Exploratory modelling of the random stratified trawl survey (RSTS) around Heard Island and McDonald Islands (HIMI) in Division 58.5.2

The random stratified trawl survey (RSTS) has been conducted annually since 1997 in waters less than 1000 m within the waters of Heard Island and McDonald Islands (HIMI) in Division 58.5.2. The RSTS provides valuable information about the abundance and composition of younger Patagonian toothfish (Dissostichus eleginoides) around HIMI. The exploration of the RSTS data presented here shows that careful consideration of stratification and structured Bayesian hierarchical modelling of the survey can provide both a more precise estimate of abundance and biomass than the current non-parametric bootstrap approach, and informative length composition estimates. The RSTS mostly provides information about length classes between 450 mm and 750 mm, with strong cohorts appearing in the RSTS approximately every three years and remain visible for about four years. The analysis of the now substantially larger RSTS dataset also supports the currently used stratification of the RSTS, developed in 2004, although there could be some minor refinements to improve biomass estimates for toothfish.