CCAMLR Science, Volume 5 (1998)
Two Monte Carlo simulation models, the krill yield model and the generalised yield model (GYM), are currently available to CCAMLR that can be used for estimating yields according to decision rules that relate spawning stock status to the median unexploited spawning biomass. These models are used in order to account for uncertainty in the biological parameters and abundance of the stock being assessed. CCAMLR currently uses results from the krill yield model as a basis for setting catch limits in the krill fishery, while the GYM is used for assessing yield in other finfish fisheries but can also be used for krill. The decision rules relate to the effects of a specified long-term annual yield on (i) the probability of the spawning stock being reduced below a set proportion of the median pre-exploitation spawning biomass during a projection trial and (ii) the median status of the spawning stock at the end of the projection trial (as a proportion of the median pre-exploitation spawning biomass). These rules require that the median pre-exploitation spawning biomass is known prior to a projection so that in each trial a direct assessment can be made of (i) whether the stock became critically depleted in that trial and (ii) the status of the stock at the end of the trial. These values from all trials are used to determine the probability of critical depletion and the median level of escapement respectively. The Monte Carlo simulations take account of uncertainty in the biological parameters used to estimate yield. In this respect, the biological parameters may change between individual projection trials. Consequently, the median pre-exploitation biomass needs to be estimated prior to each trial in order for it to be based on the parameters for that trial. The krill yield model uses a biased estimate of the median pre-exploitation spawning biomass with a method for correcting this bias in the level of escapement. The GYM uses an unbiased estimate of the median pre-exploitation spawning biomass and, thus, requires no correction. This paper examines the effects of the biased estimates of the median pre-exploitation spawning biomass on the estimates of krill yield. The results show that the biased estimate results in a bias of the probability of critical depletion, while the level of escapement is not appreciably sensitive to the method of estimating the median pre-exploitation spawning biomass. This bias in the krill yield model will result in the catch level given for a set probability of critical depletion being too high. However, this will not affect the estimates of long-term precautionary yield currently adopted by CCAMLR for the krill fishery because the level of escapement is the binding rule in this case. The results indicate that the GYM using the unbiased estimator of the pre-exploitation biomass is the preferable model for assessing long-term annual yields. |
Data from individual hauls carried out by vessels operating in the South Georgia krill fishery between 1994 and 1996 were examined and a range of descriptive measurements reflecting the fishery operation were produced. The measurements emphasise that the krill fishery at South Georgia was geographically focused, operating in a limited area along the shelf edge on the northern coast of the island. Each day several hauls were undertaken by each vessel (average 7.9 in 1993,9.9 in 1995, and 7.0 in 1996), with hauls producing higher catch rates during the middle of the day. Individual hauls were examined to establish the time required for each phase of the fishing operation. The times associated with shooting and hauling the net were usually short and showed little variation, whereas the time associated with the actual fishing period was longer and more variable. The time between consecutive hauls was almost as long as the fishing period and showed similar levels of variability. Distances moved between consecutive hauls were generally small, suggesting that little effort was spent searching for fishable swarms beyond the near neighbourhood. The range of measurements describing the fishery indicates that differences existed between years, with 1995 being a better season than either 1994 or 1996. Aspects of mesoscale variability are discussed in relation to previous attempts to model fisheries data and to derive suitable abundance indices that are sensitive to changes in biomass. The focused nature of the fishery at South Georgia suggests that fisheries-based indices may be of value for management purposes, consequently further detailed analysis would be useful. |
