Spatial and temporal distribution of krill Euphausia superba biomass in the Ross Sea (1989/90, 1994/95)

The paper, based on the results of italian expeditions to the Ross Sea (December 1989 - Januar 1990 and November - December 1994), presents how is spatial and temporal distribution of krill biomass and what is its value. The hydroacoustic observations were done in the time when investigated area was pratically covered by pack ice (November - mid December) or immediately after ice had receded (end December - January). The process of ice melting began in mid November, near Ross Ice Shelf (so called polynia area) and moved towards North. All the time daytime was prevaled. Krill in aggregations both below ice and in ice free water was observed but below ice were rather single ones. Because in the same time, many times, on the pieces of ices broke up by icebreaker, individuals of krill were observed, it can be supposed that below ice krill occurred mainly in scattered forms very close to surface. Aggregations of krill mainly in the part of Ross Sea between 70° and 75° S but in November and beginning of December closer to 75° S while in the end of December and January closer to 70° S were observed. It can suggested that temporally krill migrated horizontally towards North, parallely to receding of ice cover. It is also possible that part of observed aggregations were not migrated ones but created from krill scattered previously below ice.
Biomass density of krill was calculated per each nautical mile. For comparison the biomass from different years, the whole investigated area was divided in rectangles of 1°x30' .
The results of calculations indicate that biomass of krill in the western part of the Ross Sea (east of longitude 180°), estimated during late spring and at the beginning of the austral summer in 1994 was about 3 min. tons. The investigated area was approximately 30000 nm2 and the mean biomass density about 100 Unm2 . The highest value of biomass density (51761 t/nm2 ) was found the 23 November 1994 at position 174° 59' E, 74° 36' S where a so called superswarm was observed. In November 1994 the investigated area in 80-100% was covered by ice. The above mentioned superswarm was recorded in a small area of ice free water. It can be supposed that below ice krill occurred mainly in scattered forms (as mentioned above) and when the environmental conditions are favorable the coherent forms of aggregation can be formed very quickly.
During the 1989/'90 expedition hydroacoustic observations were done at the end of December and in January in ice-free water. During the first part of echo survey (in the end of December and at beginning of January) in an area of 13000 nm2 a total biomass of 426 thousand tons was estimated (mean biomass density around 32 t/nm2 ). During the second part of echosurvey (January) in an area of about 22000 nm2 the estimated biomass was 847 thousand tons ( mean biomass density around 39 t/nm2 ) . The highest value of biomass density (1041 t/nm2 ) was found in January at position 175° 40' E, 70° 45' S. The results of the italian expeditions indicate that biomass density of E. superba in the Ross Sea (Pacific Sector), contrary to previous opinions, is similar to the ones calculated in Atlantic and Indian Sectors.