Comparative studies on the biological and acoustical properties of krill aggregations (Euphausia superba Dana) samples during the XIII Italian Expedition to the Ross Sea (December 1997–January 1998)

During the XIII Italian Expedition to the Ross Sea in late spring and early summer (December-January 1997-98), for biological validation of hydroacoustic recordings, 35 net samplings were carried out. For all hauls both parameters: biological (length AT, sex and maturity stage of the gonads of the Euphausia superba) and acoustical (differences in volume back-scattering strength of krill aggregations MVBS at 38, 120 and 200 kHz) were examinated. Comparing the depth of the net and acoustic recordings during the hauls.
The surveyed area extended from 71° to 76°' S in latitude and from 164° E to 177° W in longitude. The aim of the paper is: (1) to compare the species/length composition of catch data and the one obtained by processing acoustic data, acquired during the net sampling; (2) to present density distribution maps of Euphausia superba and Euphausia crystallorophias obtained by processing acoustic data; (3) to analyse the structure of Euphausia superba population on the basis of biological variables, extracted from catch data;.
The analysis of the biological parameters seems to indicate statistically significant differences in them between the hauls, while within each haul the parameters are homogeneous. The maturity stage of krill in general increase with decreasing of latitude (from 76° to 71°S).
The analysis of acoustic and catch data indicate that: (1) the acoustic discrimination between the two predominant species, based on the multi-frequency method, is very close to the one based on net samplings; (2)The acoustic estimation of the krill mean length, based on the fluid sphere method, is pretty close (around 9%less) to the mean catch values; (3) The numerical abundance of the krill, estimated by the fluid sphere method, is 20-100 times higher than the one estimated from the catch.
The geographical biomass distribution of Euphausia superba and Euphausia crystallorophias, estimated acoustically, overlap in this period of year.