CCAMLR Science, Volume 1 (1994)
Acoustic estimates of krill biomass in the vicinity of Elephant Island are presented for the years 1981 to 1993 (with the exception of 1982 and 1986). Estimates for 1981 to 1991 are based on previous reports adjusted for the recently proposed definition of krill target strength (Greene et al., 1991). Biomass estimates range from 267 x 103 tonnes (March/April 1985) to 4 880 x 103 tonnes (January 1993) and biomass densities range from 2.5 g/m2 to 134.5 g/m2. Average biomass and average density over the 13-year period were 1 703 x 103 tonnes and 53.2 g/2. |
Krill stock composition and distribution patterns in the vicinity of Elephant Island during the austral summers of 1991/92 and 1992/93 are described and compared with information from previous years. The general distribution of krill length and maturity classes conformed to previous descriptions and appears to be a recurring and predictable aspect of the krill stocks in the Antarctic Peninsula region. The length frequency distributions and maturity stage composition reflected relatively good year class success from the 1990/91 spawning season but poor success from 1991/92. Year class success from these and other years appears to be associated with female maturity development and spawning during the early summer months. The overall abundance, maturity stage composition and reproductive activity of hill appeared to be affected by dense salp concentrations during 1992/93 and 1989/90. |
Trathan, P.N. and I. Everson (1994), Status of the FIBEX acoustic data from the West Atlantic Data from the FIBEX acoustic survey in the West Atlantic sector have been re-examined to check the consistency of krill abundance estimates derived from different survey vessels. There is a good level of consistency between the results from four of the vessels, Itzumi, Dr Eduardo L. Holmberg, Odissey and Walther Herwig. While there is an error factor due to the combination of data collected at 50 kHz (Walther Herwig survey) with data collected at 120 kHz (all other vessels), it is concluded that this does not materially affect the estimated biomass. The data from the Professor Siedlecki survey do not provide estimates that are consistent with the other surveys. The authors can find no explanation for this difference. |
de la Mare, W.K. (1994), Modelling krill recruitment A method is developed for using observed values of the mean proportion of recruits and its variance to model recruitment in a krill population in terms of numbers of recruits. The method includes the calculation of natural mortality and other parameters consistent with the observed proportional recruitment parameters. A procedure is given for generating families of recruitment functions which are consistent with the statistical uncertainty in the observed recruitment parameters. |
de la Mare, W.K (1994), Estimating krill recruitment and its variability A maximum likelihood method is developed for the decomposition of krill density-at-length data into the proportion of recruits in a population sampled by a net haul survey. Preliminary results from a series of five net haul surveys in the South Atlantic and the Indian Ocean sectors of the Southern Ocean give a mean recruitment rate for 1+ krill (the ratio of the number of krill aged one year to the number of krill aged one year and above) of 0.339 with a standard deviation (SD) of 0.100. The corresponding result for recruitment for 2+ krill (the ratio of the number of two-year-old krill to the number aged two and above) from nine surveys is 0.552 with a standard deviation of 0.074. A number of the assumptions needed for reliable results is discussed. |
Agnew, D.J. and V.H. Marín (1994), Preliminary model of krill fishery behaviour in Subarea 48.1 A simple model of the behaviour of the krill fishery in FAO Statistical Subarea 48.1 (South Shetland Islands and Antarctic Peninsula) is described. Parameters of the model are calculated from Chilean fishery data over the period 1989 to 1992. The distribution of catches predicted by the model, which is restricted to the four months December to March, compares favourably with the historical distribution of catches in the subarea. A number of management scenarios are considered which involve the closure of (i) a radial zone 50 km offshore from the South Shetland Islands, and (ii) zones 100 km around Livingston and Elephant Islands. The model predicts that the management option of closing the zones around Livingston and Elephant Islands in alternate years would result in an average yearly catch similar to that at present. However, in this scenario, the catch would be more concentrated in foraging areas of land-based predators during alternate years. |
Butterworth et al. (1992) put forward an extension of the harvesting model of Beddington and Cooke (1983) to relate potential krill yield to a pre-exploitation survey estimate of krill biomass. In this paper, the approach is extended further so as to incorporate most of the amendments specified by the Third and Fourth Meetings of the Working Group on Krill (WG-Krill). The most important of these extensions is integration over the ranges of uncertainty for a number of 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 possible 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). |
