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Portrait of the Minke Whales

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The common minke whale or lesser rorqual (Balaenoptera acutorostrata) is the most abundant of the baleen whales in the northern hemisphere, but our knowledge of this rather solitary, small and cryptic species is still very limited. We don’t know the size and structure nor the dynamics of most populations, there is not a single identified breeding or calving ground, migration routes are virtually unknown and there are significant knowledge gaps with regard to their behavior, ecology, reproduction, genetics and conservation. The minke whale has perhaps the most complex population structure of any whale, with evidence of considerable segregation by sex, age, and reproductive condition. The maximum length is 10.7 m, but northern minke whales are generally a bit smaller (8-9 m) than their southern counterparts, the Antarctic minke whales.

Classification and Distribution

Minke whales belong to the group of rorqual whales (Balaenopteridae), one of three baleen whale families, the other two being right whales (Balaenidae) and gray whales (Eschrichtiidae). Together, these three families form the suborder of baleen whales (Mysticetes) that comprises 13 different species.

There are currently two distinct recognized species of minke whales, the common or northern minke whale (B. acutorostrata) and the Antarctic minke whale (B. bonaerensis), which occurs throughout the southern hemisphere. The two species differ significantly in many genetic, skeletal and external features. A small subspecies of the northern minke whale, called dwarf minke whale, occurs almost exclusively in the southern hemisphere. However, South Atlantic dwarf minke whales are more closely related to North Atlantic common minke whales, than either is to dwarf minke whales from the South Pacific, indicating that their subspecies status needs further study. In addition, the common minke whales also have separate subspecies designations in the North Atlantic (B. a. acutorostrata) and the North Pacific (B. a. scammoni). DNA analysis has revealed that common minke whales and Antarctic minke whales have been separated for thousands of years and are genetically very distinct taxa. In fact, the common minke whale is genetically more closely related to the Bryde’s whale (B. edeni) than to the Antarctic minke whale, which in turn is more closely related to the much larger sei whale (B. borealis). As if this was not complicated enough, another rorqual whale discovered as recently as 2003, Omura’s whale (B. omurai), is intermediate in size between the Antarctic minke whale and the Bryde’s whale, but represents an ancient evolutionary lineage and is more closely related to the blue whale (B. musculus).

Common minke whales are distributed from the tropics to the ice edges. Like other balaenopterids, however, they tend to be found in equatorial waters during the winter and in (sub)-polar waters in summer, although the species seems to be widely distributed in all seasons and migrates in a manner hard to predict from year to year. Whilst there is little direct evidence to demonstrate long distance migration, seasonal variation in abundance and distribution suggests that the species probably undergoes some migration from higher latitudes in summer to lower latitudes in the winter. Pregnant females seem to move farther north during the summer than lactating and immature females, but in some temperate waters these animals are present year round.

Reproduction and Social Structure

Like other mammals, female cetaceans have two ovaries, a uterus, a vagina and, during gestation, a placenta. The uterus is bicornuate, i.e. with two conjoined cavities, unlike that of humans, which is composed of a single cavity. However, twins are extremely rare in whales and very unlikely to survive due to the limited milk supply from the mother. Fetal growth rates in baleen whales are the fastest in the animal kingdom, about 20 times that of primates. Gestation is about ten months in both species and the calf is born tail-first with very few exceptions, an adaptation to underwater delivery in all cetaceans, to prevent the calf from drowning by doing it’s first “breath” while still underwater. Calves can swim at birth, but the mother will push the new-born baby up to the surface for the first few breaths. Fat content in maternal milk of minke whales is above 30 %, resulting in very high growth rates during lactation, which lasts about six months. Calves weigh 150-320 kg at birth and measure 2.2 to 2.8 m, but double in size over the first six months. During this extremely fast growth period, their body weight increases 5-10 times, a process that takes 4-11 years in humans.

Normally, mother and calf are separated by the time they arrive in the feeding area, but sometimes calves may stay with their mother for up to two years. Males reach the age of reproduction at 5-8 years, females at 7-9 years. Female minke whales appear to be able to give birth every year, unlike many other baleen whales that produce only one calf every 2-3 years. This is possible because cetaceans can have a lactation estrus, i.e. they can ovulate and get pregnant while they are still lactating. This happens only if a female is in very good condition and food supply is not limited, and there are indications that the proportion of lactating-pregnant females is higher in minke whales than any other baleen whales. There doesn’t seem to be a menopause in female minke whales and life span is thought to be about 50 years for both species.

Contrary to all other baleen whales, breeding can occur year-round in minke whales, at least in the Northwest Pacific population. In the North Atlantic, conception takes place from December to May with a peak in February and birth takes place from October to March with a peak in December, although calves of an estimated age of 2-3 months have been observed in September both in the St. Lawrence Estuary as well as off the coast of Scotland. Minke whale seasonal movements are less predictable and less well defined than in many other species of baleen whales that have a very clear seasonal migration pattern between summer feeding grounds and winter breeding grounds. However, data from our long-term monitoring program in the the St. Lawrence Estuary suggest a very high level of site fidelity of individual minke whales in the feeding ground across seasons and a surprisingly high number of young individuals in the last few years. The big knowledge gap is that breeding and calving grounds are unknown in the North Atlantic, but also in the North Pacific and the southern hemisphere. Actual mating behaviour has never been observed in minke whales.