Catch per unit effort (CPUE) data are presented from krill fishing operations in CCAMLR Subareas 48.1, 48.2, 48.3 (South Atlantic) for the period 1975/76 to 1987/88. Comparisons are carried out between subareas in single years, between years on a subarea and area basis, and between oceanic and shelf regions. Results show that the CPUE may differ between subareas, years and shelf/oceanic regions. Obviously, a consistent pattern of CPUE values does not occur over time for the entire area. High average CPUE were observed in the years 1975/76 and 1980/81, whereas the lowest CPUE occurred in Subarea 48.3 in 1977/78. The validity of the CPUE index is discussed as an indicator of krill abundance/biomass and caution is expressed in this regard. Depending on the circumstances, a commercial CPUE index may seriously under- or overestimate the amount of krill available in a statistical subarea. Overall length-frequency distributions from commercial fishing operations in general tend to show little interannual variation because several factors influence the representativeness of krill smaller than 35 mm in the catches. |
The results of an analysis of haul-by-haul catch statistics from the Soviet krill fishing fleet during the 1989/90 season (sample of 3 614 hauls for the period from 1 November 1989 to 12 June 1990) are presented. During the season, fishing vessels worked one fishing ground off the northwestern tip of Coronation Island. CPUE showed variability with a minimum in November (3.2 tonnes/hour) and maximums in February and March (9.6 tonnes/hour and 11.0 tonnes/hour respectively), followed by a decrease to 6.9 and 7.1 tonnes/hour by May and June. The location of the fishing ground was observed to be stable, and it is considered that the location was determined by certain hydrodynamic characteristics of the area, for example a complex current around the islands together with topographically-induced effects. Except in November 1989, temporally and spatially sustained fishable krill aggregations did not form outside the main fishing area. During the November period the commercial fleet fished over oceanic areas, and targeted krill aggregations found in the major current flowing towards the north-east of the region. The fleet was observed drifting with the current together with krill aggregations. A maximum CPUE (3.8 tonnes/hour) was recorded at the beginning of the period, with a subsequent gradual decrease towards the minimum level acceptable to the fleet (2.5 tonnes/hour). Over the last five days of November, the aggregation disintegrated, having existed for about 25 days, and commercial vessels returned to Coronation Island. Krill drift velocity, estimated on the basis of the fishing ground shift, amounted to 7.4 km/day or 8.7 cm/sec. This is comparable to, though less than, the drift velocity off Elephant Island (11 to 13 cm/sec), evaluated earlier using the same method (Sushin and Myskov, 1992). |
Management of the mackerel icefish (Champsocephalus gunnari) at South Georgia is complicated by the likelihood of substantial periodic variations in natural mortality rates. These may be associated with increased consumption of C. gunnari by Antarctic fur seals in years of poor krill availability. Thus natural ortality of C. gunnari may, in some years, increase by a large factor (assumed here to be 4), declining to normal levels again when krill return. This paper outlines a scheme which would use information from studies on krill and predators undertaken as part of the CCAMLR Ecosystem Monitoring Program (CEMP) to interpret or modify information from commercial fisheries and research surveys leading to estimates of stock biomass. An extension of this scheme would use predictions of coming periods of krill scarcity as early warnings of increased natural mortality of C. gunnari. Full implementation of such a scheme would require greater knowledge of quantitative aspects of food web dynamics within the South Georgia ecosystem than we possess at present. There is therefore a need for an interim approach to the setting of precautionary catch limits for this fishery. An approach based on the CCAMLR generalised yield model (GYM), with periodically varying natural mortality, provides a realistic description of perceived icefish dynamics. However, the model generates a significant probability that C. gunnari populations will be depleted even in the absence of fishing, which, given the existing CCAMLR decision rules, would preclude the fishery's ever opening. Several possible modifications to the decision rules are discussed. It is concluded that while the GYM can be used to estimate a temporary, conservative long-term yield, a new approach and set of decision rules will ultimately be required for C. gunnari. |
Assessments of the abundance and potential yield of the mackerel icefish (Champsocephalus gunnari) populations at Heard Island are developed from the results of four scientific surveys conducted in 1990, 1992, 1993 and 1997. The assessments include estimates of all the parameters required, based on the data collected during the surveys, including growth curves, natural mortality, ages and sizes of maturity and a weight-length relationship. The results show that characteristics of C. gunnari populations on the Heard Plateau are sufficiently different from those on Shell Bank for them to be managed separately. Precautionary catch limits calculated using the Generalised Yield Model (GYM) are found to be 180 tonnes for the Heard Plateau and 18 tonnes for Sliell Bank. An assessment based on the results of a survey conducted at the end of the 1997 fishing season would allow for a catch limit on the Heard Plateau of 900 tonnes in the 1997/98 season and 600 tonnes in 1998/99. |
