Balaenoptera edeni, Anderson, 1879

Russell A. Mittermeier & Don E. Wilson, 2014, Balaenopteridae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 242-299 : 289-290

4.

Bryde’s Whale

Balaenoptera edeni

French: Rorqual d'Eden / German: Bryde-Wal / Spanish: Rorcual de Bryde

Other common names: Bryde's Whale Complex, Tropical Whale; Eden's Whale, Small-form Bryde's Whale (edeni type population); Large Bryde's Whale, Ordinary Bryde's Whale (brydei type population)

Taxonomy. Balaenoptera edeni Anderson, 1879 ,

Burma (= Myanmar), “found its way into the Thaybyoo Choung, which runs into the Gulf of Martaban between the Sittang and Beeling Rivers, about equidistant from each.”

Balaenoptera edeni is currently considered to be a senior synonym of the B. brydei named by Olsen in 1913; the specimen that formed the basis for this southern species was captured in waters off South Africa by whalers operating from a shore station at Saldanha Bay, Cape Province. In 1950, G. C. A. Junge proposed that the two whales represented the same species and considered B. brydei to be a junior synonym of B. edeni . Modern morphological and molecular evidence, however, suggests that there likely are at least two valid species in the Bryde’s Whale complex, a so-called “small-form” Bryde’s Whale ( edeni ) from Hong Kong, Australia, and south-western Japan and a so-called “ordinary” or “large” Bryde’s Whale ( brydei ) from pelagic waters of the western North Pacific Ocean and eastern Indian Ocean. A third taxon formerly lumped with the Bryde’s Whale complex as a so-called “pygmy” Bryde’s Whale from the Philippines was recognized in 2003 as a new rorqual species Omura’s Whale ( B. omurai ). Until more individuals of the ordinary and small-form Bryde’s Whale can be sampled and analyzed using both morphologic and molecular techniques, these taxonomic issues cannot be resolved. No subspecies are currently recognized, although broadly sympatric populations of resident inshore and migratory offshore Bryde’s Whale off South Africa may eventually prove to be distinct subspecies. Monotypic.

Distribution. Tropical and warm temperate waters of the Northern and Southern Hemispheres, generally between 40° N and 40° S.

Descriptive notes. Total length 1300-1450 cm; weight 15,000-16,600 kg (ordinary Bryde’s Whale); Total length 1050-1150 cm (small-form Bryde’s Whale). Whales of the Bryde’s Whale complex are medium-sized rorquals and were long confused with Sei Whales ( B. borealis ) in commercial whaling records because of morphological similarities. All members of the Bryde’s Whale complex, however, possess a distinctive morphological pattern of three rostral ridges consisting of a prominent median ridge running from blowhole to tip of rostrum, as in other rorquals, but complemented by shorter laterally placed auxiliary ridges, one right and one left. These laterally placed auxiliary ridges rise c.1-2 cm above adjacent rostral surface and are absent on other species of rorquals, including the Sei Whales. Whales in the Bryde’s Whale complex from the northern Indian Ocean (the type area of edeni ) are typically smaller (1100 cm) than those in the offshore waters of South Africa (the type area of brydei ) and elsewhere, which have greater mean total body lengths (1400-1500 cm). This dichotomy in body size has been reported in other ocean basins. Body coloration of Bryde’s Whale is generally dark bluish-black dorsally and white or yellowish ventrally, with a bluish gray area extending onto ventral groove blubber and flippers. The result is a rather diffuse counter-shading pattern, with a weakly differentiated boundary between dorsal and ventral surfaces. Flippers are bluish black, above and below. Several workers have noted a coloration dichotomy between pelagic Bryde’s Whales from the western North Pacific Ocean and offshore waters off South Africa and more neritic whales from coastal waters off South Africa and possibly south-western Japan. In the case of offshore whales, the body typically is heavily scarred with small oval pits from bites by cookie-cutter sharks (Isistius brasiliensis). This scarring pattern is especially concentrated over the posterior one-half of body and has been likened to the color and texture of galvanized iron. Such heavy scarring is typically absent from coastal Bryde’s Whales, which off South Africa bear thin scratches on ventral side of flukes and peduncle suggestive of direct encounters with shelf substrates. In both forms, lowerlips are typically a uniform color, although some individuals have asymmetric coloration of their throat and baleen apparatus. Head makes up 24-26% of body length, while slender and somewhat pointed flippers make up ¢.8-10% of body length and caudal flukes ¢.23-24% of body length. Dorsal fin is relatively large (up to 46 cm tall) and strongly falcate with a distinctly tapering tip;it is positioned relatively far back on body, slightly less than three-quarters the distance from tip of rostrum to flukes. Ventral groove blubber is marked by 54-56 longitudinal pleats, the longest of which extend from chin to, or beyond, umbilicus. In the Sei Whale, ventral grooves terminate well anterior to umbilicus. Baleen laminae of Bryde’s Whales number 285-350/side and typically are dark slate-gray with relatively thick grayish or brownish bristles. Researchers report some variation in shape of individual plates, noting that inshore individuals from waters off South Africa typically have plates that are more slender than those of offshore individuals. This morphological difference may correlate with different prey preferences. When swimming normally, Bryde’s Whales cruise at 2-7 km/h, but they are able to reach speeds of 20-25 km/h. At the surface, blow is columnar to bushy and up to 3-4 m high. Bryde’s Whales reportedly breach more often than most other species of rorquals.

