|Scientific Name:||Balaenoptera bonaerensis|
|Species Authority:||Burmeister, 1867|
|Taxonomic Notes:||Until the 1990s, only one species of minke whale was recognized, the Antarctic Minke Whale B. bonaerensis being regarded as conspecific with the Common Minke Whale B. acutorostrata. Most of the scientific literature prior to the late 1990s uses the name B. acutorostrata for all minke whales including Antarctic Minke Whales. Since 2000, the International Whaling Commission (IWC) Scientific Committee (SC) has recognized Antarctic Minke Whales as the separate species B. bonaerensis, while all northern hemisphere minke whales and all southern hemisphere "dwarf" minke whales are regarded as B. acutorostrata (IWC 2001). This has been followed by management and treaty bodies, such as Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). Phylogenetic analyses suggested that the two minke whale species, which are partially sympatric in the southern hemisphere, are not more closely related to each other than to other balaenopterid species (Árnason et al. 1993, Pastene et al. 1994).|
|Red List Category & Criteria:||Data Deficient ver 3.1|
|Assessor/s:||Reilly, S.B., Bannister, J.L., Best, P.B., Brown, M., Brownell Jr., R.L., Butterworth, D.S., Clapham, P.J., Cooke, J., Donovan, G.P., Urbán, J. & Zerbini, A.N.|
|Reviewer/s:||Taylor, B.L. & Notarbartolo di Sciara, G. (Cetacean Red List Authority)|
Although there is no accepted estimate of current abundance, the population size is clearly in the hundreds of thousands. The data analysed by standard methods suggest a reduction of approximately 60% between the 1978–91 period and the 1991–2004 period. However, alternative hypotheses to explain the apparent decline are still under investigation. If the decline is real, its extent and causes are currently unknown, and it may still be continuing. The corresponding population reduction thresholds (criterion A2) are 30% for Vulnerable and 50% for Endangered, measured over a 3-generation time window, which in this case is estimated to be approximately 66 years (22 years per generation). If the decline proves to be largely or mainly an artefact, or proves to have been transient in the light of analyses of more recent data, the species would qualify as Least Concern. If it were real, the species would qualify as Endangered. Pending resolution of the uncertainties relating to the apparent decline, however, the species is listed as Data Deficient (DD).
Balaenoptera bonaerensis is considered a southern hemisphere species, although there is a single record from Suriname (60°N) (Rice 1998). In summer they are abundant throughout the Antarctic south of 60°S, occurring in greatest densities near the ice edge, and to some extent within the pack ice and in polynyas. Particularly high densities have been observed in some years in high Antarctic areas such as Prydz Bay, the Weddell Sea and the Ross Sea (Kasamatsu et al. 1997). Although B. acutorostrata has been found in the Antarctic as far south as 65°S it is much less common there than B. bonaerensis (Pastene 2006a), such that all “minke whale” abundance estimates south of 60°S can for practical purposes be treated as referring to B. bonaerensis.
Minke whales are seen outside the Antarctic in summer (Kasamatsu and Miyashita 1983) but much of the summer data is ambiguous with respect to identification as B. acutorostrata or B. bonaerensis, such that it remains unclear whether significant numbers of B. bonaerensis occur outside the Antarctic in summer.
The winter distribution is less well known. There is a wintering area off Costinha, Brazil (7°S), where minke whales, almost exclusively B. bonaerensis, were the target of a whaling operation during 1964–85, with the peak abundance in October (da Rocha and Braga 1982, Holt, de la Mare and van Beek 1982). The recovery in this fishery of two whales marked in the Antarctic in Area II at 62° and 69°S (Buckland and Duff 1989) demonstrates that at least some individuals from the Brazilian population migrate to the Antarctic. Minke whales were also seen (and small numbers caught) off Durban, South Africa: the seasonal distribution was bimodal, with peaks in April/May and September/October, suggestive of migration past the area (Best 1982). There are occasional records from Peru (VanWaerebeek and Reyes 1994).
Japanese scouting vessel data indicated high abundance of minke whales (species ambiguous) in November between 10°–30°S in the central South Pacific and in much of the eastern and southern Indian Ocean from the tropics southwards to 50°S (Miyashita et al. 1995). The limited information available from low-latitude surveys from the 1987/88 season onwards, when the two minke whale species were reliably distinguished, indicates that most of the minke whales were B. bonaerensis (Nishiwaki et al. 1991), probably on route from (as yet unknown) low-latitude breeding grounds to the Antarctic. The lack of any known areas of high concentration in winter suggests that the breeding distribution is rather dispersed and largely offshore (Kasamatsu et al. 1995). The species identity of minke whales seen in Indonesian waters in November (Miyashita et al. 1995) is unclear.
At least some of the Antarctic minke whale population remains in the Antarctic in winter (Ensor 1989), but the proportion has not been quantified.
Native:Antarctica; Argentina; Australia; Brazil; Chile; French Southern Territories (the); Namibia; New Zealand; Peru; South Africa; South Georgia and the South Sandwich Islands; Uruguay
|FAO Marine Fishing Areas:||
Atlantic – southeast; Atlantic – southwest; Atlantic – western central; Indian Ocean – Antarctic; Indian Ocean – eastern; Indian Ocean – western; Pacific – Antarctic; Pacific – southeast; Pacific – southwest
|Range Map:||Click here to open the map viewer and explore range.|
As for other baleen whales, the IWC’s management of Antarctic Minke Whales has been based on six Areas, I through VI, which are longitudinal pie slices 50°–70° wide. The population structure is poorly known, but recent analyses suggest a genetic distinction between whales in the Indian Ocean sector of the Antarctic (west of 165°E) and the Pacific Ocean sector (east of this line) with presumably some overlap (Pastene 2006b). With the exception of the two marked whales mentioned above, the relationship between the Antarctic distribution and putative breeding areas is largely unknown.
