Browsing by Author "Mori, Mitsuko"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Open Access
    Model-based analyses of trends over time in the age corresponding to the transition phase for Antarctic minke whales in the JARPA research area
    (2006) Butterworth, Doug S; Mori, Mitsuko; Zenitani, Ryoko; Kato, Hidehiro
    This study applies a model-based approach similar to that of Thomson et al. (1999) to the transition phase data obtained from JARPA surveys to examine trends in the age at maturity for the I and P stocks of Antarctic minke whales. The results, which takes into account various potential biases related to examining trend in transition phase data (i.e. truncation and fringe effects, differences between readers, and readers learning over time) suggest that the age at maturity of Antarctic minke whales declined from about 11 years in the late 1940s to 7 years in the late 1960s for both stocks, and these declining trends are statistically significant at the 5% level. The analyses also suggest that the age at maturity increased slightly from the late 1960s to the late 1970s and has stabilized thereafter. These trends are consistent with the results obtained from VPA (Mori et al. 2006), which suggest that for both the I and P stocks, abundance increased from the 1940s to the late 1960s and thereafter has been stable or declined somewhat. This consistency enhances the confidence to be placed in estimates of parameters (such as natural mortality and MSYR) from such VPA analyses that may be of value for management purposes. It also serves to demonstrate the utility of age-at-maturity as an index to monitor stock status, and suggests that continued monitoring of this parameter is desirable both for this purpose and for contributing to the understanding of the dynamics of the Antarctic ecosystem.