La première campagne au large de Kerguelen à lieu en 1995, sur le petit navire océanographique de 25 m « La Curieuse » (de l’IPEV, anciennement IFRTP).
L’objectif de cette première campagne était de connaître la distribution des espèces de poissons pélagiques sur l’aire d’alimentation préférentielle du manchot royal (colonie de Ratmanoff, péninsule Courbet) en période d’alimentation des poussins. A cet effet des oiseaux avaient été instrumentés et suivis par satellite (système ARGOS) pour connaitre en temps réel l’aire géographique concernée (sud-est de Kerguelen au niveau du Front Polaire Antarctique) et les profondeurs ciblées. Une fois cette aire déterminée le navire « la Curieuse » (capitaine D. Rochard et son équipage) s’est rendu sur place (chef de mission G. Duhamel, MNHN assisté de J.M. LePetitcorps) pour effectuer des chalutages jour et nuit à différentes profondeurs. Le chalut utilisé était un IYGPT (International Young Gadoid Pelagic trawl) avec un maillage de 10 mm au cul du chalut pour retenir ces petites espèces. La durée de chaque trait de chalut était standardisée à 30 ‘ (à 3 nœuds). Différentes profondeurs furent échantillonnées répétitivement pour s’affranchir des migrations nycthémérales des différentes espèces, en particulier des poissons-lanternes Myctophidae connus pour remonter des grandes profondeurs la nuit. Deux séries de chalutages (du 1 au 5/03/1995 n = 36 et du 10 au 13/03/1995 n = 28) furent réalisées de 23 à 112 milles de la côte à quatre niveaux bathymétriques 50, 150, 250 et 300 m de profondeur, au-dessus de fonds profonds de 475 à 2400 m. Cette campagne a permis la capture de 37 espèces de poissons pélagiques (16 073 spécimens) et 8 espèces de calmars (371 spécimens). Les déterminations furent réalisées au laboratoire d’ichtyologie générale et appliquée du MNHN et les données enregistrées en collection pour constituer une base de données. Les assemblages faunistiques furent ainsi déterminés de la côte vers le large donnant des indications de leurs variations jour-nuit utiles pour les scientifiques travaillant sur la prédation.
IPEKER
Kerguelen I.
(first trial, summer 1995)
(fishing master D. Rochard)
Team: G. Duhamel, J-M LePetitcorps
The pelagic fish community of the Polar Frontal Zone off the Kerguelen islands: Assemblage and size structure, circadian changes and use by top predators.
Real time study of the Pelagic fish community present in the foraging range of a top predator: the king Penguin Aptenodyptes patagonicus, breeding on the Kerguelen Islands.
Focus on the objectives
- tracking adult birds going to sea and analyze the regurgitating food to chicks on the breeding site afterwards.
- monitoring diving sequences and depth of dives, food intake, composition of the regurgitated food of these birds.
- conduct simultaneous trawls in the foraging range of birds at stratified depths to study the pelagic fish composition, the assemblages, the circadian changes and the size structure of the species.
- tentatively establish the relationships between prey and top predator at the studied season.
Realization
- Period: summer 1995 (end of February, early March)
- Selected King Penguin breeding site: Ratmanoff colony on the eastern coast of the Kerguelen islands.
- Instrumentation (Argos satellite trackers, stomach temperature sensors, depth recorders) of 9 birds (4 specificly tracked) on the colony before the at sea ship survey, 19 food samples collected (including the tracked 9) at the return of birds.
- Survey fishing method: Midwater trawl survey (IYGT, 10 mm codend meshsize, 30 ‘ duration of each station), day and night at 4 depth horizons (50, 150, 250, 300 m) based on the known maximum depth of the King Penguin dives (300 m).
Results
- Study of a part of the foraging area: Polar Frontal Zone off the islands shelfbreak (20 - 110 miles, bottom depth: 450 - 2300 m) following the tracking of the birds.
- 51 standardized hauls (+ 11 others), minimum 5 at each depth, both day and night.
- 14 232 specimens, weight: 37 483 gr
- 33 species (including post-larvae or fingerlings of 8 demersal species)
The pelagic fish community of the Polar Frontal Zone off the Kerguelen islands: Assemblage and size structure, circadian changes and use by top predators.
Summary
A summer ornitho-ichthyological joint study (1995), in the offshore Polar Frontal Zone near the eastern coast of the Kerguelen Islands (distance: 20 - 110 miles; bottom depth: 450 - 2300 m), was the first to investigate the pelagic fish community of the upper water column. Four depths horizons were sampled (50 - 150 - 250 - 300 m) day and night which totalled 52 midwater standardized trawl hauls. Thirty four fish species were identified, including planktonic stages and pelagic juveniles of 8 known demersal species. The myctophid family (15 species) dominated the true pelagic species both in number and biomass. The species mainly belonged to the subantarctic subpattern of Hulley’s (1981) mesopelagic community.
Very large circadian changes were observed among all species. The day distribution showed a low abundance and biomass in the whole water column sampled, with patchy distribution observed in the main species. Lepidonotothen squamifrons early life stages, Muraenolepis marmoratus pelagic juveniles, Kreffichthys anderssoni, Protomyctophum bolini and P. andriashevi occupied successively deeper layers. Species diversity increased in the deepest range. Deeper living species invaded all mesopelagic depths at night with maximum abundance and biomass at the upper surface layer. A uniform horizontal distribution was observed in the whole studied area. Some species (Electrona antarctica, Gymnoscopelus braueri, G. fraseri, Protomyctophum bolini and P. tenisoni) formed the bulk of the deeper living migratory species, while others (Krefftichthys anderssoni, Electrona carlsbergi, Bathylagus tenuis) did not reach the surface. Surprising, was the high proportional biomass of Gymnoscopelus bolini, which was explained by the large individual size of the sampled specimens of this world-largest myctophid species. It contrasted with the numerous but individually small (< 1 gr) species such as K. anderssoni, P. bolini or P. tenisoni.
The pooled size structure of the species sampled was also very informative, with significant differences observed between day and night catches. Only fish under 8 cm standard length (SL) occurred by day, with different modal values in each depth stratum. The size distribution was, in contrast, very similar at each depth at night with the vast bulk of fish under 13 cm SL and a strong peak at 4 cm SL.
The area studied was recognized to include the foraging range of top predators, mainly the King penguin (Aptenodytes patagonicus) and the fur seal (Arctocephalus gazella) breeding on the Kerguelen Islands. Satellite tracking, analysis of diving sequences, fish species composition in the diet (regurgitated food to chicks or scat seals remains, mainly based on otoliths), and their rebuilt size structure allows, associated with a real time prey sampling, a better understanding of the relationships between predators and preys in this specific area of the Southern Ocean.