Zuleta, A.V. and C.A. Moreno (1992), Catch-at-age analysis applied to new fisheries: the case of Dissostichus eleginoides

Traditionally, the approach used in the assessment of new fisheries has been based on research surveys, making it possible to estimate the standing stock and potential yield of a resource by coarse methods. The main aim of such surveys is to monitor fishing operations in order to develop a comprehensive sampling design which will provide data in line with assessment procedures that are quite similar to those commonly used in the statistical sampling theory. These procedures are, to a certain extent, limited by the complex fishing strategies of the operators and by the movements of fish stocks, which seriously affect model assumptions. This paper demonstrates that for the new fishery, analyses using catch-at-age data derived from research cruises may provide an alternative to other methods of estimating the parameters of fish stocks. This alternative method is applied to the new fishery of Dissostichus eleginoides off the south of Chile which is being conducted in accordance with CCAMLR regulations.

Robotham, V. H. and Z. Young U. (1992), An iterative model to construct an age/length key to assess the age composition of a new fishery for Dissostichus eleginoides in Chilean waters

Traditional estimation of age composition of catch requires that age/length data and random length frequency have been sampled from the same population. If such sampling is not possible the estimates of the age distribution could be biased. By using an iteration algorithm it is possible to overcome this limitation. The age composition of catch for Dissostichus eleginoides was obtained with the application of a single age/length key to the monthly length distribution using the non-linear model (Hoenig and Heisey, 1987). We found that age distribution derived from the model is not significantly different from the one obtained by using traditional methods

Moreno, C.A. and P.S. Rubilar (1992), Remarks on the natural mortality of Dissostichus eleginoides in Subarea 48.3

Natural mortality (M) of Dissostichus eleginoides was calculated after Sparre (1989) from the data obtained during the Chilean longline fishery in Subarea 48.3 (N = 7 848. The von Bertalanffy growth parameters obtained independently by three authors were used. Three cases were considered: fishing site, fishing depth and hook type. It was determined that there were no significant differences in M estimated by fishing depth and hook type with different growth parameters although there were differences in M between fishing sites. On the other hand, when comparing various mean values of M for the different cases, it was found that there were significant differences between fishing sites and between curve hooks No. 14 and straight hooks No. 22. The general mean value of M calculated for the 44 analysed situations was 0.14 ± 0.03.

Agnew, D.J. and C.A. Moreno (1992), The 1991/92 fishery for Dissostichus eleginoides in Subarea 48.3

The 1991/92 longline fishery for Dissostichus eleginoides in Subarea 48.3 was open from 4 November 1991 to 10 March 1992. A total of 3 382 tonnes was taken by one Bulgarian, five Russian and eight Chilean vessels. The maximum catch taken in a five-day period was 375 tonnes. Catch rate was not influenced by fishing depth, time of day or soak time, but was dependent on type of hook and geographical position. Fish caught around Shag Rocks were larger than to the north and southeast of South Georgia. CPUE varied markedly among fleets and was about 1 kg/hook in the Chilean fleet, 0.2 to 0.4 in the Russian and 0.2 in the Bulgarian. These differences were probably a result of the different hook types used by the fleets.

Parkes, G. (1992), Notes on the use of virtual population analysis for stock assessment of the mackerel icefish, Champsocephalus gunnari (Lönnberg, 1906) in Subarea 48.3 for the 1990/91 and 1991/92 seasons

Following on from the apparent failure of the CCAMLR Working Group on Fish Stock Assessment (WG-FSA) to satisfactorily assess in 1991 the status of the Champsocephalus gunnari population in Subarea 48.3 using VPA, attempts were made to re-work the analysis using Laurec-Shepherd and ADAPT tuning techniques, from 1991 back to 1977. The predicted age structure, dominated in recent years by the 1987 year-class (1 year olds in 1988), was quite robust, despite the use of various combinations of survey and CPUE indices for tuning. According to the VPA the population in 1991/92 was composed of a large proportion of 5 year olds, which was not observed during the survey on Falklands Protector in January 1992. Breakdown in the credibility of the VPA results in most recent years is attributed to the invalid assumption of constant M and contradictions in the input data. A conservative approach to management for 1992/93 is recommended, based on the results of surveys by Falklands Protector in 1990/91 and 1991/92.

