Browsing by Subject "Antarctic minke whales"

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    Estimated density of Antarctic minke whales obtained from simulated IDCR/SOWER survey data using the “standard” method
    (2005) Branch, Trevor A
    Simulated survey datasets provided by Palka and Smith were analysed using standard line transect methods that assume g(0)=1. Estimates of mean school size were generally positively biased by >10% when cue production was asynchronous, but otherwise typically unbiased. Overall estimates of whale density were negatively biased for nearly all scenarios considered, with mean bias of -23% for “2004” scenarios and -10% for “2005” scenarios. In the “2004” scenarios, bias was greatest when the detection function used to generate the simulated sightings included school size or weather as a covariate; in the “2005” scenarios, the greatest bias occurred when weather and whale density were correlated and when survey was in IO mode only. Implied values of g(0) ranged from 0.48 to 1.02 (mean 0.77) for the “2004” scenarios and from 0.78 to 1.03 (mean 0.90) for the “2005” scenarios, and were higher than previous estimates from the IDCR surveys (generally 0.5–0.7).
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    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.
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    On the compatibility of results from ADAPT-VPA and transition-phased-based trends in age-at-maturity for the I-Stock of Antarctic minke whales
    (2007) Butterworth, Doug S; Mori, Mitsuyo
    A counter example is provided to show that inferences of incompatibility between population trends from ADAPT-VPA and independently estimated changes in the age-at-sexual-maturity of Antarctic minke whales over the late 1940s to late 1960s are not robust to plausible variants of the ADAPT-VPA assessment (specifically in this case admitting the possibility of a small negative slope in selectivity at larger ages). Final conclusions on this matter should await fuller investigations on the ability to the ADAPT-VPA estimator to reliably estimate the shape of the time-trend in carrying capacity over this period, upon which inferences of compatibility or otherwise are critically dependent.
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    Preliminary abundance estimates for Antarctic minke whales from three completed sets of IDCR/SOWER circumpolar surveys, 1978/79 to 2003/04
    (2005) Branch, Trevor A
    Abundance estimates are provided for Antarctic minke whales from the ship-based IDCR-SOWER surveys using standard distance sampling methodology. The methods of pooling strata and of estimating mean school size have altered since the most recent re-assessment of all these surveys. The IDCR-SOWER surveys are grouped into three completed circumpolar sets of cruises: 1978/79–1983/84 (CPI), 1985/86–1990/91 (CPII) and 1991/92–2003/04 (CPIII), which respectively covered 63.1%, 79.5% and 99.9% of the ice-free area south of 60°S. Abundance estimates from individual surveys were added for CPI and CPII and combined using the ‘surveyonce’ method for CPIII to obtain circumpolar abundance estimates. When closing and independent observer mode estimates were inverse-variance weighted, circumpolar abundance estimates were 594,000 (CV = 0.128), 769,000 (CV = 0.094) and 362,000 (CV = 0.080) for CPI, CPII and CPIII respectively. These estimates are known to be negatively biased because some Antarctic minke whales may be north of 60°S or inside the pack ice during the surveys, and because some whales on the trackline were likely missed. After simple extrapolation to account for differences in the latitudes surveyed during each circumpolar set (but not for increasing proportions of ‘like minke’ sightings), the ratio of the circumpolar estimates was 0.92:1.00:0.39, echoing previous findings of appreciably lower CPIII estimates. These results should be considered preliminary.