CCAMLR Science, Volume 9 (2002)
We used the otoliths from a sample of Dissostichus eleginoides pre-recruits, whose length density was distinctly polymodal, to see whether ages estimated by reading otoliths were congruent with the length modes observed. Length densities by age were compared graphically with the overall length density observed. Additionally, ages were predicted for each fish based on length, and compared with ages estimated from reading otoliths in a goodness-of-fit test. The majority of the otoliths (83.6%) were estimated to be from fish 1+ or 2+ years old, with mean total lengths of 32.5 cm and 41.3 cm respectively. No difference was found between predicted and estimated ages. We conclude that the two dominant modes observed in the length density represent different age cohorts separated by one year. |
The history of the fishery for Patagonian toothfish (Dissostichus eleginoides) in the Prince Edward Islands Exclusive Economic Zone (EEZ) is reviewed briefly. It is characterised by very large illegal catches and a sharply declining longline catch-per-unit-effort (CPUE) trend. Application of a simple age-structured production model (ASPM) provides a robust indication that the spawning biomass has been depleted to, at most, a few percent only of its pre-exploitation level. Projections suggest that the annual catch limit should be reduced to a maximum of about 400 tonnes. Implications for surveillance are discussed. |
Stocks of Patagonian toothfish (Dissostichus eleginoides) in different sectors of the Southern Ocean are considered to be genetically distinct. However, in the Indian Ocean, it is largely unknown whether stocks are separate between shelves and banks separated by deep water. More particularly, the separation of stocks on the Kerguelen Plateau has not been investigated. This paper examines the assumptions of stock separation using tagged fish from the fishery around Heard Island and McDonald Islands (HIMI) in the Australian Exclusive Economic Zone (EEZ). Movement, growth and trends in fish abundance are examined using these data. The bulk of the data show that fish disperse only a very short distance, up to 15 n miles in most cases, from their point of release. This may be explained in part because of the concentration of fishing in the established grounds, which makes recaptures more likely in the same place as marking. The concentration of effort should also result in a high probability of detecting fish movements between grounds, if they occur, but so far no such movement has been detected. It appears that fish tend to be locally resident during their phase in the depth range of the fishing grounds, but they move on once they approach maturity at about 850 mm total length and become unavailable to the fishery. Four fish have been recorded as moving a long distance, and thus provide contradictory evidence. Three fish moved from ground B of HIMI to Crozet and were all within the size range normally found in the Heard Island fishing grounds, and so appear to be behaving differently from those fish that follow the normal pattern of residency in the grounds. One fish has been recorded as moving from ground A to Kerguelen and was at the upper end of the size range normally found on the grounds. These four long-range movers provide the first documented direct evidence of toothfish moving such distances and of fish moving from one fishery to another. The significance of this is discussed. Trends in abundance of fish show that the number of fish in ground B and in the population as a whole has been reduced since 1998. The implications of these results are discussed. The overall results show that mark–recapture experiments provide important information for the management of toothfish stocks and that trends in abundance derived from these experiments might be useful adjuncts to the assessment process in the future. |
The distribution of mackerel icefish (Champsocephalus gunnari) around South Georgia (FAO Statistical Subarea 48.3) at various stages of its life cycle was studied using material collected by AtlantNIRO from 1970 to 1991. This study took into account the spatial–temporal variability of C. gunnari. Young fish of the first age group (<15 cm in length) were mainly found in the southern part of the study area, immature fish (15–25 cm) in the east and southwest, and adults (25–39 cm) in the north. The distribution of C. gunnari changes during its life cycle. Fish larvae inhabit the upper 100 m layer of pelagic waters. Young fish (<6 cm) were mainly observed near the bottom and at depths of up to 75 m from the bottom. Young fish undertake vertical migrations into the water column during the day and return to the bottom layers at night. Immature fish were found to be distributed throughout the water column at all times of the day, but they also undertake vertical diurnal migrations similar to those of mature fish. Mature fish were found near the bottom during the day and in the water column at night. Larger fish (>40 cm) did not undertake vertical migrations. Variations found in the depth distribution of C. gunnari relate both to fish length and season. |