For the same reason, there is hardly any information available about the mating system of minke whales, but like other rorqual whales, they are probably polygamous. While common minke whales are mostly solitary in the feeding grounds except for mother-calf pairs and occasional feeding aggregations of 3-10 animals, Antarctic minke whales can often be observed in groups of 10 animals or more, probably a strategy against a particular ecotype of killer whales which is their only predator in Antarctic waters. Surprisingly, unlike any other rorqual whales, common minke whales seem to segregate by sex when returning to their feeding grounds, at least in the Gulf of St. Lawrence, where virtually all observations and stranded animals as well as all our biopsies collected so far suggest that adult minke whales in these waters are females. We don’t know where the males go during the summer season, but satellite tags on females could lead us to the breeding ground during the winter and consecutive tags on a few males would potentially reveal the males’ summer feeding ground. Ultimately, such tagging data could shed light on the question whether minke whales in the North Atlantic are migrating between the four recognized populations, being the Canadian east coast, west Greenland, central North Atlantic, and northeastern North Atlantic stocks.

Ecology and Behavior

Minke whales are opportunistic feeders and typically found in coastal waters, sometimes in areas as shallow as three meters, thus occupying an ecological niche that is not accessible to most other rorqual whales. Their diet ranges from krill and small schooling fish (capelin, sand lance, herring) to larger fish such as maquerel, haddock and mature Arctic cod. They have 230-360 comparatively short baleen plates on each side (200-300 in B. bonaerensis) and 50 to 70 accordion-like ventral pleats (22-38 in B. bonaerensis) which allows them to engulf a large mouthful as the whales lunge into schools of prey with their mouth open by expanding their buccal cavity. They then filter the prey from the water by closing their jaws except for a small opening, contracting the ventral pleats and expelling the water between the baleen plates by moving the tongue from the back of their throat like a piston in a cylinder and the baleen acting as a comb. Once filtering completed, the whale uses the tongue to collect the prey, along the inside of the baleen to the back of the throat, to finally swallow it. This gulp-feeding behavior, found in all rorqual whales, is a very complex biomechanical process and young animals have to learn this technique to master it. An adult minke whale can consume up to 400 kg of prey per day.

A common minke whale performing a lunge-feeding behavior in the St. Lawrence Estuary (© Mériscope / Brian Kot)

The scientific name of the common minke whale, acutorostrata, refers to the acutely pointed shape of their head. The most streamlined of all rorqual whales, minke whales are enduring racers and capable of maintaining a speed of 15-30 km/h for an hour or two. Speed-swimming is an efficient anti-predator strategy against killer whales in open water, but if the orcas manage to drive a minke whale into a bay or a cove, predation is unavoidable.

Being mostly solitary, minke whales are not as vocally active as humpback whales or other cetaceans with a complex social structure (belugas, killer whales). However, they produce a variety of sounds across their range of occurrence, such as low-frequency downsweeps, low-frequency pulse trains and higher frequency clicks. In the North Pacific, the mysterious ‘boing’ sounds have been attributed to common minke whales and off Australia, dwarf minke whales produce a complex and stereotyped sound pattern called the “star-wars” vocalization, which spans a wide frequency range from 50 Hz to 9.4 kHz and is composed of distinct and repeated components. In the Gulf of St. Lawrence and the estuary, minke whales produce frequency-modulated downsweeps from 120-80 Hz and low-frequency pulse trains in the range of 40-400 Hz. These pulse trains typically consist of 30 or more pulses, each one lasting 0.2-0.25 seconds and occurring at intervals of 0.15-0.25 seconds, regularly spaced throughout the sequence, contrary to pulse trains recorded elsewhere in the North Atlantic that are either ‘slow-down’ or ‘speed-up’ sequences. Pulse trains in the Gulf of St. Lawrence have energy peaks at 110-180 Hz and 300-340 Hz and are associated with the proximity of minke whales performing sub-surface feeding behaviour in patches of capelin or sandlance, suggesting that minke whales produce these sounds to concentrate schooling fish into a dense prey patch before engaging in an engulfement manoeuvre. 

In certain areas, e.g. around Iceland and the Antarctic Peninsula, minke whales tend to be very curious in the presence of small boats, spy-hopping and sometimes hovering around whale-watching zodiacs for up to an hour, but this behaviour is very rare among minke whales in the St. Lawrence Estuary.

