A review of rattail (Macrourus spp.) and skate by-catch and analysis of standardised CPUE, for the exploratory fishery in the Ross Sea (CCAMLR Subareas 88.1 and 88.2) from 1997/98 to 2004/05

A standardised CPUE analysis was used to determine factors affecting bycatch rates of rattails (Macrourus spp.) and skates in the exploratory fishery for toothfish in the Ross Sea (CCAMLR Subareas 88.1 & 88.2). The analysis was based on fine-scale haul-by-haul (C2) data and observer data from all vessels in the fishery from 1997/98 to 2004/05. The major factors influencing rattail bycatch were vessel, area, and depth. Catch rates of rattails were highest along the shelf edge (SSRUs 88.1E, 88.1I, 88.1K and 88.2E) in depths from 600 to 1000 m, and there was an order of magnitude difference in rattail catch rates between different vessels. Examination of vessel characteristics showed that catch rates of rattails were lower with the Spanish line system than with the autoline system. Russian and Korean vessels had extremely low catch rates compared to other vessels. Standardised CPUE of rattails has shown a general increasing trend since 1998/99, with a peak in 2002/03.
It was not possible to reliably determine factors influencing catch rates of skates from either C2 or observer data because a proportion of skates are cut free and released at the surface and these are not accurately recorded or reported in either dataset. There are a number of inconsistencies within the observer data which need to be resolved before these data can usefully be used for estimating bycatch. These include a consistent definition of a set across the observer and C2 datasets, accurate recording of the number of hooks observed for bycatch, and completion of the L11 form on the fate of skates for all sets.