The diet composition of the mackerel icefish (Champsocephalus gunnari) caught in CCAMLR Subarea 48.3 in late summer from 1994 to 1997 was analysed using frequency of occurrence (F%) and dietary coefficient (Q%) methods. Krill (Euphausia superba), followed by the amphipod hyperiid Themisto gaudichaudii, was the main food item around South Georgia and Shag Rocks in the period investigated, except in 1994, when the order of importance of these two species in the diet of C. gunnari was reversed. The consumption of other prey items such as mysids and Thysanoessa sp., which are known to be important alternative food in years of krill scarcity, as well as fish, was only occasional or negligible and varied from year to year. The spatial distribution of the main prey items in the diet of C. gunnari, as well as of the proportions of stomachs with/without food in Subarea 48.3, did not show a consistent pattern from year to year. Likewise, in the four surveys by the Dr Eduardo L. Holinberg from 1994 to 1997, high proportions of fish had empty stomachs, a phenomenon which could be caused by several concurrent factors. The variation of the diet according to the length of C. gunnari did not reveal any selectivity pattern for any of the prey items. The availability of krill in Subarea 48.3 during the period investigated can be defined as low in 1994 (equivalent to years of krill scarcity), intermediate in 1995 (below years of historically high abundance), and high in 1996 and 1997 (in line with years of high krill abundance). This information is well in accord with independent information from acoustic surveys and krill-dependent species monitored under the CCAMLR Ecosystem Monitoring Program (CEMP). |
This paper presents the results of randomly stratified pelagic trawl census surveys conducted in Subarea 48.3. From 1984 to 1990 daily sampling stations aimed at studying the vertical migrations of some of the most abundant species were carried out during the six pelagic surveys. Horizontal distribution and length composition of fish, as well as the comparative distribution of krill and juvenile Champsocephalus gunnari were studied. The pelagic component of the ichthyocenosis in the subarea consisted of pelagic fishes (mainly Myctophidae), distributed at a depth range of 250 to 300 m, as well as demersal fishes in the pelagic stage of their life cycle. Catches contained both young and adult specimens of 42 species from 16 families. Numerically, catches were dominated by C. gunnari, Lepidonotothen larseni, Chaenocephalus aceratus and Gymnoscopelus nicholsi. Nototheniidae (12 species), Myctophidae (9 species) and Bathydraconidae (5 species) were the most common families. Adults and juveniles of some demersal species inhabited different areas, i.e. juveniles were distributed mainly in the south, whereas adults were found in the northern shelf area. Diurnal vertical distribution of young and adult fish was also different. Large fishes were most often taken near the bottom, while small ones were caught in the water column. The main food item (up to 60-80%) of both juvenile and adult fish taken by the sampling trawl in the water column was found to be krill. Aggregations of juvenile C. gunnari were observed only outside dense concentrations of Antarctic krill. |
Finfish stocks in the Antarctic Peninsula region (Subarea 48.1) were exploited from 1978/79 to 1988/89, with most of the commercial harvesting taking place in the first two years of the fishery. Results of bottom trawl surveys conducted by Germany in the vicinity of Elephant Island in the 1980s showed that stocks of Champsocephalus gunnari, Notothenia rossii, Gobionotothen gibberifrons and Chaenocephalus aceratus have been considerably affected by fishing. Stocks of G. gibberifrons and C. aceratus had apparently recovered to a large extent by the second half of the 1980s, while C. gunnari stocks remained at a low level. The status of N. rossii is still unclear, although some recovery was apparent from length compositions obtained in the 1980s. The Antarctic Peninsula region was closed for finfishing from 1989/90 onwards. Results of the first bottom trawl survey carried out after the closure of the area in November/December 1996 suggested that the biomass of the standing stock of finfish may have further declined. Given the low abundance of C. gunnari and other species and the difficulties encountered in managing fisheries which exploit mixed-species assemblages, there appears to be little prospect at present of re-opening the fishery around Elephant Island. |
