Minke Whale

Minke whales are the smallest of the baleen whales reaching a maximum length of around 10m.

Classification

Class: Mammalia
Order: Cetacea
Suborder: Mysticeti
Family: Balaenopteridae
Genus: Balaenoptera
Species: acutorostrata
Common names: Minke whale, piked whale, pikehead, lesser rorqual
Irish Name: An droimeiteach beag 
 

Key Identification Features

Body length: Up to 10m - ♂9m, ♀10m (females may be slightly larger than males)
Weight: Up to 9,200kg
Blow: Quick and short reaching up to 3m - generally not visible unless seen at close range or in calm conditions when it can be heard.
Head: One of the most distinctive features of the minke whale - narrow/sharply pointed with a single ridge running down the centre of the triangular shaped rostrum beginning in front of the blowholes. The ridge resembles that of the fin whale, except it is much sharper. Rostrum is flat, baleen is short and sometimes visible when a whale is feeding.
Dorsal/pectoral fins: Dorsal fin is tall, falcate and highly variable positioned slightly less than 2/3 of the way back from the tip of the rostrum.
Pectoral fins are small and pointed, about of 1/8 the total body length, and have a conspicuous white diagonal band (variable) on the upper surface.
Colouration/markings: Generally black, brown, or dark grey on their backs and whitish on belly/underside of flippers. Conspicuous feature of these animals in the Northern Hemisphere is a diagonal band of white on the upper surface of each flipper. The extent and orientation of the white band on the flipper varies individually. Minke whales sometimes have a pale chevron on the back behind the head and two regions of light grey on each side - one just above and behind the flipper, and another just in front of and below the dorsal fin. The tail flukes may be pale grey, bluish-grey or white on the underside, usually with a dark margin. Identification of individual minke whales is possible using a combination of naturally occurring features (shape of dorsal fin, scars, pigmentation) which vary between individuals and has facilitated photo-identification studies of minke whales in some areas.

http://marinebio.org/species.asp?id=230

Field identification: at sea (behaviour)

The dive sequence of the minke whale is quite distinctive - if the animal seen is within the 20-30ft range, is clearly too large to be a dolphin, showing no obvious blow, has a fair sized dorsal fin and does not fluke upon diving, then you are most likely looking at a minke whale. The sharply pointed snout breaks the surface first and the sight of both the blowhole and dorsal fin simultaneously distinguishes the minke whale from all other rorqual whales, with the exception of the sei whale. The back and tail stock arch more strongly than the sei whale as the minke prepares for a long dive - the flukes do not appear above the surface. 

Minke whales are typically seen once and then may disappear for up to five minutes before reappearing in an unpredictable location, earning them the nickname '‘slinky minke'. A typical breathing sequence consists of 5-8 blows at intervals less than one minute, followed by 3-8 shallow dives before a deep dive which typically lasts 3-5 minutes, but may last up to 20 minutes.

They are often seen alone but may be seen in small groups of 3-5 individuals, although on rare occasions they have been seen in groups of up to 100 in rich feeding areas. Minke whales are often difficult to approach but some, especially juveniles, may be inquisitive and approach boats.

Breaching behaviour is occasionally seen, particularly in stormy weather. They usually leave the water dorsal-side up at and angle of about 45° and re-enter, without twisting or turning their bodies, head first or with a belly flop. Most of the body may leave the water in the initial breach and the tail may sometimes be seen. Breaches are often repeated two or three times but may be repeated for long sequences. They can often be seen feeding in the presence seabirds.
 

Field identification: on land (strandings)

The general stout form of this slender whale aids in identification of stranded animals. There are 50-70 thin ventral pleats on the throat and belly, the longest of which end slightly anterior to the navel. There are between 230-360 short (up to 30cm long, 12cm wide), fine bristled baleen plates which are mostly creamy white in colour with occasional black streaking on the lighter plates. 

Minke whales have stranded on every coastline in Ireland with the highest incidences of strandings occurring on the south and south west coastlines - they are the 5th most frequently stranded cetacean in Ireland.

Species similar in appearance

May be confused with sei or fin whales. Both sei and fin whales lack the pale patches on the pectoral fins and have very visible blows, and the fin whales are visibly much larger.

Distribution and abundance

The minke whale is the most abundant of all baleen whales and is found virtually worldwide both over continental shelves and in deeper water. They are less common in the tropics than in cooler waters. Some evidence suggests they may winter near the equator and migrate north to higher latitudes in the summer months. There are three geographically isolated populations currently recognised –in the North Pacific, North Atlantic and Southern Hemisphere.

A summer population population of c30,000 - 40,000 animals was estimated for northwest European waters, and there are thought to be between 500,000 to one million worldwide. The highest relative abundance  of minke whales was recorded off the south and southwest coasts of Ireland in the autumn and in the western Irish Sea in the spring - peaks are thought to have occurred due to the presence of large concentrations of pelagic schooling fish. The whales were present in the Irish Sea from late April through to early August but largely absent for the remainder of the year.

Where and when best seen in Ireland

Minke whales are the most frequently recorded baleen whale in Irish waters can be seen off most headlands throughout the year along the entire Irish coast, although most sightings are recorded from the south and west coasts between May and October. They are mostly seen in shallow waters (<200m) over the Irish Shelf, as well as shallower areas such as the Porcupine and Rockall Banks. They are rarely seen beyond the continental shelf.  There appears to be a seasonal inshore migration of minke whales off Loop Head during September and October. 

