A toothfish fishery has operated during the Antarctic summer (December - May) from 1997 to 2003, in Subareas 88.1 and 88.2 in the region of the Ross Sea. A preliminary standardised analysis of toothfish CPUE (catch (kg) per baited hook per set) carried out last year compared three alternative toothfish CPUE analyses for the 1998 to 2002 seasons. The analysis of a data subset including only the two main vessels and the 3 main fishing grounds was the best fit to the data.
This report updates the main grounds and vessels analysis to include the 2003 season. Seasonal indices were variable with no trend, and error bounds on the estimates were low, but the main grounds analysis model was a relatively poor fit to the data. The variables depth, soaktime, length of line, area, and season, entered the model, which explained 37% of data variability. Because of the poor diagnostics, the model was fitted to sets excluding zero catches of Antarctic toothfish as a sensitivity. The diagnostics for this model were substantially improved and the seasonal indices were slightly more optimistic. As fishing effort has extended to other grounds since 2001, a further sensitivity analysis was completed for the two main vessels. The seasonal indices were similar to the main grounds analysis. While the model explained 48% of data variability, it was influenced by the unbalanced nature of the data, and model diagnostics indicated a poor fit to the data.
Data are available from seven seasons, but the analysis was confined to the two vessels with a consistent fishing history, and as such only represents a small area of the entire Ross Sea toothfish fishery. The variables included in the analysis are plausible, but the models may be influenced by extreme values of soaktime, depth and length of line set, that reflect the exploratory nature of the fishery. The descriptive power of the main grounds model is reasonable, but it is still influenced by the unbalanced nature of the fishery and records of zero catches.
Seasonal indices show no trend between 1998 and 2003, but the relationship between these indices and relative abundance is unknown, and fishery independent data are unavailable to validate this relationship. Standardised CPUE indices have been validated for overseas longline fisheries, including Alaskan sablefish (Anoplopoma fimbria), Atlantic halibut (Hippoglossus hippoglossus), and Atlantic cod (Gadus morhua), using tagging, acoustic survey, and quantitative longline survey methods. These species have generally similar biology, distribution, and fisheries operation to Antarctic toothfish. Therefore, trends in standardised CPUE indices could be directly related to relative abundance for toothfish. The toothfish standardised CPUE data provide an index of fishery performance. Continued monitoring of CPUE for the main grounds, main vessels is recommended, and further research on other possible CPUE models, and inclusion of first order interactions in the analysis is suggested. Research into suitable methods for validating the relationship between CPUE seasonal indices and the relative abundance of toothfish is also recommended.