Scale and otolith age readings were compared for 105 specimens of the Patagonian toothfish (Dissostichus eleginoides) from the South Georgia region (Subarea 48.3). In 44% of specimens, age readings from both structures were identical. A Wilcoxon test for paired data (Ts = 0.256, n = 59) conducted on the remaining 56% showed that differences in age estimates between the two methods were not significant. Otoliths frequently appeared totally opaque, making it impossible to read them, while scale images were always clear. The scale method is considered most appropriate for determining the age of D. eleginoides because it does not underestimate age and is the most effective with regard to time and cost. Values of fish growth parameters using the generalised von Bertalanffy equation estimated from 1 000 data records are: Lt = 207.01 cm [1 - e - 0.0748{t + 0.2898}]; where Loo = 207.01 cm, K = +0.0748 and to = -0.2898. The maximum age of D. eleginoides was found to be 24 years. This is very close to the maximum age of 22.8 years calculated from Taylor's formulae which relates maximum observed age to natural mortality. In this case we used M = 0.13, which is the value recommended by CCAMLR. |
Salp by-catch and krill discolouration ('green' krill) caused by active feeding on phytoplankton were analysed using logbook data from Japanese krill trawlers operating near the Antarctic Peninsula. Interannual and seasonal variability of the timing, duration and intensity of salp blooms and the presence of green krill were analysed. No relationship between salp density and the proportion of green krill in the catches was evident when both salps and krill were taken together. In the Livingston Island area, the proportion of green krill was high only when salp density was extremely low. However, no clear relationship was observed in the Elephant Island area. Possible reasons for these phenomena are discussed. |
Longlining operations for Dissostichus eleginoides off South Georgia were assessed for interactions with seabirds and the effectiveness of measures employed by CCAMLR to mitigate seabird mortality. Following an agreement between the Governments of the United Kingdom and Chile, a UK observer was placed aboard a Chilean longliner, the BF Cisne Verde, fishing in CCAMLR Subarea 48.3 during the 1996/97 season. In this study 61 lines were laid from March to May 1997 using the Spanish double-line method. In accordance with CCAMLR conservation measures, lines were set at night, weights (6-7 kg) were set at 50 m intervals along the line, all deck lights were extinguished and no offal was discarded during setting. A streamer line made to CCAMLR specifications was also used, except during an experiment that attempted to assess the effectiveness of the streamer line in mitigating incidental mortality of seabirds during night-time setting. Data were recorded during hauling using a randomised cluster sampling method, developed to allow representative data to be collected when 100% observer coverage could not be achieved: A total of 12 dead seabirds were recorded, giving an average mortality rate/night of 0.099 birds/l 000 observed hooks, substantially lower than other vessels fishing around South Georgia during the same period. Mortalities consisted of nine white-chinned petrels, two black-browed albatrosses and one unidentifiable bird caught on hooks. One giant petrel was also killed as a result of flying into the side of the vessel. Generally, few birds were seen following the vessel during setting operations. During April, however, large numbers of white-chinned petrels were seen occasionally, and large numbers of black-browed albatrosses were seen when the moon was full. Live birds were observed to become caught on hooks during hauling on 23 occasions; all birds were released alive. Black-browed albatrosses concentrated on taking returning bait off the line and accounted for 18 birds caught; giant petrels concentrated on taking discarded offal, and accounted for five birds caught. |
This paper assesses the potential for the commercial trawl fisheries in the Heard Island area (Statistical Division 58.5.2) to significantly affect, in the long term, spawning biomass of three taxa caught as by-catch: Channichthys rhinoceratus, Lepidonotothen squamifrons and Bathyraja spp. (deep-water skates). The long-term annual yield for each of the three taxa was estimated using the generalised yield model (GYM) used by the CCAMLR Working Group on Fish Stock Assessment (WG-FSA). Input data included biomass estimates from surveys in the region from 1990 to 1993 and a limited amount of biological data from the local area. Where possible, values for missing input parameters were obtained from the literature, for the same species from other areas, otherwise proximate data for similar species elsewhere in the world were used. The second part of the analysis examined the amount of each species caught during commercial operations and the effectiveness of by-catch provisions (the 5% rule) set by CCAMLR in 1996 to ensure that the status of these commercial species is not affected by