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Minke Whale

Minke Whale © Martin Kitching

The Minke Whale Balaenoptera acutorostrata, also called the Lesser Piked Whale or Lesser Rorqual, is a relatively small rorqual whale and the most frequently seen around the seas of northeast Britain. It has a streamlined body up to around 9.8 metres long and weighs up to 10 tons. The snout is sharply pointed and there is a single, sharp rostrum ridge leading to a triangular-shaped rostrum. A relatively tall sickle-shaped dorsal fin is situated almost two thirds of the distance along its back. The upperparts are generally dark grey-brown with paler underparts and usually 62 white rorqual grooves on the throat and belly (Watson, 1981). Paler areas reach up the sides behind the shoulder to form a vague chevron above the flanks, while the flippers have dark upper surfaces with conspicuous broad white bands. The fluke (tail) is dark, concave and with a median notch. Their mouth contains 230-360 creamy-white baleen plates, each about 30 cm long (Carwardine, 1995).

Minke Whales usually surface showing their lower jaw and head with a low indistinct blow up to about two metres, followed by a shallow rolling action revealing the back and the large dorsal fin before the sleek body slips smoothly back into the sea, with the flukes staying submerged at all times. Normal speed is around five-seven mph but they can reach 17 mph if pressed (Hoyt, 1984). In normal feeding mode, there are usually three or four blows but sometimes up to eight at intervals of less than a minute before the deep dive in which the whale’s back arches much more steeply before the dive, but still without showing the tail flukes. The period at the surface is usually about three seconds and in calm seas it is often possible to plot where the next blow is likely to occur due to the even spacing of the “footprints” left by the previous dives. Dives typically last 3-12 minutes but can be up to 20. When feeding on surface-shoaling fish like Herring Clupea harengus, Minke Whales frequently lunge feed, rolling on to their sides with mouths agape to scoop up the fish. Their flukes and white-banded flippers may well show under these circumstances together with their white chin and rorqual grooves (Carwardine, 1995).

Minke Whales are generally solitary but are occasionally found in small groups of two to three, rarely more when in productive feeding areas. They breach quite frequently, leaping almost vertically and nearly clearing the water before falling sideways back into the sea. During these breaches the white throat and belly together with the white-banded flippers can be very conspicuous, as is the relatively slender streamlined profile of the whale with its pointed snout. Active feeding is often accompanied by flocks of birds which frequently serve as a cue to their presence. They may sometimes be inquisitive around boats, even spy-hopping to get a better view (Leatherwood et al, 1976; Carwardine, 1995).

Minke Whales become sexually mature at three to eight years old when about seven metres long. Females give birth to a single calf every one to two years after a gestation of 10-11 months and lactation of four to six months. The newborn calves are 2.4 to 3.5 metres long. They can live for about 50 years if they avoid the attentions of the whaling nations, including Norway and Iceland, which have targeted this species for commercial as well as for “scientific purposes” despite the International Whaling Commission’s moratorium on commercial whaling (Shirihai and Jarrett, 2006).

Minke Whales are widespread in the Northern Hemisphere, mainly in the North Atlantic and North Pacific with different sub-species in the Southern Hemisphere and Pacific. They are most abundant in the temperate to polar waters, generally moving north in summer. In the Northern Hemisphere there is thought to be segregation in summer with males moving further north in open waters and females remaining south in more coastal waters, with immatures even further south. It is thought that the Atlantic population is around 185,000. Off north western Europe, surveys in the Bay of Biscay suggest Minke Whales seem to prefer the shallower waters of the continental shelf to the deep water canyons or abyssal plain (Walker and Cresswell, 2008).

Minke Whale by Chris Bielby

Around Britain there are several “hotspots” for sightings of Minke Whales in summer, including especially around the Western Isles and the northwest coast of Scotland, but also around the Moray Firth. Sightings off the coasts of Northumberland, Durham, Cleveland and North Yorkshire are becoming annual but are still infrequent, and in the North Sea the southern limit of its range seems to be the southern Yorkshire coast (Reid et al, 2003; Carwardine, 2003).

Although never common in the North Sea reported sightings of Minke Whales have become more regular in recent years, partly due to the increased survey activity from commercial ferries as well as sea-watching activity by birdwatchers from headlands and bird observatories. However it may be that changes in migratory patterns of prey species in the North Sea, possibly due to the effects of climate change, are also causing genuine increases in the cetacean population. Other factors to be considered include the increase in interest and willingness to report sightings by the general public, as well as the improvement in optical equipment and expertise of observers.

There are 13 relatively recent records of strandings of Minke Whales in our region together with a further four records of unidentified and decomposed Mysteceti whales which may also have been Minke. These have mainly occurred during the summer months, with later records often relating to animals that are severely decomposed and have been dead for some time. One whale showed signs of entanglement in discarded fishing gear, a common hazard. Most large whales which strand have little hope of survival as their own body weight results in suffocation and effective crushing which releases toxins into their bloodstream which cause irreparable damage. Similarly, their body mass is so great that the heat generated by their normal metabolic processes becomes lethal as there is insufficient cooling effect when they are out of water. In the North Sea many stranded whales also show signs of emaciation due to starvation, which ironically leads to dehydration as whales need to metabolise fresh water from their food. This means that whales which strand have a very short window of opportunity to be refloated before irreparable damage occurs and under current practice (British Divers Marine Life Rescue, pers. comm., 2012) they would be euthanized to prevent further suffering. It is thought that gently sloping beaches and mudflats can confuse whales’ sonar navigation systems leading them to strand, but loud noises from shipping, sonar and seismic surveys have also been linked to strandings in many species.

Records are sparse prior to 2000, reflecting the lack of observers and facilities to record sightings rather than a definite absence of Minke Whales in our region. Apart from stranded individuals, one live sighting involved two animals actively feeding approximately five miles east of the Farne Islands on 12 September 1993, viewed at very close range from a chartered boat on a birdwatching cruise (author’s own observation).

The majority of post-2000 records emanate from headlands where there is a lot of birdwatching activity and also where dedicated cetacean watchers tend to concentrate. This “observer bias” is based around North Northumberland, Tynemouth, Whitburn Bird Observatory, Hartlepool Headland, Whitby, Scarborough, Filey and Flamborough Head.

The records suggest that Minke Whales are migratory and appear off the North East coast mainly in the summer months with records concentrated from June to September. This observed trend of seasonality is probably accurate as it is free from the “observer bias” of some distributional data, as seawatching is a pursuit which continues throughout the year, and is not concentrated during the summer months.

Following a peak in records in 2007, numbers have declined steadily towards the 2005 level; however recent surveys to the south of our region have revealed larger numbers of Minkes possibly associating with returning shoals of Herring to the North Sea. It seems likely that they follow the shoals of smaller fish into the North Sea where they have been seen feeding with accompanying flocks of seabirds. An old name for Minke Whale was “Herring Hog” which suggests the fishermen were well aware of their feeding habits. A perceived increase in abundance in recent years may be a result of changes in the migratory habits of their prey species, which may also be responding to observed changes in the species and timing of plankton abundance in the North Sea, probably as a result of climate change.

References

Written by Chris Bielby (last updated Nov 12)