CCAMLR

This paper reports on a feasibility study into a tag and recapture experiment for the purpose of stock assessment of the Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea. An exploratory fishery for D. mawsoni has been operating since 1997-98. Catches have increased from ~40 t in 1997-98 to nearly 1800 t in 2002-03. Although some Patagonian toothfish (D. eleginoides) are taken in the more northern areas, catches have been predominantly D. mawsoni.
To date nearly 2000 toothfish (about 90% D. mawsoni) have been tagged and released in the area, from vessels operating in the exploratory fishery during the last 3 seasons. A good number of tag recoveries have been reported, mainly within the same season but also between successive seasons, which indicates that fish are surviving the tagging event.
It is proposed to continue tag releases each season as part of the exploratory fishery for the purpose of determining the stock size of D. mawsoni and sustainable yields for the fishery. Ongoing annual tagging will provide data suitable for estimating natural survival, abundance and recruitment using the Jolly-Seber or variant estimators. The utility of this approach is investigated here by applying a Jolly-Seber estimator to simulated tagging data generated by an operating model over a range of assumed population sizes and tagging strategies. The study indicates the number of releases required to achieve various levels of precision of the population estimate based on:
• assumed survival rates of released fish;
• current exploitation rates in the fishery; and
• known growth and natural mortality parameters.
Although this initial attempt considers the simplest population structure (a homogeneous stock and fishery), further complexity can be added to the model to explore the heterogeneity in the population using length and spatial strata.

This report presents results from a pilot study to determine the feasibility of conducting acoustic surveys for toothfish and rattails in the Ross Sea. Acoustic data were collected using a Simrad ES60 38 kHz echosounder on the New Zealand commercial longliner FV Janas during the 2002–03 exploratory fishery. Data were recorded continuously from 28 December to 2 February 2003, then during line setting only during 5–22 February 2003. Analyses were carried out to assess data quality, describe different mark types, and quantify acoustic backscatter by echo integration and echo counting. These analyses focused on the subset of acoustic data collected when setting longlines so acoustic recordings could be compared with longline catches. Each ‘line’ recording was between 20 and 50 min long, corresponding to 2–4 nmi.
Data quality was generally good. Of the 84 line recordings, 68 were considered suitable for acoustic analysis. The other 16 files were rejected because data quality was too poor (11 files), the file was corrupted (1 file), or the longline was lost so there were no corresponding catch data (4 files). Poor data quality was associated with strong winds (greater than Beaufort 5) and/or high seas (swell heights greater than 2 m): conditions that led to bubble interference on the hull-mounted transducer. Other issues with data quality were interference from another echosounder before 11 January 2003, and the occurrence of a double bottom echo caused by too high a ping rate from 23–30 January.
All line recordings were in water over 1000 m deep. Because of the spreading of the acoustic beam, the acoustic deadzone at these depths is relatively large, especially if the bottom is rough or sloped. Simulations indicated that at 1500 m depth, the acoustic deadzone would be over 50 m high for a sea-bed with a slope of 20º. The problem of the acoustic deadzone was worsened by the occurrence of side-lobe echoes, produced as longlines were set on steep slopes parallel to the depth contours. Measurements indicated that side-lobe could create a deadzone of 50–100 m on apparently flat ground. Because both toothfish and rattails are considered to be demersal species, the inability of the acoustics to ‘see’ close to the bottom is a major limitation that could only be avoided with the use of an acoustic system deployed at depth.
Two types of pelagic layers were present in most acoustic recordings: a dense shallow layer between 30 and 200 m; and a more diffuse deep scattering layer between 300 and 800 m. Pelagic schools were also present in some recordings and these tended to occur at 150–400 m depth, between the layer marks. The most common demersal mark was single targets, which were present in 84% of line recordings. Most single targets occurred in a surface-referenced band between 800 and 1100 m depth, and were up to 500 m off the bottom. There was a significant positive correlation between the number of single targets counted from the echogram and the catch of rattails in the accompanying longline set. Bottom-referenced layers were present in 18% of line recordings and were also associated with higher catches of rattails. Demersal schools were present in 16% of recordings and were associated with higher catches of toothfish. Despite these associations, no acoustic marks could be reliably identified as being rattails or toothfish. It seems unlikely that the schools were toothfish or the single targets were rattails, as these were often more than 300 m off the bottom…[please contact the Secretariat for the full version of the abstract]

This paper provides a review of the boundaries of the small scale research units used to manage the exploratory fishery for D. mawsoni in Subarea 88.1. In determining appropriate SSRU boundaries we considered the physical and geographical features of the Subarea including the impact of sea ice on fishing practices, as well as the distribution and abundance of the target and bycatch species (rattails and skates). We recommend that the northern SSRU boundary (at 65°S) remain in place but that the other boundaries of the other four SSRUs are changed to reflect the underlying bathymetry, species distributions and ice conditions.

Age, growth rate and age composition of icefish catches in subareas 48.3, 48.2, 48.1 were studied for different periods. The analysis of age composition of icefish in subarea 48.3 showed that the catches consisted mainly of individuals of 2-4 age groups, where more than 60% were represented by fish of two contiguous age groups. It was shown that in the 1980s during a number of years the catches were based on fish of two contiguous year classes, strong 1984 year class and middle 1983 year class. These year classes appeared against a background of the intensive icefish fishery, and krill biomass, judging from the catches, was rather low. Possibly, the abundance of one or another icefish year class in subarea 48.3 depends on many factors, among which it it difficult to determine the most important factors now. A comparative analysis of icefish growth in subarea 48.3 based on data collected in different periods of fishery revealed a substantial interannual variability. The comparison of the growth rate of fish caught in 1978 in subareas 48.3 and 48.2 and that of fish caught in 1986 in subareas 48.3 and 48.1 did not reveal any significant differences. The relatively high growth rate of icefish in subareas 48.3 and 48.2 in 1978 compared with 1985 may indicate on the common factors affected the icefish growth in different regions of the South Atlantic in that time.

Calculation of age composition of density-at-length data produced during inventory surveys using the method of W.K. de la Mare (1994) is a conventional procedure in the WG-EMM and WG-FSA practice.
Verification of the method was made on the materials on the Baltic cod collected in the ICES Subarea 26. Age-length keys for 1995-2001 and length frequency data obtained during the surveys carried out by research vessels of Russia and other countries were used.
It has been shown that divergence in the estimates of age composition proportions calculated using the CMIX method from those determined from age-length keys (ALK) computed based on the materials of direct age reading may be significant reaching 100 per cent or more. Individually, recruitment estimates also differ considerably from the estimates based on ALK.

Each year, WG-FSA and WG-EMM undertake assessments of stocks and other parameters. The WG-FSA Subgroup on Assessment Methods (WG-FSA-SAM) is to consider the assessment methods to be used by WG-FSA in 2003. This paper aims to summarise the existing assessment process for each species considered by WG-FSA. It concludes with a suggested format for these summaries in the future.