Three separate acoustic surveys of Antarctic krill (Euphausia superba) were conducted around South Georgia in the 2000/01 season: in October 2000 (early-season); during late December/early January 2000/01 (mid-season), and in March 2001 (late-season). The surveys were the first in a newly planned five-year series of observations designed to complement and extend an existing time series regularly maintained by the British Antarctic Survey since 1996 (and on a more ad hoc basis since the early 1980s). We hoped that conducting several surveys in one season would provide information on short-term variability that could be used to set data from more restricted once-per-season ‘snap-shot’ cruises in a broader context. The early- and late-season surveys were associated with logistic support voyages to South Georgia and were restricted to four transects within a box to the northwest of South Georgia. The dedicated mid-season survey covered that box in more detail (twice as many transects) and, in addition, examined boxes to the north, northeast and southwest of the island. Together these surveys provided temporally and spatially extensive coverage around South Georgia. Krill density in the western box in the early-season survey was very low (3.5 g m-2) but rose significantly (P = 0.020) by midseason (to 34.7 g m-2). In a pattern that is consistent with observations from previous years, krill density in the western box in the mid-season survey was less than that in the eastern box (80.4 g m-2). Analysis of transect data obtained in the western box in the midseason survey revealed no significant difference in the mean krill density derived from only those four transects surveyed early- and late-season, or from the full eight transects. Our first occupation of a survey box off the central north coast of South Georgia in midseason revealed a krill density of 47.2 g m-2 that was intermediate between the eastern and western areas. The size structure of the krill in the central region also reflected a mix of those to the east (generally small) and west (generally large). Krill density to the southwest of South Georgia was 32.1 g m-2 in mid-season. By March, krill density in the western survey area had fallen significantly (P = 0.037) from the mid-season high to 7.8 g m-2. Our multiple surveys at South Georgia have revealed major intra-annual changes in krill density at the island and have shown that the timing of the acoustic survey can affect significantly the estimate of krill density. The multiple estimates of krill density will allow reproductive performance indices for top level predators to be compared to prey availability at more appropriate time-scales than previous single ‘snap-shot’ acoustic survey data have allowed. This is a crucial step in the elucidation of response functions of dependent species to changes in krill abundance, and could be a useful contribution to ecosystem management. |
The distribution and internal structure of commercial krill concentrations were investigated in the areas of the South Shetland, Elephant, South Georgia and South Orkney Islands in the spring and summer from 1997 to 1999. The density of concentrations varied with area and season. The densest concentrations were found near the South Shetlands. Concentrations at night were several times less dense than those observed during the day. Between February and April the density of concentrations increased, while in May and June it decreased. From February to June the period of daylight becomes progressively shorter, therefore it must be assumed that there are factors other than daylight which have an impact on the density of krill concentrations. The internal structure of concentrations was very diverse, and mean density varied from 11 to 31 370 specimens per 1 000 m3. At night, krill concentrations were scattered throughout the water column, however no clear evidence of vertical migrations was found. Between February and April concentrations occurred in much shallower waters at night than during the day, while in May and June they occurred at the same depths at night as during the day or sometimes even deeper. Between February and April concentrations during the day were found in shallower waters, while between April and June they were found in deeper waters. The size structure of the krill varied during all periods and in all areas, however krill size was observed to decrease as the season progressed. |
Jarman, S.N. and S. Nicol (2002), Sources of variance in studies of krill population genetics There has been a long-term interest in the population genetics of Antarctic krill species because of their ecological and economic importance. The possibility that there are distinct genetic stocks of these species would affect the design of management strategies for their conservation. A recent resurgence of interest in identifying distinct stocks of swarming krill species has been driven by the development of genetic technologies that are more sensitive to subtle population structure than older methods. Previous studies of the population genetics of the Antarctic species Euphausia crystallorophias and E. superba and the boreal species Meganyctiphanes norvegica that used allozymes found no evidence of genetic population structure. More recent investigations using sequence variation in mitochondrial DNA (mtDNA) have exposed genetic differentiation between samples taken from different parts of each species’ range. However, the underlying assumption of these studies that differentiation between samples is caused primarily by restricted gene flow between widely separated sampling sites may be incorrect. Our recent study of E. crystallorophias mtDNA variation has indicated that there is significant genetic differentiation between samples taken within one region. This has important implications for the design of future studies of krill population genetics, which must be able to accommodate this sympatric variance component as well as variance attributable to differences between regions. Genetic differentiation between stocks of krill in different regions therefore can not be adequately assessed unless multiple samples are taken from each region. |
