CCAMLR Science, Volume 6 (1999)
During February and March 1998, a 43-day cruise on FV Tierra del Fuego was conducted as part of the New Fisheries Projects developed by Chile, primarily to establish the presence of Dissostichus spp. in Subareas 48.1, 48.2 and 88.3. Industrial longlines (Spanish system) were used for the operation, with variable quantities of fish hooks (1 440 to 4 320), No. 9 Mustad Kirby type, set between depths of 600 and 2 550 m. Sardines (Sardinops sagax) and squid (Illex argentinus) were used as bait. Antarctic toothfish (Dissostichus mawsoni) were recorded around Peter I Island (Latitude 68°49'S), and from the Bellingshausen Sea (70°38'S) to Clarence and Elephant Islands (61°14'S), while Patagonian toothfish (Dissostichus eleginoides) were caught in waters from King George Island in the South Pacific Ocean (61°24'S) to the Scotia Sea in the South Atlantic (58°03'S). Yields of Dissostichus spp. were very low in the three subareas, with values of only 5.7 g/hook (Subarea 88.3), 19.1 g/hook (Subarea 48.1) and 3.0 g/hook (Subarea 48.2), and an average over all areas of 11.1 g/ hook. |
Trends in mean length and maturity stage of Patagonian toothfish (Dissostichus eleginoides) were examined using data from approximately 7000 fish sampled by observers from the 1998 fishery around South Georgia and Shag Rocks (Subarea 48.3). D. eleginoides were distributed down the shelf slope with an approximately linear relationship existing between depth and fish length. A statistically significant pattern of changing mean length at depth with month was detected, suggesting movement of animals both up and down slope at particular times of year. Months with a high mean length at depth are May and August, and with a low mean length at depth - April and July. Examination of maturity data suggests that in addition to a major spawning event in late July/August, there may be a small spawning event in April/May. Detailed examination of maturity-at-depth data suggests that mature males move down slope and females move up slope to meet at breeding areas defined by slope position (between 800 and 1200 m depth) and not by geographical position. Pre-spawning animals appear to be distributed all around the South Georgia and Shag Rocks shelf slopes. Spent animals of both sexes appear to move up slope to shallower water, and are concentrated to the northeast of Shag Rocks. Few spent fish were found around South Georgia. This concentration of large post-spawning fish may have some implications for the management of the fishery. |
Analyses of size at sexual maturity were undertaken on samples of Patagonian toothfish (Dissostichus eleginoides) caught in the commercial fishery within Subarea 48.3. The results for 1996 indicate that Lm50 occurs at 78.5 cm total length for male and 98.2 cm for female fish. In 1997 the results for male fish were essentially the same. For female fish there is evidence that in 1997 a significant proportion, 25 to 43%, of sexually mature fish were not coming into spawning condition. This introduces a bias leading to the overestimation Lm50 of calculated by conventional methods. Further analysis allowing for this bias indicates that Lm50 for female fish in 1997 was no different to that for 1996. The implications of this phenomenon are discussed. |
Acoustic estimates of densities of Antarctic krill, Euphausia superba, at South Georgia during 11 austral summers between 1981 and 1998 are presented. Krill density at the island fluctuated widely from year to year over that time, ranging from ≈2 to ≈150 g m-2(fresh mass). The 1982, 1991 and 1994 austral summer seasons were characterised by particularly low densities of krill (upper 95% confidence limits <15 g m-2). For five of the summers between 1990 and 1998 it was possible to calculate separate density estimates for northeastern and northwestern South Georgia, and in four of these seasons density was higher to the east. |
In this paper we consider the results of three mesoscale surveys covering a wide area of krill distribution between the Antarctic Peninsula and South Georgia Island. Surveys including from 158 to 202 stations were carried out by Soviet research vessels in the summer-autumn periods of 1983/84 and 1987/88 and in the spring-summer of 1984/85 using a grid of stations according to the standard method for Isaacs-Kidd midwater trawls operating in the layer 0 to 100 m. Overall krill biomass distribution (g/1 000 m3) and its mean values were analysed by subarea (Subareas 48.1, 48.2 and 48.3) and for the whole study area. Survey results confirm current understanding of the general pattern of krill distribution and drift in the western part of the Atlantic sector of the Antarctic (e.g. Priddle et al., 1988; Makarov, 1996). The greatest degree of spatial-temporal stability in the krill distribution field with high biomass of aggregations was observed in the western part of the area stretching from the Antarctic Peninsula to the eastern part of the South Orkneys area. The greatest variability of krill distribution on the spatial-temporal scale of the survey, which is characterised by a change in biomass of more than two orders of magnitude in different seasons, was observed in the South Georgia region and the adjacent waters of the Scotia Sea (Subarea 48.3). The possibility of krill transportation into the South Georgia region is considered. It is shown that the absence of krill in the island region during the 1983/84 season was caused exclusively by unfavourable oceanological conditions (lack of water flow from southern areas). It is also demonstrated that the absence of krill in the eastern part of the Scotia Sea may cause it to be absent from the South Georgia region regardless of oceanological conditions (spring-summer period 1984/85). The possibility of recruitment of the krill stock to the South Georgia region from the central part of the Scotia Sea is considered. Analysis of variability of mean biomass by area and season indicates that patterns of krill abundance and biomass recently observed in Subarea 48.1 (Siegel et al., 1997, 1998) are not representative of the survey area as a whole. The total biomass index in the study area during all seasons remained at a constant level (76.5-101.7 g/1 000 m3). |
