The IWC recognizes four stocks of minke whales in the North Atlantic: Northeast Atlantic, Central North Atlantic, West Greenland, and Canadian East Coast. The last includes the US east coast. Population estimates were last reviewed by the IWC SC in 2003 (IWC 2004a), but a new estimate for West Greenland was accepted in 2006 (IWC 2007a). The best/most recent available estimates are listed in Table 1 in the linked PDF document (which constitutes an integral part of this assessment). These total about 182,000.
No abundance estimate is available for the Newfoundland area where there was a small-scale fishery for minke whales during 1948-72 (Mitchell 1974).
Minke whales have been exploited in the North Atlantic, mainly since the 1940s, and recorded catches total about 140,000 (IWC 2006a). The largest catches have been by Norwegian “small-type” whalers who have taken about 120,000 since 1948, mainly in the Northeast Atlantic. Annual catches peaked at over 4,000 in the late 1950s, declining to about 2,000 annually in the early 1980s. Catches were phased out from 1984 to 1987. Commercial minke whaling resumed in 1993 at a lower level and continues to the present.
About 4,000 minke whales were taken off Iceland during 1941-85, but recent abundance estimates imply that this would have had no discernible effect on the population. About 8,000 common minke whales have been caught by small-type whaling off West Greenland, mainly since 1960. The present catch limits (up to 175 annually for the years 2003-7) were set by the IWC in the absence of advice from its SC. Some concerns have been expressed by the IWC SC over the sustainability of the catch levels given the uncertainty over the size of the population available to the hunters off West Greenland (IWC 2007a) although sex ratio information suggests that the population has not been significantly reduced by the catches (Witting 2006).
North Pacific
The IWC recognizes three management stocks in the North Pacific: Sea of Japan – Yellow Sea – East China Sea; Okhotsk Sea – West Pacific (west of 180°); and “Remainder” (east of 180°). Considerable research has been undertaken since the IWC designation indicating the potential for further stock structure both within the Okhotsk Sea - West Pacific area and in the "Remainder" area.
Okhotsk Sea – West Pacific
The IWC SC conducted an assessment of the Okhotsk Sea – West Pacific stock in 1991. Surveys in the summers of 1989 and 1990 yielded an abundance estimate of 25,049 (CV 0.316) of which the bulk (19,209; CV 0.339) was in the Okhotsk Sea (Buckland et al. 1992). Although the Okhotsk Sea was surveyed again in 1992 and 2003 (Miyashita 2004), an abundance estimate from the last of these surveys has yet to be presented.
About 13,000 minke whales have been recorded caught by Japanese coastal whaling during 1948-87 (IWC 2006a), of which probably all but about 1,000 were from the Okhotsk Sea-West Pacific stock. Annual catches peaked at over 500 in 1973, declining to 300 in 1987 when commercial whaling was suspended. Catches resumed in 1994 under scientific permits issued by the Government of Japan and have increased since then. The catch in 2006 was 195 from a permit for 220 (Miyashita and Kato 2007).
The above catch figures do not include net catches, which are not subject to whaling regulations. Reported net catches of minke whales off Japanese coasts averaged only about 5 per year in the 1980s, but Tobayama et al. (1992) estimated that the true level in 1989 was about 100. Based on genetic analysis of samples of whale products collected in commercial markets during 1993-1999, Baker et al. (2000) also estimated the net catch to be about 100 per year. During the 1990s, reported net catches averaged about 20 per year. After new regulations were introduced in 2001 that provided an incentive to report (only those catches that are reported and genetically sampled can be legally marketed), reported net catches have averaged 127 per year (Japan 2002-2006). About 55% of the reported net catches during 2001-05 have been on the Sea of Japan and East China Sea coasts (IWC 2007b), and would be mainly from the “J stock” (see below), with the remainder from the Okhotsk Sea-West Pacific Stock.
The issue of subpopulation structure within the Okhotsk Sea – West Pacific Stock has been discussed extensively by the IWC SC in the context of preparations for implementing the Revised Management Procedure (RMP) for western North Pacific minke whales, but with inconclusive results (IWC 2004b).
