CCAMLR Science, Volume 3 (1996)
The concept of precautionary measures for krill is discussed. A precautionary catch limit, based on the potential yield model developed by the CCAMLR Scientific Committee's Working Group on Krill (WG-Krill) is discussed and it is concluded that such a method is appropriate in setting an overall precautionary catch limit in large areas, such as Statistical Area 48, but the approach requires considerable refinement to provide a sufficient safeguard for land-based predators. A new approach is described which takes account of the requirements of land-based predators. Implementation of the approach is discussed and the question of how the method relates to the current CCAMLR Ecosystem Monitoring Program (CEMP) is considered. |
Interannual variability in the Southern Ocean ecosystem has long been noted and physical factors appear to dominate the dynamics. There are, however, few ecological time series available to assess this variability and it is only recently that physical datasets have been generated for the Southern Ocean. In this study, time series of krill abundance (CPUE) indices were derived from Soviet fishing operations in the northeastern Scotia Sea around South Georgia for the period from 1974 to 1992. These indices were examined in relation to the variation in the physical environment. There are links between the ice, ocean and atmospheric components of the system, and there are also correlations with CPUE data. The CPUE index generally agrees with other data which give direct or indirect indications of krill availability, suggesting it can be a useful index for some regions. Associations with ice-edge position and atmospheric components were only expressed strongly in years of extreme conditions. The correlation with water temperature was more consistent, supporting suggestions that the variability in krill abundance around South Georgia is strongly influenced by the oceanographic regime of the Scotia Sea. |
This paper presents a general fish stock projection model for assessing the long-term annual yield which satisfies objectives for the maintenance of the spawning stock biomass in accordance with CCAMLR criteria. These specify a bound on the probability that the spawning biomass will become depleted to below some specified level over a specified period and set a further constraint on the long-term status of the stock relative to the pre-exploitation biomass. The model provides a flexible method for assessing the influence of different patterns of growth, natural mortality, spawning and fishing on estimates of yield and yield per recruit. It can also be used to evaluate stochastic stock trajectories under a specified catch regime. The model uses an adaptive Runge-Kutta algorithm to calculate stock trajectories and catch rates over a specified simulation period. The procedure numerically integrates a set of differential equations which incorporate functions that specify growth, mortality, age-dependent selectivity and seasonal patterns in fishing mortality. Results from the model are compared with existing analyses from the krill yield model. The model can include a known catch history and thus allow assessments of yield to be made for existing fisheries. An example is presented for the Patagonian toothfish, Dissostichus eleginoides, around South Georgia Island. |
The Leslie depletion model has been used to examine local patterns in catch per unit effort (CPUE) in the longline fisheries for Patagonian toothfish (Dissostichus eleginoides) around South Georgia and the Pacific coast of Chile. This paper presents a re-working and synthesis of previous analyses to examine whether declines in CPUE can be modelled using a linear regression of catch per hook against cumulative catch, when single vessels operate in localised areas for periods of at least three days. The results suggest that there is greater potential for application of the model in some areas than in others, but in general there is no sufficiently consistent signal in the data for the depletion model to be used to estimate local abundance. A number of possible explanations are considered, including movements of fish, the behaviour of fishermen, variable catchability and insufficiently high catch to cause local depletion. Suggestions are made for further work. |
This paper analyses all incidental mortality of seabirds recorded on Argentinian and Chilean vessels fishing for Patagonian toothfish (Dissostichus eleginoides) in Subarea 48.3 during the 1995 season. The data was obtained from the fine-scale reports submitted by each vessel to its national fishing authority and supplemented with the information collected by international scientific observers from each vessel. It was thus possible to evaluate almost all of the incidental mortality of seabirds observed during the season. The total mortality recorded was 1 428 birds of which white-chimed petrels (Procellaria aequinoctialis) comprised 77.8%, southern giant petrels (Macronectes giganteus) 10.8%, black-browed albatrosses (Diomedea melanophris) 8.1%, wandering albatrosses (D. exulans) 2.0% and grey-headed albatrosses (D. Chrysostoma) 0.7%. A total of 56 seabirds captured during hauling operations and released alive was also recorded. Within this group, the most abundant were M. giganteus (50%), D. melanophris (14.3%) and P. aequinoctialis (8.9%). A study of the variables affecting incidental mortality shows that the distance from land, the lunar phase and the use of streamer lines and hook size were important sources of variation, both in vessels' analyses of catch of birds per unit effort (BPUE), as well as of the haul-by-haul data. |
