Antarctic Minke whale
Introduction to Minke Whales
The Antarctic minke whale is one of seven species of baleen whales that are also called rorquals. The word rorqual is Scandinavian in origin—some say Norwegian and others say Danish—and depending on where you hear it, it can mean wrinkled or furrowed, tubed or pleated. In any case, it refers to the expandable throat grooves on the whales underside that allow them to take in massive gulps of seawater and prey while expelling the water out through the baleen, trapping the prey against the inner surface and allowing it to wash down the digestive tract.
The Antarctic minke whale (Balaenoptera bonaerensis) is the largest of two minke species. The other, Balaenoptera acutorostrata (the common minke whale) has a subspecies, B. acutorostrata ssp. (the dwarf minke whale). It wasn’t until the 1990s that genetic analyses confirmed that the differences in skeletal and morphological characteristics were a result of being separated from the minke for thousands of years. It was first described in Buenos Aires and thus the species name, bonaerensis.
Antarctic Minke Whale Identification and Distribution
Antarctic minke whales can reach up to 35 feet in length with no difference (for the boat-based observer) between males and females. Their blow is low and often indistinguishable and, when they dive, their flukes are not visible, only their falcate (sickle-shaped) dorsal fin. They frequently approach vessels, crossing their bow and, in some Antarctic harbors, their curiosity draws them to small boats, including Zodiacs— instantly making them one of the favorite whale species in the Antarctic.
Distinguishing between the species is made slightly easier by the fact that the common minke whale is most often found in the northern hemisphere and the Antarctic minke is found in the southern hemisphere, circumglobally. They do, however, overlap considerably with the range of the dwarf minke whale, which is the southern hemisphere subspecies of the minke. Three things that set the Antarctic minke apart are its larger size, a dorsal fin that is set farther back on the body, and asymmetrical coloration of its baleen. There are more anterior white plates of baleen on the right side of the Antarctic minke than on the left side, which is dominated by dark gray baleen. Also the dwarf minke whale has a very distinctive white patch on its flipper, unlike the light gray flipper of the Antarctic species. Both the tail fluke and the underside of the flippers are white.
Antarctic Minke Whale Behavior
Spyhopping is not unknown, particularly when in the vicinity of pack ice. It is apparently used not for prey location but for orientation. Minkes are active swimmers and their sleek profiles allow for fast swimming.
Antarctic Minke Whale Migration
As with many whales, migration is highly dependent on the life history of their prey and there is no question that the presence of the majority of the Antarctic minke population in the Southern Ocean in the austral summer is linked to the profusion of krill in the surface waters. They lunge through the bioluminescent schools of the shrimp-like crustaceans and gulp large quantities as do other rorqual whales. It is thought that as the harvest of humpback and blue whales peaked in the early 20th century, the abundance of Antarctic minkes increased as a result of the increased food availability. Early on the considerably smaller minkes were less desirable as a harvestable species. Some current ecological models suggest that, as blue whales gradually increase in number with decreased harvest pressure, the Antarctic minke will gradually decrease.
In Antarctica minke density increases beginning in November, peaks in January and then declines again in February. After that their movements are less understood. They are known to generally be in the pelagic, or open ocean, beyond the continental shelf in the lower latitudes around Australia from April to September and have been noted off Brazil from June through December, but age classes and sexes differ. And there have been numerous records of minkes over-wintering in the Antarctic as well.
Antarctic Minke Whale Breeding
While the biology of their breeding cycle is incomplete, it is generally agreed that they breed in the lower latitudes with mating occurring June through December. There does not appear to be much feeding in these months, with the Antarctic minkes assuming the life style of many of the rorquals in that they bulk up when the food is plentiful and slack off in the breeding season. Males reach sexual maturity when they are approximately eight years old (24 feet long) and females when they are seven to eight years old (26 feet long). With gestation lasting about 10 months, calving is at its peak in May and June, generally in the vicinity of the relatively warmer waters of the Antarctic Convergence. Newborns are about nine feet long and weigh 1,000 pounds. By the time they are weaned five months later they have reached at least 18 feet in length.
Their life span is even less understood, with most information coming from harvest data. It is estimated that the Antarctic minke whale provides up to 85% of the orca, or killer whale, diet in the Southern Ocean.
Antarctic Minke Whale Population
Issues with distinguishing one species from the other have hampered attempts to accurately estimate the population of the Antarctic minke. However, in 1989, the population of all minke whales in the southern hemisphere was estimated to be 761,000.