CCAMLR

Acoustic estimates of krill biomass in the vicinity of Elephant Island for the years 1981-1993 (with the exception of 1982 and 1986) are presented. Estimates for 1981-1991 are based on previous reports adjusted for the recently proposed definition of krill target strength. Biomass estimates range from 81 xl03 t (March/April 1985) to 4,880 xl03 t (January 1993) and areal biomass densities range from 2.5 g/m2 to 134.5 g/m2 Average biomass and average areal density over the 13-year period was 1,692 xl03 t and 52.8 g/m2.

The diel vertical migration of Antarctic krill (Euphausia superba) can greatly bias the results of qualitative and quantitative hydroacoustic surveys which are conducted with a down-looking sonar and independent of the time of day. To demonstrate and quantify these negative biases on both the estimates of biomass distribution and abundance, a time-depth-density analysis was performed. Data for this study was collected in the vicinity of Elephant Island, Antarctica during the Austral Summer of 1992. The data includes five surveys conducted from mid-January to mid March. The first and fourth surveys covered a 105 by 105 n.mile study area centered on Elephant Island; the second and third surveys covered a 60 by 35 n.mile area immediately north of Elephant Island; the fifth survey covered a 1 n.mi.2 area centered on a large krill swarm to the west of Seal Island. Average krill volume densities were calculated for each hour as well as for three daily time periods; day, twilight and night; these data were normalized and presented as a probability of daily average density. A function was fit to the probability of average daily biomass versus local apparent time. This function was used to create a temporal compensation function(TCF), for upwardly adjusting acoustic biomass estimates due to diel vertical migration. The TCF was then applied to the original survey data; the resulting biomass estimates are 2.3 to 99.6% higher than those calculated disregarding biases due to diel vertical migration.

The diving behavior of 7 chinstrap penguins (Pygoscelis antarctica) (N=12,171 dives) was measured concurrently with a hydroacoustic assessment of the vertical distribution and abundance of their primary prey, krill (Euphausia superba) in the vicinity of Seal Island, South Shetland Islands, Antarctica between January 19 and March 10 1992. Krill was found to show a distinct diel migration pattern, being dispersed in the upper portion of the water column at night and more concentrated and deeper during the day. Penguin foraging effort was found to be concentrated around noon and midnight, with a reduction in effort around dawn and dusk (local apparent time). The mean and maximum depth of chinstrap penguin dives was found to follow the diel migration pattern of the krill. On average, chinstrap penguins dove to the shallow limit of the distribution of krill. The maximum depth of penguin dives did not exceed the maximum depth distribution of krill. These patterns in diving behavior may result from diel changes in the methods used by penguins to locate and capture prey. Our results suggest that penguins do not require extremely dense aggregations of prey in order to successfully capture sufficient krill to meet their energetic needs. We hypothesize that the diel migration pattern found in krill which has been found to be variable in different study locations at different times may in part be determined by the intensity of predation pressure by predators which feed in the upper portion of the water column (i.e. seabirds and marine mammals).

The materials on Euphausia superba biological state and size composition in the Indian Antarctic seas for the period from 1985 to 1990 are analyzed to form an opinion about its growth, life duration, as well as about the interannual variability in the above parameters.
According to our data, duration of E.superba life cycle somewhat exceeds 5 years in the Sea of Cosmonauts and 6 years in the Sea of Sodruzhestvo. E.superba growth rate ranges from 0.126- 0.133 mm per day in the first year 0f life to 0.028-0.041 mm per day in the fifth year under condition that E.superba grows 180 days per year.
Bertalanffy growth curves calculated for different areas are close to that obtained by Australian researchers (Hosie et al., 1988) for the Prydz Bay by data for 1981-1985.
Basing on long-term observations, relations are noted between E.superba age composition and its spawning efficiency in coastal areas of the Seas of Sodruzhestvo and Cosmonauts. The presence of relatively independent (self-reproducing) groupings of E.superba is assumed in these areas.

Data on the size and age composition of Euphausia superba were collected in the Cooperation and Cosmonaut Seas from 1985 to 1990.
The coefficients of instantaneous natural mortality of E.superba in the Indian sector of the Southern Ocean were calculated according to Alverson-Carney’s (1975), Richter-Efanov’s (1977) and Beverton-Holt’s (1958) methods. Values varyed from 0.72 to 0.87, from 0.41 to 0.59 and from 0.94 to 2.30, respectively.
The estimation of the annual extinction rate of E. superba was obtained using the method of Zikov,Slepokurov (1982) and results were best fitted by a parabolic equation. The coefficients of natural mortality of E. superba derived with this method range from 0.49, during the maturation period, to 1.17 and 3.22 during the first and last years of life, respectively.

An initial attempt is made to develop the model1ing framework suggested by the Joint Meeting of the CCAMLR's WG-Kril1 and WG-CEMP in 1992 to address this issue. First. estimates are made of the parameters of predator survival rates as functions of krill abundance, by considering a krill dynamics model incorporating recruitment fluctuations together with information on adult survival and breeding success patterns for certain krill predator species. A "one-way" interaction model is developed, in which krill abundance fluctuations impact the predator population, but not vice versa. Computations based on this model indicate that variability in the annual recruitment of krill results in predator populations being less resilient to krill harvesting than deterministic evaluations would suggest. However, the analyses also raise a number of questions about the proper interpretation of the available predator population dynamics information in the context of the models developed, and about the model1ing of the predator survival rates as functions of krill abundance. It is suggested that these questions merit discussion at the forthcoming WG-Kril1 and WG-CEMP meetings. A formalism for a "two-way" interaction model (including also the effect of differing predator consumption levels on krill) is developed, but computations based on this approach are deferred pending clarification of the Questions raised above at the WG-Krill and WG-CEMP meetings.

The results of Butterworth et al. (1992) relating potential krill yield to a pre-exploitation survey estimate of kril1 biomass are extended to incorporate most of the amendments specified by the Third and Fourth Meetings of Working Group on Krill. The most important of these extensions is integration over the ranges of uncertainty for a number of the model parameters. Results are provided for the probability of spawning biomass falling below various fractions of its median pre-exploitation level (Ksp), as a function of the fraction of the biomass estimate which is set as the catch for a 20-year period. Three alternative fishing seasons are considered. The model extensions requested by the Third Meeting make little difference to the results of Butterworth et al. (1992). Winter fishing is marginally preferable to a summer harvest. However, the imposition of an upper-bound of 1.5 yr-1 on the effective annual fishing mortality, as specified by the Fourth Meeting, results in marked reductions in the probabilities of krill spawning biomass falling below specified fractions of Ksp.

Data from several summer surveys to the Antarctic Peninsula region were analysed to calculate length and age at maturity for the Antarctic krill Euphausia superba. L50 values of 34.62 to 35.90 mm for female krill are the best estimates for the high spawning season. Males attain sexual maturity later at a length between L50 = 43.33 and 43.79 mm. Length at maturity, and length at first spawning are identical for krill. Comparisons with length at age data show that females mature in the third year of the life cycle (age class 2 +), while males reach maturity in the fourth year as age class 3 +. Both sexes show knife-edge maturity.