Balaenoptera acutorostrata, Lacepede, 1804

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

1.

Common Minke Whale

Balaenoptera acutorostrata

French: Petit Rorqual / German: Nordlicher Zwergwal / Spanish: Rorcual aliblanco

Other common names: Lesser Rorqual, Little Piked Whale, Sharp-headed Finner Whale: North Atlantic Minke Whale (acutorostrata); North Pacific Minke Whale (scammoni)

Taxonomy. Balaenoptera acutorostrata Lacépede, 1804 ,

France, “pris aux environs de la rade de Cherbourg,” Manche .

Although distinct populations of Minke whales are known to occur in different ocean basins, only a single, cosmopolitan species was officially recognized until the 1990s. A taxonomic review in 1998 concluded that the population of larger-bodied B. acutorostrata in the high latitude regions of the Southern Ocean should be separated based on genetic and morphological grounds and assigned to a different species, B. bonaerensis . This species is distinctly larger than B. acutorostrata and generally sympatric with an unnamed form in the Southern Hemisphere, generally called the dwarf Minke whale. Two subspecies recognized.

Subspecies and Distribution.

B.a.acutorostrataLacépede,1804—NAtlanticOcean.

B. a. scammoni Demére, 1986 — N Pacific Ocean.

An unnamed form is found in the Southern Hemisphere.

Descriptive notes. Total length 650-880 cm; weight 2000-2700 kg. Adult female Common Minke Whales are slightly larger than adult males. Total body length at physical maturity varies between ocean basins: 850 cm (females) and 790 cm (males) in the North Pacific Ocean; 850-880 cm (females) and 780-820 cm (males) in the North Atlantic Ocean; and 700 cm (females) and 650 cm (males) in the Southern Ocean. Coloration is also quite variable; populations of the unnamed southern form display the most complex body-color pattern of any species of rorqual. The more general coloration of the Common Minke Whale consists of a bluish dark-gray back and ivorywhite ventral surface, with streaks or lobes of lighter transitional shading, including dark thorax and flank fields and light thorax and flank patches. Dark coloration extends onto lower lips, while white coloration extends onto underside of pectoral flipper and caudal flukes. Tips and trailing edges of flukes are trimmed in dark gray. Many populations of Common Minke Whales have a roughly symmetrical pair of prominent pale gray, posteriorly convex, arched streaks that extend from just above flippers to midline of back. The most distinctive marking of the Common Minke Whale is a brilliant white patch or diagonal band that extends across middle of dorsal surface of pectoral flipper. In populations of the unnamed southern form, this flipper patch even extends onto flanks as a broad, white shoulder patch; there are dark gray, right and left, throat patches that cross ventrally onto the throat region but do not meet at the midline. In this form, there may also be a light-gray nape streak that extends from just above eyes from one side to the other, while near blowholes a pair of lightgray blowhole streaks curves posteriorly and to the left. Head of the Common Minke Whale is 22-23% oftotal body length and is marked dorsally by a single, very prominent median rostral ridge extending from blowholes to tip of snout. Lateral margins of rostrum, viewed from above, are straight and together form a narrowly triangular and pointed head—as in the slightly larger, Antarctic Minke Whale ( B. bonaerensis ). As in nearly all species of rorquals, lateral profile of head of the Common Minke Whale is relatively flat and not arched. Dorsalfin is relatively large, typically falcate in shape and positioned on back slightly less than two-thirds the distance from tip of rostrum to flukes. Pectoral flipper is relatively small and slender, with a distinctly pointed apex, and measures c.12% of total body length. As with all species of rorquals, flipper is composed of only four elongate digits (digit I is lost). Caudal flukes are also relatively small (compared with Fin Whales, B. physalus ), with width measuring only ¢.23-28% of total body length. Relatively curvilinear trailing edge of flukes lacks deep median notch. Ventral groove blubber is marked by 50-70 grooves, the longest of which terminate just anterior to umbilicus. In most rorquals (except of the Sei Whale, B. borealis , and the Antarctic Minke Whale), ventral throat grooves extend to and often beyond umbilicus. Baleen apparatus of the Common Minke Whale consists of relatively short and broad main plates, with an average length-to-width ratio typically greater than 3-3:1. Largest main baleen plates are only ¢.21 cm in length. Individual plates have slightly sinuous, cross-sectional shapes and relatively coarse bristles (¢.0-3 mm in diameter). Northern subspecies of the Common Minke Whale have baleen that is generally yellowish white in color, while baleen in the unnamed southern form appears dark gray or brown posteriorly due to dark fringing bristles. This coloration pattern is quite different from asymmetrical coloration pattern of baleen apparatus of the Antarctic Minke Whale. There are generally 230-290 baleen laminae/side or rack in the “North Pacific Minke Whale” (B. a. scammoni) and 270-325 baleen laminae/side in the “North Atlantic Minke Whale” (B. a. acutorostrata ). The Common Minke Whale is not the swiftest rorqual, with normal swimming speeds of 5-25 km/h and short bursts of up to 38 km/h. When an individual whale is surfacing at sea, blowhole and prominent dorsal fin are often visible at the same time. Blow is low (1-2 m high), bushy, and rather inconspicuous. When diving, the Common Minke Whale typically archesits tailstock (caudal peduncle) high but rarely raises tail flukes above the water. Prominent dorsal and ventral keels characterize caudal peduncle region. The Common Minke Whaleis known to breach more often than any other species of rorquals and performs full body breaches that end in either smooth reentries or spectacular splashes.

