CCAMLR Science, Volume 4 (1997)
Analyses of the distribution of krill fishing based on fine-scale (0.5o latitude x 1o longitude) data show that harvesting in the Scotia Sea has been mainly in shelf and shelf-break areas on the northern side of the Scotia Arc. Regional analyses reveal that there are restricted parts of shelf and shelf-break areas where high catches have been obtained. Although these data give a general idea of where the fishery has operated, they do not reveal the localised nature of the fishing operation. Over the last three winter fishing seasons (1993 to 1995) individual trawl statistics have been recorded in the South Georgia area. Analyses of these data show marked interannual variability in the distribution of trawls. The data for 1993 were only available from August but the fishery was restricted in that period to an area on the western edge of the shelf break where negligible fishing occurred in the following two seasons. In 1994 the fishery was almost totally based over a large shallow bank area on the northeast shelf edge. During 1995 the fishery was still predominantly in this area but also operated further west on a range of banks associated with submarine valleys. The results are discussed in relation to the ecology of krill and the fishery's interaction with local predator colonies. |
Krill distribution and density are reviewed for the Elephant Island area with regard to the representativeness of the study area (60°-62o30´S and 53o-57o30´W) for proportional recruit and density indices. Proportional recruitment indices were re-calculated applying the delta distribution approach introduced by de la Mare (1994a). The high interannual variability of krill recruitment is confirmed by the present analysis. Results are compared for one- and two-year-old krill (R1 and R2 respectively). Statistically significant fluctuations in krill density over the period 1977 to 1994 are also confirmed by this study using randomisation tests on an analysis of variance. |
Four indices of prey availability are calculated from surveys conducted in the vicinity of the Seal Island CEMP site during the austral summers of 1990 to 1996. The indices are measures of average prey density, depth, distance from Seal Island, and persistence over time. Acoustic data from two AMLR surveys each year were subsampled to include the foraging range of predators breeding at Seal Island. Indices derived from these data indicate that although average prey density varied by a factor of almost five between 1990 and 1996, prey were consistently distributed along the edge of the shelf break north of Elephant Island, and between 25 and l00 m depth. Indices of average prey density agree with demographic indices of krill abundance derived from net samples. Indices of persistence over time reflect the relative biomass of juvenile krill near Seal Island during the first survey of each year as compared to the biomass of post-spawning adults near Seal Island during the second survey of each year. It is hoped that these indices may be useful in modelling krill predator breeding success as a function of prey availability. |
Proportional recruitment indices and CPUE in Subarea 48.1 (Antarctic Peninsula region) from 1980 to 1996 were calculated based on logbook data from Japanese commercial krill trawlers. Proportional recruitment rates calculated from fisheries data were similar to those from scientific data (Siegel et al., 1997) and showed a close relationship to sea-ice indices. CPUE (catch/towing volume) in the Livingston Island area showed a decreasing trend during the study period. The use of different fishing strategies in response to a demand for higher product quality and a general decrease in krill density in the study area were considered as possible reasons for this. On the other hand, CPUE in the Elephant Island area showed greater interannual variation without any trend. |
Analysis of seasonal sea-ice cover of an area measuring 1.25 x 106 km2 off the northwestern side of the Antarctic Peninsula indicates that there were four periods of extensive ice cover during the last 18 years. Variability in seasonal timing, areal extent, seasonal duration and persistence of sea-ice over multiple years is apparent and is consistent with features described by Stammerjohn and Smith (1996) for ice cover in the Bellingshausen Sea. Periods of extensive winter ice cover peaked in 1980, 1986, 1991and 1995 and were foreshadowed by the late retreat of ice during the springs of 1985, 1990 and 1994. Ice cover during the summer months and winter ice cover during years between periods of extensive ice cover decreased during the second half of the time series. Maximum ice cover during the periods of extensive ice cover did not change. Annual curves of sea-ice cover were integrated over time to produce an annual index of sea-ice cover in units of 106 km2/month. This index may be used in conjunction with studies of variability in biological production, particularly the reproductive success of Antarctic krill, in the Antarctic Peninsula area. |
This paper presents a summary of the foraging locations of Adélie penguins, determined by satellite tracking, at seven widely-spaced breeding colonies along the coast of eastern Antarctica between 55oE (Enderby Land) and 175oE (Ross Sea). Adélie penguins feeding chicks regularly travelled up to 120 km offshore to the continental shelf and returned with krill, Euphausia superba. Locations of penguins on foraging trips were compared with historical records of fine-scale krill catch data for the regions of interest. The potential for overlap with krill fishing occurs particularly where ice conditions permit the fishing fleet to approach the edge of the continental shelf or where penguins forage downstream from the fishery. It is suggested that the potential for overlap between foraging areas of Adélie penguins and fishable concentrations of krill be investigated before a decision is made to establish a new CEMP site. |
