Candy, S.G and A.J. Constable (2008), An integrated stock assessment for the Patagonian toothfish (Dissostichus eleginoides) for the Heard and McDonald Islands using CASAL

An integrated stock assessment for the Patagonian toothfish (Dissostichus eleginoides) for the Heard and McDonald Islands (CCAMLR Division 58.5.2), using CASAL and data consisting of multi-year random stratified trawl survey (RSTS) abundance estimates by length bin, commercial catch-at-length data, standardised CPUE series for the trawl grounds, and tag releases and recoveries by length bin, is described. The annual surveys are spatially representative of the main plateau, where juvenile fish are found, but are of relatively low intensity in effort compared to the commercial shots. In contrast, the commercial shots are very restricted in space, consisting of three main grounds. The model implemented in CASAL is a simple, single-sex, single-area population model, but spatial complexity in the fishery was modelled using separate fishing selectivity functions for each ground by gear (trawl and longline) combination. Various combinations of dataset weighting were investigated using haul-level estimated effective sample sizes with, additionally, iteratively estimated process error for the catch-at-length data and the inclusion versus exclusion of the tag data. A key uncertainty is the number of ages fully selected by the main survey series. With all the data included in the model, age-4 and 5 fish are fully selected, whereas when the survey data have greater influence and without the tag data, the selectivity of these ages was reduced. The method of quantifying process error used in this assessment removes ‘systematic lack-of-fit’ (SLOF) from population/fishery model predictions. Extension of this method of estimating process error to RSTS abundance data and commercial catch CPUE data is given, but incorporation of process error for the RSTS data was not considered appropriate, since SLOF could not be removed to an acceptable degree. The issues concerning the effect of the tension between survey data and mark–recapture data on parameter estimation are discussed.

Hanchet, S.M., G.J. Rickard, J.M. Fenaughty, A. Dunn and M.J. Williams (2008), A hypothetical life cycle for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region

Aspects of the reproduction, size distribution and movements of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region are reviewed. Based on the presumed location and timing of spawning and the probable early life-history characteristics of toothfish, the drift of eggs and larvae over a 6–24 month period were investigated using an oceanic circulation model linked to the high-resolution global environmental model (HiGEM). Model outputs indicated that the locations of toothfish larvae after an 18–24 month period were moderately consistent with the distribution of the smallest toothfish taken in the toothfish fishery.

The hypothesis presented is that D. mawsoni in CCAMLR Subareas 88.1 and 88.2 spawn mainly on the ridges and banks of the Pacific-Antarctic ridge to the north and east of the Ross Sea. The spawning appears to take place during the austral winter and spring. Depending on the exact location of spawning, eggs and larvae become entrained by the Ross Sea gyres, and may move west, settling out around the Balleny Islands and adjacent Antarctic continental shelf; south onto the Ross Sea shelf; or eastwards with the eastern Ross Sea gyre, settling out along the continental slope and shelf to the east of the Ross Sea in Subarea 88.2. As the juveniles grow in size, they move west back towards the Ross Sea shelf and then move out into deeper water. As they mature, the fish gradually move deeper out onto the continental slope where they gain condition before undergoing a northwards spawning migration to the Pacific-Antarctic Ridge to start the cycle again. Toothfish probably remain in the northern area for 6–18 months before migrating back to the slope to regain condition.

McKinlay, J.P., D.C. Welsford, A.J. Constable and G.B. Nowara (2008), An assessment of the exploratory fishery for Dissostichus spp. on BANZARE Bank (CCAMLR Division 58.4.3b) based on fine-scale catch and effort data

In 2006, the CCAMLR Scientific Committee expressed concern as to the status of the stock on which the exploratory Dissostichus spp. fishery on BANZARE Bank (CCAMLR Division 58.4.3b) was based. This paper presents a summary of fine-scale catch and effort data held by CCAMLR for BANZARE Bank, as well as descriptive analyses of biological data submitted by scientific observers on board vessels in the fishery. Results of a Leslie depletion analysis of stratified catch and effort data indicate that there is strong evidence for depletion of toothfish at the scale of individual fishing grounds in the 2004/05 and 2005/06 seasons. This result, coupled with high estimated levels of illegal, unreported and unregulated (IUU) fishing, and evidence of poor recent recruitment, indicates that the stock is likely to have been overfished. Several inconsistencies are noted between historical data and those submitted in the 2006/07 season, with Patagonian toothfish (D. eleginoides) reportedly replacing Antarctic toothfish (D. mawsoni) as the dominant toothfish species in catches in one ground for the first time. Incomplete reporting of biological information for by-catch species by on-board scientific observers was noted, limiting meaningful analysis of patterns in by-catch catch and effort. Further research, particularly research surveys, would greatly assist with assessing the status and stock structure of Dissostichus spp. in this area. The potential for similar analyses in other exploratory toothfish fisheries should be investigated, as this study indicates that severe depletion can occur in the space of just two or three seasons.

