A discussion of the feeding habits of Minke Whales Balaenoptera acutorostrata along the Filey coastline in summer 2013
Minke Whales are the smallest and most abundant baleen whale found within the North Sea region. Baleen whales include the largest whales in the world, notably other Roquals such as the Blue Whale Balaenoptera musculus, Fin Whale Balaenoptera physalus and Sei Whale Balaenoptera borealis, as well as the scarce North Atlantic Right Whale Eubalaena glacialis. These whales do not have teeth, but instead have complex comb-like structures attached to the upper surface of the mouth, known as baleen plates. The whale feeds by engulfing a large amount of water containing prey items, as the mouth closes water passes back out into the sea through the baleen plates, which effectively acts as a sieve leaving the larger prey items in the mouth.
The (Common) Minke Whale is almost exclusively a predator of small fish in the North Sea, with our warmer waters unable to support large amounts of Euphausiids (Krill) and Pteropods (swimming molluscs), both of which become more prevalent in the diets of North Atlantic populations. Fish prey in the North Sea typically comprise Sandeel, Herring, Sprat, Mackerel, Cod, Whiting and similar species. The Minke Whale is predominantly an inshore species, regularly observed coming into bays and estuaries unlike its larger congeners. Foraging typically occurs over banks, small islands and, notably in our context, headlands and upwellings.
Minke Whales (like most Roqual whales) typically spend the summer in high latitudes feeding in prey-rich waters before migrating south to breed and give birth and then foraging during the winter. A clear increase in sightings occurs between May and September, although records occur sporadically throughout the year around the British Isles. Minke Whales in the north-east Atlantic are far less migratory in comparison to other Roquals, due mainly to the lesser foraging pressure on their smaller bodies.
During July 2013 locally exceptional numbers of Minke Whales where observed foraging in the waters to the north-east of Carr Naze. Large numbers of this species are seldom recorded together in the southern/central part of the North Sea, with the species becoming increasingly infrequent south of Yorkshire. The preponderance of activity occurred for about three weeks, and in many cases allowed for close and sustained observations, with individuals occasionally coming within 100 metres of the Carr Naze cliff edge.
A peak of ten animals were observed at any one time, with regular counts of six individuals or more in the same stretch of water. Foraging would appear to be consistent between 0900 and 1600hrs on most days, with a clear peak in activity one hour before high tide and up to three hours after high tide. Early morning observations were surprisingly quite scarce (before 0700); this, combined with a trend for animals moving north each day during the late afternoon or evening, would suggest deeper or more sheltered waters were sought for an alternative food source and/or rest each evening.
Whilst watching the localised foraging area off the Northern Side of Carr Naze it quickly became apparent by following surfacing whales that water currents were used almost exclusively for travel. These appear as silver/light blue lines/streaks within the more stable dark blue water. Anecdotally 90% of all surfacing activity occurred within these areas, making recording, counting and observing surprisingly predictable, with whales often travelling along these lines to reach localised feeding sites.
The most notable foraging activity observed within the area was bird associated/assisted. Whales evidently caught onto the fact that a commotion of birds would more than likely provide a feeding opportunity. In most cases, bait balls (tightly-packed clusters of small fish typically consisting of Clupeids, i.e. Sprat and Herring, and Sandeels) developed without the whales; either schooling Mackerel or diving Guillemot, Razorbill and Puffins (or a combination thereof) would drive the smaller fish to the surface, trapping them against the water and air interface. Here the presence of bait balls became evident even at considerable distance as Kittiwakes, auks, Herring Gulls, Great Black-backed Gulls, Fulmars, Gannets and occasionally Shearwaters and Skuas homed in the activity and plundered the spoils.
It is at the point when the fish are ‘trapped’ that Minke Whales would regularly be recorded lunging through the bait ball and catching prey. This happened with great regularity, with individual whales going from bait ball to bait ball and consistently crashing through the feeding birds. Bait balls were strictly ephemeral and commotions of birds and whales would appear and disappear just as quickly.
Perhaps the most interesting foraging observation was when there were no birds or schooling Mackerel and the whales had to herd the smaller prey fish themselves. One such whale gave prolonged and spectacular views within 100 metres of the Brigg, diving below the schooling fish and then proceeding to tightly circle below the fish (allowing its white ventral surface and white pectoral fin flashes to be seen), driving the fish to the surface in a bid to escape. The whale then realigned itself and lunged through the now tightly packed bait ball catching its prey. Understandably this behaviour soon attracted gulls and gannets, which quickly mopped up any stunned fish the whale had missed.
Curiously, while observing the above behaviour a large upwelling of water occurred, presumably caused by the whales circling body motion, fluke and fins – likely to have further confused and disorientated the prey. The likelihood of this being an air bubble is slim, as to the author’s knowledge this is an unrecorded behaviour in Minke Whales. In contrast, Humpback Whales Megaptera novaeangliae regularly blow bubbles to confuse prey in a communal foraging strategy across localised parts of its range.
Despite a range of engulfing strategies being recorded across this species range, Filey’s visitors almost always lunge-fed – holding the body parallel with the water’s surface, with no significant twists occurring and with just a small part of the head showing during each lunge attempt. Occasionally an individual would rear its head out of the water, showing the throat grooves and baleen. On no occasion was more than one whale observed at a given feeding area, indeed even travelling whales seldom joined up with others.
This lack of interaction suggests that the main factor attracting the whales to the area was the prey-rich foraging, and not social interaction, although it is likely that the presence of such a large number of whales drew other transient individuals in from the wider area.