Habitat. Cosmopolitan oceanic distribution and found in all ocean basins in varying numbers. Migratory patterns of the Common Minke Whale are rather poorly known, and consequently its habitat preferences, especially on breeding and birthing grounds, are only generally defined. Habitat preference related to feeding grounds, however, is better understood, and numerous studies suggest that Common Minke Whales occur more frequently in inshore coastal waters than other species of rorquals, and it is known to swim into bays,inlets, fjords, and even up larger rivers. This behavioris well demonstrated by Common Minke Whales that summer in the high-productivity waters of the Gulf of Saint Lawrence and the adjacent Saint Lawrence River, where individuals are seen foraging in relatively shallow water on various species of schooling fish. In the eastern Bering Sea, Common Minke Whales seem to prefer neritic environments of the middle and outer shelf and coastal habitats and fjords along the Alaskan Peninsula. Like the Antarctic Minke Whale, the Common Minke Whale is more pagophilic than most rorquals and is known to penetrate into ice floes and “polynyas” (open water surrounded by ice) in polar seas. Spy hopping is common when Common Minke Whales are in icy areas.

Food and Feeding. The Common Minke Whale is relatively euryphagous compared with other rorquals and is known to feed on a variety of prey species depending on location of feeding grounds, available prey, season, and year. At least one study has found that the relatively high metabolic needs of Common Minke Whales are matched by a diet that is focused on prey that has a generally higher caloric content than prey of other species of rorquals. This is reflected in their apparent preference for feeding at higher trophic levels on schooling fish rather than planktonic crustaceans. The Common Minke Whale is generally characterized as a surfaceor near-surface feeder, in contrast to the majority of other species of rorquals that forage at greater water depths. In the Barents Sea, north of Norway and Russia, Common Minke Whales primarily feed on schooling fish including Atlantic herring (Clupea harengus) and capelin (Mallotus villosus). In years when these fish stocks collapse and spawning is well below normal, they switch to feeding almost exclusively on euphausid krill (7hysanoessa spp. and Meganyctiphanes norvegica). There are also some regional differencesin this system, with populations in the northern part of the Barents Sea switching to krill and those in the southern part feeding on a mixed-fish diet. Interestingly, there also seems to be a predator-prey linkage between planktivorous schooling fish (herring and capelin) and krill. When fish populations collapse, krill populations rebound, and Common Minke Whalesshift to feeding at a lower trophic level. In the Norwegian Sea, Common Minke Whales appear to feed in more pelagic habitats where they primarily consume sand eel ( Ammodytes spp. ), with lesser amounts of mackerel (Scomber spp.), herring (Clupea spp-), and Norway pout (Trisopterus esmarkii). In waters around Iceland, the diet of Common Minke Whales includes schooling fish (59%) and planktonic crustaceans (41%). One study has suggested that Common Minke Whales are a major predator of schooling fish in Icelandic waters, consuming more than one million tons annually. In the western North Atlantic Ocean, Common Minke Whales killed off Newfoundland were found to have been primarily eating capelin. In the North Pacific Ocean, prey preference of Common Minke Whales varies seasonally, with one study from the western North Pacific reporting that whales preferred Japanese anchovy (Engraulis japonicus) in May—June, Pacific saury (Cololabis saira) in July-August, and Pacific krill (Euphausia pacifica) in September. This study also found that Common Minke Whales avoided feeding on krill unless it was the only available prey. In other areas of the North Pacific Ocean, Common Minke Whales seasonally prefer schooling fish like sand lance ( Ammodytidae ) and walleye pollock (Theragra chalcogramma). In the Southern Ocean, the diet of the unnamed southern form is more restricted and reportedly consists of myctophid fish in the Antarctic and krill (E. similis) in other areas. When feeding, Common Minke Whales are known to practice a variety of specialized strategies, depending on location and available prey species. In waters of the Gulf of Saint Lawrence and the Saint Lawrence River, they reportedly swim into rather restricted fjords where they appear to expend a significant amount of energy concentrating schooling fish near the water’s surface prior to a feeding strike. Taking advantage of the air-water interface as a barrierto fish attempting to flee, individual whales use various surface maneuvers that seem to effectively cause the fish prey to form temporary aggregations. Such maneuvers include “head slaps” where a whale slowly raisesits head out of the water and then quickly bringsits head downward to create a large splash and audible slap; “chin-up blow” where a whale vigorously raises its head above the water as it surfaces to breathe and then continues swimming as it submerges without making a splash; and “exhale-on-the-dive” where a whale forcefully exhales air just below the air—water interface to create a water-filled spout blast. These disparate behaviors seem to generate mechanical or percussive acoustic cues that stimulate an aggregation response in the prey just prior to a surface lunge feeding strike. Although the general foraging pattern is lunge feeding, some observers report seeing Common Minke Whalesalso practicing a form ofskim feeding reminiscent of the predominant feeding strategy of the Bowhead Whale ( Balaena mysticetus) and right whales ( Eubalaena spp. ).

