Browsing by Author "Mori, Mitsuyo"
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- ItemOpen AccessApplication of ADAPT-VPA to Antarctic minke whales in the JARPA research area(2006) Mori, Mitsuyo; Kitakado, Toshihide; Butterworth, Doug SThe ADAPT-VPA assessment methodology originally developed by Butterworth et al. (1999) has been greatly improved by taking into account various comments made during a series of IWC-SC meetings and is applied here to abundance estimates (from both IDCR/SOWER and JARPA surveys) as well as catch at age data (both commercial and scientific) for the I and P-stocks of Antarctic minke whales. The improvements to the methodology allow account to be taken of various further aspects, primarily: 1) inter-annual differences in the distribution of the population between different management Areas, 2) a stock-recruitment relationship, and 3) the effects of possible ageing-error. Furthermore sensitivities to various functional forms for selectivity and natural mortality with age are explored. The general pattern shown by the results for both stocks is of a minke whale abundance trend that increased over the middle decades of the 20th Century to peak at about 1970, and then stabilized or declined somewhat for the next three decades. The recruitment trend is similar, though with its peak slightly earlier. The annual natural mortality rate, M, is estimated to be 0.056 with a CV of 0.16 for the I-stock, and 0.069 with a CV of 0.15 for the P-stock for the “Reference case” assessments. When only the JARPA abundance estimates are used for tuning, M is estimated as 0.038 and 0.060 for the I- and P-stocks, respectively. The estimation of M is fairly robust to the various assumptions of the model. The CVs of these M estimates for the “Reference case” assessments, when compared with those of typically 0.35 for the Areaspecific assessments of Butterworth et al. (1999) which were based on fewer data, indicate an improvement in the precision of these estimates due to the accumulation of data over the long-term of the JARPA surveys. The fits of the stock-recruitment model generally require a carrying capacity for minke whales that first increased and then stabilized or declined somewhat during the last century, and suggest MSYR(1+) values in the 4-6% range. The improved precision in the estimation of M may contribute in the improvement of management and assessment of this species on a stock-specific basis, since it can reduce the uncertainty concerning the value of M and can provide an improved prior distribution for MSYR. The latter in particular, in the context of providing a measure of the productivity of which the species is capable, is essential information for effective RMP implementation through reduction of the range of plausible scenarios which need to be considered in Implementation Simulation Trials.
- ItemOpen AccessApplication of the D&M management procedure to the core and sensitivity trials to assist identify factors to which MP performance is likely to be the most sensitive(2005) Butterworth, Doug S; Mori, MitsuyoThe D&M MP is tuned to the Core set OM and then applied to constituent scenarios of the Core set and the Sensitivity trials. The presence or absence of appreciable changes in anticipated performance as factors are varied in these trials suggests that Indonesian selectivity might be included in the final Reference set, but the M0 factor excluded.
- ItemOpen AccessFurther progress on application of ADAPT-VPA to Antarctic minke whales(2007) Mori, Mitsuyo; Butterworth, Doug S; Kitakado, ToshihideThe ADAPT-VPA assessment methodology originally developed by Butterworth et al. (1999) has been appreciably advanced by taking into account various comments made during a series of IWC-SC meetings and is applied here to abundance estimates (from both IDCR/SOWER and JARPA surveys) as well as catch at age data (both commercial and scientific) for the I and P-stocks of Antarctic minke whales. The improvements to the methodology allow account to be taken of various further aspects, primarily: 1) inter-annual differences in the distribution of the population between different management Areas, 2) a stock-recruitment relationship, 3) the effects of possible ageing-error, and 4) the effects of possible change in age-at-sexual maturity over time as indicated by analyses of readings of transition phases in ear plugs. Furthermore sensitivities to various functional forms for selectivity and natural mortality with age are explored. The general pattern shown by analysis for both stocks is of a minke whale abundance trend that increased over the middle decades of the 20th Century to peak at about 1970, and then stabilized or declined somewhat for the next three decades. The recruitment trend is similar, though with its peak slightly earlier. The annual natural mortality rate, M, is estimated to be 0.056 with a CV of 0.16 for the I-stock, and 0.069 with a CV of 0.15 for the P-stock for the “Reference case” assessments. When only the JARPA abundance estimates are used for tuning, M is estimated as 0.037 and 0.060 for the I- and P-stocks, respectively. The estimation of M is fairly robust to the various assumptions of the model. This analysis is considered preliminary, as conclusions relating to estimates of M and trends in abundance and recruitment obtained using this approach await further revisions of 1) abundance estimates obtained from IDCR and JARPA surveys, and 2) error-models for the catch-at-age data, particularly those obtained from the commercial harvests.
