Progress on application of ADAPT-VPA to minke whales in Areas IV and V given updated information from IDCR/SOWER and JARPA surveys

Working Paper


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University of Cape Town

The ADAPT-VPA assessment methodology of Butterworth et al. (1999) is applied to abundance estimates (from both IDCR/SOWER and JARPA surveys) and catch at age data (both commercial and scientific) for Areas IV and V. The methodology is extended to be able to take account of inter-annual differences in the distribution of the population between the two Areas when they are assessed jointly. An important feature of these updated results is that revised JARPA estimates of abundance are shown to be statistically comparable with estimates from the IDCR/SOWER programme (i.e. calibration factor not significantly different from 1). The general pattern shown by results is of a minke whale abundance trend that increased over the middle decades of the 20th Century to peak at about 1970, and then declined for the next three decades. The recruitment trend is similar, though with its peak slightly earlier. The factor to which the results are most sensitive is the value of natural mortality M. The assessments do show retrospective patterns, primarily related to changes in the best estimate of M as time has progressed. This in turn seems linked to the IDCR/SOWER survey trends suggesting higher, and the JARPA survey trends lower estimates of M. For the assessment of the two Areas combined, M is estimated at 0.068 with a CV of 0.12; this compares with CVs of typically 0.35 for the Area-specific assessments of Butterworth et al. (1999), which were based on eight seasons’ fewer data. The paper reflects an account of work in progress, and suggestions are made of areas where further analyses would be desirable.