Acoustic Characterization of Middle Trophic Level Species in the Ross Sea Ecosystem (Antarctica)

By far the largest components of the Ross Sea Middle Trophic Level are the oceanic (E. superba) and the shelf (E. crystallorophias) krill species. In spite of their abundance and key role in Ross Sea trophic chain, they have received very little attention from the scientific community before the six large scale acoustic surveys conducted during the Italian expeditions to Antarctica of 1989-90, 1994, 1997-98, 2000, 2003-04. The region sampled acoustically is roughly a rectangle of 131000 nm2, 64% of the Ross Sea. A three frequency method, adequate to the biological and environmental situation of the Ross Sea, much more complex than that of Atlantic sector areas, was developed. The estimation of E. superba biomass was around 2.5 millions of tons in Nov. 1994, around 2 millions of tons in Dec. 1997 and 1 million of tons in Jan. 2000. The decreasing of E. superba biomass in the Ross Sea from Nov. to Feb. occurred in parallel with the movement northwards of the polynya ice edge. This suggests that E. superba habitat enlarges to the Pacific Ocean, and portions of this population of the Ross Sea spread into the Pacific waters, beginning from the second half of Dec. On the other hand the total biomass of E. crystallorophias was rather stable (200000 t) in the five surveys. It suggests that ice krill population is segregated within the Ross Sea.