- ItemOpen AccessModelling the krill-predator dynamics of the Antarctic ecosystem(2005) Mori, Mitsuyo; Butterworth, Doug SThe main objective of this thesis is to model the krill-predator dynamics of the Antarctic ecosystem so as to determine whether predator-prey interactions alone can broadly explain observed population trends of the species considered in the model without any appeal to systematic effects possibly caused by environmental change. The history of human harvesting in the Antarctic is summarized briefly, and the central role played by krill is emphasized. The background to the hypothesis of a krill surplus in the mid 20th Century is described, and the information, particularly regarding population trends, that has become available since the postulate was first advanced is discussed. By reviewing the consumption and abundance estimates for various species in the Antarctic, it is evident that among the baleen whales, blue, fin, humpback and minke whales feed mainly on krill, and could collectively be consuming up to 120 million tons of krill in this region for each of the years around 1990. Of the seals, the Antarctic fur seals and crab-eater seals also feed mainly on krill, and these two species could be consuming up to 70 million tons of krill each year. Consumption estimates for other krill predators (birds, fish and cephalopods) are relatively poorly determined by comparison. Of these four baleen whale species, minke whales currently make the greatest impact on krill due to their large number at present compared to the other larger whale populations which are still depleted. Trend information suggests that the large baleen whales that were heavily depleted during the commercial whaling period are now recovering at rates in the vicinity of 10% per year, but there are some indications of a recent decrease in minke whale numbers. Thus, the consumption of krill by these large baleen whales has probably been increasing over recent years, though decreasing for minke whales. Updated and refined catch-at-age analyses of minke whales for the International Whaling Commission (IWC) Management Areas IV and V suggest an increase in abundance of this species in the middle decades of the 20th Century to peak at about 1970, followed by a decline for the next three decades. Fitting the recruitment time trend obtained from these analyses to a stock-recruitment model suggests that minke whale carrying capacity first increased from about 1940 to 1960 followed by a 60% decrease from the 1960s to the present. General trends in the biological parameters of this species are consistent with such a decline. A predator-prey interaction model is developed including krill, four baleen whale (blue, fin, humpback and minke) and two seal (Antarctic fur and crab-eater) species. The model commences in 1780 (the onset of fur seal harvests) and distinguishes the Atlantic/Indian and Pacific sectors in view of the much larger past harvests in the former. A reference case and six sensitivities are fit to available data on predator abundances and trends, and the plausibility of the results and the assumptions on which they are based is discussed, together with suggested areas for future investigation. Amongst the key inferences of the study are that: i) species interaction effects alone can explain observed predator abundance trends, though not without some difficulty; ii) it is necessary to consider other species in addition to baleen whales and krill to explain observed trends, with crab-eater seals seemingly playing an important role and constituting a particular priority for improved abundance and trend information; iii the Atlantic/Indian region shows major changes in species abundances, in contrast to the Pacific which is much more stable; iv) baleen whales have to be able to achieve relatively high growth rates to explain observed trends; v) species interaction effects impact the dynamics of these predators in ways that differ from what might be anticipated in a conventional single-species harvesting context, and they need to be better understood and taken into account in management decisions, and vi) Laws' (1977) estimate of some 150 million tons for the krill surplus may be appreciably too high as a result of his calculations omitting consideration of density dependent effects in feeding rates. . A priority for future work is to obtain improved estimates of the amount of krill consumed by other species, such as birds, cephalopods and fish as well as to obtain consensus on current abundance estimates for crab-eater seals and baleen whales (especially minke whales and also the associated abundance trend). Once such information is improved, more thorough sensitivity tests to the assumptions of the model and uncertainties in the abundance estimates of the species considered need to be explored. With such further development, it is hoped that such a model may ultimately assist in providing scientific advice for appropriate sustainable harvesting strategies for the Antarctic marine ecosystem taking species interactions into account, as this is a matter of key importance for the IWC and for the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR).
- ItemOpen AccessObserver experience and minke whale sighting ability in the IWC/IDCR-SOWER surveys(Journal of Cetacean Research and Management, 2003) Mori, Mitsuyo; Butterworth, Doug S; Brandão, Anabela; Rademeyer, Rebecca A; Okamura, Hiroshi; Hiroyuki, MatsudaThe relationship between observer experience and the number of minke whale schools sighted on International Whaling Commission/International Decade of Cetacean Research-Southern Ocean Whale and Ecosystem Research (IWC/IDCR-SOWER) surveys from 1993/94 to 1998/99 is investigated for Independent Observer (IO) mode survey. Observer experience is defined as the number of past sightings surveys in which the observer participated. During the third circumpolar set of surveys (from 1991/92 onwards), about half of the observers had participated in fewer than five previous sightings surveys. Based upon the QAIC model selection criterion, the observers are classified into two groups depending on their experience: ‘Beginners’ (0-4 surveys) and ‘Experts’ ( > 4). The sighting rate for minke whale schools by Beginners is estimated to be 42% lower than that by Expert observers. Furthermore, perpendicular distances to the sightings do not show significant differences in relation to observer experience. These results jointly indicate that the probability of detection on the trackline, g(0), may be less than one when Beginners are amongst those observing. Abundance estimation for minke whales in IO mode involves the sightings made by triple observer combinations, with two observers in the barrel and one observer in the Independent Observer Platform (IOP) all searching simultaneously. Surprisingly, given the result above, no significant trend in sighting rate with the combined experience of this three-observer combination is detected. This might be an artifact of small sample size for some observer combinations, such as Experts in all platforms. When observer combinations in the barrel are pooled across, the estimated trend in the sighting rate with combined observer experience becomes steeper. Furthermore, when like-minke sightings are also taken into account, the trend becomes steeper still. In this case, when observations are pooled across observer combinations in the barrel, a model for sighting rate that includes an observer effect is selected in terms of the QAIC criteria. These analyses thus provide suggestive evidence that the introduction of Beginner observers during the third circumpolar set of surveys may have reduced g(0) and hence negatively biased abundance estimates for minke whales, both in absolute terms and compared with estimates from the second circumpolar set of surveys.
- ItemOpen AccessOn 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, MitsuyoA 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.