At sub-Antarctic Marion Island, annual breeding success of macaroni (Eudyptes chrysolophus) between 1979/80 and 1995/96 and rockhopper penguins (E. chrysocome) between 1985/86 and 1995/96 was measured. For macaroni penguins, averages of 0.48 eggs and 0.35 chicks were hatched and fledged respectively for each clutch laid. Corresponding averages for rockhopper penguins were 0.68 eggs and 0.48 respectively. The largest of the three macaroni penguin colonies investigated decreased in size over the study period, while the other two remained stable. The only significant relationship between inter-season trends in the number of pairs breeding at the three colonies was a negative correlation between two adjacent colonies, which suggests inter-colony transfer. The larger two of the three rockhopper penguin colonies investigated decreased over the study period, while the other remained stable. Trends in the number of pairs breeding at the three rockhopper penguin colonies were all significantly correlated. The proportion of rockhopper penguins attempting to breed may have varied as a result of some environmental signal. Inter-season trends in breeding success of macaroni penguins were significantly correlated in all three inter-colony comparisons over the entire study period. Inter-season trends in breeding success and chick survival of rockhopper penguins were significantly correlated in only one of the three inter-colony comparisons. The greater coherence in the performance of macaroni penguin compared to rockhopper penguin colonies suggests that breeding success of macaroni penguins may be influenced by a wider-scale phenomenon than is applicable to rockhopper penguins. Trends in breeding success, hatching success and chick survival of macaroni penguins and rockhopper penguins were not significantly correlated to each other, even for nearby colonies. This suggests that factors influencing the reproductive performance of the two species are not the same. |
The diet of Patagonian toothfish (Dissostichus eleginoides) was investigated in two offshore regions in the southwestern Atlantic: on the Argentinian continental shelf/slope between 46°-55°S and 60°-64°W, and around South Georgia between 53°-55°30' and 3°-45°W. The South Georgia area was divided into two zones: the shelf zone and deep-water zone. On the Argentinian continental shelf/slope, the length of D. eleginoides ranged from 30 to 95 cm total length (TL) which corresponds to the juvenile stage. Around South Georgia, the size distribution included both juvenile and adult stages, and ranged between 18 and 212 cm TL. The diets of fish of various sizes and both sexes were compared, and the feeding status of this species is discussed. D. eleginoides is a mixed-species carnivore, feeding principally on fish and secondarily on crustaceans and cephalopods. The principal fish species in the diet of D. eleginoides on the Argentinian continental shelf were nototheniids (e.g. Patagonotothen ramsayi). Around South Georgia, adult D. eleginoides preyed mainly on fish (Zoarcidae, Moridae, Bathydraconidae, Channichthyidae) and on Decapoda, while juveniles in coastal waters consumed krill and, in deeper waters, various species of fish (Nototheniidae, Myctopliidae, Channichthyidae). It was also found that the diet of D. eleginoides varied in relation to its size and water depth in both the shallow and deep-water zones. |
In anticipation of the development of a new fishery for the ommastrephid squid Martialia hyadesi in the Scotia Sea, this paper presents a revision of annual consumption of the species by higher predators and provides a brief review of information about the life cycle and distribution of the species obtained from research fishing and commercial catches. This species is eaten by seals, whales and seabirds, the latter being the most reliable source of consumption data because comprehensive sampling can be carried out during their breeding seasons. A conservative estimate for total annual consumption of M. hyadesi by higher predators in the Scotia Sea is 245 000 tonnes, with an upper estimate of 550 000 tonnes if less reliable data are included. M. hyadesi spawns between autumn and mid-summer with peak hatching in winter/spring. Its life span has not been established. Data from the CCAMLR Convention Area suggest that M. hyadesi may live for two years, but this may vary. In common with other ommastrephids, the species is probably semelparous. It is proposed that the timing and catches of the fishery should be highly conservative and set taking into account the timing of breeding and consumption rates of the most sensitive of the dependent species. Most Antarctic predators which have been studied consume relatively small and immature specimens of M. hyadesi. Fishing for M. hyadesi after the chick-rearing period of the most sensitive predator (grey-headed albatross) would minimise competition locally and ensure that the fishery only exploited the stock after escapement from most higher predator species. It would also allow seabird predation of the stock to be monitored prior to the fishing season as a way of assessing numbers of pre-recruits. Closing the fishery before recruitment of the next generation of squid would ensure availability of prey to higher predators during the following chick-rearing period. Preliminary data from a squid jigger which undertook research fishing around South Georgia in June 1996 provided the basis for determining realistic potential catch rates. |