An assessment of the competition between the Japanese krill (Euphausia superba) fishery and penguins during the penguins' breeding season (December to March) in the South Shetland Islands (Subarea 48.1) was made based on available information on the fishery, penguins and krill. The krill catch is very low in December while the catch rate for the following three months is roughly constant (3 000 to 10 000 tonnes/10-day period) due to an increase in fishing intensity in areas closer to the northern shelf of either Livingston or Elephant Island. In contrast, food consumption by penguins is estimated to be high to the north of King George, Nelson and Robert Islands (9 746 tonnes/10-day period), and around Low (6 102 tonnes/10-day period) and Clarence (5 663 tonnes/10-day period) Islands, but small to the north of Livingston (1 921 tonnes/10-day period) and Elephant (1 991 tonnes/10-day period) Islands. Thus, the overlap between the main fishing and foraging areas is insignificant and implies a low level of competition between the fishery and penguins. Krill biomass was estimated to be as large as 200 to 1 500 x 103 tonnes within the preferred fishing areas during the breeding season. Compared with the level of biomass (≥200 x 103 tonnes) and its variability (the order of 100 x 103 tonnes/10-20-day period), the present catch rate (≤10 x 103 tonnes/1O-day period) is smaller by one or more orders of magnitude within the localised areas. Thus, the present fishery is very unlikely to have an adverse impact on the local krill biomass, and hence on penguins, when catch levels are also taken into account. |
This paper discusses an investigation of juvenile Antarctic fish caught during krill fishing by the FV Grigory Kovtun in the area around South Georgia from May to July 1992. Two species were predominant in the by-catches: Champsocephalus gunnari and Nototheniops larseni. The frequency of occurrence of juvenile fish was 18.2% when all krill tows were included and 45.5% when only tows made in shelf waters were considered. The abundance of juvenile fish in catches, normalised to one tonne of krill, ranged from 700 to 18 900 individuals. In the case of C. gunnari, average values were 966 ± 225 ind./tonne krill and 2 434 ± 579 ind./tonne krill for all trawls and for shelf trawls, respectively. For N. larseni the corresponding averages were 557 ± 103 and 1 388 ± 248. The mean standard length of C. gunnari was 68.5 to 79.7 mm in May and June and 93 mm in late July. The mean growth rate of this species over this period is estimated to be 0.37 mm per day. In May to June, N. larseni juveniles were represented by fingerlings (mean length of 42.4 to 47.4 mm) and yearlings (72.5 to 73.7 mm). In late July the mean length of fingerlings increased to 48.5 mm. The mean daily length increase in N. larseni is estimated to be 0.08 mm. The diet, by mass, of both C. gunnari fingerlings and N. larseni yearlings was dominated by juveniles of Euphausia superba. The food bolus in N. larseni fingerlings consisted mainly of Chaetognatha, Copepoda and furcilia of Thysanoessa spp. According to the data obtained from the Ukrainian krill fishing fleet in the South Georgia area from May to August 1992, the total removals of C. gunnari and N. larseni resulting from by-catches are estimated to be 34.3 ± 8.0 and 19.8 ± 3.7 million individuals, respectively. |
During seven days of operations involving the setting and hauling of longlines to catch the toothfish Dissostichus eleginoides around the South Sandwich Islands, actual and potential interactions with seabirds were assessed. Bird numbers increased rapidly after dawn and large numbers of Cape, giant and storm petrels and smaller numbers of white-chinned petrels and black-browed albatrosses were present during day hauling operations. No incidental mortality was seen and only one bird was caught on a hook; nevertheless many species in these aggregations of birds are clearly potentially vulnerable to setting operations in daylight hours. Several species of the seabirds present presumably originated from South Georgia populations; however wandering and grey-headed albatrosses, whose populations are in serious decline at South Georgia, were rare; their vulnerability to longlining operations in the South Sandwich Islands may, therefore, be low. Quantitative data and observations of longline vessels fishing around South Georgia, however, indicate significant catch rates of albatrosses. |
Clarke, J. and K. Kerry (1994), The effects of monitoring procedures on Adélie penguins The effects of flipper bands, implanted tags, gastric lavage and external instrument attachment on the performance of Adélie penguins were investigated over three seasons at Béchervaise Island, Mac. Robertson Land, Antarctica. The return rates of birds carrying bands and/or implanted electronic transponders were compared to investigate the contribution of bands to bird mortality and to determine rates of band and tag loss. There was a slight reduction in return rates of birds banded for more than one season, but no evidence of band or tag loss over a single winter. The attachment of satellite tracking devices during the incubation period resulted in increased foraging trip durations and reduced breeding success. Attachment for single or multiple foraging trips post-hatching caused no significant increase in foraging trip durations. No reduction in fledging rates of chicks from nests of stomach-lavaged birds was detected over two breeding seasons. The implications of these findings for the CCAMLR Ecosystem Monitoring Program are discussed. |
