CCAMLR

Maps of krill (Euphausia superba) density derived from acoustic survey data and the distribution of fishing effort in the Chilean krill fishery indicate an area of high krill density wrapping around the northwestern end of Elephant Island during the austral summer of 1992. In this area, the distribution of catch-per-fishing-time and krill density (measured acoustically) show similar forms. Search time could not be used to estimate other aspects of krill distribution pattern because fishing operations are limited by processing efficiency rather than availability of krill. Analysis of the acoustic survey data suggested characteristic distribution pattern scales of 1.7 and 4 n.mile. Estimates of a composite index of krill abundance (SC-CCAMLR-VIII, 1989) were derived for two surveys from these data sets. In addition, the data suggest that the abundance of krill in the Elephant Island area can change rapidly, and when krill do come into to the area they are most often found in water 100m-500m depth along the shelf break north of Elephant Island, particularly in the area where it wraps around the western end of the island.

Target strengths (TS) of various zooplankton were measured at 200 kHz, 420 kHz and 1 MHz and the dependence of these data on animal volume versus cross-sectional area was explored The 420 kHz and I MHz data were collected with a dual-beam sonar system and the 200 kHz data with a split-beam system. Experiments were conducted with live, tethered individuals in an enclosure filled with filtered seawater. The data were compared to both empirical and theoretical models of reduced target strength (TS normalized by the square of the animal length) versus ka (the product of wave number and equivalent cylindrical radius). The theoretical models chosen for this comparison were two versions of a high-pass bent-cylinder model (Stanton, 1989b) that indicate TS is dependent on animal volume, and the ray bent-cylinder model (Stanton, 1993a) which implies TS is dependent on the cross-sectional area. The dependence of acoustic backscattering on animal volume or area was tested by fitting regression lines for TS versus the logs of ka, length (L), wet weight (WW) and dry weight (DW). Contrary to an empirical model derived from similar experiments (Wiebe et al., 1990), and to the high-pass models, the regressions indicated that TS is proportional to the cross-sectional area of the animal. However, neither Wiebe et al. (1990) nor this experiment directly accounted for animal orientations. Simulations using a Distorted Wave Born Approximation Model (Chu et al., 1993), indicated that animal behavior is an important factor in the scattering characteristics of zooplankton. In addition, because scattering from individual zooplankton is highly non-linear, especially in the geometric scattering region (ka>l), linear regressions of TS versus the log of ka, L, WW or DW are inappropriate and misleading.

In-situ measurements of target strength (TS) were made of Antarctic zooplankton (Euphausia superba and Salpa thompsoni) at 120 kHz and 200 kHz. Concurrently, a two meter Isaacs-Kidd midwater trawl was used to sample the zooplankton populations and animal length-frequency data were recorded. The TS and length-frequency data were combined to corroborate theoretical scattering models for both species. The individual TS measurements were collected at 120 kHz with a split-beam echosounder and a single-target detection algorithm. Because the two transducers were essentially collocated, range-bin and off-axis angles from the 120 kHz detections were used to extract the corresponding TS from the 200 kHz single-beam data. The backscattering cross-sectional areas (σbs) of salps are shown to fit a fluid sphere model [V.C. Anderson, J.Acoust. Soc. Am. 22:426-431 (1950)]; presumably, predominant scattering is from each animal's spheroidal nucleus. Consistent with the measurements of encaged krill [K.G. Foote et al., J. Acoust. Soc. Amer. 87(1):16-24 (1990) and D. Chu et al., J. Acoust. Soc. Am. 93(5):2985-2988 (1993)], the TS of in-situ krill are shown to fit a deformed cylinder model [T.K. Stanton, et al., J. Acoust. Soc. Am. 94(6):3454-3462 (1993a) and 94(6):3463-3472 (1993b)]. Utilizing these scattering models and empirically derived distributions of animal sizes, a technique is developed for acoustically delineating the two species. The method uses nonparametric Kolmogorov-Smirnov tests to evaluate cumulative distributions (CDF's) of Δσbs derived from the differences in abs measurements at two frequencies. About 15% of the tests indicated the presence of salps without krill and about 3% revealed krill without salps. The salp/no krill echograms were characterized by diffuse scattering layers which were much higher in volume backscattering strength (Sv) at 200 kHz than at 120 kHz. Conversely, the krill/no salp echograms included dense swarms with virtually equivalent Sv at the two frequencies. The ability for this method to delineate salps from krill is highly dependent on the degree to which the two CDF's differ. A simulated combination of 200 kHz and 38 kHz resulted in highly distinguishable CDF's, indicating that these frequencies may be more useful for distinguishing salps from krill. Data were collected as part of the United States Antarctic Marine Living Resources Program, near Elephant Island, Antarctica, during the Austral Summer of 1994.

Poland began krill fishing operations in the 1976/1977 season. It has been engaged in krill commercial fishery since the 1986/1987 season. Maximum catches, equalling 15.9 thous tonnes. were, attained in the 1992/1993 season. The report presents an analysis of Polish krill catches in the 1991/1992 and 1992/1993 seasons. CPUEs by month in individual subareas of the Atlantic sector of the Antarctic were calculated. The results of biological investigations of krill on the fishing grounds of the South Orkney and South Georgia subareas are also given.

In 1994 the group of above mentioned authors has completed the first stage of researches for creation of an imitation model of krill distribution and search in the Cooperation and Cosmonauts Seas (30–80°E).
At this the following main problems were addressed:
- Systematization of krill echograms (types of aggregations, their main characteristics),
- Assessment of distribution of aggregations (length, volume of concentrations, occurrence frequency, stability on time-scale,
- Assessment of results of fishing activity of Soviet FVs in the synoptic, seasonal and annual aspects,
- Working out of algorithms for automatic processing of materials by means of computing devices and mathematic analysis of distribution of krill concentrations and catch size.