Moreno, C.A., R. Castro, L.J. Mújica and P. Reyes (2008), Significant conservation benefits obtained from the use of a new fishing gear in the Chilean Patagonian toothfish fishery

A new fishing technique, adapted from the artisanal trotline fishery for Patagonian toothfish (Dissostichus eleginoides) in Chile is described. The modified artisanal system, which includes a net sleeve that is placed on secondary vertical lines, has practically eliminated depredation of fish by killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus). The performance of this fishing technique with regard to seabird mortality and depredation by sperm and killer whales on fish catch rate was assessed during September–December 2006. The results were then compared with similar data obtained in 2002 in the same fleet in the same fishing grounds prior to the implementation of the modification. The number of seabirds killed in 2002 was 1 542 compared to zero in 2006; there was also a reduction in depredation of the catch from a maximum of 5% in 2002 to a maximum of 0.36% of the total catch in 2006. The fishers who developed the net sleeve modification called it ‘cachalotera’ (from ‘cachalote’ meaning sperm whale in Spanish). The term ‘Chilean longline’ is preferred in this paper because it was developed in 2005 in the Chilean toothfish fishery in the Magellan region.

Robertson, G., C.A. Moreno, E. Gutiérrez, S.G. Candy, E.F. Melvin and J.P. Seco Pon (2008), Line weights of constant mass (and sink rates) for Spanish system Patagonian toothfish longline vessels

CCAMLR Conservation Measure 25-02 requires Spanish-system longline vessels to attach 8.5 kg weights at 40 m intervals on longlines to minimise interactions with seabirds. The weights typically used are collections of rocks enclosed in netting bags. During fishing operations the netting bags abrade on the seabed causing rocks to be lost and the weights to become progressively lighter, requiring ongoing repair. This problem can be solved by use of hydrodynamically shaped (e.g. torpedo-shaped) steel weights, which are smaller for equivalent weight, and require no maintenance. An experiment was conducted on a Spanish-system longline vessel to determine the relationship between the sink rates of longlines equipped with bags of rocks (4, 6 and 8 kg) and those with steel weights of equivalent masses. The purpose of the experiment was to provide vessel operators with the option of substituting steel weights for rock weights while remaining in compliance with the sink rates associated with the line-weighting provisions of Conservation Measure 25-02. Both the Spanish system and the newly developed Chilean longline (a modified version of the former method to avoid fish loss by toothed whales) were tested in the experiment. Spanish-system longlines with 8 kg rock weights every 40 m averaged 0.22 m s^–1 to 2 m depth, which would be equal to, or exceeded by, lines with 5 kg steel weights. Sink rates of Chilean longlines greatly exceeded those of the Spanish system, ranging from 0.68 m s^–1 (4 kg rocks) to 1.31 m s^–1 (8 kg steel) in the shallow depth ranges. Hydrodynamically shaped steel weights weighing 5 kg would be an appropriate substitute for 8.5 kg rock weights irrespective of fishing method.

Robertson, G., J. Williamson, M. McNeill, S.G. Candy and N. Smith (2008), Seabird by-catch by autoline vessels: do line setters increase the sink rate of integrated weight longlines?

Line setters are used with integrated weight (IW) (50 g m^–1 lead core) longlines by some autoline vessels in the Kerguelen and Crozet Islands Patagonian toothfish (Dissostichus eleginoides) fisheries to deter seabirds, ostensibly by expediting gear sink rates. A trial was conducted in the Ross Sea to determine the effectiveness of line setters in increasing the sink rates of IW longlines. Time-depth recorders were deployed on lines set with and without a line setter using a paired-treatment design. Sink rates of longlines set with and without a line setter were identical, including in the first few metres of the water column where seabird interactions are likely to be most intense. Longlines deployed with the line setter entered the water several metres closer to the stern of the vessel and commenced sinking sooner, thus increasing slightly (<0.5 m) the depth of longlines for given distances astern. This increase in depth is minor and unlikely to result in substantial reductions in interactions between longlines and seabirds in the Kerguelen and Crozet fisheries.

