CCAMLR

Acoustic scattering experiments involving simultaneous acquisition of broadband echoes and video footage from several Antarctic krill were carried out to determine the effect of animal orientation on echo spectral structure. A novel video analysis technique, applied to extract krill angle of orientation corresponding to each insonification, revealed that echo spectra from krill near broadside incidence relative to the incident acoustic wave exhibited widely spaced, deep nulls, whereas off-broadside echo spectra had a more erratic structure, with several closely spaced nulls of variable depth. The pattern of changes in echo spectra with orientation for the experimentally measured acoustic returns was very similar to theoretically predicted patterns based on a distorted wave Born approximation (DWBA) model. Information contained in the broadband echo spectra of the krill was exploited to . invert the acoustic returns for angle of orientation by applying a newly developed Co variance Mean Variance Classification (CMVC) approach, using generic and animal-specific theoretical and empirical model spaces. The animal-specific empirical model space was best able to invert for angle of orientation. The CMVC inversion technique can be implemented using a generic empirical model space to determine angle of orientation based on broadband echoes from individual zooplankton in the field.

Refinements have been made to the multiple-frequency method that was recently developed to improve the accuracy and precision of in-situ target strength (TS) measurements using split-beam echosounders (Demer et al., J. Acoust. Soc. Am. 105(4) 2359:2376 (1999)]. The multiple-frequency method improves the rejection of unresolvable and constructively interfering target multiples by combining synchronized signals from two or more adjacent split-beam transducers of different frequencies which are not integer multiples of each other. In this study, the method itself was improved by: 1) optimizing the accuracy and precision of the angular and range measurements of the individual frequency detections; 2) more precisely determining the relative three-dimensional (3-D) locations (x, y, and z) and angular orientations (pan and tilt) of the transducers and thus the positional transformation; and 3) increasing the range resolution of one or more of the frequencies. These improvements are demonstrated through controlled test tank experiments using 38 and 120 kHz split-beam transducers and a 200 kHz single-beam transducer. Tolerances for matching target positions at two or more frequencies were determined to be ±0.5 m in radial range and ±0.7° off-axis (compared to ±l.0 m ±l.5° in the previous study). These experimental results indicate that such careful application of the multiple-frequency TS method can reject 100% of multiple targets while allowing 90% of the resolvable single-targets to be measured

Variability in the Southern Ocean is frequently reflected in changes in the abundance of Antarctic krill Euphausia superba and subsequent effects on dependent predators. However, the nature and consequences of changes in krill population dynamics that accompany l1uctuations in its abundance are essentially unknown. A conceptual model, developed froll1 quantitative measures of krill length ill the diet of predators at South Georgia from 1991-1997, allowed predictions to be made about the abundance and population structure of krill in 1998 and the consequences for predators. Consistent with model predictions, in 1998 there was a serial change in krill population structure, low krill biomass and low predator reproductive performance. The change in modal size of krill, from 56 mm in December to 42 mm in March was apparently a result of the transport of krill into the region. This is the first occasion when the future status and structure of the krill population at South Georgia has been successfully predicted. By representing local krill population dynamics, which may also reflect large-scale physical and biological processes, predators have a potential key role as indicators of Southern Ocean environmental variation at a range of spatial scales.

Archival data loggers were used to collect information about depth, swimming speed, and heart rate in 23 free-ranging antarctic fur seals. Deployments averaged 9.6 ± 5.6 days (SD) and totaled 191 days of recording. Heart rate averaged 108.7 ± 17.7 beats/min (SD) but varied from 83 to 145 beats/min among animals. Morphometrics explained most variations in heart rate among animals. These interacted with diving activity and swimming speed to produce a complex relationship between heart rate and activity patterns. Heart rate was also correlated with behavior over time lags of several hours. There was significant (P

This study examined the relative contribution of environmental variation and the seasonal demands of pup rearing on the foraging behaviour of female Antarctic fur seals Arctocephalus gazella at Bird Island, South Georgia (54° S, 38° W), during 3 austral summers (1994 to 1996). Time-depth recorders measured the diving behaviour of 72 individuals during a total of 385 foraging trips totalling 1964 d at sea. The frequencies of krill, fish and squid in the diet were estimated from prey items contained in scats. In 1996, a year of high krill abundance, females made shorter foraging trips, fewer dives and spent more time ashore than in 1994 when krill was scarce. Females fed exclusively on krill in 1996, and frequent shallow daytime diving indicated that krill were close to the surface during the day. In 1994 and 1995 deeper and longer-duration daytime dives were associated with a higher proportion of fish and squid in the diet. Foraging trip duration, ashore duration and dive frequency increased through the course of the 1995 and 1996 lactation seasons. Females, therefore, appeared to match pup demands by increasing both time feeding at sea and energy delivery to the pup on land. However, the importance of sea surface temperature and duration of night in multiple regression models suggested that physical factors were also important in explaining the seasonal pattern of fur seal foraging behaviour.