Balaenoptera bonaerensis, Burmeister, 1867

Russell A. Mittermeier & Don E. Wilson, 2014, Balaenopteridae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 242-299 : 242-299

2.

Antarctic Minke Whale

Balaenoptera bonaerensis

French: Rorqual antarctique / German: Sudlicher Zwergwal / Spanish: Rorcual austral

Other common names: Southern Minke Whale

Taxonomy. Balaenoptera bonaerensis Burmeister, 1867 ,

near Belgrano, Prov. Buenos Aires, Argentina.

The type specimen was found at the mouth of Riachuelo Medrano, on the western shore of La Plata Estuary, in the South Atlantic Ocean. Until the 1990s, B. bonaerensis was considered to represent a large-bodied Southern Hemisphere subspecies of B. acutorostrata . Monotypic.

Distribution. Southern Hemisphere, from tropical waters (7° S) to the edge of and within pack ice (65° S) around the Antarctic continent. One record from off the coast of Suriname (4° N) in the Northern Hemisphere.

Descriptive notes. Total length 840-1020 cm; weight 6800-11,000 kg. Adult female Antarctic Minke Whales may be up to 100 cm longer than males. Mean lengths of sexually mature females and males are 900 cm and 840 cm, respectively. The Antarctic Minke Whale is the second smallest species of rorquals, after the Common Minke Whale ( B. acutorostrata ), and it has a dark-gray to black back and an almost all white ventral area. Along sides of body, back, and underbelly, colorations meet to form a diffuse, light-gray zone oftransition, generally with a diffuse boundary between gray and black. A dark thorax field separates discrete, light-gray lateral patches on anterior portion of thorax and posterior flank. Typically, there are two light-gray wavy streaks extending posterolaterally from near blowhole. The Antarctic Minke Whale has slim and pointed pectoralflippers that typically are solid gray, with white leading edges and lacking distinctive brilliant white pectoral band that characterizes Northern and Southern Hemisphere forms of the Common Minke Whale. Undersides offlippers and caudal flukes of the Antarctic Minke Whale are white with a dark-gray trim. Head is marked by a single, prominent, median rostral ridge that extends from blowhole to tip of snout. Like the head of the Common Minke Whale, the Antarctic Minke Whale has a narrowly triangular dorsal outline with a sharply pointed rostrum. In lateral view, head is relatively flat, with a straight (not arched) dorsal profile. Dorsal fin is relatively tall, typically falcate, and compared with larger species of rorquals,it is positioned more anteriorly and slightly less than two-thirds the distance from tip of rostrum to flukes. Pectoralflipper is relatively slender and lanceolate in shape, with a distinctly pointed apex, and is c.13% of total body length. As with all species of rorquals, there are only four elongate digits in the flipper (digit I is lost). Caudal flukes are also relatively small, with a width of only ¢.25% oftotal body length. Viewed from above, flukes are distinctly triangular with linear anterior and posterior margins. Median notch is variably developed but never deeply inset. Ventral groove blubberis marked by c.60-70 grooves,the longest of which terminate up to 30 cm anterior to umbilicus. In other species of rorquals (except the Sei Whale, B. borealis , and the Common Minke Whale), ventral grooves extend to and often beyond umbilicus. Baleen apparatus is very distinctive in its asymmetrical coloration pattern, in which plates on left side are yellowish-white only at the very front of rack, which is mostly composed of black plates. On the rightside, anterior one-third of plates is yellowish-white and posterior two-thirds of plates are black. This coloration is quite different from the symmetrical, uniformly white baleen coloration pattern seen in the Common Minke Whale. In general shape, baleen plates are relatively short and broad, with longest plates measuring 28-31 cm in length. Fringing baleen bristles are relatively coarse (c.0-3 mm in diameter) and intermediate in size between bristles of the Sei Whale and the Blue Whale ( B. musculus ). Baleen laminae generally number 260-360/side or rack. The Antarctic Minke Whale can cruise at speeds up to 20 km/h and is known to dive for up to 20 minutes at a time, although its dives typically last for just a few minutes. When swimming at sea, blowhole and large dorsalfin typically are visible together when an individual surfaces. Blow is low, generally 1-2 m high, and bushy. Like the Common Minke Whale, sharply defined dorsal and ventral keels mark the caudal peduncle area of the Antarctic Minke Whale Habitat. Mainly pelagic and beyond the continental shelf break; shelf waters inshore of the shelf break, with or without sea ice, on summer feeding grounds. This small rorqual has a very broad distribution in the Southern Hemisphere, and although details of its migratory pattern are poorly known, it seems to follow a typical seasonal migration cycle from a high-latitude summer range to a low-latitude winter range. The Antarctic Minke Whale is perhaps the most pagophilic ofall species of rorquals, and helicopter surveys have reported them as far as 350 km south of the edge of the pack ice during austral summer. Some researchers suggest an evolutionary link between the slim, compact body and relatively small pectoral flippers of the Antarctic Minke Whale and its ability to invade narrow leads between ice floes. They further suggest that its narrowly pointed, stout rostrum facilitates breaking through newly formed sea ice to create breathing holes far from the pack-ice edge. These breathing holes are in turn used by other air-breathing marine predators (seals and penguins), which otherwise would likely not be able to live in pack ice. In this context, the Antarctic Minke Whale may function as a type of keystone species in the pack-ice ecosystem around Antarctica.

