An Integrated Assessment of Southern Blue Whiting (Micromesistius australis) from New Zealand Using Separable Sequential Population Analysis
S. Hanchet, V. Haist, and D. Fournier
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The southern blue whiting (Micromesistius australis) fishery is one of the largest fisheries in New Zealand waters, with landings peaking at over 75,000 t in 1992. Early assessments of the Campbell Island stock were carried out in 1992 and 1993 using Virtual Population Analysis. There was concern over these assessments because diagnostics suggested that the survivors were poorly estimated due to the noisy CPUE data. In addition, estimates of recruited biomass from acoustic surveys could not be fitted and confidence intervals could not be estimated using the existing software.
A separable Sequential Population Analysis was developed to simultaneously analyze all the available data and to address these issues. The 17 years of catch-at-age data were fitted as a multinomial distribution with a median sample size of 100, weighted annually between years. Three acoustic indices of spawning stock biomass were fitted as relative abundance indices, with a coefficient of variation (CV) of 0.3. Eleven years of standardized CPUE indices were converted to relative effort and fitted with a CV of 0.5. Simulated data were used to estimate confidence limits, and included uncertainty in the annual catch, catch-at-age, CPUE, and acoustic data.
The model results suggest that the stock underwent a major decline during the 1980s and early 1990s but has since recovered, due mainly to the recruitment of the strong 1991 year class. However, the extent of the recovery is uncertain largely because of observation error in the tuning indices and the sensitivity of the model to the selectivity assumptions.
- Item number: AK-SG-98-01h
- Year: 1998
- DOI: https://doi.org/10.4027/fsam.1998.08