Ship-based summer surveys of the area south of 60°S have been conducted each summer since 1978/79, under the auspices of the IWC SC, under the International Decade of Cetacean Research (IDCR) —later Southern Ocean Whale and Ecosystem Research (SOWER) — programme, and covering a different Area (or part Area) each year (Matsuoka et al. 2001). These have been used to estimate minke whale population sizes, on the assumption that the bulk of the population is found south of 60°S in the survey season (Branch and Butterworth 2001). The survey vessels do not enter the pack ice, where minke whales are known to occur to some extent (Shimada and Kato 2006).
The IWC SC conducted a major assessment of Antarctic Minke Whales in 1990, and a population estimate of 760,000 was adopted, based on results of the IDCR surveys conducted in the seasons 1982/83 through 1988/89 (IWC 1991). Results of subsequent surveys indicated lower abundances (see Table 1 in linked PDF document, which constitutes an integral part of this assessment), leading the Committee to conclude in 2000 that the estimate of 760,000 was no longer a valid estimate of current abundance (IWC 2001, p. 31). The Committee has not yet adopted a new current estimate, pending the development, testing and implementation of improved analysis methods. Estimates calculated using the standard methodology for comparative purposes (Branch 2006, IWC 2007) were:
After adjusting for the different coverage of the three sets of surveys (which is not simply a multiplication, because the areas unsurveyed in the earlier years tended to be further north), the ratio of the three abundance estimates for the three periods was 0.97:1.00:0.39 (see Table 1 in attached PDF document).
The Committee has to date (January 2007) been unable to determine whether the apparent decline was real or artifactual. The Committee considered the two most likely confounding factors to be: (i) a reduction in sighting efficiency (e.g., due to smaller school sizes and possibly less experienced observers) and (ii) changes in ice extent, such that fewer whales occurred in surveyable open water.
Two additional points were noted (IWC 2007):
(i) The decline was specific to minke whales; estimates for other species (blue, fin, killer, humpback) increased over the period; an explanation for the decline would need to account for this.
(ii) Abundance estimates for Areas IV and V obtained by the Japanese scientific permit whaling programme (JARPA) for the seasons 1989/90 through 2004/05 showed no statistically significant trend.
Further work on the issues of sighting efficiency and whale/ice distribution is underway. The Committee expects to adopt revised abundance estimates in the near future (2008) and to determine whether and to what extent the apparent decline is real.
Some authors have proposed that the population of Antarctic Minke Whales had been increasing up to the late 1960s (e.g. Mori and Butterworth 2006), but this is controversial.
|Habitat and Ecology:||
While in the Antarctic, minke whales feed almost exclusively on krill, primarily Euphausia superba, but also E. crystallorophias (ice krill), E. frigida, and Thysanoessa macrura (Tamura and Konishi 2006). Observed densities of minkes are highest near the edge of the pack ice, but these whales also occur within the pack ice (Shimada and Kato 2006). It is not known whether Antarctic Minke Whales also feed to any significant extent while outside the Antarctic on their wintering grounds or migration routes. Best (1982) found a very low level of feeding, almost entirely on euphausiids, by Antarctic minke whales taken in winter off Durban, South Africa. Antarctic minke whales may themselves be an important prey for type-A Killer Whales Orcinus orca (Pitman and Ensor 2003).
The generation time is estimated to be 22 years (Taylor et al. 2007).
Whaling on this species has not been as intensive as on the larger baleen whales. Substantial catches of Antarctic Minke Whales, apart from some experimental catches in the late 1960s, have been made by pelagic expeditions only since 1971, following depletion of the larger baleen whales (IWC 2006). Nearly 100,000 minke whales have been taken by pelagic whaling expeditions in the Antarctic, in addition to over 14,000 taken from the Brazilian land station at Costinha during 1964–85. Since 1987, pelagic catching has continued under scientific permit – at a much reduced, but increasing, level. The catch in the 2005/2006 season was 853 (Miyashita and Kato 2006). Otherwise, Antarctic minke whales are not subject to any substantially known direct anthropogenic threats. Bycatches in fishing gear have been recorded (e.g. van Waerebeek and Reyes 1994) but there is no indication that the numbers are significant.
During this century, a profound reduction in the extent of sea ice in the Antarctic is expected, and possibly a complete disappearance in summer, as mean Antarctic temperatures rise faster than the global average (Turner et al. 2006). The implications of this for Minke Whales are unclear but warrant monitoring.
|Conservation Actions:||Catches of Antarctic Minke Whales were subject to IWC catch limits soon after exploitation started. Catch limits for commercial whaling became zero from 1986 with the coming into effect of the IWC moratorium on commercial whaling. The summer range of Antarctic Minke Whales is also nominally protected by the IWC Southern Ocean Sanctuary, adopted in 1994, which prohibits catches south of a boundary located mainly at 40°S. Neither the moratorium nor the sanctuary’s proscription applies to taking under scientific permits issued by IWC member governments, which have been ongoing from 1987 to the present. Antarctic Minke Whales are listed on Appendix I of CITES, but Japan has held a reservation against this listing since July 2000. Japan also holds a reservation on the Sanctuary provision and therefore is not bound by it. The species is listed in Appendix II of CMS. The species is covered by generic wildlife conservation measures adopted under international treaties.|
|Citation:||Reilly, S.B., Bannister, J.L., Best, P.B., Brown, M., Brownell Jr., R.L., Butterworth, D.S., Clapham, P.J., Cooke, J., Donovan, G.P., Urbán, J. & Zerbini, A.N. 2008. Balaenoptera bonaerensis. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>. Downloaded on 20 June 2013.|
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