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Food and Feeding. Unlike the more euryphagous Common Minke Whale, adult Antarctic Minke Whales primarily consume Antarctic krill (Euphausia superba) on summer feeding grounds, although smaller krill species (E. crystallorophias and E. spinifera) are also eaten. Amphipods (Themisto gaudichaudi) and occasionally perciform fish (Pleuragramma antarctica) have also been reported as prey species. Antarctic Minke Whales often lunge at the surface when feeding, but they also feed at depths generally shallower than those of other species of rorquals on Antarctic feeding grounds. There appears to be some ecological control of the diet of Antarctic Minke Whales, with Antarctic krill the primary prey species in offshore waters such as in the Weddell Sea and E. crystallorophias the primary prey species in coastal shelf waters such as the Ross Sea and Prydz Bay. Immature and mature male Antarctic Minke Whales reportedly spend up to 90 days on feeding grounds, while mature females spend up to 120 days feeding. From whaling postmortem statistics, it has been proposed that volume of krill consumed by a foraging Antarctic Minke Whale is 83.7-325.5 kg/day (c.2:7-4% of body weight) and that mean per capita krill consumption for the entire summer feeding season is 7500 kg and 16,400 kg for immature and mature males, respectively, and 12,500 kg and 39,100 kg for immature and mature females. This volume of built-up energy stores apparently is sufficient to sustain Antarctic Minke Whales through the austral winter because they appear to forage very little or not at all during this period. Reports of limited winter-feeding activity off South Africa suggest that even atthis time of year Antarctic Minke Whales are essentially stenophagous on krill.

Breeding. Information about reproduction of the Antarctic Minke Whale is very limited and primarily comes from postmortem whaling records, which suggest that females reach sexual maturity at 7-8 years of age and males at c.8 years. Some evidence suggests that age of sexual maturity has been declining since the 1950s due to commercial exploitation. Antarctic Minke Whales in the South Atlantic Ocean are reportedly polygamous and have separate breeding and birthing areas—the former in waters off Brazil and the latter at higher latitudes (25-40° S). There is little information about composition of breeding groups, although some data suggest that males dominate group structure on breeding grounds. Mating occurs in the austral winter (June-December), with a peak in August-September. In the western South Atlantic Ocean, most sightings of Antarctic Minke Whales were of single individuals, although groups of one male and two females or two males and three or more females were also noted. Gestation usually lasts ¢.10 months, and birth peaks appear to occur during late May and earlyJune in waters north of the Antarctic Convergence. Typically, only one young is born, although twins and triplets have been reported. Young are ¢.280 cm in length and weigh 455 kg at birth. Young grow at a rate of c¢.1 cm/day while nursing and are weaned at 3-6 months on summer feeding grounds at ¢.570 cm in length. Weaned young may remain with their mothers for up to two years. Female Antarctic Minke Whales ovulate c.4 months after giving birth, suggesting a 14month birth interval. Pregnancy rates reportedly are ¢.90% for most ofthe year, indicating a rapid reproductive capacity of the Antarctic Minke Whale. It has been proposed that the extended mating season contributes to this near annual reproductive cycle. Nevertheless, whaling records for female Antarctic Minke Whales killed on winter breeding grounds off Durban, South Africa, suggest a lower reproductive rate closer to 78%. Mortality rate of young is unknown. Studies of growth layers formed in wax plugs taken from external ear canals of dead Antarctic Minke Whales suggest that they live up to 73 years.

Activity patterns. There is limited information on daily activity patterns of Antarctic Minke Whales, but one study reported a diurnal feeding pattern, with feeding activity reaching a peak early in the morning before 05:00 h, with a possible second feeding episode in the late afternoon. Like other species of rorquals, annual activity patterns of Antarctic Minke Whales are primarily related to migration between summer feeding grounds and winter breeding, birthing, and nursery grounds.