Food and Feeding

Minke whale feeding, West Cork © Pádraig Whooley, IWDG

Minke whales have the most varied diet of all baleen whales feeding on various small fish including capelin, sand eel, herring and cod, as well as small squid. In the Southern Hemisphere they feed exclusively on krill. They hunt in a variety of ways often herding or chasing fish into a group before capturing them and driving them from the deep to the surface or rushing through a group of fish and gulping them as they swim through resulting in the capture of larger species such as cod and haddock.

When actively foraging, minkes in Irish waters have generally been seen in groups of 2-3, and groups of up to 7 individuals have been recorded foraging in the Irish Sea. Off the south coast the outer Dingle Bay is a particularly important foraging area for minke whales between April and October as well as from Cape Clear to Hook Head between April and November. 

Reproduction and Life Cycle

Life expectancy: Up to 50 years - average 45 years
Age of sexual maturity: 3-8 years - generally 6 years ♀7 years
Gestation period: 10 months
Calf weight / length at birth: 350kg / 2.8m
Calving frequency: One calf every 2 years 
Calves born: December - January
Calves weaned: Weaned after 6 months - earliest known weaning period for any baleen species

Mating occurs between January and May. There is no evidence of minke whales calving in Irish waters and it is believe that they migrate to lower latitudes to breed in the winter months. They are thought to calve on two breeding grounds, although these locations have yet to be identified. One possibility is the Caribbean as vocalisations known as "thump trains" believed to be associated with reproduction were recorded in the late winter and early spring. 

Social Structure and communication

Minke whales are common in Irish waters and typically seen alone or in small groups. When actively foraging, minkes in Irish waters are generally seen in groups of 2-3, although groups of up to 7 individuals have been recorded foraging in the Irish Sea. 

Minke whales have been recorded emitting clicks. It is generally believed that clicks from mysticetes are not used as a means of echolocation. They emit various vocalisations, one of which is known as a 'thump train' and is believed to be associated with reproduction. 

Status and conservation issues

Status: Migratory

IUCN status: Least concern

Conservation status in Ireland: Good 

Protection:

  • EU Habitats Directive (92/43/EEC) Annex IV
  • Wildlife Act, 1976 and Wildlife (Amendment)  Act 2000
  • No SACs listed

The minke whales only known predators are killer whales and man. Although not endangered, some stocks of minke whales have been significantly reduced by whaling - particularly those in the central and eastern North Atlantic and the western North Pacific. The incidental capture of minke whales in fishing nets and traps may also be a cause for concern. The problem has been given little attention and its impact on populations has not been assessed. Contamination of the oceans by toxic, man-made chemicals and other pollutants may impact minke whales as has been suggested for other species.

The effects of oil and gas exploration, other industrial activities, development of offshore wind turbines and shipping in the minke whale's habitat along the Irish coastline are largely unknown warranting further study.

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References

Anderwald, P., Daníelsdóttir, A.K., Haug, T., Larsen, F., Lesage, V., Reid, R.J., Víkingsson, G.A. and Hoelzel, A.R., 2011. Possible cryptic stock structure for minke whales in the North Atlantic: Implications for conservation and management. Biological conservation144(10), pp.2479-2489.

Beamish, P. and Mitchell, E., 1973, April. Short pulse length audio frequency sounds recorded in the presence of a minke whale (Balaenoptera acutorostrata). In Deep Sea Research and Oceanographic Abstracts (Vol. 20, No. 4, pp. 375IN5381-380IN10386). Elsevier

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Department of the Environment, Heritage and Local Government (2009) Conservation Plan for Cetaceans in Irish Waters. http://www.npws.ie/sites/default/files/publications/pdf/2009_Cetaceans_CP.pdf

Edds-Walton, P.L., 1997. Acoustic communication signals of mysticete whales. Bioacoustics8(1-2), pp.47-60.

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Mellinger, D.K. and Clark, C.W., 2000. Recognizing transient low-frequency whale sounds by spectrogram correlation. The Journal of the Acoustical Society of America107(6), pp.3518-3529.

Shirhiai H, Jarret B (2006) Whales, dolphins and other marine mammals of the world. Princeton University Press, Princeton

Todd V, Todd I, Gardiner J, Morrin E (2015). Marine Mammal Observer & Passive Acoustic Monitoring Handbook. Exeter: Pelagic Publishing. ISBN 978-1-907807-66-4

Wall D, Murray C, O’Brien J, Kavanagh L, Wilson C, Glanville B, Williams D, Whooley P,  Berrow S (2013) Atlas of the Distribution and Relative Abundance of Marine Mammals in Irish Offshore Waters: 2005-2011. Irish Whale and Dolphin Group

Wall, D., O'Brien, J., Meade, J. and Allen, B.M., 2006, August. Summer distribution and relative abundance of cetaceans off the west coast of Ireland. In Biology and Environment: Proceedings of the Royal Irish Academy (pp. 135-142). Royal Irish Academy.

Wilson J, Berrow S (2006) A Guide to the Identification of Whales and Dolphins in Ireland. Irish Whale and Dolphin Group. ISBN 0-9540552-2-5

Winn, H.E. and Perkins, P.J., 1976. Distribution and sounds of the minke whale: with a review of mysticete sounds. Biological Systems

Würtz, M. and Repetto, N., 1998. Whales and dolphins: a guide to the biology and behaviour of cetaceans. Swan Hill