Bioacoustics of Balaenoptera borealis

Movements, Home range and Social organization. Because of anti-tropical distribution and out-of-phase seasonal migration patterns of Sei Whales, northern and southern populations are seldom, if ever, sympatric. Complete allopatry would eliminate gene flow between the two regions, thus lending support to the recognition of separate northern and southern subspecies. Sei Whales in the North Pacific Ocean are distributed mainly north of 40° N during boreal summer, extending into the Gulf of Alaska and even the Bering Sea. During boreal winter in the North Pacific Ocean, Sei Whales are more widely dispersed, with some individuals known to winter in subtropical waters off the Bonin Islands (27° N) in the west and as far south as the Revillagigedo Islands (18° N) in the east. Probably because of whaling related mortality, the majority of Sei Whales in the North Pacific Ocean occur today east of 180° W. During summer in the North Atlantic Ocean, Sei Whales typically occur north of 44° N in the west and up to 70° N in the east. Their winter distribution is poorly known, but it is reported to extend to ¢.28° N in the west (reports of Sei Whales in the Caribbean and Gulf of Mexico may represent misidentifications of Bryde’s Whales) and off Mauritania in the east. In the Southern Hemisphere, Sei Whales occur throughout the Southern Ocean during summer feeding season, generally between 40° S and 50° S. Austral winter distribution of Southern Sei Whales is poorly known and primarily based on whaling catch records, which reported Sei Whales off north-eastern Brazil (7°S) in the western South Atlantic Ocean and off Angola (8° S) in the eastern South Atlantic Ocean. During austral spring migration, Sei Whales tend to arrive on feeding grounds after Blue Whales and Fin Whales have already been there and typically do not forage as far south as the latter species, remaining well north of the Antarctic pack ice. It is unknown, however,if all Sei Whales in the Southern Hemisphere actually make a full southern migration; some individuals may remain in more temperate waters as year-round residents. The pole-ward summer migration of Sei Whales in both hemispheres typically begins with pregnant females leaving breeding and birthing groundsfirst, followed by the remaining adult whales and then immature individuals. Densities are not well known and vary with time of year and geographical location. During austral and boreal winters when Sei Whales are in warmer waters, groups can consist of 2-5 mature individuals dominated by males. In contrast, migrating whales are often solitary, except for mother—offspring pairs. Solitary whales also occur on feeding grounds, although large groups of 20-100 individuals have been reported. Social structure of large groups is not well known, and associations may simply be passive and a function of prey abundance rather than actual social interaction. Whaling records suggest a 1:1 sex ratio in the North Atlantic and North Pacific oceans, while males seem to outnumber females by 6:4 in Antarctic waters. Based on limited genetic data and vocalization patterns, it is assumed that discrete populations of Sei Whale inhabit each ocean basin, except perhaps for the Southern Ocean where several populations may occur. Although Sei Whales are the least acoustically known, they vocalize, like all species of rorquals, at infrasonic frequencies mainly below 1 kHz. In the Southern Ocean, Sei Whales produce broadband sounds described as growls and “whooshes™ (100-600 Hz, 1-5 seconds in duration), as well as low frequency tonal (100-40 Hz, lasting 1 second) and down-sweep calls (39-21 Hz lasting 1-3 seconds). The latter calls often have multiple parts, with a frequency step in between. Calls of Sei Whales recorded in the western North Atlantic Ocean off Cape Cod, USA, consist of down-sweep sounds (82-34 Hz, lasting 1-4 seconds) and are made as a single call. This vocalization pattern might represent a contact call that allows dispersed individual Sei Whales to coordinate activities such as feeding or breeding. Acoustic studies in the Gulf of Maine found a correlation between diel movement of copepods, rate of Sei Whale vocalization, and feeding behavior. Vocalization rates were lower at night when Sei Whales were feeding close to the surface on copepods that had moved from deeper in the water column. Conversely, during the day when copepods had returned to deeper water and feeding was more difficult, Sei Whales vocalized possibly due to increased social activity.

Activity patterns. Limited information exists concerning daily activity patterns of Sei Whales. Acoustic studies, however, conducted in the Gulf of Maine in the western North Atlantic Ocean suggest a correlation between diurnal movements of copepods and rates of vocalization and feeding activity of Sei Whales. Nocturnal movement of copepods to near the sea surface was associated with decreased rates of vocalization in Sei Whales feeding at the same depth. Conversely, during the day when copepods had migrated into deeper water, vocalization of Sei Whales increased possibly due to increased social interactions and greater difficulty in locating prey. Like all species of rorquals, annual activity patterns of Sei Whales are primarily related to migration between summer feeding grounds and winter breeding, birthing, and nursery grounds.