Agnew, D.J. (1992), The choice of procedure for deciding when to close fisheries regulated by CCAMLR: a simulation model

The methods, described in CCAMLR Conservation Measures, used for deciding the closure date for fisheries monitored by the Secretariat of CCAMLR, have been difficult to implement because of the variation in catch rates shown by the fisheries. Non-fluctuating random and fluctuating random catch histories are simulated and the performance of four models for making closure decisions is investigated under a variety of circumstances.

The model described in the existing conservation measures is shown to have a high probability of allowing large over- or under-shoots of the TAC. The most successful model determines the trend of catch rates using linear regression over the latest four reporting periods, and closes the fishery if these rates indicate that the TAC will be taken before the next report is received by the Secretariat. The probability of large over-shoots of the TAC is reduced if reporting periods are small (five days) and the reporting delay is minimal.

It is recommended that in future conservation measures, methodologies for deciding the date of closure of fisheries should incorporate a formulation of Model 4, given in this paper.

Foote, K.G., D. Chu and T.K. Stanton (1992), Status of krill target strength

Empirical estimates for the target strength of krill are extracted from publications. These are confined to measurements on aggregations of live euphausiids and should not be affected by a frequent cause of bias in single-animal measurements, namely thresholding. Theoretical estimates for the target strength are derived from the deformed-cylinder scattering model assuming specific sets of physical and orientational parameters, for which there is an empirical basis. The theoretical estimates show a non-monotonic dependence of target strength on both animal size and transmit frequency, notwithstanding admitted shortcomings. Some recent single-animal measurements of target strength for live euphausiids and euphausiid-related species, made under high signal-to-noise-ratio conditions, are consistent with the general pattern. Several specific recommendations are made for future, improved determinations of krill target strength. Based on the comparisons, general prediction curves for the target strength are presented that are applicable to a wide range of lengths, acoustic frequencies and orientation parameters.

Demer, D.A. and R.P. Hewitt (1992), Calibration of an acoustic echo-integration system in a deep tank, with system gain comparisons over standard sphere material, water temperature and time

This paper outlines the theory and procedures for calibrating an echo integration acoustic system with a standard sphere. It presents the results of an extensive calibration of a Simrad EK500 scientific echosounder with a 120 kHz split-beam transducer in a refrigerated 10 m deep tank. Calibration parameters are studied in relation to sphere material (WC and Cu), water temperature (0.5 to 5.5°C), transmitted pulse length (0.1, 0.3 and 1.0 ms), target depth (0.8 to 7.5 m), and time (149 days). The total range in TS gain, including the effects of temperature, standard sphere and time, is 2.9 dB. Gain values calibrated with the Cu sphere are, with the exception of the long pulse length, lower than those from the WC sphere. The general trends are consistent between SA gain and TS gain. The total spread in SA gain, including the effects of temperature, standard sphere and time is 2.1 dB. Thus, the accuracy of the standard sphere as a reference TS value, the pulse length, the water temperature range, and equipment instabilities during the duration of a survey can contribute significant errors to the accuracy and precision of an echo integration acoustic survey. To minimise these effects, the TS gain and SA gain parameters should be meticulously measured, measured frequently, and matched to the pulse length used and the water temperature in the survey area.

Siegel, V. (1992), Review of length-weight relationships for Antarctic krill

Length-weight relationships for krill, Euphausia superba (as well as relationships for other Antarctic euphausiid species), are listed for ash-free dry weight, dry weight, and wet weight. The accuracy of length-weight relationship calculations is improved when information on sex and dominant maturity stages is taken into account. The influence of seasonal changes on length-weight relationship parameters is discussed. Recommendations for possible uses of reviewed length-weight relationships are presented.

