Standardised CPUE analysis of Antarctic toothfish (Dissostichus mawsoni) fishery in the Ross Sea for the years 1997/98 to 2004/05

The Ross Sea toothfish fishery has operated during the Antarctic summer (November–May) since 1997 in CCAMLR Subareas 88.1 and 88.2. Previously, standardised analysis of toothfish CPUE (catch per hook) have been carried out in 2004 calculated CPUE indices for the 1997–98 to 2003–04 fishing years in Subarea 88.1 and 88.2. This report revises and updates the previous analysis with the addition of data from the 2005 season for the Ross Sea (Subarea 88.1 and SSRUs 88.2 A–B) using data sets based on (a) all vessels and (b) the two main New Zealand vessels that have been in the fishery over most of the period. We estimate indices using both lognormal generalised linear models and a Tweedie mixed model, in which data were modelled using the methods described by Candy (2003)
The variables included in the analysis appeared reasonable and were consistent between the models presented here and CPUE analyses for previous years. The yearly indices were relatively stable between 1998 and 2003, with a decline in 2004, and a sharp increase in 2005. Possible explanations for the decline in 2004 were described by Phillips et al. (2004) as gear conflict and competition between vessels to set lines, in addition to extreme ice conditions that limited where lines could be set. None of these factors were believed to be important in 2005.
Investigation of additional explanatory terms included an index of ice cover and an index of vessel crowding. Neither of these terms was found to be significant or explain the decline in 2004 or the increase in 2005. Lack of a balanced experimental design and possibly the choice of parameterisation of the variables are likely to be the reasons why neither of these terms were found to be significant.