Although population models suggest that, when considered as a single biological unit, the Okhotsk Sea-West Pacific stock has declined little under the influence of past or present catches (IWC 2004c), the situation should be kept under review; in particular, the more recent 2003 survey data should be analyzed to update the 1990 and 1992abundance estimates for the Okhotsk Sea.
Sea of Japan (East Sea) – Yellow Sea – East China Sea stock (“J stock”)
The reproductive cycle of this stock, usually referred to by biologists as the “J stock”, appears to be four months out of phase with other Northern Hemisphere minke whales, with conceptions occurring in October-November instead of February-March for the Okhotsk Sea – West Pacific stock (Omura and Sakiura 1956, Kato 1992). This is the only known case of breeding asynchrony in baleen whale populations from the same hemisphere. A degree of reproductive isolation between the two stocks is also suggested by frequency differences at selected allozyme loci (Wada 1984). A segregation of maternal lineages is indicated by marked differences in the frequencies of mtDNA haplotypes (Goto and Pastene 1997; Baker et al. 2000).
About 16,000 whales are recorded to have been taken from this stock by commercial whaling based in South Korea during 1940-86, in addition to about 1,000 from western Japan (IWC 2006a). Catches peaked at 1,033 in 1977. Catches were phased out with a complete moratorium to take effect from 1986 (IWC 1984).
However, net catches (which are not subject to whaling regulations) began in Korea in the late 1980s (Kim 1999). From 1996 onwards, a regulation has been in effect that requires the reporting of such catches. During 1996-2005, reported catches averaged 90 per year. However, a genetic analysis of samples of whale products on Korean commercial markets yielded an estimate of 827 (SE 164) whales entering the market in the period 1999-2003, as compared with a reported catch of only 458 whales (Baker et al. In press). This suggests that reporting is still far from complete, and that the total J-stock catch from South Korea and Japan (see above) has exceeded 200 per year over the last 10 years. Minke whales are also reported to be a common bycatch in China, but no figures are available (IWC 2006c).
Surveys in the range of the J-stock have been conducted from 1999 in the waters of the Republic of Korea (Sohn et al. 2005) and from 2002 in Japanese waters (Miyashita 2004), but no estimate of total population size is available. The IWC in 2005 (IWC 2006b) endorsed plans for joint research by the range states (Republic of Korea, China, Japan, Russian Federation), and a comprehensive survey in the waters of all four countries is planned for 2007.
Eastern North Pacific (east of 180°E)
Population estimates are available only for parts of the eastern North Pacific, e.g. 1,015 (CV 0.73) for the west coast of the US during 1991-2001 (Anon. 2003) and 810 (CV 0.36) and 1003 (CV=0.26) respectively in the central and southeastern Bering Sea (Moore et al. 2002), and 1,232 (CV=0.34) for coastal waters of the northern Gulf of Alaska and the eastern and central Aleutian Islands (Zerbini et al., 2006). Minke whales are apparently quite abundant in the offshore Gulf of Alaska (Miyashita et al. 1995) but no abundance estimate is available for that region.
Differences in vocalizations during the breeding season strongly suggest further population structure within this area. Different calls are found on either side of about 135 degrees W longitude (Rankin and Barlow 2005).
Southern Hemisphere
It is not possible at this time to estimate the abundance of B. acutorostrata in the Southern Hemisphere, because most of the available quantitative sighting data do not distinguish it from the much more numerous B. bonaerensis with which it is partially sympatric. B. acutorostrata has not been subject to significant exploitation in the Southern Hemisphere.
Generation time
No satisfactory method of age determination has been developed for this species to date. Therefore the value of 22 years from Taylor et al. (2007) was used. Unlike Antarctic minke whales, common minke whales tend not to develop ear plugs with readable layering (Lockyer 1984). A method based on layer counts in tympanic bullae (Christensen 1981) could not be reproduced by later workers. Other age-estimation methods are being investigated, but none is yet at the stage where it can be applied reliably to this species (Olsen and Øien 2002).