Krill biomass estimates for two survey boxes to the north east and north-west of South Georgia in January 1996: the beginning of a five-year monitoring program

Acoustic surveys were carried out in January 1996 within two 100 x 80 km boxes located over the shelf-break to the north-east and north-west of South Georgia. Surveys were conducted from RRS James Clark Ross using a Simrad EK500 echo-sounder operating at 38, 120 and 200 kHz. These surveys were the first in a new five-year British Antarctic Survey programme initiated in part to monitor inter-annual variability in the abundance of the Antarctic krill Euphausia superba in the South Georgia region, and to provide krill biomass estimates for management purposes. Survey box 1 was located to the north-east of Cumberland Bay and encompassed Charlotte Bank, an area where historically whale catches were high and where numerous observations of elevated krill abundance have been made. Box 2 was located to the north of Bird Island, within a foraging area of prime importance for breeding krill predators. Together the two boxes therefore provide data of value to fishery scientists, ecologists and environmental modellers. The acoustic surveys were of randomised design and each nominally constituted 10 randomly spaced parallel transects, 80 km in length, running perpendicular to the major direction of shelf-break in the area; poor weather however necessitated some modification of this plan during the 1996 surveys. All transects were steamed during the hours of daylight in order to avoid biases in krill biomass estimation caused by diel vertical migration. Echoes were integrated in 2 m depth bins from 2 to 250 m below the transducers, over 100 second intervals (≈0.5 km at 10 knot survey speed). Two transects were run per day, and at night net hauls were carried out at a location toward the centre of the previous pair of transects to obtain krill for estimates of length frequency distributions. Weighted mean krill lengths of 29.5 mm for box 1 and 32.0 mm for box 2 were determined. Thresholded (mvbs 120 kHz minimum -100 dB) 120 and 38 kHz echo signal pairs were partitioned using a dB difference technique (δmvbs = mvbs 120 kHz - mvbs 38 kHz) into those attributable to krill (δmvbs between 2 and 12 dB), small zooplankton (δmvbs > 12 dB) and nekton (fish/squid, δmvbs < 2 dB), and a mean weighted biomass and variance estimate for each fraction was derived from the 120 kHz signal using a generic target strength (TS) to weight relationship (TS of 1 kg of krill in box 1= -39.13 dB, in box 2 = -39.03 dB). Weighted mean krill density estimates (and their weighted variances) for the January 1996 surveys were 40.57 gm-2 (13.37) and 26.48 gm-2 (54.30) for boxes 1 and 2 respectively. Density estimates for the 1996 season were high compared to those obtained from similar regions during acoustic surveys conducted in January 1994. The previously published 1994 estimates were re-calculated here using the same target identification and thresholding procedures as those applied to the 1996 data to facilitate direct comparison. The resulting 1994 box 1 and 2 density (and variance) estimates were 1.87 gm-2 (0.14) and 7.43 gm-2 (1.33) respectively. In 1994 the low krill abundance around Bird Island resulted in greatly reduced breeding success in most habitual krill predator species there. In the 1996 season however breeding success of Gentoo penguins, Black-browed Albatrosses and Antarctic fur seals, species particulary dependant upon krill, were normal. Instantaneous estimates of krill abundance using acoustic techniques are therefore supported by measurements from predators, the breeding performances of which provide a longer-term indication of prey abundance in the surrounding pelagic ecosystem within a particular season.