CCAMLR

An updated Antarctic toothfish (in the Ross Sea) stock assessment was done by means of the TISVPA model (2006 data on catch-at-age and cpue and the data on tag recaptures were included). All the three sources of information taken separately (and together) indicate historical increase in stock biomass, perhaps, due to development (broadening) of the fishery. The results show that, according to the Constable & de la Mare (1996) decision rules, annual catches of 23 000 tonnes per year are suitable.

In 2006 the CCAMLR scientific committee noted several features of exploratory Dissostichus fisheries in the southern Indian Ocean (58.4) which gave cause for concern as to the status of the resource in this area, and the lack of a scientific basis for setting catch limits in these areas (SC-CAMLR XXV, paragraphs 4.184-4.192).
In its management advice for this and other exploratory fisheries, the Scientific Committee requested urgent consideration by Members on of methods for collecting of data and of assessing these stocks. We outline a methodology to assess the BANZARE fishery including:
• Identification of grounds through analysis of spatial pattern in catch and effort;
• Construction of standardised CPUE series for each ground;
• DeLury/Leslie analysis of standardised CPUE of Dissostichus and major bycatch groups to provide initial estimates of biomass and rates of depletion in each ground until such time as they can be improved; and
• Estimation of the proportion of the stock which can be harvested (?) to satisfy the CCAMLR decision rules.
• Analysis of relative catch rates of bycatch groups and target species
The input data for this assessment is the C2 catch and effort data held by CCAMLR for the fishery in this Division since it began in 2003/04.
The results presented in this paper indicate that this methodology will produce an improved understanding of the trends in this fishery and greatly assist in developing management advice for the stocks of Dissostichus and major bycatch species on BANZARE Bank.
We welcome recommendations from WG-SAM as to the most productive way forward to an assessment for this Division to be considered at CCAMLR XXVI.

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 in age class or length class frequencies. The ESS applied within the definition of the negative log-likelihood contribution to the objective function in CASAL determines the implicit weight given to the commercial catch-at-age or catch-at-length frequency data relative to the other types of data used in integrated assessments of toothfish stocks. An appropriate and accurate estimate of the ESS for catch frequency data for each fishery and fishing year is therefore 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. Either between-year “process” or “systematic” error in these probabilities was also generated by varying one of the selectivity function parameters across years randomly or linearly, respectively. Five alternative methods of estimation of effective sample size were compared using this simulation model. Two existing methods are 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 a conditional profile maximum likelihood estimate of the D-M dispersion parameter. This last method generally gave the best estimator of an ESS that is appropriate for haul-level heterogeneity with another of the haul-level methods giving similar estimates. The year-level methods gave very inaccurate estimates of this ESS when process error variance was set to zero with relative mean square error an order of magnitude worse than the best two haul-level methods. When process error was incorporated one of the year-level methods gave reduced estimates of ESS. An appropriate distributional model that incorporates process error in addition to haul-level heterogeneity while giving a marginal variance relationship which allows an ESS to be defined does not appear to be available so heuristic arguments and simulation results are used to discuss the issue of estimating ESS in the presence of process error. It is shown that care should be taken to avoid year-to-year model lack-of-fit due to systematic deviations in observed versus predicted class frequencies being mistaken for process error and used to reduce the ESS inappropriately.

In this paper, we address a number of aspects of the model inputs and parameters of the Antarctic toothfish stock assessment for the Ross Sea fishery. In particular we review catch history, length-weight relationships, catch-at-length and catch-at age. In addition, we report some preliminary model runs that investigate the sensitivity of the 2006 stock assessment to changes in these model inputs and parameters.
Tree-regression methods were used to investigate the areal structure of the length distribution of Antarctic toothfish. While tree-regressions suggested strong evidence of a high degree of small-scale areal complexity, we were unable to provide a stratification that resulted in improved or consistent patterns in length frequencies over the duration of the fishery. Including terms for nation, vessel, or vessel type did not provide any additional information as these tended to be highly correlated with the location variables.
The catch and CPUE indices for the Ross Sea Antarctic toothfish fishery were updated, as are some modelling parameters, and methods for calculating age- and length-frequencies. Most of these changes did not have a significant impact on the assessment results.
We also provide an update of the numbers of fish scanned at length by New Zealand vessels, and the numbers of tagged fish recaptured. Inclusion of observations of the 2006 fish recaptured in 2007 had the greatest impact on the assessment model results. Dunn et al. (2007) noted that the locations of the 2007 recaptures were highly aggregated and were mostly located on four key locations in the Ross Sea, and most had moved only short distances. This confirms the concern that the key uncertainty underlying the current model is the impact of movements and spatial structure in the Antarctic toothfish population. In particular, the level and nature of the bias from non-homogeneous mixing assumptions of tagged fish.