Candy, S.G (2008), Estimation of effective sample size for catch-at-age and catch-at-length data using simulated data from the Dirichlet-multinomial distribution

The incorporation of ‘effective sample size’ (ESS) in integrated assessments is an approximate but simple way of modelling the distribution of catch-at-age or catch-at-length frequencies using a multinomial likelihood when there is extra-multinomial heterogeneity. Accurate estimation of ESS for catch-frequency data for each fishery and fishing year is important for such assessments, and this issue is studied using simulation. Between-haul heterogeneity within fishing year was simulated using samples from the Dirichlet-multinomial (D-M) distribution, with marginal class probabilities generated using a simple age-structured model incorporating fishing selectivity. Four methods of estimation of effective sample size were compared using this simulation model and its variants. One of the methods is based on the lack-of-fit of predictions of class probabilities using aggregate year-level frequencies. The other three estimators use the haul-level frequencies, including a method based on an approximate profile maximum likelihood estimate (PMLE) of the D-M dispersion parameter. The remaining two estimators based on haul-level frequencies are derived from models for the empirical coefficient of variation (CV) in the proportions, with one being based on an existing CV model used for CCAMLR fisheries while the other is a new method. The methods that use haul-level frequencies gave accurate estimators of an ESS that is appropriate for haul-level heterogeneity with increasing accuracy in the following order: (i) the estimator based on the existing CV model; (ii) that based on the new CV model; and (iii) that based on the PMLE. The year-level method gave very inaccurate estimates of this ESS with relative mean square error two orders of magnitude worse than the best haul-level method.

To account for process error in the calculation of the ESS, the lack of fit of the age-structured model in predicting class/bin by year frequencies is used to obtain a single, across-years, over-dispersion parameter. The ESS is then rescaled by dividing by the over-dispersion parameter, and the model refitted, giving a two-step iterative procedure. The ESS will be over-corrected if there is a systematic component to the lack of fit. A simple generic model of systematic lack-of-fit (SLOF) is presented, and its performance, in terms of providing unbiased estimates of ESS when SLOF is either present or absent, is studied using perturbations of the age-structured model. These perturbations consisted of either systematic or random variation across years in one of the selectivity function parameters and similarly for the mortality rate parameter when combined with systematic or random variation in recruitment. The SLOF model substantially reduced the bias when SLOF was present and is useful when its source is not clear or cannot be rectified by changing the underlying age-structured assessment model.

Sutton, C.P., M.J. Manning, D.W. Stevens and P.M. Marriott (2008), Biological parameters for icefish (Chionobathyscus dewitti) in the Ross Sea, Antarctica

Icefish (Channichthyidae) specimens were randomly collected by observers during the 2005/06 fishing season. These observers were placed on board three longline vessels targeting Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea (CCAMLR Subareas 88.1 and 88.2). Biological data from 303 specimens were collected. These data included species identification, fish length, weight, sex, meristics, reproductive biology, diet and age estimation. All of the icefish sampled were identified as Chionobathyscus dewitti, and showed no significant difference in sex ratio. Meristic, diet and age data were consistent with previous research.

Regression equations for converting standard length to total length and for defining length–weight relationships were calculated and presented for both male and female fish. Gonad maturity stage data showed that most fish were either immature or resting (mature). Gonadosomatic indices (GSIs) were calculated and plotted against sample month. There was a weak positive trend in GSI between December and February, but this was limited, probably due to the short temporal distribution of the data. Length-at-maturity and age-at-maturity ogives indicated that 50% of the fish sampled were mature at about 340–360 mm total length (TL) and about 3–4 years of age, and that 95% were mature at about 370–400 mm TL and 6–8 years of age.

Counts of growth zones in sectioned otoliths were used to determine ages and von Bertalanffy growth parameters. Fish growth was rapid for both sexes, and females approached a significantly larger mean asymptotic maximum size than males. Maximum ages of 8 and 11 years were obtained for male and female fish respectively.

Diet analysis showed most icefish stomachs were empty and the few prey items recovered were generally in advanced stages of digestion. This may be due to regurgitation of prey during capture.

Lockhart, S.J. and C.D. Jones (2008), Biogeographic patterns of benthic invertebrate megafauna on shelf areas within the Southern Ocean Atlantic sector

Bioregionalisation of Antarctic and Southern Ocean shelf communities ideally incorporates a range of data on physical, environmental and biological properties. Analysis of the benthic invertebrate megafaunal assemblages of shelf habitats within the Atlantic sector, from scientific survey trawl catches, reveals distributional patterns. For the northern Antarctic Peninsula and the South Shetland Islands, the data indicate a two-layered pattern based on standardised total biomass data and the composition of phyla that contribute to that biomass. An examination of physical oceanographic data reveals a pattern of shelf faunal zonation: the benthic invertebrate communities on the northern shelves of the South Shetland Islands and the northern Antarctic Peninsula can apparently be separated into zones based on the physical properties of the Antarctic Circumpolar Current and the Weddell water masses that meet and mix in this region. Evident at smaller spatial scales are the effects of disturbance regimes, whether by iceberg scouring or historical commercial bottom trawling. Patterns of benthic invertebrate biomass are also described for the South Orkney Islands, as well as general patterns of phylum-level composition for South Georgia, the South Sandwich and Bouvet Islands. These regions are generally echinoderm-dominated, compared to the hexactinellid sponge-dominated northern Antarctic Peninsula region.