CCAMLR

A standardised CPUE analysis was used to determine factors affecting bycatch rates of rattails (Macrourus spp.) and skates in the exploratory fishery for toothfish in the Ross Sea (CCAMLR Subareas 88.1 & 88.2). The analysis was based on fine-scale haul-by-haul (C2) data and observer data from all vessels in the fishery from 1997/98 to 2004/05. The major factors influencing rattail bycatch were vessel, area, and depth. Catch rates of rattails were highest along the shelf edge (SSRUs 88.1E, 88.1I, 88.1K and 88.2E) in depths from 600 to 1000 m, and there was an order of magnitude difference in rattail catch rates between different vessels. Examination of vessel characteristics showed that catch rates of rattails were lower with the Spanish line system than with the autoline system. Russian and Korean vessels had extremely low catch rates compared to other vessels. Standardised CPUE of rattails has shown a general increasing trend since 1998/99, with a peak in 2002/03.
It was not possible to reliably determine factors influencing catch rates of skates from either C2 or observer data because a proportion of skates are cut free and released at the surface and these are not accurately recorded or reported in either dataset. There are a number of inconsistencies within the observer data which need to be resolved before these data can usefully be used for estimating bycatch. These include a consistent definition of a set across the observer and C2 datasets, accurate recording of the number of hooks observed for bycatch, and completion of the L11 form on the fate of skates for all sets.

Age estimates of mackerel icefish from different laboratories obtained during a CCAMLR ototlith exchange programme in the late 1980’s revealed considerable differences between readers when fish older than 3 or 4 years were aged. These differences could not be reconciled at the end of the exchange. A second age determination workshop on mackerel icefish to improve the accuracy of age determination was agreed upon at the last meeting of the ‘Working Group on Fish Stock Assessment’. Our report briefly summarizes the state of knowledge on age determination in mackerel icefish and assesses the validity of ageing. Possible ways forward were described to improve the validity of age determinations of the species.

This report presents results from a desktop study to consider approaches to monitoring and assessing rattails and skates, which are major bycatch species in the exploratory fishery for toothfish in the Ross Sea. We review standardised CPUE analysis, quantitative research longline surveys, experimental manipulation of fishing effort, catch-curve analysis, tagging programmes, bottom trawl surveys, and acoustic surveys.
We recommend that a random bottom trawl survey would be the best approach towards obtaining abundance estimates for rattails and skates in the Ross Sea. The major advantage of this approach is that preliminary stock assessments could be carried out for both species groups after only one successful trawl survey. Simulations have indicated that only 35–40 trawls would be required in the depth range 600–1500 m to obtain a precise estimate of bycatch abundance in the area of highest densities (SSRUs 881E, G, H, I, J, and K). A trawl survey could also be used in conjunction with other methods of monitoring abundance, e.g., skates caught during the trawl survey could be tagged and released, rattails could be aged for catch-curve analysis. The main limitations of this approach is the variable ice cover in the Ross Sea, which may restrict access to some areas, the rough bottom topography, and concerns about the environmental impact of bottom trawling on benthic communities. Tag-recapture experiments for skates and experimental manipulation of fishing effort are alternative methods which show some promise for monitoring abundance.

This report presents risk categorization tables for Macrourus whitsoni and Amblyraja georgiana, which are the major bycatch species in the exploratory fishery for toothfish in the Ross Sea.
Amblyraja georgiana were categorized as risk status 3 – species that are exploited as bycatch, and have a limited reproductive potential, and/or other life history characteristics that make them especially vulnerable to overfishing. The risk to A. georgiana is mitigated due to a CCAMLR programme to cut all skates from longlines whilst still in the water and release them. Macrourus whitsoni were categorized as between risk status 2 and 3 – although life history characteristics may make this species vulnerable to overfishing, catch rates in the toothfish fishery have not declined, juveniles are not selected by the fishery, and comparison of longline and trawl catch rates with other Antarctic areas suggest that population in the Ross Sea may be relatively large.

Juveniles of the macrourid rattail Macrourus whitsoni were collected by the NIWA research vessel Tangaroa during the BioRoss survey of the Western Ross Sea and Balleny Islands, with the purpose of analyzing the otoliths to generate more accurate age estimates for this species. Intensive analysis of otoliths from small specimens greatly increased confidence in the interpretation of the zone structure displayed in the early growth rings. The findings supported the interpretation protocols used in previous work on this species. Von Bertalanffy growth curves were generated and compared to previous results finding no significant differences between the years. Von Bertalanffy parameters for the pooled dataset with unsexed juveniles are L? 76.12, K 0.065 and t0 -0.159 for males and L? 92.03, K 0.055 and t0 0.159 for females. Revised estimates of the mean total length–at– maturity (38.8cm, 46.4cm) and mean age–at–maturity (10.6 years, 13.6 years) are presented for males and females respectively, using a reduced probit model.

