diomedea exulans, the white-chinned petrel Procellaria aequinoctialis and the giant petrels Macronectes spp. Only the black-browed albatross Thalassarche melanophris was present and abundant at all stations on the plateau and at the edge of the plateau, but almost absent at M4, the deepest and furthest station on the plateau. In contrast, light-mantled albatrosses Phoebetria palpebrata and white-headed petrel Pterodroma lessoni were more abundant at M4 than on the plateau. Albatrosses and medium and large petrels are less dependent on crustaceans in their diet. For example, albatrosses eat mostly squid, large fish and fishing waste.

        Data collected through this observational protocol will be coupled with dietary data and used to parameterize the contribution of top predators to the carbon fluxes of the Kerguelen ecosystem.

Photos of wandering albatross, Antarctic giant petrel, white-chinned petrel and black-browed albatross taken by Olivier Crispi during the MOBYDICK 2018 cruise.

Team “trawl/acoustic” involved in this study during the MOBYDICK campaign: 

  • Yves Cherel (CEBC, Chizé)
  • Cédric Cotté (LOCEAN, Paris)
  • Clara Péron (LOCEAN, Paris)
  • Anna Conchon (LOCEAN/CLS Paris/Toulouse)
  • Jérémie Habasque (LEMAR, Brest)
  • Boris Espinasse (Univ. of British Columbia, Canada)
  • Natasha Henschke (Univ. of British Columbia, Canada)

Laisser un commentaire