CCAMLR Science, Volume 2 (1995)
Division 58.5.2 encompasses the southern part of the Kerguelen plateau, and contains Heard Island and the McDonald Islands, an Australian Territory around which a 200-mile EEZ has been declared. In order to assess the potential of this area for commercial fishing, and to obtain some preliminary data for its management, three random stratified surveys were conducted to investigate the distribution, abundance and biology of the most important species, covering a number of years and seasons. Five species were found to be abundant. There were two consistent areas of aggregation of Champsocephalus gunnari, but total abundance varied greatly between years. Channichthys rhinoceratus has a similar distribution, but abundance is lower and less variable with location and between surveys. Dissostichus eleginoides is widespread over those parts of the plateau where waters are more than 300 m deep, but is nowhere very abundant, although the total biomass is relatively high by virtue of the large area it occupies. There was one aggregation area of Lepidonotothen squamifrons on an outlying bank during two surveys, but this had disappeared by the third survey. Rays were widespread and moderately abundant in waters shallower than 300 m. From these results, it seems that a small fishery of a few hundred tonnes each for C. gunnari and D. eleginoides may be possible. |
Combined studies on the inner and outer northeastern part of the Kerguelen shelf during the 1989/90 to 1993/94 period provide new information on the stock of Champsocephalus gunnari. A quarterly program of ichthyoplanktonic and bottom trawl surveys over the inner shelf has enabled a new cohort of fish to be identified each year. However, annual variations in abundance have been observed. During the sampling period the 1988 cohort was dominant on the outer shelf. The three-year cycle observed previously seems to be related to the maturation stage of each current cohort and the relative abundance of spawners-at-age during each winter spawning period. Only once have spawners been observed in large numbers on the inner shelf: during the winter of 1991/92. These fish were the first of the 1988 cohort to spawn. |
A histological analysis was carried out on ovarian development in Champsocephalus gunnari caught by the RV Dr Eduardo L. Holmberg in the South Georgia and Shag Rocks areas in February and March 1994. Six phases of oocyte development were identified, and these are similar to those described for other species. A gonad maturation scale for studies using microscopes (microscopical scale) was elaborated and adapted to correspond with the macroscopic (i.e., unaided visual observation) scale commonly used. The first five stages correspond with those described macroscopically for various species of Channichthyidae. Stage six includes ovaries undergoing oocyte resorption processes, which macroscopically resemble a maturity phase but microscopically conform to a regression stage. In some cases these ovaries presented few atretic oocytes; in others a generalised regression was found, which was observed in 41% of mature fish. A high proportion of juvenile individuals (45%) was found, together with a relatively low occurrence of females that spawned or were in condition to spawn in the current season (32%). |
Methods for calculating the horizontal opening (and, thus, swept area) of a bottom trawl are discussed. In situ measurements of the FP-120 trawl used during the 1993/94 UK fish stock assessment survey around South Georgia on the MV Cordella are presented. Regression equations for the estimation of horizontal opening from other, more easily measured variables, such as tow speed and trawl depth, are proposed. Biomass estimates for the major fish species around South Georgia are estimated using a new regression equation. The results of this calculation are compared to values obtained from a previous equation. The resulting difference in biomass estimates is of the order of a 10 to 14% increase, depending upon species. |
Results of research into the trophic relationships of myctophids demonstrate that this group of mesopelagic fish plays a significant role in the community of Southern Ocean marine organisms. Myctophids have the trophic status of zooplankton consumers. The large amount of meso- and macroplankton consumed by myctophids indicates high abundance and biomass of these fishes. According to a preliminary estimate, the yearly consumption of zooplankton by Electrona carlsbergi ranges from 157 to 364 million tonnes. Myctophids occupy the third level in the Southern Ocean trophic system and are consumers of the second order. Myctophids play a major role as producers, supporting the production of organisms higher up the food chain (certain species of squid, notothenioids, seabirds and mammals). |
An initial attempt is made to develop the modelling framework suggested by the Joint Meeting of CCAMLR's Working Group on Krill (WG-Krill) and Working Group for the CCAMLR Ecosystem Monitoring Program (WG-CEMP) in 1992, in order to investigate the possible effects of krill fishing on krill predators. 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 preliminary 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 on 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. The analyses have also facilitated discussion on the interpretation of the preliminary estimates of adult survival rate provided for some krill predator populations, and on the possibility of biases in their estimation. These matters will need to be taken into account in future developments of this approach, before the latter can claim sufficient realism to provide a possible basis for management recommendations. A mathematical framework 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 further development of the 'one-way' analyses. |
CCAMLR has been using the total catch of krill taken within 100 km of penguin colonies in their breeding season (December to March) as an index of overlap between the potential foraging areas of penguins and the distribution of the krill fishery in Subarea 48.1 (South Shetland Islands). As this index has proven unsuitable for a number of reasons, an alternative method of calculating an index of fishery-predator interaction has been developed. The new index reflects the functional interaction between these two utilisers of the krill resource and is based on a detailed model of penguin foraging patterns combined with catch positions. This analysis shows that the overlap between the fishery and chinstrap penguins is much greater than for other penguins, and that this overlap has been decreasing since 1988. |
Longlining operations for Dissostichus eleginoides off South Georgia were assessed for incidental mortality and interactions with seabirds. Twenty-seven line sets were observed and 98 deaths of seabirds were recorded over 20 sets; no deaths occurred during hauls. The 16 sets made at night contributed 15% of overall mortality, all of white-chinned petrels (Procellaria aequinoctialis); the four day sets contributed 85% of deaths, with giant petrels (Macronectes sp.), grey-headed albatrosses (Diomedea chrysostoma) and black-browed albatrosses (D. melanophrys) predominating. Grey-headed albatrosses, whose populations (at South Georgia are in serious decline, were disproportionately affected in relation to their numbers near the fishing vessel; giant petrels may be disproportionately affected in relation to the size of their breeding population at South Georgia when compared to albatrosses. The average mortality rate for the 20 sets was 0.48 birds/1 000 hooks and maximum mortality 3.12 birds/1 000 hooks. Mortality and interactions of birds with operations varied with site and time of day, and was affected by behavioural interactions between birds. Setting only at night would dramatically reduce the number of albatross deaths, and restrict mortality to white-chinned petrels. A streamer line made to CCAMLR specifications may reduce mortality but may be less effective during calm weather, intense feeding activity by seabirds, or when incorrectly constructed. |
Experiments were conducted to determine the effects of bait size (large versus small) and bait condition (frozen versus thawed) on the sink rate of five species of bait used in the Japanese longline tuna fishery in Tasmanian waters. The effect of adding lead sinkers to baited hooks was also examined. While all factors (bait species, size, condition) were statistically significant, the most powerful effect was due to bait condition: thawed bait sank whereas frozen bait floated. Fish with inflated swim bladders floated, however, even when thawed. The addition of a 20 g lead sinker to a conventional 20 g tuna hook reduced the sink time of bait bearing a hook alone by 50% to 2 sec. m-1. The results highlight the importance of careful bait selection (to avoid fish species with swim bladders which cause the bait to float), the proper thawing of bait and the addition of small amounts of weight to baited hooks before deployment from longline tuna vessels. |
Areas of seabed within 50 m depth contours to a depth of 100 m, and within 100 m depth contours from 100 to 500 m, have been estimated from soundings obtained during plankton and bottom trawl surveys around Elephant Island (the northernmost island of the South Shetland Islands) from 1977/78 to 1992/93. The results are presented in table format. |