Trathan, P.N., D.J. Agnew, D.G.M. Miller, J.L. Watkins, I. Everson, M.R. Thorley, E. Murphy, A.W.A. Murray and C. Goss (1992), Krill biomass in Area 48 and Area 58: recalculations of FIBEX data

FIBEX acoustic and length frequency data held in the BIOMASS database were used to provide estimates of mean density and biomass for the Indian Ocean sector and the West Atlantic sector as well as for FAO Statistical Area 41, and CCAMLR Subareas 48.1, 48.2, 48.3, 48.6 and Division 58.4.2. Density estimates were calculated using the target strength relationships used at the original FIBEX acoustic workshop. Estimates for the different areas were also calculated using the target strength relationships of Green et al. (1990). The new estimates were on average 4.76 times larger than the old estimates for those cruises (seven out of the nine considered) that used an echosounder frequency of 120 kHz.

Ichii, T., H. Ishii and M. Naganobu (1992), Abundance, size and maturity of krill (Euphausia superba) in the krill fishing ground of Subarea 48.1 during the 1990/91 austral summer

Acoustic and net sampling surveys for krill were conducted in the krill fishing ground north of the South Shetland Islands from 18 January to 3 February 1991. Distinct offshore-inshore heterogeneities in abundance and maturity of krill were observed. The survey area was divided into four zones: oceanic, slope frontal, neritic and nearshore zones. The mean density of krill was low in the oceanic zone (8.5 g/m²), intermediate in the frontal (37.3 g/m²) and neritic (28.1 g/m²) zones, and extremely high in the nearshore zone (134.7 g/m²). The last zone corresponds to the shelf break: or the shelf area where topographic eddies were generated, suggesting that hydrodynamic convergence might be responsible for accumulation of krill in this zone. The total biomass over the survey area was estimated to be 1.59±0.45 million tonnes (95% confidence limit), of which 1.22±0.42 million tonnes was concentrated in the fishing ground (frontal + neritic + nearshore zones). Information from other studies indicated that krill biomass in this region had been lower than expected until early February 1991. As for maturity stages of krill, spawning krill (modal body length 49 mm) were dominant in the oceanic and frontal zones, whereas less mature krill (modal length 45 mm) dominated in the neritic and nearshore zones. Juveniles, which were scarce in the survey described, were found restricted mainly to the nearshore zone. Gravid females were exceedingly abundant in the slope frontal zone, having a mean density of 23.9 g/m² (411 000 tonnes), as contrasted with a low 3.7 g/m² (163000 tonnes) in the oceanic zone. Gravid females were nearly absent in the neritic and nearshore zones. This indicates that slope frontal features may be important for the formation of favourable conditions for krill spawning.

Vagin, A.V., R.R. Makarov and L.L. Menshenina (1992), Diurnal variations in biological characteristics of krill, Euphausia superba Dana, to the west of the South Orkney Islands, 24 March to 18 June 1990 - based on data reported by a biologist-observer

Investigations of diurnal variations in the size composition of Euphausia superba were carried out on the commercial trawler Grigory Kovtun near the South Orkney Islands from March to June 1990. Observations at six daily stations were carried out in various locations inside the fishing area. Each station consisted of a series of catches made using a standard commercial trawl (9 to 12 tows per station, over one day). An increase in the total average size of animals caught in periods of light or darkness was noted at several stations. Increases in the proportion of males in the catch, as well as an increased difference between average length of males and females in the layer fished were indicative of these variations. Diurnal variations of females’ size composition were usually less evident. These changes were related to diurnal vertical migrations of krill which were noted on echo sounder recordings. Trawling depths usually corresponded to the depth of the largest concentration of krill. Within swarms males initiate diurnal vertical migrations. In the absence of diurnal migrations (particularly late in the season), diurnal variations in size composition of krill catches were less evident or non-existent. The significance of these observations in relation to krill size composition data obtained from standard surveys (only one sample of krill per station) is discussed. A gradual decrease in the average size of krill from the end of March to June was observed in the fishing area. The following three causes of the observed variations in krill composition are considered: (i) post-spawning mortality of large specimens; (ii) body shrinkage of krill due to a decrease in food availability; and (iii) size selectivity of krill by commercial fisheries.

