Revision of biological and population parameters for Dissostichus eleginoides on the Heard Island Plateau (Division 58.5.2) based on a comprehensive survey of fishing grounds and recruitment areas in the region

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.