This paper assesses the status and abundance of M. carinatus on BANZARE Bank (Statistical Division 58.4.3/1). The long-term precautionary yield was estimated using the Generalised Yield Model (GYM) used previously by the CCAMLR Working Group on Fish Stock Assessment. Estimates of population and biological parameters were not available for this species at BANZARE Bank. Length and weight data were taken from a trawl survey conducted at Macquarie Island in 1999. Where parameters were not directly available for M. carinatus, estimates were obtained from the literature for similar species elsewhere in the world. The trawl survey data was used to calculate estimates of biomass and density for the area and showed that M. carinatus did not appear to be aggregated over any part of BANZARE Bank. The long-term annual yield calculated for M. carinatus was 550 tonnes, based on a critical value of y of 0.033 (the proportion of the estimate of pre-exploitation biomass) found using the CCAMLR decision rules. The binding rule for this assessment was the decision rule on the probability of depletion below 0.2 of the median pre-exploitation spawning biomass. This assessment could be used as a basis for setting catch rates of M. carinatus in longline fisheries targeting Dissostichus eleginoides. Applying the critical y to the mean density observed in the survey results gives a catch rate of 5.81 kg/km2 which translates into a precautionary yield of 17.9 tonnes per fine-scale rectangle. Such a yield represents 18 % of the total catch allowed for D. eleginoides in fine scale areas in new and exploratory fisheries. This catch rate may be useful in setting general by-catch rules for M. carinatus to protect the species from localised depletions.
Abstract:
Based on a survey of Dissostichus eleginoides in the Heard Island region in April 1999 and information from an observer program on the commercial fishery begun in 1997, this paper aims (i) to provide an assessment of biomass and stock structure in the vicinity of Heard, (ii) to examine the relationship between commercial fishing grounds and the wider plateau and banks where surveys have been undertaken previously, and (iii) to revise the population parameters, where possible, including fishing selectivity, growth, maturity and recruitment. The results demonstrate clear differences between populations of Dissostichus eleginoides at Heard Island in the southern Indian Ocean and South Georgia Island in the South Atlantic. For example, Patagonian toothfish at Heard Island are considerably older and slower growing than those at South Georgia. This raises questions as to the appropriateness of the current range of natural mortality used in the assessment of yield at Heard Island. The revised estimates of recruitment indicate that the variability of recruitment at Heard Island is much greater than originally estimated and that, in recent years, there has been an influx of a large number of young fish compared to the period during the first surveys in the early 1990s. A notable outcome of the 1999 survey is the identification of Ground B as the most important area for 7 to 8 year old fish in the Heard Island region. Careful thought will need to be given to how best to manage a stock that aggregates in this manner while still juvenile and with some years before becoming reproductively mature.