Mujica R., A., E. Acuña S. and A. Rivera O. (1992), Krill population biology during the 1991 Chilean Antarctic krill fishery

Population biology of Antarctic krill, Euphausia superba, was studied from samples taken in 1991 during krill fishing operations around the South Shetland Islands on board the Chilean factory vessel Kirishima. Hauls were made using a commercial midwater trawl (mouth area approximately 40 x 40 m and mesh size from 1.5 to 3 cm). Two subsamples of 100 specimens each were taken from 50 samples and analysed. The fishing ground was divided into two areas: Area A, north of the South Shetland Islands; and Area B, north of Elephant Island. The samples were grouped by time of capture: daytime, twilight and night-time. The specimens were measured (total length, TL) to the nearest millimetre and weighed (wet weight) to the nearest 0.01 g. Mean catch-per-hour and mean catch-per-towing time were determined from a total of 419 hauls. In Area A, a unimodal size frequency distribution was found; the size range was between 30 and 55 mm TL, with a mean TL of 45 mm for females and 48 mm for males. A very weak mode for juvenile specimens between 26 and 36 mm TL was also found. The sex composition was 65.1% females, 34.4% males and 1.4% juveniles. Of the females sampled, 25.2% bore spermatophores. Although the smallest specimen found with a spermatophore had a 36.5 mm TL, 80% of the females with spermatophores had a TL larger than 45 mm. In Area B, a bimodal size frequency distribution and a larger size range were found, with one mode between 32 and 55 mm TL (mean length 43 mm for females and 46 mm for males), and the other (modal length 32 mm) for juvenile specimens between 20 and 39 mm TL. Females comprised 47.1%, males 40% and juveniles 12.9%. Of the females sampled, 27.1% bore spermatophores, with a size range between 35.4 and 56 mm TL, although 80% of them had a TL larger than 45 mm. The size frequency distribution showed no significant differences between the three time periods. However, when the sex composition is considered, males are more abundant in night-time catches while females are more abundant during daytime catches, thus showing a different trend. Considering all catches, the yield in terms of tonnes-per-mile and tonnes-per-hour was higher during the daytime than during twilight and night in both fishing areas. These daytime catches were also made at consistently greater depths. Research was financed by INACH (Chilean Antarctic Institute).

Watters, G. and R.P. Hewitt (1992), Alternative methods for determining subarea or local area catch limits for krill in Statistical Area 48

CCAMLR Conservation Measure 32/X sets a 1.5 million tonne precautionary catch limit on krill (Euphausia superba) in Statistical Area 48. The measure also implies an application in future of precautionary limits to subareas or local areas of this area. Nine alternative methods of determining subarea or local area krill catch limits are evaluated relative to six criteria: (i) the degree to which information on biological relationships is considered; (ii) the cost of data collection; (iii) the reliability of required information; (iv) the ease of enforcement; (v) the effects on current fishing patterns; and (vi) the potential for delay in implementing the alternative. An alternative is less likely to adversely impact dependent species (e.g., penguins and seals) if the ecological relationships between krill and their predators are explicitly considered and the potential for delayed implementation is low. Therefore, we consider the following trade-off to be important: choosing a biologically explicit alternative and delaying implementation, or choosing a biologically unrealistic alternative and implementing a management scheme immediately. We recognise that other trade-offs may be equally important. Alternatives that allocate the 1.5 million tonne limit by evenly dividing the catch among subareas or by using historical catches to set limits can be categorised as having a low potential for delaying implementation, but they ignore information on biological relationships. Alternatives based on protective zones, critical periods, predator censuses, and predator-prey models include large amounts of biological information, but may not be practical in the near future. Alternatives based on continental shelf area, simple pulse fishing, and krill surveys are not biologically explicit and result in delayed implementation. None of the alternatives are categorised as being both biologically explicit and immediately available for implementation. However, two of the alternatives (i.e., protective zones and critical periods) are unsatisfactory only because they would alter current fishing patterns. These two alternatives could be implemented immediately if the CCAMLR Member nations are willing to tolerate changes in current fishing patterns.