Breeding. Information about reproduction of the Common Minke Whale primarily comes from whaling records, which suggest that females reach sexual maturity at c.6 years of age and males at c.7 years. In the North Pacific Ocean, breeding occurs throughout the year, with peaks in January and June. An exception to this pattern has been documented for the reproductive cycle of resident Common Minke Whales in the inshore waters of the Sea of Japan, Yellow Sea, and East China Sea, where conceptions typically occur in October-November. In the North Atlantic Ocean, mating generally occurs in October—March. There is no reliable information on the composition of breeding groups. Gestation usually lasts c.10 months, and birth peaks occur in December andJune in the North Pacific Ocean and in November-March in the North Atlantic. Young are ¢.220 cm in length at birth and weigh ¢.350 kg. Typically only one young is born, but twins and triplets have been reported. Weaning occurs after 4-5 months when young have reached ¢.570 cm in length. Pregnancy rates of 87% and 97% have been reported for adult females in the western and eastern North Atlantic Ocean, respectively. This implies an annual reproductive cycle for mature female North Atlantic Minke Whales. Although little is known about the reproductive cycle of the unnamed form in the Southern Hemisphere, limited data suggests similar patterns to those of the northern subspecies. Studies of growth layers formed in wax plugs from external ear canals of dead individuals suggest that Common Minke Whaleslive up to 60 years.

Activity patterns. There is no specific information for this species, but as with most species of rorquals, annual activity patterns of the Common Minke Whale are primarily related to migration between summer feeding grounds and winter breeding, birthing, and nursery grounds.

Movements, Home range and Social organization. The Common Minke Whale has a cosmopolitan distribution and occurs in all ocean basins. Whaling mortality data suggests that, like all species of rorquals, most populations of the Common Minke Whale undergo an annual migration cycle from high-latitude feeding grounds to lowlatitude breeding and birthing grounds. Details of this migration pattern and especially locations of summer breeding grounds are poorly documented. In the North Pacific Ocean during summer, the Common Minke Whale ranges as far north as the Chukchi Sea, the Sea of Okhotsk, and the Bering Sea and south to the East China Sea in the west and the coast of central Baja California in the east. There is some evidence that Common Minke Whales off California and Baja California (including the Gulf of California) form a year-round resident population. Summer range in the central Pacific Ocean extends to ¢.30° N, with rare reports around the Hawaiian Islands. Winter distribution of the Common Minke Whale in the North Pacific Ocean is largely unknown, and even concerted efforts by Japanese “scientific expeditions” failed to locate winter breeding grounds in the south-western region of this ocean basin. The fact that Common Minke Whales show a seasonal increase in numbers in the Yellow Sea serves as evidence that they do undergo an annual migration. In the North Atlantic Ocean during summer, the Common Minke Whale is widespread and occurs as far north as Baffin Bay and the Greenland Sea in the west and Svalbard and Franz Josef Land in the east. Winter distribution in the North Atlantic Ocean is poorly known, but it extends at least to the Caribbean in the west and the Straits of Gibraltar in the east. Common Minke Whales are considered visitors to the Mediterranean Sea. They occasionally have been reported in the waters off Western Sahara and Mauritania and more frequently in waters around the Canary Islands (29° N). The unnamed southern form in the Southern Hemisphere seems to be seasonally sympatric with the Antarctic Minke Whale. This is certainly the case on austral summer feeding grounds, although the unnamed southern form tends to remain north of 60° S, while the Antarctic Minke Whale ranges farther south to 65° S and higher latitudes. Austral winter range of the unnamed southern form reportedly extends at least to 11° S in the western South Pacific Ocean off Australia and to 2° S off Brazil in the eastern South Atlantic Ocean. Individuals of the unnamed southern form are frequently observed during this season in waters off South Africa where their distinctive morphology was first recognized and described. Some populations of this unnamed southern form appear to have more resident home ranges and are not highly migratory. The Common Minke Whale typically occurs singly or in small groups of 2-3 individuals. Larger groups have been reported, especially on summer feeding grounds. As with most species of rorquals, social structure of large groups is poorly known and likely the result of high prey densities rather than true social interaction. On these occasions, groups of Common Minke Whales may occur with other species of rorquals.

