Balaenoptera bonaerensis
Antarctic minke whale

Balaenoptera bonaerensis occurs in polar to tropical waters of the southern hemisphere. It occurs in large numbers south of 60º S, throughout the Antarctic. The distribution is more difficult to assess north of the Antarctic because of its co-occurrence with Balaenoptera acutorostrata. As a result, the boundaries of the species’ winter distributions remain largely undefined. Balaenoptera bonaerensis is observed off the coast of Brazil and South Africa and there have been occasional sightings in Peru. An unknown proportion of the species remains in Antarctic waters during the winter. (Mead and Brownell Jr, 2005)

Balaenoptera bonaerensis can be found in marine waters from polar to tropical regions, generally within 160 km of the edge of pack ice. While mostly found at the ice edge, B. bonaerensis can also be found within the pack ice and in polynyas. Association with pack ice is especially pronounced during winter. (Friedlaender et al., 2004; Mead and Brownell Jr, 2005; Schueller, 2004)

Balaenoptera bonaerensis is among the smallest rorqual species. Mature males average 8.36 m in length and weigh 6.85 tons, but they can reach a total length of 9.63 m and a weight of 11.05 tons. Females are slightly longer with a mean total length of 7.57 m and a maximum measured length of 10.22 m. On average, B. bonaerensis is slightly longer than all forms of B. acutorostrata. Similar to common minke whales, Antarctic minke whales are dark grey on the back with a pale ventral side. The main recognition character that allows for the distinction of Antarctic minke whales from common minke whales is the absence of a white patch on the flippers in Antarctic minke whales. The rostrum is narrow and pointed. The dorsal fin is hook-shaped and located about two-thirds the length of the body from the anterior. Baleen plates are black on the left side and on the posterior 2/3 of the right side, while the remaining baleen plates are white. The baleen plate filaments average about 3.0 mm in diameter. Antarctic minke whales have larger skulls than common minke whales. (Konishi et al., 2008; Perrin and Brownell Jr, 2002)

In an 18-year study of energy storage in Antarctic minke whales, Konishi et al. (2008) aged a total of 4,268 mature whales (mature males and pregnant females). They used measurement of the earplug, a layered keratinized and fatty structure inside the external auditory canal, to determine age. The oldest whale aged in their study was 73 years old according to this technique. (Konishi et al., 2008; Lockyer, 1972)

Minke whales in Antarctic feeding areas can be solitary or form small groups. They are generally seen in groups of 2 to 4 individuals. Distribution of individuals within these groups is relatively random. There seems to be an enhanced amount of clustering in relatively enclosed areas (e.g., bays) as opposed to open water habitats. Antarctic minke whales sometimes uses the rostrum to break ice several centimeters in thickness to create breathing holes. The distance between two neighboring holes usually ranges from 200 m to 300 m. The species is often said to actively avoid moving ships and uses “porpoising” behavior in doing so. However, minke whales are also notorious for their curiosity and are one of the most frequently observed rorquals because of their habit of approaching stationary boats. Fleeing behavior is not as commonly observed in the pack ice. Antarctic minke whales are fast swimmers, capable of speeds up to 20 km/hr. They typically dive for 2 to 6 minutes, with 1 minute at the surface during which they blow 5 to 8 times. They can dive for up to 20 minutes. Minke whales breach more often than other baleen whales. Most populations seem to migrate between summer and winter grounds, but some populations appear to remain in Antarctic waters throughout the year. (Ainley et al., 2007; Kasamatsu, Ensor, and Joyce, 1998; Schueller, 2004)

Home Range

Home range sizes have not been estimated.

Balaenoptera acutorostrata in the North Atlantic produces an extensive range of sounds. Mellinger and Carson (2000) analyzed pulse trains of minke whales in the Caribbean and classified them into two categories: the “speed-up” trains and the less common “slow-down” trains. Pulse trains are sequences of pulses produced at regular or irregular intervals. There is limited information, however, as to whether this type of vocalization is also present in B. bonaerensis and what function it serves in B. acutorostrata. (Mellinger and Carson, 2000)