Marschoff, E. and B. Gonzalez (1992), Homogeneity of Adélie penguins as krill samplers

A nested ANOVA design was used to measure the variance component due to differences between individual Adélie penguins in the length of krill eaten, using data from Esperanza Bay. The variance component -0.26 was not significantly different from zero (F = 0.093; P = 0.54).This finding supports the argument for using individual penguins to estimate parameters of the prey population without discriminating by sex, weight or other factors pertaining to the predator.

Agnew, D.J. (1992), Can we use discriminant function analysis to sex penguins prior to calculating an index of a morphometric characteristic?

In sexually dimorphic species, morphometric characteristics have separate distributions for males and females, and these often overlap. Whilst discriminant analysis can be used to determine the sex of individuals, it is only able to correctly sex a certain proportion of birds. Two overlapping normal distributions are used to show that there is a difference between the real mean characteristic for a sex, and the apparent mean derived by sexing the birds using discriminant analysis.

When discriminant functions are able to correctly determine the sex of birds with greater than 80% success, the difference between the true and apparent mean is likely to be undetectable when fewer than 600 birds are sampled.

Therefore, under most normal sampling regimes a discriminant function with greater than 80% success may be used to derive statistically robust estimates of male and female characteristics.

Combining all data for both sexes is considered as a procedure for avoiding the necessity of sex determination. However, uncertainty in sex ratios can lead to considerable Type I and Type II errors. Lack of knowledge about the sex ratio between years makes combining the data a very doubtful procedure and use of a discriminant function to determine sex is recommended as being most practically robust.

Ichii, T., H. Ishii, J.L. Bengtson, P. Boveng, J.K. Jansen and M. Naganobu (1992), Differences in distribution and population structure of krill (Euphausia superba) between penguin and fur seal foraging areas near Seal Island

Shipboard tracking studies of krill-eating predators (penguins and female fur seals) near Seal Island were conducted to identify and evaluate their foraging areas during early January 1990/91. Penguin foraging areas were found in inshore regions where krill frequently occurred but higher density areas of krill (~250 g/m²) were rather limited. In contrast, fur seal foraging areas were found in offshore regions where krill occurred only occasionally but in large aggregations (surface length about 2 to 3 km) of higher densities (~250 g/m²). In the inshore foraging areas krill undertake diurnal vertical migrations, tending to be at a deeper range from 50 to 100 m in the day while at a shallower range from 20 to 50 m at night. In the offshore foraging areas krill do not undertake any diurnal vertical migrations, staying close to the surface throughout the day. With regard to body size and maturity of krill in the inshore foraging areas, middle-sized krill (modal length 43 mm), which consisted mainly of non-gravid krill, were dominant with occasional occurrences of juveniles (modal length 21 mm). In contrast, in the offshore foraging areas large krill (modal length 47 mm) were dominant, the majority of which were gravid females. Thus, horizontal and vertical distributions and population structure of krill were totally different in the foraging areas of penguins and fur seals. The reasons why fur seals chose offshore foraging areas instead of inshore foraging areas are discussed.