This paper discusses the principles and approach required for developing small-scale management units to take account of predators’ needs when managing the fishery for Antarctic krill. It provides a theoretical foundation for considering the scales of management units involving the integration of local populations of harvested species, foraging areas of predators, fishing grounds and the potential influences of the environment, including oceanography and metapopulation structure of the harvested species. The integration of these components involves two different types of unit: the ‘harvesting unit’, which is at the scale of the metapopulation of the harvested species, and the ‘predator unit’, which does not have to be a relatively self-contained ecosystem but should be sufficiently independent for fishing in that unit not to inadvertently affect predators being monitored in other units. The South Atlantic region (Area 48) is used to illustrate how to define predator units. The derived conceptual model is then used to formulate an approach for developing fisheries on prey species, notably krill, in other harvesting units. The manner in which predator units can be used to help the CCAMLR Ecosystem Monitoring Program (CEMP) provide strategic advice on the effects of fishing is discussed. In general, the early acquisition of information on the distribution of local populations of krill and the potential foraging densities of predators from within a harvesting unit (i.e. abundance of predators, distribution of colonies and foraging range) will provide a means of circumscribing predator units as well as undertaking an assessment of long-term annual yield. It is proposed that the early development of the fishery could be concentrated in a small number of units in such a way that the relative fishing intensities in those units are equivalent, although not necessarily equal, to the intensity expected across all units once the catch limit had been reached. Other units in which fishing was not occurring could be monitored as well. This process could help determine whether or not the catch limit is likely to cause undesirable effects on predators in any of the predator units. In this way, it is possible to determine, well in advance of the catch limit being reached, whether or not local restrictions on harvesting are necessary, as well as the overall requirements for the monitoring program. |
A large sample of otoliths from bigeye grenadier (Macrourus holotrachys) caught as by-catch in the Patagonian toothfish (Dissostichus eleginoides) fishery in Subarea 48.3 was measured and weighed, and the usefulness of otoliths as predictors of fish size was determined. Otolith mass provides good estimates of fish length, whereas otolith length and width measurements provide less accurate estimates of fish length. Seasonal variations in fish mass with reproductive condition need to be considered if predictions of fish mass from otolith mass are undertaken. Otolith size/fish size models should be derived for each fish population under investigation. The length of fish chosen in such studies should be representative of the size range consumed by predators. |
During the survey of Subarea 48.4 from 17 January to 1 February 2000 it was observed that krill was present in non-aggregated and scattered forms, as well as in swarms. Differences between spatial distribution patterns of different krill aggregation forms in relation to water mass structure and dynamics were revealed. Distributional features and physical characteristics of some 2 400 krill swarms detected in the study area were sized acoustically and are described in this paper. Results of comparisons of swarm parameters by area and season are given. The effect of spatial distribution patterns of krill aggregations on the horizontal and vertical distribution of krill biomass is shown. About 64% of the krill biomass was concentrated in krill swarms in the upper 80 m depth layer within meander and eddy zones of the Weddell Sea (14% of the study area). Spatial distribution patterns of krill aggregations and distribution of krill biomass density were analysed with a view to detecting potential fishing grounds where the recommended precautionary catch limit could be taken. Such fishing grounds could be located within the zones in which swarms were concentrated and where the biomass density was greater than 1.5 g m-3. The biomass in these potential fishing grounds amounted to about 1.7 million tonnes. It was shown that the removal of biomass up to a recommended precautionary limit could be possible from such grounds despite a predicted low catch per hour of trawling. |