Habitat. Generally warm temperate to tropical waters, varying from neritic (shelf) habitats to pelagic habitats. Bryde’s Whale is rarely reported north of 40° N or south of 40° S. Some populations, such as the inshore Bryde’s Whale stock off South Africa (the “South Africa Inshore Stock”), prefer neritic (shelf) habitats within 30 km of the coast, while other populations, such as the “South-east Atlantic Stock,” prefer pelagic habitats in waters more than 80 km from the coast. There are limited data to suggest that a similar dichotomous pattern of inshore—offshore habitat preference holds for some populations of Bryde’s Whales in the Northern Hemisphere.

Food and Feeding. Bryde’s Whales are considered opportunistic feeders, switching their prey preference depending on prey availability, geographical location, season, and year. In the waters off South Africa, individuals of the South Africa Inshore Stock primarily feed on pelagic fish (Engraulis capensis, Trachurus trachurus, and Sardinops sagax ocellatus), while individuals of the South-east Atlantic Stock reportedly prefer krill (Euphausia lucens and E. recurva) but also eat mesopelagic fish (Mawrolicus mueller and Lestidium sp.). A study of the stomach contents of Bryde’s Whales from the Arabian Sea found mostly lantern fish ( Myctophidae ), spotted mackerel (Scomber australasicus), and horse mackerel (Trachurus spp.). Individuals in this more northern population appearto be genetically distinct from Bryde’s Whales occurring farther south off the southern coast of Madagascar. In December-March, these whales primarily feed on krill (Euphausia recurva and E. diomedeae). Seasonal variations of prey preference were recorded from the population of ordinary Bryde’s Whales in the western North Pacific Ocean where krill (E. pacifica) and pelagic schooling fish such as the Japanese anchovy (Engraulisjaponicus) are commonly consumed in May-June, the Pacific saury (Cololabis saira) in July-August, and krill again in September.

Breeding. Information on reproduction of Bryde’s Whale is limited and based almost entirely on whaling postmortem records. Perhaps the most comprehensive data set comes from South Africa and suggests that females and males of the South-east Atlantic Stock reach sexual maturity at slightly less than seven years of age and c.1280-1370 cm. Length at sexual maturity for the inshore population (South Africa Inshore Stock) was smaller by ¢.60-91 cm. Seasonality of breeding in these populations, as suggested by data from slaughtered pregnant females, suggests almost year-round reproduction for the offshore population, with a peak in conceptions in March—May. Yearround reproduction was also suggested for the inshore population, but there were no obvious seasonal peaks. Data also reveal that inshore individuals had a much higher rate of ovulations per reproductive cycle. This degree of a non-seasonal, year-round breeding is unusual in a rorqual and has been proposed as possibly reflecting a major behavioral shift in this non-migrating, resident population, which is apparently able to exploit year-round availability of prey (pelagic fish) in the continental shelf waters of this region. In a sense, this inshore population of Bryde’s Whales may be behaving more like an “income” breeder than a “capital” breeder. Gestation in Bryde’s Whales last 11-12 months, with young ¢.400 cm in length at birth. Weaning occurs after ¢.6 months, when the young is 700 cm in length. In general, there is a 2year interbirth interval consisting of 11-12 months of gestation, six months of nursing, and a six-month anestrous period.

Activity patterns. Little is known about daily activity patterns of Bryde’s Whales. Although it is clear that some populations undergo an annual, seasonal migration between low-latitude winter breeding and birthing grounds and mid-latitude summer feeding grounds,it is also clear that some populations are non-migratory, remaining year-round in high-productivity, mid-latitude settings.