Antarctic minke whales feed mainly on krill (Euphausia superba). Euphausia superba comprises 100% of stomach contents of minke whales caught at the ice edge and 94% (by weight) of the stomach contents of minke whales in the offshore zone. Euphausia crystarollophias was also found in smaller quantities in the stomachs of Antarctic minke whales caught in coastal areas. Other prey include Euphasi frigida and Thysanoessa macrura. This is in contrast to common minke whales, which feed on a more diverse array of fish and invertebrates. Antarctic minke whales feed primarily in the early morning and late evening and most feeding activity is observed at the edge of pack ice. Daily food consumption in the summer was estimated at 3.6 to 5.3% of body weight, representing an important proportion of krill biomass in the study area. It is likely that Antarctic minke whales eat much smaller quantities of food during the austral winter or perhaps forage very little at all on wintering grounds (Best 1982 as cited in Reilly et al., 2008). The blubber layer thickens as the feeding season progresses but mean blubber thickness in individuals has decreased over the 18 year period between 1987 and 2005. This might suggest a decrease in food availability in Antarctic waters. (Ichii and Kato, 1991; Konishi et al., 2008; Reilly et al., 2008; Tamura and Konishi, 2006)

Antarctic minke whales seem to be the main prey for type-A killer whales in Antarctica (Orcinus orca). Antarctic minke whales also represent a large proportion of Japanese whaling catches and have become an increasingly important in recent years with the decline of larger Balaenoptera species populations. (Pitman and Ensor, 2003; Reilly et al., 2008)

Antarctic minke whales are predators of krill, mainly Euphasia superba and are preyed on by killer whales (Orcinus orca). They may compete with other mammals and birds feeding on krill, such as penguins (Spheniscidae), crabeater seals (Lobodon carcinophaga), and blue whales (Balaenoptera musculus). Antarctic minke whales may also be the host of several types of microorganisms. For instance, they are often observed to have a film of diatoms on their skin. The extent of this film is thought to be correlated with the amount of time a whale has spent in cold waters. (Konishi et al., 2008; Pitman and Ensor, 2003; Tamura and Konishi, 2006)

There are no recognized direct negative impacts of Antarctic minke whales on human populations. One could hypothesize, however, that potential food competition with other economically important whale species, such as blue whales (Balaenoptera musculus), could have a negative economic impact on their harvest. Larger whale species are considered more valuable in that they provide more meat per unit catch.

Since 1986, commercial whaling has been prohibited by the International Whaling Commision. Balaenoptera bonaerensis is currently taken for scientific whaling by the Japanese. The animals caught in that process can then be sold on the market for food. Before the decline of larger whale species, such as fin and blue whales (Balaenoptera physalus and Balaenoptera musculus, respectively), B. bonaerensis was rarely targeted by whalers due to its comparatively smaller size. Consequently, whaling on Antarctic minke whales has only started relatively recently, since 1971 (IWC 2006 as cited in Reilly et al., 2008). (Reilly et al., 2008)

The International Union for the Conservation of Nature and Natural Resources (IUCN) currently lists the species as “Data Deficient”. However, it has been suggested that the species has declined by about 60% between the periods 1978–91 and 1991–2004. If the decline is shown to be an artifact or to have been transient, the species would then be classified as “Least Concern” whereas it would be classified as “Endangered” if it was demonstrated to be an actual decline. The Peru population was added to Appendix I of CITES in 1986 and withdrawn from it in 2001. The predicted substantial decline in the extent of Antarctic sea ice may dramatically effect krill populations and the Antarctic minke whale populations that depend on them. (Reilly et al., 2008)

Antarctic minke whales used to be considered a subspecies of Balaenoptera acutorostrata, B. a. bonaerensis. In the late 1990s, however, it came to be recognized as a separate species, Balaenoptera bonaerensis (Burmeister, 1867). Phylogenetic analysis of the mitochondrial DNA control region led Àrnason et al. (1993) to elevate B. bonaerensis to full species status. Their analysis also led to the conclusion that B. bonaerensis and B. acutorostrata are sister taxa more closely related to each other than to any other species of the genus Balaenoptera. (Àrnason, Gullberg, and Widegren, 1993)

Although the geographic ranges of both minke whale species overlap in the Southern Hemisphere, Balaenoptera bonaerensis is numerically dominant in high southern latitudes (south of 60ºS). By analysis of mitochondrial DNA, Pastene et al. (2007) estimated the divergence of B. acutorostrata and B. bonaerensis to have occurred about 5 million years ago in a period of global warming during the Pliocene. (Pastene et al., 2007)

A relatively large amount of research has been conducted on the reproductive biology of Antarctic minke whales. Tetsuka et al. (2004) found that there was a very high level of variation in the size of ovaries of the 94 immature females they captured. In addition, they found that ovarian size almost doubled during the prepubertal period. Over time, the developed ovary contains larger but less abundant follicles. Some researchers have even attempted to perform in vitro maturation of Antarctic minke whale oocytes (Iwayama et al., 2005). (Iwayama et al., 2005; Ohsumi et al., 2004)