Movements, Home range and Social organization. The Antarctic Minke Whale essentially is a Southern Hemisphere endemic, with a single sighting reported from tropical waters in the western North Atlantic Ocean off Suriname (4° N). An additional Northern Hemisphere report is based solely on mtDNA and microsatellite DNA analyses of tissue samples collected from a whale captured during boreal summer in the northeastern Atlantic Ocean in the Arctic (70° 57° N), suggesting that rare vagrants may make it into boreal high-latitude waters. The Antarctic Minke Whale,iike other species of rorquals, has an annual migration cycle that includes traveling between high-latitude summer feeding grounds and low-latitude winter breeding, birthing, and nursery grounds. Some data indicate that individuals may overwinter in Antarctic waters. During austral summer, Antarctic Minke Whales typically occur in high numbers at 60-65° S and even as far as 70-75° S in Prydz Bay, the Ross Sea, and the Weddell Sea. During austral winter, most populations of Antarctic Minke Whales migrate north to poorly defined breeding grounds at middle latitudes (10-30° S) in the subtropical Pacific Ocean, off north-eastern and eastern Australia, off western South Africa, and off north-eastern Brazil. Analysis of whaling records suggests some degree of sexual segregation of Antarctic Minke Whales on austral summer feeding grounds, where mature males dominate populations early in winter before the arrival of mature females later in the season. Populations ofAntarctic Minke Whales occur south of 40° S in the austral summer; densities increase from November onward, peaking in January and declining in February. During austral winter when Antarctic Minke Whales are in warmer waters, groups can consist of 2-5 mature individuals with males predominating. In contrast, migrating individuals are often solitary, except for mother—offspring pairs. Antarctic Minke Whales have been reported off Durban, South Africa (29° 53’ S), from April to at least September, but they generally occur in June-December in the western South Atlantic Ocean off Brazil (7° S). Like most species of rorquals, Antarctic Minke Whales are not gregarious and tend to swim alone or in small groups of 2—4 individuals,although large feeding aggregations of up to 60 individuals may form in polar waters near the ice edge. Social structure of these larger groups is not well known, and associations may simply be passive and a function of prey abundance rather than actual social interaction. No accounts of cooperative feeding behavior have been reported for the Antarctic Minke Whale. Whaling records suggest an austral winter sex ratio of c.2:1 off of South Africa until about September,after which sex ratio may drop to 1-4:1.

Status and Conservation. CITES Appendix I. Classified as Data Deficient on The [UCN Red List. This IUCN classification is based on uncertainty surrounding the cause of an apparent 61% decline in populations of Antarctic Minke Whales in the Southern Ocean from an estimated 843,000-869,000 individuals in 1978-1991 to ¢.339,000 individuals in 1991-2004. If the decline turns out to be a sampling artifact or temporary, the Antarctic Minke Whale would be classified as Least Concern. If it is an actual decline, however, the ranking would be changed to Endangered. Antarctic Minke Whales in the Southern Ocean largely escaped intense commercial exploitation until the 1970s because of their small size and former abundance of larger species of rorqual like the Blue Whale, the Fin Whale ( B. physalus ), and the Sei Whale in these waters. Whaling records indicate that between the 1957-1958 season and cessation of commercial whaling at the close of the 1985-1986 season, ¢.98,000 Antarctic Minke Whales were killed on summer feeding grounds around Antarctica and 14,600 individuals on winter breeding grounds off Brazil. During this same period, an additional 1113 individuals were killed in waters off South Africa. Although protected from commercial exploitation by the international whaling moratorium, Antarctic Minke Whales continue to be hunted byJapanese under a “scientific permit” that allows ¢.935 ind/year to be killed. No information on population trends is currently available for the Antarctic Minke Whale. Besides continuation of “scientific research” kills of Antarctic Minke Whales by Japanese, other threats include entanglement in fishing gear (bycatch), fatal encounters with vessels (ship strikes), pollution, habitat decline, and anthropogenic competition for resources. Possible effects of climate change on the extent and duration of pack ice are unknown, butit could pose a major threat to this pagophilic rorqual. Killer Whales (Orcinus orca) are a natural predator of Antarctic Minke Whales, which in at least one report are suggested to make up 85% of the diet of these apex marine predators.

Bibliography. Ainley et al. (2012), Arnold et al. (2005), Best, PB. (1982, 1985), Delarue et al. (2013), Glover et al. (2010), Horwood (1990), Kasamatsu etal. (1998), Lucena (2006), Perrin & Brownell (2009), Reilly et al. (2008b), Ruegg et al. (2010), Scheidat et al. (2007), Tamura & Konishi (2009), Thiele & Gill (1999), Williamson (1959).

Figures

1. Common Minke Whale (Balaenoptera acutorostrata), 2. Antarctic Minke Whale (Balaenoptera bonaerensis), 3. Sei Whale (Balaenoptera borealis), 4. Bryde’s Whale (Balaenoptera edeni)

Distribution. Southern Hemisphere, from tropical waters (7° S) to the edge of and within pack ice (65° S) around the Antarctic continent. One record from off the coast of Suriname (4° N) in the Northern Hemisphere.