A curious Antarctic minke whale spy-hopping beside an expedition zodiac (© Mériscope / Quark Expeditions)

Population Estimates and Conservation Status

Because of their small size, minke whales were not hunted much during the early ages of industrial whaling, but as larger species became depleted particularly in the second half of the 20th century, hunting of minke whales has increased. In 1983, the International Whaling Commission (IWC) has adopted a moratorium on commercial whaling that was implemented by most nations except Japan, Norway and Iceland. While today’s whaling nations are focussing on Antarctic minke whales as well as common minke whales in the Northwest Pacific and the Northeast Atlantic, all populations are suffering from habitat degradation induced by global climate change and other anthropogenic impacts, such as interactions with fisheries, pollution, man-made noise and ship strikes. The cumulative effect of multiple factors may cause a significant impact on all cetacean populations including minke whales.

A) Common minke whale (Balaenoptera acutorostrata):

There is no estimate of global population size of common minke whales, but estimates covering most of the summer range in the North Atlantic and the North Pacific total around 200’000 individuals. While declines occurred in some regions in the past due to overexploitation, it can be assumed that the population has not been reduced by more than 50% relative to three generations ago (Red List criterion A1 for status ‘Vulnerable’), given that the estimated catches, including net catches, taken during 1951-2017 (three generations of 22 years) sum to less than 180’000. The models used by the International Whaling Commission (IWC) Scientific Committee to assess the populations predict that the population has been recovering in the North Atlantic and is still declining in the western North Pacific. In 2018, the International Union for the Conservation of Nature (IUCN) labelled the common minke whales as ‘Least Concern’.

Since January 2017, elevated minke whale mortalities have occurred along the US Atlantic coast from Maine through South Carolina. Mortality counts where 27 (2017), 30 (2018) and 22 (2019), compared to a preceding annual average of 12.5 (2011-2016). Full or partial necropsy examinations were conducted on more than 60% of the whales. Preliminary findings in several of the whales have shown evidence of human interactions or infectious disease. Minke whales off the US East Coast are considered to be part of the Canadian East Coast stock, as are minke whales in the Gulf of St. Lawrence, but it is uncertain if there are separate stocks within the Canadian East Coast stock.

Currently, the best available abundance estimate is obtained from an aerial survey conducted in August 2006 which covered 10’676 km of trackline in the region from the 2000 m depth contour on the southern edge of Georges Bank to the upper Bay of Fundy and to the entrance of the Gulf of St. Lawrence. The abundance estimate generated from this aerial survey was 3’312 minke whales for the Canadian East Coast stock. In 1995, an abundance estimate for the entire Gulf of St. Lawrence obtained from aerial surveys had yielded 1’020 minke whales. New population estimates are needed given the unusual mortality events along the US Atlantic coast and the time lapse since the last surveys. In April 2006, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) has put both the Northeast Pacific and the Canadian East Coast stocks in the ‘Not At Risk’ category.

B) Antarctic minke whale (Balaenoptera bonaerensis)

The Antarctic minke whale is considered pagophilic (ice-loving) in the sense of being better able than the larger baleen whales to use habitat with high pack ice densities. The proportion of the population found within the pack ice is not well known but has been estimated at 10-50% in Area IV (southeast Indian Ocean sector) in summer. Due to climate change, sea ice cover in the Antarctic is predicted to decline by 50% in winter and 30% in summer and there is concern that this could negatively impact species such as Antarctic minke whales for which areas with sea ice constitute a substantial part of their habitat.

Antarctic minke whales were subject to IWC catch limits soon after exploitation started. Catch limits for commercial whaling became zero from 1986 with the coming into effect of the IWC moratorium on commercial whaling. The summer range of Antarctic minke whales is also nominally protected by the IWC Southern Ocean Sanctuary, adopted in 1994, which prohibits catches south of a boundary located mainly at 40°S. Neither the moratorium nor the sanctuary provision applies to takes of whales under Special Permits issued by IWC member governments. Such catches continued from 1987 until 2014 when the International Court of Justice ordered a stop to the permit programme on the grounds that it was not for purposes of scientific research. Catches resumed from the 2015/16 season under a new programme.

Antarctic minke whales are listed on Appendix I of the Convention on International Trade in Endangered Species (CITES), but this does not apply to products landed in Japan because the party holds a reservation on this species under CITES. Japan also holds a reservation on the IWC Sanctuary provision and therefore is not bound by it. The species is listed in Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals.

The Antarctic minke whale was previously listed as ‘Data Deficient’ pending clarification of abundance and trends. The IWC Scientific Committee has since accepted circumpolar population estimates of about 500’000 based on surveys conducted during 1993-2004. The population was estimated to have declined by 31% relative to the previous circumpolar surveys (1986-1991, population estimate 750’000), but imprecision in the abundance estimates means that the decline is not statistically significant. In addition, an unknown proportion of the population would have been in unsurveyed pack ice habitat at the time of the surveys. The imprecise abundance and unknown proportion of whales in pack ice contributes to an overall lack of confidence in status determination based on decline rate. However, because the decline may not have ceased and its causes are poorly understood, the IUCN has attributed the Antarctic minke whale population the status of ‘Near Threatened’ in 2018.

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