Bioacoustics of Balaenoptera acutorostrata

Food and Feeding. The Common Minke Whale is relatively euryphagous compared with other rorquals and is known to feed on a variety of prey species depending on location of feeding grounds, available prey, season, and year. At least one study has found that the relatively high metabolic needs of Common Minke Whales are matched by a diet that is focused on prey that has a generally higher caloric content than prey of other species of rorquals. This is reflected in their apparent preference for feeding at higher trophic levels on schooling fish rather than planktonic crustaceans. The Common Minke Whale is generally characterized as a surfaceor near-surface feeder, in contrast to the majority of other species of rorquals that forage at greater water depths. In the Barents Sea, north of Norway and Russia, Common Minke Whales primarily feed on schooling fish including Atlantic herring (Clupea harengus) and capelin (Mallotus villosus). In years when these fish stocks collapse and spawning is well below normal, they switch to feeding almost exclusively on euphausid krill (7hysanoessa spp. and Meganyctiphanes norvegica). There are also some regional differencesin this system, with populations in the northern part of the Barents Sea switching to krill and those in the southern part feeding on a mixed-fish diet. Interestingly, there also seems to be a predator-prey linkage between planktivorous schooling fish (herring and capelin) and krill. When fish populations collapse, krill populations rebound, and Common Minke Whalesshift to feeding at a lower trophic level. In the Norwegian Sea, Common Minke Whales appear to feed in more pelagic habitats where they primarily consume sand eel ( Ammodytes spp. ), with lesser amounts of mackerel (Scomber spp.), herring (Clupea spp-), and Norway pout (Trisopterus esmarkii). In waters around Iceland, the diet of Common Minke Whales includes schooling fish (59%) and planktonic crustaceans (41%). One study has suggested that Common Minke Whales are a major predator of schooling fish in Icelandic waters, consuming more than one million tons annually. In the western North Atlantic Ocean, Common Minke Whales killed off Newfoundland were found to have been primarily eating capelin. In the North Pacific Ocean, prey preference of Common Minke Whales varies seasonally, with one study from the western North Pacific reporting that whales preferred Japanese anchovy (Engraulis japonicus) in May—June, Pacific saury (Cololabis saira) in July-August, and Pacific krill (Euphausia pacifica) in September. This study also found that Common Minke Whales avoided feeding on krill unless it was the only available prey. In other areas of the North Pacific Ocean, Common Minke Whales seasonally prefer schooling fish like sand lance ( Ammodytidae ) and walleye pollock (Theragra chalcogramma). In the Southern Ocean, the diet of the unnamed southern form is more restricted and reportedly consists of myctophid fish in the Antarctic and krill (E. similis) in other areas. When feeding, Common Minke Whales are known to practice a variety of specialized strategies, depending on location and available prey species. In waters of the Gulf of Saint Lawrence and the Saint Lawrence River, they reportedly swim into rather restricted fjords where they appear to expend a significant amount of energy concentrating schooling fish near the water’s surface prior to a feeding strike. Taking advantage of the air-water interface as a barrierto fish attempting to flee, individual whales use various surface maneuvers that seem to effectively cause the fish prey to form temporary aggregations. Such maneuvers include “head slaps” where a whale slowly raisesits head out of the water and then quickly bringsits head downward to create a large splash and audible slap; “chin-up blow” where a whale vigorously raises its head above the water as it surfaces to breathe and then continues swimming as it submerges without making a splash; and “exhale-on-the-dive” where a whale forcefully exhales air just below the air—water interface to create a water-filled spout blast. These disparate behaviors seem to generate mechanical or percussive acoustic cues that stimulate an aggregation response in the prey just prior to a surface lunge feeding strike. Although the general foraging pattern is lunge feeding, some observers report seeing Common Minke Whalesalso practicing a form ofskim feeding reminiscent of the predominant feeding strategy of the Bowhead Whale ( Balaena mysticetus) and right whales ( Eubalaena spp. ).