Monitoring of sea-surface temperatures (SSTs) in Subarea 48.3, which includes satellite surveys with GOES-E, Meteosat and in situ monitoring, together with further analysis of SST maps, provides continuous information on hydrological conditions in the area. Analyses of the data at the beginning of each summer season could enable us to evaluate the potential for conducting a krill fishery in the area during the entire year. The reliability of these forecasts could be appraised by comparing them with krill catches obtained under various hydrological conditions. In comparison with the early 1990s, the period from December to February in the late 1990s and early 2000s showed a considerable decrease in mean SST off South Georgia. This observation is especially obvious for the northern shelf waters where the mean SST dropped from 3.96°C in 1990/91 to 2.05°C in 2000/01. During that period, the SST anomaly sign changed from positive to negative: on average, the December–February anomaly was up to +0.71°C in 1990/91, whereas it was -1.2°C in 2000/01 (for the 1999/2000 season the SST anomaly was -0.62°C). The observed cooling of South Georgia waters had a negative effect on fi shing activities in the area. The total catch of krill was 81 369 tonnes in 1989/90 and 123 562 tonnes in 1990/91. However, it dropped to 39 766 tonnes in 2000/01. The SST maps for 1999–2001 showed a prominent |
Miller, D.G.M (2002), Antarctic krill and ecosystem management – from Seattle to Siena This paper outlines CCAMLR’s development of a management approach for the Antarctic krill (Euphausia superba) fishery and associated ecosystem component between 1984 and 1995. The approach is shown to be consistent with the objectives of the CAMLR Convention, particularly its Article II. Emphasis is given to the initiation of the CCAMLR Ecosystem Monitoring Program (CEMP) and the deliberations of the Working Group on Krill (WG-Krill). Particular prominence is also attached to the modelling approach developed in order to calculate a precautionary catch limit for the krill fishery in various CCAMLR statistical areas. This paper complements those of Constable (2002 – this volume) and Everson (2002 – this volume), with the three papers documenting developments during CCAMLR’s first 20 years of existence. |
Everson, I (2002), Consideration of major issues in ecosystem monitoring and management This paper outlines the requirements of an ecosystem approach to the management of Southern Ocean resources and highlights the need for information on harvested and dependent species, their interactions and the manner in which their populations vary naturally. Large-scale interactions are catered for in the Krill Yield Model (KYM). Smaller-scale interactions centre around three main categories: the availability of krill, variation in vital rates of the dependent species and the overlap between commercial fishing and predator foraging. The CCAMLR Ecosystem Monitoring Program (CEMP) provides a good framework within which to investigate krill availability. Vital rates can be investigated directly and also by means of CEMP. The overlap between fishing and predator foraging is being monitored. A mechanism for bringing these various components together as an ecosystem approach to management is discussed. |
Constable, A.J (2002), CCAMLR ecosystem monitoring and management: future work Harvesting of marine living resources in the Southern Ocean is managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). CCAMLR is widely known for its ecosystem approach to managing fisheries with the maintenance of ecological relationships included in the conservation objectives. In the late 1980s, CCAMLR’s precautionary approach was developed, incorporating principles of how to use scientific evidence in the decision-making process. Even though this approach was based on the management of a single species (Antarctic krill, Euphausia superba), it takes account of the needs of predators in the assessment of catch limits. The success of this process is due to the formulation of a management procedure which includes decision rules that specify how harvest controls will be adjusted based on the scientific information available and the assessments that arise from such information. To assist the Commission in meeting its objectives, the CCAMLR Ecosystem Monitoring Program (CEMP) was set up to detect the effects of fishing on krill predators. CCAMLR needs to adopt a management procedure which has a good chance of maintaining ecological relationships and meeting the needs of predators, incorporating: (i) operational objectives that articulate the target conditions of relevant aspects of the system, (ii) methods for assessing the status of the system, (iii) decision rules on how to adjust harvest controls given the difference between the assessment and the agreed objectives, and (iv) methods for dealing with uncertainty. This paper reviews progress in developing ecosystem-based management procedures by summarising the gaps in the existing approach, the types of models developed for the Antarctic marine ecosystem, the implications of the large scale of the fishery for designing a monitoring program to detect the effects of fishing, and the types of procedures already proposed for management of the krill fishery. It highlights the need to focus future work in these areas, including the need to evaluate candidate management procedures in advance of the expansion of the krill fishery. Most importantly, operational objectives for dependent species and feasible management options need to be clearly articulated to guide this work. |