Status and Conservation. CITES Appendix I, except the West Greenland population listed in Appendix II. Classified as Least Concern on The IUCN Red List. Although the global population of the Common Minke Whale is not known, the current estimate for stocks in the North Atlantic Ocean alone is ¢.182,000 individuals, including 80,487 individuals in the eastern North Atlantic, 95,943 individuals in the central North Atlantic, 3474 individuals in waters off West Greenland, and 4018 individuals along the east coasts of Canada and the USA. Given this population size, the Common Minke Whale does not meet the census thresholds for a threatened category. Nevertheless, certain populations in the North Pacific Ocean, like the so-called J-stock (Sea of Japan—Yellow Sea—East China Sea) and O-stock (Pacific waters and Sea of Okhotsk) have been subjected to intense overexploitation and are in decline and considered threatened. Active commercial whaling of Common Minke Whales did not begin in earnest until about the 1930s in the western North Pacific Ocean, the 1940s in the North Atlantic Ocean, and the 1950s in the Southern Hemisphere. This shift to increased hunting of Common Minke Whales was driven in large part by precipitous declines in populations of larger species of rorquals (the Blue Whale, the Fin Whale, and the Sei Whale). Whaling records document the killing of ¢.140,000 Common Minke Whales in the North Atlantic Ocean since the 1940s and another 29,000 in the eastern North Pacific (Sea of Okhotsk, Sea of Japan, and East China Sea) over this same period. This level of commercial whaling ended in 1987 under the international moratorium on whaling imposed by the International Whaling Commission (IWC). Nevertheless, commercial whaling by Norway resumed in 1993 under a formal objection to the moratorium. The current quota set by Norway is 1286 Common Minke Whales/year, although the actual number of whales killed in 2011 was 533. Iceland and Russia have filed similar objections, and Iceland resumed whaling of Common Minke Whales in 2006, with a national quota of 100 ind/year. Japan resumed killing Common Minke Whales in the North Pacific Ocean in 1994 under a “scientific research” permit that set an annual catch limit of 220 individuals in 2006. Aboriginal whaling off Greenland also has continued to kill Common Minke Whales from shore stations, with annual quotas of 178 ind/year and 12 ind/year set for West Greenland and East Greenland waters, respectively. Besides resumption of commercial whaling, other threats to Common Minke Whales include entanglementin fishing gear (bycatch), fatal encounters with vessels (ship strikes), anthropogenic impacts (whale watching harassment), and habitat decline. Possible effects of climate change on the extent and duration of pack ice, and its effect on the distribution of the Common Minke Whale, are unknown.

Bibliography. Arnold et al. (2005), Christiansen et al. (2013), Friday et al. (2012), Haug et al. (2002), Kuker et al. (2005), Murase et al. (2007), Perrin & Brownell (2009), Reilly et al. (2008a), Rice (1998), Spitz et al. (2012), Stewart & Leatherwood (1985), Tamura & Fujise (2002), Watanabe et al. (2012).

Figures

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

Subspecies and Distribution. B.a.acutorostrataLacépede,1804—NAtlanticOcean. B. a. scammoni Demére, 1986 — N Pacific Ocean. An unnamed form is found in the Southern Hemisphere.