During the 2005 fishing season experiments on the survivorship of post-tag toothfish were carried out on 8 different vessels fishing in 48.3. Toothfish were selected for tagging as normal, and then were kept in tanks with varying degrees of seawater replacement for at least 12 hours after tagging. On one vessel fish with a variety of injuries were selected to see if this affected recovery. 395 animals were included in the final analysis, with an overall survivorship of 89%. There were significant differences between vessels, and smaller animals and animals in better initial condition had a slightly higher survivorship.
There are indications that survivorship rates may be lower while observers are learning how to tag effectively and, perhaps, in experiments such as this if handling stress is increased. The results suggest that experienced observers using animals in good condition would normally achieve a survivorship of 95% or more. An assumption of 90% post-tagging survivorship would appear to be an appropriate, conservative, parameter to use in population estimators such as CASAL and mark-recapture.

The derivation of all parameters used in the assessments of toothfish using CASAL and the Petersen mark-recapture estimator in papers WG-FSA-05/16 and 17 are described.

The mark-recapture experiment continued at South Georgia in 2005. In total some 8000 fish have now been tagged in 48.3, and both tagging effort, fishing effort and recaptures were well – distributed over the whole of the fishable grounds in 48.3 this year. The Petersen mark-recapture estimator, which has been applied previously and described in detail by Agnew et al (2004) and Payne et al (2005), was recalculated for Subarea 48.3 based on updated parameter estimates and four different selectivity regimes: the historically used Kirkwood shallow selectivity, the single- and double- fleet selectivities estimates by CASAL (see WG-FSA-05/16), and a selectivity estimated directly from the tagging data.
Estimates from 2004 and 2005 were very similar to each other, whichever selectivity was used: about 62,000 t of vulnerable biomass using the Kirkwood shallow selectivity; 52-55,000 t using the CASAL derived selectivity functions; and 41,000 t using the tag-derived selectivity function. Current vulnerable biomass (BV2005) was (naturally) lower when CASAL determined selectivity and tag-derived selectivity were used, since these selectivity functions are more peaked than the Kirkwood shallow selectivity used in 2004. Estimates of sustainable yield, made adjusting mean recruitment in GYM so that BV2005 corresponded to the current estimates of vulnerable biomass, resulted in consistent estimates of sustainable yield of about 4,800 t, irrespective of the selectivity used.

The CASAL software has been successfully applied to an assessment of the stock of toothfish around South Georgia in Subarea 48.3. Both point estimate (“MPD”) and MCMC runs were carried out, with an extensive range of sensitivity tests and examination of diagnostics.
A baseline assessment model was selected to best represent the fishery and the available data sources. This incorporated two fleets with different selectivities, and fitted to catch length frequencies, tag recapture data, CPUE data, and the survey recruitment series (treated as a relative index). This produced excellent fits to the catch length frequencies, but a poor fit to the early CPUE data, for which a substantial process error CV was estimated (0.39). A possible alternative assessment model was also identified using a single fleet, as it is possible that overall trends in the CPUE series were not fully taken into account in the two fleet assessment. This alternative assessment produced worse fits to the catch length frequencies, and a poor fit to the CPUE data, for which an even higher process error CV was estimated (0.46).
For the baseline assessment incorporating two fleets, the 2005 vulnerable biomass was estimated to be 45,893 t, and the spawning stock biomass was estimated to be 120,360 t, around 67% of its unexploited level. For the alternative single fleet assessment, the 2005 vulnerable biomass was estimated to be 49,943 t, and the spawning stock biomass was estimated to be 100657 t, some 64% of its unexploited level. These estimates of current biomass are somewhat lower than the modified Petersen tagging estimates alone, which were 53,600 t and 55,600 t respectively (Agnew and Payne 2005: WG-FSA-05/17) but the confidence intervals for the estimates overlap.
The current implementation of CASAL is unable to reproduce all the features of GYM projections to estimate long term yields according to the CCAMLR decision rules. Using GY, the estimated long term yield is 4910 t for the baseline two-fleet assessment, while for the one-fleet assessment, the estimated long term yield is 4560 t.

At the request of the Scientific Committee, the Secretariat has prepared a single reference document which provides guidelines for the submission of meeting documents to the Scientific Committee, WG-EMM and WG-FSA (including ad hoc WG-IMAF).
In doing so, the Secretariat noted some Working Group-specific differences in relation to: submission deadline; exception to the deadline; and approach to accepting revised documents.
WG-EMM agreed that standardising the Working Groups’ guidelines in relation to the submission of meeting documents would simplify and unite the guidelines which participants to both WG-EMM and WG-FSA are required to follow. Standardisation would also simplify the Secretariat’s work in preparing information and documents for meetings. Consequently, WG-EMM agreed to a proposal to standardise the specific differences which related to the submission of documents to its meetings (WG-EMM-05/10).
WG-EMM also agreed to 3 changes to maximum size of meeting documents, submission of published papers, and availability of meeting documents in locked and unlock pfd. If WG-FSA adopted the changes agreed by WG-EMM, then the requirements for WG-FSA and WG-EMM would be identical and this would result in a standard set of guidelines for the submission of meeting documents to working groups.