The behaviour of seabirds and the rate of incidental mortality were observed during commercial trawling operations for Patagonian toothfish (Dissostichus eleginoides) in the 1994/95 season near Macquarie Island and Kerguelen Island. Longlining for D. eleginoides and trawling for mackerel icefish (Champsocephalus gunnari) were also observed at Kerguelen. Many seabirds were constantly observed near the vessels, especially giant petrels (Macronectes halli and M. giganteus), black-browed albatrosses (Diomedea melanophris) and Dominican gulls (Larus dominicanus). White-chinned petrels (Procellaria aequinoctialis) were common only at Kerguelen. Bird mortality during trawling was zero at Macquarie Island and very low at Kerguelen; most was associated with a single vessel using a netsonde cable while targetting C. gunnari. Most mortality associated with both longliners and trawlers was of white-chinned petrels, with a lesser mortality of albatross species. Mortality rates associated with longlining in the Kerguelen region were lower than those reported for the South Georgia area, probably because the birds at the former site are distracted from the longlines by the discharge of offal from the side of the vessel opposite to where the longline is set. |
We examine the extent of overlap between South Georgia wandering albatrosses (Diomedea exulans) and local longline fishing for Patagonian toothfish (Dissostichus eleginoides), using satellite-tracking data and precise haul locations, respectively. We conclude that D. exulans breeding at South Georgia have a relatively low potential risk of interaction with longline fisheries around South Georgia between December and February (incubation period) and between late May and October (main chick-rearing period). However during the chick-brooding period, from March through mid-May, adult birds of both sexes spend most of their time at sea over the South Georgia continental shelf in areas very similar to where the longline fishery operates and are therefore at serious risk from these fisheries at this time. Until fishing methods which do not catch albatrosses are in comprehensive use, we recommend that the South Georgia longline fishery for D. eleginoides should be managed in such a way as to avoid fishing between the end of February and mid-May. |
Armstrong (1995) presented a final report of scientific observations conducted aboard the Japanese krill fishing vessel Chiyo Maru No. 2. Unfortunately, Armstrong's fish by-catch data were not presented in a standardised manner. This paper presents a brief re-analysis of the by-catch data collected on board the Chiyo Maru No. 2; means and variances are calculated from the delta distribution. In numbers, an average of about 14 fish were caught per tome of krill, and an average of about 25 fish were caught per hour of towing. In weight, an average of about 1.2 kg of fish were caught per tonne of krill, and an average of about 2.2 kg of fish were caught per hour of towing. Confidence intervals for mean by-catch estimates are wide suggesting that, in the future, more hauls should be sampled for by-catch. The proportion of hauls containing fish was much higher for the Chiyo Maru No. 2 than it was for other Japanese and Ukrainian krill trawlers fishing in the Atlantic sector of the Southern Ocean. This difference may be related to differences in the amount of krill that was subsampled for making observations on fish by-catch. The by-catch data from the Chiyo Maru No. 2 did not provide a clear picture of the relationship between haul-specific fish by-catch and the catch rate of krill, and this observation is different from those made in most other studies. It would be worthwhile to estimate an appropriate sample size (in numbers of hauls) and subsample size (as a fraction of haul-specific krill catches) for observers collecting data on fish by-catch. |
This paper describes tracks followed by six surface drifting buoys that were deployed in shelf and oceanic waters to the north of the South Shetland Islands. Each buoy was deployed with a drogue attached at a depth of 30 m to assess flow patterns at the depth range where krill tended to be most abundant. The buoys were tracked using the ARGOS satellite positioning system. Based on buoy tracks, the surface circulation to the north of the South Shetlands shelf was described as a slow sluggish southwest current with a series of small-scale circular and semi-circular flows in the shelf region, whereas the circulation in the oceanic region was described as the strong northeast current. Between these counter-flowing currents a shear current was found in the slope region. The direct current measurements demonstrated that the oceanographic regime in these waters is not well described by geostrophic calculations. The picture obtained of the surface circulation was considered in conjunction with the distribution of krill catches taken in the area by Japanese vessels during the period from 1981 to 1994. It is suggested that the sluggish and shear currents observed to the north of the islands may have properties which contribute to the formation of stable krill concentrations in the area. The observed long-distance movements of three buoys also suggested that oceanic currents may transport krill from the South Shetland Islands to the South Georgia and South Orkney Islands on a time scale of 110 to 120 and 85 days respectively. |