Movements, Home range and Social organization. Whales of the Bryde’s Whale complex are unique among species of rorquals in that they do not undergo long-distance annual migrations into high-latitude (above 50-60° N or S) summer feeding grounds. For those populations that do migrate, migrations never take individuals beyond temperate regions. For example, the South-east Atlantic Stock of Bryde’s Whale ranges only as far south as 34° S (Cape Agulhas) during austral summer where it feeds in offshore pelagic waters. In austral autumn, they travel north paralleling the west coast of Africa to winter breeding groundsin waters off Cameroon (3° S). In the western North Pacific Ocean, Bryde’s Whales range along the Pacific shores ofJapan, Taiwan, and the Philippines, extending from ¢.40° N in boreal summerto c¢.2° N in boreal winter. The central tropical Pacific Ocean hosts a distinct population of Bryde’s Whale, especially west of 150° W. Migration patterns of other populations of Bryde’s Whale are poorly known, although there are reports of non-migratory, resident populations in some regions. In the eastern North Pacific Ocean, a resident population of Bryde’s Whale reportedly lives year-round in the Gulf of California. The distribution of Bryde’s Whale in the South Pacific Ocean is also widespread, occurring in a zone between the Equator and ¢.30° S and extending from the eastern coast of Australia across the central Pacific Ocean to ¢.120° W. The Indian Ocean is home to more than one population of Bryde’s Whale, including both “ordinary” and “small-form” types. Distribution of these whales extends from ¢.37° S to the northern shores of the Bay of Bengal and the Arabian Sea. A population of Bryde’s Whales off the southern coast of Madagascar may or may not undergo a seasonal migration. Little is known about the social organization of Bryde’s Whales, and although they typically occuras solitary individuals, small groups of 2-3 whales have been reported.

Status and Conservation. CITES Appendix I. Data Deficient on The IUCN Red List. The Data Deficient category for Bryde’s Whale is applied here because of uncertainty over taxonomic composition of the Bryde’s Whale complex. If it turns out that this taxon represents a single, polymorphic species, the category should be changed to Least Concern. If further genetic work, however, showsthis taxon to consist of two (or more) separate species and subspecies, the smaller populations may be considered threatened. Population decline documented for most other species of rorquals is difficult to determine for the Bryde’s Whale complex because prior to 1972, whaling statistics did not differentiate it from the Sei Whale. Nevertheless,it is clear from geographical locations and time of year data recorded for particular catches, that Bryde’s Whales suffered some degree of commercial overexploitation, especially in the western North Pacific Ocean and eastern South Atlantic Ocean. During the 1970s and into the 1980s, commercial harvesting of Bryde’s Whales increased, but it ceased after 1987 with the international moratorium on commercial whaling. Certain populations, such as the western North Pacific stock, are estimated to have experienced a 49% decline from 1911 to 1987. Current population estimates for the Bryde’s Whale complex are mostly old and of questionable accuracy, but they suggest that the cumulative global total from the eleven recognized stocks is at least 68,000 individuals. This figure may be meaningless given growing molecular evidence for recognition of separate species. Despite the moratorium on commercial whaling of rorquals, Japan resumed pelagic whaling of Bryde’s Whales in 2000 undera “scientific research” permit and has a quota of 50 ind/year. There also is a small artisanal fishery in the Philippines and Indonesia that kills Bryde’s Whales every year. Other threats include entanglement in fishing gear (bycatch), fatal encounters with vessels (ship strikes), and habitat decline and pollution. Threats posed by climate change to this warm-water species complex are probably notas great as for more pagophilic species of rorquals.

Bibliography. Best (2001), Cummings (1985a), John et al. (2012), Junge (1950), Kato & Perrin (2009), Olsen (1913), Penry et al. (2011), Reilly et al. (2008d), Rice (1998), Sasakiet al. (2006), Tamura & Fujise (2002), Yamada et al. (2008), Yoshida & Kato (1999).

Figures

1. Common Minke Whale (Balaenoptera acutorostrata), 2. Antarctic Minke Whale (Balaenoptera bonaerensis), 3. Sei Whale (Balaenoptera borealis), 4. Bryde’s Whale (Balaenoptera edeni)

Distribution. Tropical and warm temperate waters of the Northern and Southern Hemispheres, generally between 40° N and 40° S.