CCAMLR

As part of a wider research project aimed at developing management strategies based on the current toothfish assessment methodology using the Generalised Yield Model (GYM), a Java version of the GYM has been developed. The new version has been developed directly from the specifications kindly provided by Dr Andrew Constable. Some routines have been translated from the GYM FORTRAN code into Java. As such, if sufficiently similar results can be obtained from the JGYM and the current GYM given identical input, then this would be a major step towards a verification of the GYM. This paper, which will be given in the form of a Power Point presentation, will discuss how the JGYM was developed, indicate differences from the GYM where they exist, and compare results from the two programs with identical inputs.

This note describes the most recent additions to Fish Heaven and provides illustrative examples of its application as a tool for evaluating fisheries management systems. As in previous years there have been numerous small changes and additions to the functionality of the program. This note describes the large structural additions which have been made to Fish Heaven and which now allow a very wide range of models to be constructed and explored. The application of Fish Heaven as an evaluation tool is illustrated by some examples of how Catch Per Unit Effort performs as an indicator of abundance under simple stock scenarios.

The Generalised Yield Model (GYM) was first developed in 1995 as a generalised form of the Krill Yield Model, which was based on the method for evaluating yield developed by Beddington & Cooke (1983). The first version incorporated options for assessing long-term annual yield according to catches set by a proportion of an estimate of pre-exploitation biomass (as in krill), a specified catch in the units of biomass and relative to the recruitment parameters (as in toothfish) or according to a constant fishing mortality (F). It also included the capacity to evaluate yield per recruit. The latest version of GYM (Version 5.01b) differs from earlier versions in 2 main ways: (i) improved storage of output, population characteristics and presentation, and (ii) new features to allow specifying the starting biomass and/or age structure of the population obtained from surveys during a year. In addition, S-plus scripts have been developed to help with output diagnostics. The GYM User’s Manual, Specifications and Examples are also vastly improved. These features now provide the flexibility to undertake a wide range of assessments on stocks, not just specific to CCAMLR. In CCAMLR, the latest version of GYM can used on assessments for Antarctic krill, Patagonian toothfish, and mackerel icefish. This paper presents the specifications for the Generalised Yield Model Version 5, detailing the population model used in the projection program, the algorithm for evaluating yields and the requirements for inputting parameters into the model. It also details how different parts of the model can be manipulated to explore alternative functions. Finally, some examples are presented to show how the GYM can be validated by the user. The input and output files for these examples are available. The latest version of the Generalised Yield Model (GYM), Version 5.01b can be downloaded from the Australian Antarctic Division Website, www.aad.gov.au/marine_eco_software.

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.