Descriptive analyses of the toothfish tagging programme carried out in Subareas 88.1 and 88.2 since 2001 are updated. The paper provides a preliminary update of the tag-release and tag-recapture data that were presented at the October 2006 meeting of WG-FSA by including data from New Zealand vessels and preliminary data for other vessels that fished in 2007.
Release and recapture data that previously were unavailable for about half of the non-New Zealand vessels for 2004 are now available and described in this paper for the first time. Overall, a reported total of 12 177 Antarctic toothfish have been released and 333 recaptured, and 859 Patagonian toothfish released and 29 recaptured since 2001.
The number of tags recaptured in the Ross Sea in 2007 by New Zealand vessels was the highest annual recapture to date and double the number caught in 2006, although the nature of these recaptures suggests that assumptions of homogeneous mixing may need to be investigated. For the first time, long distance movements of Antarctic toothfish were observed from fish tagged by fishing vessels. A total of four fish moved significant distances from the slope fisheries in SSRUs 88.1H, 88.1I, and 88.1K to Terra Nova Bay in SSRU 88.1J. There was also some evidence that more fish are recaptured after a longer time at liberty on the slope than in the North.
However, we note that data from the 2007 season for the non-New Zealand vessels were incomplete at the time of this analyses and will need to be updated in future analyses.

This report presents the data and preliminary results from developmental model for Antarctic skates in SSRUs 88.1H, 88.1I, 88.1J, & 88.1K of the Ross Sea. The developmental model attempted to create a catch history of all skates and rays in the Ross Sea, and integrate these data with the available observational data (including tag-recapture data) into a single integrated stock assessment model.
We conclude that aspects of the catch history were very uncertain, including the species composition, the weight and number of skates caught, the proportion discarded, and the survival of those tagged or discarded. The size composition of the commercial catch was also very uncertain because of the low numbers sampled each year. Most aspects of the tagging data were also uncertain including the actual numbers of skates released, the initial mortality of tagged skate, the tag loss rate, and the numbers of skates scanned for tags. While updated summaries of the numbers of skate tag releases and recaptures have been reported, these data are still preliminary, and further work is required. Lastly, there is great uncertainty over the biological parameters including age and growth, natural mortality, steepness, and size and age at maturity.
The applicability of a general model, such as presented here, to a multi-species catch has not been investigated. While is it plausible that a general model may be adequate if the productivity parameters of the different species of skates and rays are similar, we conclude that additional research is required to investigate the usefulness of such models. We also make a number of suggestions for areas where better data are required. These include recommending work that would improve species identification, increasing the detection rate of tagged skates, increasing the number of skates measured and sexed, validating estimates of age and growth, revising the skate tagging protocols, and undertaking more extensive skate survivorship experiments.

Antarctic krill, Euphausia superba Dana, has a heterogeneous circumpolar distribution in the Southern Ocean. Krill have a close association with sea ice which provides access to a critical food source and shelter, particularly in the early life stages. Advective modelling of transport pathways of krill have until now been on regional scales and have not taken explicit account of sea ice. Here we present Lagrangian modelling studies at the circumpolar scale that include interaction with sea ice. The advection scheme uses ocean velocity output from the Ocean Circulation and Climate Advanced Modelling (OCCAM) project model together with satellite-derived sea ice motion vectors to examine the potential roles of the ocean and sea ice in maintaining the observed circumpolar krill distribution. We show that the Antarctic Coastal Current is likely to be important in generating the large-scale distribution and that sea ice motion can substantially modify the ocean transport pathways, enhancing retention or dispersal depending upon location. Within the major krill region of the Scotia Sea, the effect of temporal variability in both the ocean and sea ice velocity fields is examined. Variability in sea ice motion increases variability of influx to South Georgia, at times concentrating the influx into pulses of arrival. This variability has implications for the ecosystem around the island. The inclusion of sea ice motion leads to the identification of source regions for the South Georgia krill populations additional to those identified when only ocean motion is considered. This study indicates that the circumpolar oceanic circulation and interaction with sea ice is important in determining the large-scale distribution of krill and its associated variability.