Data collected on board the FV American Champion during Phases 1 and 2 of the experimental crab fishery were analysed using generalised additive models and depletion estimators. Results from the generalised additive models show that the density of fishable Paralomis spinosissima is highest off the northern coast of South Georgia and at depths of about 100 to 300 fathoms. The Phase 1 results suggest that it would not be appropriate to extrapolate local estimates of abundance to the whole of Subarea 48.3 solely on the basis of depth-specific seabed area; extrapolations must consider location. Linear models fitted to catch per unit effort (CPUE) and cumulative catch data from the Phase 2 depletion experiments did not have significant negative slopes. The insignificant regressions were probably a result of small catches, inter-haul variability in CPUE, and crab movement and suggested that depletion estimators will not be appropriate tools for estimating local abundances of P. spinosissima. It was not possible to determine whether depletion estimators can be used to estimate crab abundance when larger areas are considered and larger catches are taken. Approximately 6 000 crabs were tagged and released during Phase 2. Four of the tagged crabs were recovered, and these crabs were at liberty for about one to five weeks. The recaptured crabs had minimum movement rates of 0.08 to 0.25 n miles/day. Mark-recapture estimates of standing stock and density were made for the area around Phase 2's third depletion square. The assumptions of the mark-recapture model were probably violated, but attempts were made to account for biased sampling of recaptured crabs and the movement of crabs away from their release sites. Density estimates from the mark-recapture data were in the order of 50 000 to 100 000 legal-sized male crabs/n mile2, and there was a lot of uncertainty associated with these estimates. In general, the experimental harvest regime was successful. The regime provided important information about crab distribution and facilitated evaluation of local depletion estimators for use in stock assessment, but the CCAMLR Working Group on Fish Stock Assessment (WG-FSA) may wish to re-evaluate the design of Phase 2 and consider the implementation of a wide-scale, intensive tagging study. |
Kasatkina, S.M (1997), Selectivity of commercial and research trawls in relation to krill This paper describes the results of fishing a single krill aggregation with a commercial trawl RT 72/308 and an Isaacs-Kidd midwater research trawl (IKMT). The commercial trawl catches contained more large krill (35-58 mm) than catches of the research trawl (30-54 mm). The maximum difference in mean length in catches made by both trawls was ∆L = 6.2 mm. Statistically significant variability in krill length composition was observed between catches made by the research trawl, while in commercial trawl catches krill length composition hardly varied at all. Statistical analysis of catches showed that when it comes to obtaining representative trawl samples, estimates of length composition in krill catches made using the commercial RT 72/308 trawl are more accurate than those obtained using the IKMT research trawl. The two types of trawl have different catchability characteristics: the research trawl is better at retaining small krill, while the opposite is true for the commercial trawl. Consequently, estimates of krill length based on data obtained using either commercial or research trawls will be biased and contain systematic errors, mainly due to the selectivity properties of the trawls. The various selectivity properties of the trawl used in acoustic surveys have different effects on the accuracy of krill abundance and biomass estimates. Estimates of krill mass density (g/m2) and biomass are less dependent upon the selectivity properties of trawls used in acoustic surveys than are estimates of krill numerical density (numbers/m2) and abundance. |
The only interaction observed between longliners fishing for Patagonian toothfish (Dissostichus eleginoides) to the west of the Kerguelen Islands during the 1995/96 cruise and marine mammals occurred with the Antarctic fur seal (Arctocephalus gazella). These seals are responsible for the loss of 0.75 % of toothfish during the longline sets in which interaction occurred, as against 2.2 % of fish lost from the hooks during line hauling. |
By-catches of fish taken during a commercial fishing cruise and an experimental deep-sea longline fishing cruise targeting Patagonian toothfish (Dissostichus eleginoides) off the Kerguelen Islands (Division 58.5.1) during the 1995/96 season were analysed. Eleven species were identified, of which the grenadier Macrourus carinatus, the skates Bathyraja eatonii and B. irrasa and the morid Antimora rostrata were dominant. The others were mainly shelf-dwelling species found at the limit of their bathymetric range. An abundance index was used to show bathymetric and geographical distributions of the most common by-catch species. The size of some by-catch species shows that they could be of interest commercially if longline fishing operations expand in this part of the Southern Ocean in the future. |
This paper describes the results of an experiment conducted from 19 February to 7 March 1996 to assess krill flux factors in Subarea 48.2. The study area (80 x 240 n miles) was located in the zone of interaction between waters of the Antarctic Circumpolar Current (ACC) and the Weddell Sea. The study area covered sites where high krill concentrations had often been observed in the past. The geostrophic transport of krill across boundaries of the study area was calculated as a product of two variables integrated over the depth range 0 to 200 m: krill density (g/m3) and water mass transport (m3/sec) per nautical mile of the study area boundary. Also given are the results of krill density assessment, evaluations of geostrophic and wind-induced water transport along the study area perimeter. Mean krill transport rate was 7.2 tonnes/hour/n mile with a standard deviation of 15.5 tonnes/hour/n mile. Significant variability of krill transport across the study area boundary was observed, both in terms of the amount of biomass transported and the direction of flux. Due to the location of the study area, it was only possible to assess krill flux caused by waters of the southern periphery of ACC. Based on estimations of krill density and geostrophic |