This paper provides information on Antarctic crabs obtained during the 1986 Spanish survey ANTARTIDA 8611, which covered the shelf around all the archipelagos in the Scotia Sea. A total of 345 hauls was conducted, of which 29 took place around Shag Rocks, 104 at South Georgia, eight around the South Sandwich Islands, 93 around the South Orkneys, 46 at Elephant Island and 65 around the South Shetlands. Depths surveyed ranged from 63 m down to 643 m. In the whole of the area surveyed, the two crab species of the genus Paralomis (P. spinosissima and P. formosa) were found only around Shag Rocks and South Georgia. P. spinosissima was caught in seven hauls of the 29 made around Shag Rocks and in 20 of the 104 hauls made at South Georgia. P. formosa was caught twice, once in each zone. The bathymetric distribution of P. spinosissima ranged from 160 to 627 m and that of P. formosa, far less frequent in the catches, from 320 to 350 m. Field observations showed that these two species were absent from the southern archipelagos of the Scotia Arc (South Orkney, Elephant Island and South Shetland), at least in the depth range surveyed. The location of the hauls where Paralomis spp. were found, together with the qualitative composition of the fish fauna usually accompanying them in the catches, suggest that P. spinosissima and P. formosa tend to concentrate in areas close to the shelf break, where environmental conditions show some degree of stability. |
Relatively high levels of infection by a rhizocephalan barnacle have been found in the population of the crab Paralomis spinosissima, which has been harvested by a new fishery around South Georgia (Subarea 48.3) since 1992. One of the main effects of the parasite is to render infected animals sterile. Parasitism can therefore reduce the effective spawning stock biomass, and this clearly has implications for stock management. The possible implications of parasitism for management are investigated by considering a population model that captures the main characteristics of the system. An age-based host-parasite model is constructed, incorporating two scenarios for the stock-recruit relationship of the host: (i) resilience; and (ii) sensitivity to declines in spawning stock. The effects of harvesting both healthy and infected animals are contrasted with the effects of harvesting healthy animals only. Equilibrium and transition dynamics of the models are explored for ranges of parameter values. Results indicate a need to harvest or remove infected animals from the population. The need is stronger when the population is sensitive to declines in spawning stock. Results highlight the importance of incorporating parasitism in the design of a management strategy for this crab population. The main data requirements for improving on this preliminary study are identified. |
A new fishery for the lithodid crab, Paralomis spinosissima, started in Subarea 48.3 in 1992. The fishery is also new to CCAMLR, both in terms of the species and fishing gear. We provide an overview of the management options for crab fisheries in general and then consider these options in the context of this crab fishery with specific reference to the principles of the CCAMLR Convention. We outline the assessment methodologies and data requirements for the implementation of the various management options. All the features of the fishery and the objectives of the Convention point to the need for a conservative approach to management. Such an approach includes the use of catch/effort controls in addition to the 'size-sex-season' controls. Implementation of both sets of controls relies on information on the growth and life-history of P. spinosissima. Implementation of the catch/effort controls also requires estimates of stock abundance, which could be obtained by depletion methods. Consideration should also be given to the creation of refuges, though careful attention must be given to their design. One of the main aims in designing a management plan is to allow the fishery to develop at a rate compatible with the expansion of the information base available for assessment. |
A method is developed for calculating asymptotic confidence intervals for estimates of abundance obtained from trawl surveys conducted by means of the swept area method, using likelihood ratios from Aitchison's delta distribution. Simulation tests of the method show that unbiased estimates of density and biomass can be obtained and that the estimated confidence intervals have close to the nominal coverage probability. Performance deteriorates in cases where few of the hauls contain fish and the coefficient of variation (CV) is high. The upper confidence bound appears to be slightly less reliable than the lower. |