Agnew, D.J. (1992), Distribution of krill (Euphausia superba Dana) catches in the South Shetlands and South Orkneys

A set of concentric zones of 20 km width was defined around selected colonies of penguins distributed around the coasts of the South Shetland and South Orkney Islands. Krill catches in these zones are shown to have a consistent pattern in Subarea 48.1 but an unpredictable distribution in Subarea 48.2, probably as a result of more variable hydrographic conditions. About 50% of the catch in Subarea 48.1 from December to March was taken within 40 km of the coast, and 90% within 80 km in all years 1988 to 1991. In 1987, 1988 and 1991, 75% of the catch in Subarea 48.2 between December and March was taken within 80 km of colonies in the South Orkneys. Estimates of consumption rates, foraging ranges and population sizes from the literature are used to show that for some years catches within 100 km of predator colonies between December and March may be up to 45% of the land-based predator consumption. Whilst the normal ratio of catch to consumption is relatively low (less than 27%), and the fishery may have to increase by a factor of 2 or 3 before ratios of catch to consumption approach maximum sustainable levels, any competition between the fishery and predators as a result of large increases in catch is likely to emerge in areas of high overlap between predators and the fishery earlier than would be expected considering the fishery as a whole.

Sushin, V.A. and A.S. Myskov (1992), Location and intensity of the Soviet krill fishery in the Elephant Island area (South Shetland Islands), 1988/89

This paper analyses fine-scale data from the Soviet krill fishery off Elephant Island (Subarea 48.1) between 59°-62°S and 53°-57°W during the period from 21 November 1988 to 25 March 1989. Although the total catch of the USSR in this season reached a maximum, for the last nine seasons the total fishing intensity by the USSR around Elephant Island has been low. In 1988/89 only one standard fishing vessel operated in the area over 40% of the time. The highest catch-per-unit-effort was observed in January 1989 (7.7 tonnes per hour of trawling on average), and the lowest in November 1988 (3.5 tonnes per hour of trawling on average). Fishing strategy in the Elephant Island area conforms to the following simple pattern: (i) vessels enter the island near-shore zone (north of Elephant Island) and start searching for krill concentrations; (ii) krill concentrations are fished and followed as they drift from the island with the current; and (iii) vessels return to position (i) when aggregations are dispersed or lost due to storms and other factors. The velocity of the northeast drift of krill concentrations, calculated on the basis of vessel relocation, was from 9.7 to 11.1 km/day (11 to 13 cm/sec). An analysis of the location of fishing grounds by five-day periods showed that the areas in which the fleet operated overlap a minor part of the foraging zones of krill predators. Based on this, and taking into account the low fishing intensity, it was concluded that the current krill fishery does not significantly affect krill-eating seals and birds.

Kerry, K.R., J.R. Clarke and G.D. Else (1992), The foraging range of Adélie penguins at Béchervaise Island, Mac. Robertson Land, Antarctica, and its overlap with the krill fishery

The foraging ranges of six female and four male Adélie penguins breeding at Béchervaise Island near Mawson Station (Mac. Robertson Land) were determined by satellite tracking using the ARGOS system. Birds were tracked over four foraging trips (two females and four males) during the incubation period (November to December 1991) and 17 trips (four females and two males) throughout January 1992 when birds were feeding chicks. Most birds made foraging trips to the continental shelf break (1 000 m isobath) approximately 110 km distant at its closest point. Birds feeding chicks also made journeys of one to two days ranging up to 12 km after 17 January when the sea became ice free to the coast. Concentrations of krill, Euphausia superba, which have in the past been the subject of a fishery, occur along the shelf break zone where the birds were foraging. There is potential for overlap between the foraging range of Adélie penguins breeding along the Mac. Robertson Land Coast (approximately 150 000 pairs) and any future harvest of krill in the region. The foraging range of the birds at Béchervaise Island considerably exceeds the 15 to 50 km determined for birds in the South Shetland and South Orkney Islands and reflects the distance offshore of krill, one of their major food sources.

Constable, A.J. (1992), CCAMLR ecosystem monitoring and a feedback management procedure for krill

The CCAMLR Ecosystem Monitoring Program has been developing a technique which might detect short-term declines in land-based predator performance (e.g., reproductive performance) that may be attributable to loss of prey through fishing activities. The principal fishery in the CCAMLR Convention Area is the krill fishery and this paper examines ways in which the information being obtained from the Ecosystem Monitoring Program might be incorporated into a feedback management strategy for this fishery.