Relationships between distribution of two euphausiid species and oceanographic characteristics in the Ross Sea (January–February 2000)

During austral summer (January 21st to February 5th) 2000 an oceanographic cruise, devoted to study two krill species (Euphausia superba and Euphausia crystallorophias), was carried out in the Ross Sea area. Activities included acoustic, fishery and physical measurements. More than 2370 nautical miles were acoustically sampled to determine the euphausiid biomass; during the echosurvey, every 6 hours a haul and a CTD cast (or a XBT launch) were performed, and a XBT was launched between consecutive hauling stations. This allowed to collect 34 CTD stations and 73 temperature profiles (XBT), so identifying main water masses and oceanographic features. Acoustic data were processed in order to distinguish between the two species. Euphausia superba and Euphausia crystallorophias swarms were recognized, and average length estimated, by means of the three-frequency method, based on the fluid sphere model. Net samples were considered the ground truth data, being compared to the acoustic estimates of krill species and size. Characteristics (dimensions, volume, weight, krill mean length) for each krill swarm were determined, and the krill average biomass per squared nautical miles was computed along the ship route track. Adopted methodologies for cruise execution and for acoustic data post-processing allowed to obtain for the first time a detailed description of the krill distribution in the Ross Sea area related to oceanographic characteristics. Horizontal distribution of krill average biomass is showed separately for each species, and associated to thermohaline properties. Highest krill density biomass areas are closely investigated, and vertical sections with krill swarms weight and sea temperature are reported. Results indicate that the Euphausia superba detected biomass was about one order of magnitude greater than Euphausia crystallorophias one. The first species was almost exclusively present in the northern area, interesting only a limited portion of the continental shelf, while the second one dominated the southern area, starting from the Ross Ice Shelf region until the zone close to the shelf break, with some presence in the open ocean region too. In the proper shelf break area, Euphausia crystallorophias was practically absent, while Euphausia superba was relatively abundant. The two species had a very limited, but with relatively high biomasses, overlapping area in the northernmost part of the Joides basin. Few other minor overlapping areas were detected. Swarms of both species were mostly found in the surface water layer (Euphausia crystallorophias being located at quite deeper depths, often close to the seasonal thermocline zone) and they appeared to prefer cold waters, avoiding the warm Modified Circumpolar Waters and the warmest portion of the surface waters.