A composite recruitment index based on density data derived from acoustic surveys and on length-frequency data obtained from either target or station net hauls is described. This composite index is compared with an index where density and length data are derived from standard station net hauls only. Comparison of these indices showed substantial differences, due in part to bias arising from a large number of krill in just one of the station net hauls. Therefore, when the number of station net hauls is limited, bias is likely to be reduced if target net haul data can be included by using the composite recruitment index. This index is used to describe the population structure of krill around South Georgia during seven cruises between 1989 and 1998. Before 1994 the number of 1+ krill found around South Georgia was very low (proportion of 1+ krill <7%), while between 1994 and 1998 the number of 1+ krill found in the population increased with a very successful recruitment year due to krill spawned in 1994/95. |
Marine debris found in association with seabirds at sub-Antarctic Marion Island for the period May 1996 to April 1998 is reported. Standardised searches and incidental finds show a large increase between the 1996/97 and the 1997/98 field seasons. Fishing gear increased at a rate 10 times higher than 'other' (non-fishing) marine debris over this period. Sixty 'rope nooses' apparently used for suspending Patagonian toothfish (Dissostichus eleginoides) in blast freezers and 23 toothfish hooks were found. Prior to this study only three hooks (all from the southern bluefin tuna longline industry) had been found in association with seabird nests at Marion Island. Fishery-related marine debris was found most frequently in association with wandering albatross nests, while 'other' (non-fishing) marine debris was found most frequently in association with grey-headed albatrosses and southern giant petrels. Three seabirds (a southern giant petrel, a northern giant petrel, and a sub-Antarctic skua) were found entangled in fishing gear, while five seabird carcasses (three wandering albatross chicks, one white-chinned petrel chick and a southern giant petrel adult) were found to contain ingested fishing gear. |
A bottom longline survey took place between 22 October and 1 December 1997, and several seamounts were surveyed to the south of Africa: close to the Antarctic Polar Front (Meteor Bank) and in the CCAMLR region (Shona and Spiess seamounts, the western slope of Bouvet Island, Ob and Lena Banks). Samples were collected from longlines set perpendicularly to the isobaths and in depths down to 1 500-2 000 m. Numerically, catches were dominated by Patagonian toothfish (Dissostichus eleginoides), with 2 822 individuals caught (83% of total catch). Grenadiers of the genus Macrourus were the second most abundant taxon (248 individuals). The total catch by weight was 21.430 tonnes, of which D. eleginoides comprised 20.502 tonnes (96%). The seamounts differed substantially in relation to seabed topography, species composition of catches, and relative abundance and size structure of D. eleginoides caught. Lengths of individual D. eleginoides generally increased with depth although this trend may be confounded by the presence of local seabed features, and juvenile fish in nearby shallow water. The sex ratio of immature D. eleginoides less than 90 cm total length did not significantly vary from 1:1. Females accounted for 70% of individuals of more than 90 cm in total length. The stratum with the highest catch (weight) per unit effort was 600 to 800 m, although absolute values varied between areas. Catch per unit effort ranged from a maximum value of 359 kg/1 000 hooks on Ob and Lena Banks to a minimum of 185 kg/1 000 hooks on Meteor Bank. |
A large sample of otoliths from the mackerel icefish (Champsocephalus gunnari) was measured and weighed and their effectiveness as predictors of fish length and fish mass determined. The two measures, otolith length and otolith mass, provide good predictors of fish length, the latter being slightly better. The same measures did not predict fish total mass as accurately. |
Acoustic data from the Polish FIBEX survey in the West Atlantic were analysed using recent acoustic target strength estimates of krill (SC-CAMLR, 1991a). This analysis indicated that the acoustic densities of krill in the oceanic parts of the Drake Passage were low (3 g m-2); however, the results also indicated that spatial variability was important and that parts of the shelf to the north of the South Shetland Islands were areas of high density. High acoustic densities of krill (77 g m-2) were also present in the Bransfield Strait. These results are discussed in the context of spatial and seasonal variation which has previously been reported for the area and shown to be important. |
Bathymetric maps of two major areas within the South Shetland Islands were generated using several integrated bathymetric databases. These areas comprise the lower South Shetland Island chain from King George Island to Smith Island, and the region around Elephant Island. From the integrated datasets, areas of seabed within the 500 m isobath for six levels of depth strata (0-50 m, 51-100 m, 101-200 m, 201-300 m, 301-400 m and 401-500 m) were computed. Areas were calculated based on planar and interpolated surface area of seabed incorporating seafloor slope. Differences in calculated seabed areas from previous studies were likely due to both improved methodology and georeferencing of contours. |