these commercial fisheries. The ranges of estimates of long-term annual yields for C. rhinoceratus, L. sqamifrons and skates were 62 to 87 tonnes, 7 to 911 tonnes and 50 to 210 tonnes respectively. While refinement of these estimates is needed, they enable a preliminary examination of the potential for the fishery to be exceeding long-term sustainable annual yields. For C. rhinoceratus, L. squamifrons and skates (principally B. eatoni), the total by-catch during commercial activities in 1996/97 was approximately 5%, 58% and 11% of the respective lowest estimates of yield. These results indicate that the current commercial trawl fisheries around Heard Island are unlikely to be negatively affecting these stocks. Notably, by-catch species appeared as a high proportion of the catch (greater than 5%) when only small amounts of fish were caught (less than 100 kg of by-catch), which was usually during prospecting. By-catch was usually very low as a proportion of the total catch when large catches of target species were taken, indicating that CCAMLR's 1996 by-catch rules need revising in order that prospecting may be undertaken. Thus, to enable prospecting for commercial aggregations of Champsocephalus gunnari or Dissostichus eleginoides while protecting the by-catch species from excessive fishing effort, the rules governing by-catch for the Heard Island area could be a combination of (i) limiting the total by-catch in a year to the estimates of long-term annual yield, (ii) allowing catches of by-catch species of up to 100 kg in any one haul and (iii) changing fishing grounds if the catch of any one of the by-catch species exceeds 100 kg and the total by-catch is greater than 5% of the total catch in a single haul. |
Information on natural mortality rates of the mackerel icefish (Champsocephalus gunnari) from the period prior to known large-scale commercial harvesting is discussed. After careful consideration of the data and available methods, several errors are noted in earlier estimates. Unfortunately, because none of the datasets are sufficiently comprehensive, it is impossible to unequivocally provide an estimate that is unaffected by confounding effects mainly due to recruitment variation, but also possibly due to age-dependent mortality and commercial fishing. |
During a research cruise of the British Antarctic Survey ship RRS James Clark Ross in February 1996, more than 2 000 individuals of the ommastrephid squid, Martialia hyadesi, were caught in a single haul made by a pelagic trawl near the Antarctic Polar Frontal Zone in CCAMLR Subarea 48.3. This paper describes the analysis of acoustic data gathered during this haul and a survey of the immediate vicinity, in order to characterise an acoustic signature that may be used to identify this species of squid. Distinctive echoes that did not occur when other, unsuccessful hauls were undertaken, were the only strong echoes recorded from the part of the water column traversed by the net. The trawl was restricted to a single depth zone which reduced the potential number of echoes that might otherwise have been attributed to the squid that were caught. |
Accurate estimation of krill biomass by acoustic techniques is dependent on a number of factors of which one of the most fundamental is accurate echosounder calibration. The Simrad EK500 scientific echosounder used aboard RRS James Clark Ross is calibrated regularly at South Georgia before and after krill surveys there, and exhibits temporal stability in system gain settings. Between Antarctic seasons this echosounder has also been calibrated in temperate European waters. Under these warmer conditions, calibrated gain settings differ markedly from those applied in the Antarctic, even after adjustments have been made to account for differences in sound speed between locations. Here we present results from multiple Antarctic and European calibrations which suggest that echosounder transducer performance is dependent on ambient water temperature. Highly significant differences in volume backscattering (Sv) and target strength (TS) transducer gains were detected at both 38 and 120 kHz between calibrations conducted at the two locations. At 120 kHz, the required Sv transducer gains at South Georgia (sea temperature at depth of transducer = 2.3oC) were on average 1.4 dB less than in European waters (16.6oC), and a similar trend was detected at 38 kHz. If European calibration parameters were to be employed during surveys around South Georgia, and no account were taken of the differences in gain settings, then integrated 120 kHz echo signals would be under-reported by 2.8 dB, leading in turn to an under-estimation of krill biomass by 52.5%. Every effort should therefore be made to ensure that echosounders are calibrated at temperatures as close as possible to those prevailing within the area in which surveys are conducted. In addition, the implications for biomass estimation of temperature variation across a survey area should be considered carefully |