Balaenoptera acutorostrata Lacepede 1804

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

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