CCAMLR

Laboratory studies have shown that Antarctic krill Euphausia superba shrink if maintained in conditions of low food availability. Recent studies have also demonstrated that E. superba individuals may be shrinking in the field during winter. If krill shrink during the winter, conclusions reached by length-frequency analysis may be unreliable. In this study, the correlation between the body-length and the crystalline cone number of the compound eye was examined. Samples collected in the late summer show an apparent linear relationship between crystalline cone number and body-length. From a laboratory population, it appears that when krill shrink, the crystalline cone number remains relatively unchanged. If crystalline cone number is little affected by shrinking, then the crystalline cone number may be a more reliable indicator of age than body-length alone. The ratio of crystalline cone number to body-length offers a method for detecting the effect of shrinking in natural populations of krill. On the basis of the crystalline cone number count, it appears that E. superba shrink during winter.

The reproductive state and size composition of Euphausia superba collected in the Indian sector of the Southern Ocean from 1985 to 1990 were analyzed to estimate its growth, life span and mortality rates. The duration of the life cycle of E.superba exceeded 5 years in the Cosmonaut Sea and 6 years in the Cooperation Sea. Assuming growth for only 180 days per year, growth rates ranged from 0.120-0.133 mm.d-1, during the first year of life, to 0.019-0.022 mm.d-1 during the fifth year. Von Bertalanffy growth curves calculated for different areas are similar to those obtained by Australian researchers in the Prydz Bay region for 1981-1985. In mid summer, E.superba of age 2+ to 4+ were predominant in all hauls made south of the Antarctic Divergence, while north of the Divergence the krill stock was clearly dominated by individuals of age 4+. The coefficients of natural mortality (M) of E.superba in the Indian sector of the Southern Ocean, calculated by the methods of Alverson & Carney, Richter & Efanov and Beverton & Holt, varied from 0.72 to 0.87, from 0.52 to 0.57 and from 0.76 to 2.92, respectively. The value of age-dependent natural mortality of E.superba derived using Zikov & Slepokurov’s method ranged from 0.52, during the maturation period, to 1.1-2.41, during the first and last years of life. Based on long-term observations, the relationship between E.superba age composition and its spawning success is examined for the coastal areas of the Cooperation and Cosmonaut Seas.

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.