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The primary objectives of the bioacoustic sampling program were to map the mesoscale (10s of kilometers) dispersion of krill (Euphausia superba) near Elephant Island, to estimate their biomass, and to determine their association with predator foraging patterns, water mass boundaries, spatial patterns of primary productivity, and bathymetry. Secondary objectives were to describe cross-sections of volume backscattering strength along transects through Admiralty Bay, across the shelf break to the north and south of the South Shetland Islands, and across the Antarctic Convergence in Drake's Passage. Directed net sampling was also conducted to verify classification of acoustic targets. In situ target strength (TS) measurements of individual zooplankton were made; these data will be used to develop or enhance TS models for various macrozooplankton and nekton.
Acoustic data were collected using a multifrequency echo sounder configured with downlooking 38, 120, and 200 kilohertz (kHz) transducers mounted in the hull of the ship. System calibrations were conducted before and after the surveys using standard sphere techniques while the ship was at anchor in Ezcurra Inlet, King George Island. For the purposes of generating distribution maps and biomass estimates, 120-kHz volume backscattering strength attributed to krill was integrated over depth from 10 meters (m) to 250 m and averaged over transect intervals of 185 m (18 pings assuming nominal vessel speed of 10 knots and 2-second ping interval).
Integrated volume backscattering strength per unit sea surface area was converted to estimates of krill biomass density by applying a factor equal to the quotient of the weight of an individual krill and its backscattering cross-sectional area, summed over the sampled length frequency distribution for each survey (Hewitt and Demer 1993). Total biomass was estimated by treating the mean biomass density on each transect as an independent estimate of the mean density over the survey area (Hewitt and Demer 1993). The entire survey area was treated as a single stratum.
Survey A (23 January to 4 February) and Survey D (24 February to 8 March) were conducted to map the mesoscale dispersion and estimate the biomass of krill in 15,000 square nautical miles near Elephant Island and the eastern end of King George Island. Each survey consisted of 12 north-south transects with 15-nautical-mile spacing between lines. Stations were 15 nautical miles apart and included a conductivity-temperature-depth cast and an Isaacs-Kidd Midwater Tow (IKMT) plankton tow.
During both surveys, areas of high krill density were mapped in wide bands along the north side of King George and Elephant Islands, where water depth was greater than 200 m ( figure 1). The bands widened north of Elephant Island, where water flowing north from Bransfield Strait impinged on the general northeast flow along the north side of the South Shetland Islands. This distribution pattern is similar to that observed during previous surveys.
Krill biomass, estimated from the acoustic data for the area outlined in the box on figure 1, was the highest estimated since 1992 (table). High biomass estimates during Survey A represent large numbers of juvenile krill that were spawned in 1995; high biomass estimates during Survey D represent moderate numbers of sexually mature adults.
Acoustic data were collected along transects through Admiralty Bay during the approach (Transect AB1) and again during the departure (Transect AB2) from the anchorage at Ezcurra Inlet (see Martin, Hewitt, and Holt, Antarctic Journal, in this issue). Four IKMT tows were conducted during the transects as well; two on the way in and two on the way out. The diets of chinstrap, Adélie, and gentoo penguins were sampled concurrently at colonies along the shores of Admiralty Bay by Nina Karnovski at the Copacabana field camp.
The acoustic and IKMT data indicate a layer of dense swarms existed between 50 and 150 m depth throughout the extent of the Admiralty Bay but not extending into Bransfield Strait ( figure 2). Krill length-frequency distributions from the IKMT tows indicated that the krill in the area comprised two modal size groups centered around 26 and 48 millimeters (mm). These modes are similar to that observed last year in Admiralty Bay, except that the larger mode is about 5 mm longer. The krill length frequency distributions from penguin diet samples indicated no significant differences between penguin species and also that the penguins may have been feeding selectively on larger krill.
Krill aggregations encountered during Transect AB2 appear to be slightly higher in the water column than during Transect AB1, particularly during the latter portion of Transect AB2. This increase is likely due to the vertical movement of krill into shallower waters beginning at dusk. It has been suggested that krill reflect less sound during times of vertical migration when their body orientation with respect to the vertical is steeper. This explanation may be an alternative to the apparent decrease in krill between the two transects.
References
Hewitt, R.P., and D.A. Demer. 1993. Dispersion and abundance of antarctic krill in the vicinity of Elephant Island in the 1992 austral summer. Marine Ecological Progress Series, 99, 29-39.
Martin, J.E., R.P. Hewitt, and R.S. Holt. 1996. The U.S. Antarctic Marine Living Resources (AMLR) program: 1995-1996 field season activities. Antarctic Journal of the U.S., 31(2).