Lagenorhynchus obscurus (Gray, 1828)

Russell A. Mittermeier & Don E. Wilson, 2014, Delphinidae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 410-526 : 492

6.

Dusky Dolphin

Lagenorhynchus obscurus

French: Dauphin obscur / German: Dunkler Delfin / Spanish: Delfin de Fitzroy

Other common names: Beakless Dolphin; African Dusky Dolphin (obscurus); Fitzroy's Dolphin, South American Dusky Dolphin (fitzroyi); Chilean Dusky Dolphin, Peruvian Dusky Dolphin (Peru and northern Chile population); New Zealand Dusky Dolphin (New Zealand population)

Taxonomy. Delphinus obscurus Gray, 1828,

“Inhab. Cape of Good Hope,” Western Cape Province, South Africa.

Taxonomy of Lagenorhynchus is currently in dispute; recent molecular analyses have revealed that the genus is not monophyletic. L. obscurus may form a sister lineage with L. obliquidens to Cephalorhynchus . The two species may be classified into this or another genus ( Sagmatias ) in the near future. Two subspecies have been recognized based on patchy discontinuous distributions, but more recent genetic evidence suggests that L. o. fitzroyi from South America consists of two separate stocks. The population of L. o. fitzroyi off northern Chile and Peru (the “Peruvian/Chilean Dusky Dolphin”) is likely distinct from the Patagonian population, and it was assigned to a separate subspecies, posidonia, named by Philippi in 1893, but not recognized here. A still undescribed form (the “New Zealand Dusky Dolphin”) is present in the southwestern Pacific Ocean. Two subspecies are recognized.

Subspecies and Distribution.

L.o.obscurusGray,1828—coastalwatersofSAfrica(Namibia,SouthAfrica),PrinceEdwardandAmsterdamIs.

L. o. fitzroyi Waterhouse, 1838 — coastal waters of S South America as far N as Peru in the Pacific Ocean and N Argentina in the Atlantic Ocean.

Dusky Dolphins of a still undescribed form (New Zealand Dusky Dolphins) are present near New Zealand, Campbell, Auckland, and Chatham Is, and an uncertain subspecies has also been observed infrequently off the Falkland Is, Gough I, S Australia, and Tasmania.

Descriptive notes. Total length up to 210 cm; weight 70-85 kg. Neonates are 80-100 cm long and weigh 9-6 kg. Dusky Dolphins are not sexually dimorphic in size. They have robust body shape, short well-defined beak, and falcate dorsal fin with a pointed tip positioned slightly behind mid-body. Flippers are small and curved with rounded tips. Skin is dark gray to black dorsally; it fades into white on belly and throat. There are large pale-gray patches on lower anterior flanks that extend onto most of face. Most of beak and lips are dark gray. Wide, pale-gray streak, also on lower posterior flanks, extends from tailstock and tapers into a point at mid-body just in front of dorsal fin. Thinner streak often branches off from dorsal margin behind dorsal fin and extends along upper flanks and onto back, sometimes as far as blowhole. There is also dark-gray ring around each eye. Flippers are pale gray and fade into dark gray around edges. Dorsal fin may also have a faint pale-gray patch on its posterior one-half. Some individuals in populations of “Fitzroy’s Dusky Dolphin” (L. o. fitzroyi) are larger in size than individuals in populations of the African Dusky Dolphin (L. o. obscurus ). There are 27-36 pairs of small, conical teeth in each jaw.

Habitat. Coastal species, most abundant over continental slopes. The Dusky Dolphin prefers deep (up to 2000 m), cool waters of 10-18°C. They are associated with the cool Benguela Current off South Africa and the cool Humboldt Current off South America.

Food and Feeding. The Dusky Dolphin is an opportunistic feeder whose diet varies according to geographic region and prey availability throughout the year. They feed on a variety offish species throughout the water column, including Argentine anchovy ( Engraulis anchoita), hake ( Merluccius spp. ), and lanternfish ( Myctophidae ). Off Peru, Peruvian anchoveta (E. ringens) is their most common prey, but they will feed on other species when anchoveta are not abundant. Some populations of Dusky Dolphins also feed on squid, so they forage nocturnally in association with the rise of the deep scattering layer. Common squid prey species include Patagonian squid ( Doryteuthis gahi) and jumbo flying squid ( Dosidicus gigas). Off New Zealand, there are seasonal changes in prey preferences. In May-July, Dusky Dolphins feed on more pelagic species, but in August=November, they focus on small schooling surface fish. Dusky Dolphins may forage cooperatively. Large groups off Argentina herd schools of anchovy into tight balls near the surface for easier capture; larger groups are more successful at this herding technique than smaller groups.

Breeding. Breeding of the Dusky Dolphin peaks in August-October off Peru and New Zealand, but elsewhere in November—February, during the Southern Hemisphere’s summer. Both sexes mature at 4-6 years. Gestation lasts c.13 months, and young individuals are weaned after about one year. Breeding females give birth to a single offspring every 1-2 years. Male testes size is known to increase during the breeding season, so given the lack of observed male-male aggression and sexual dimorphism, the mating system of the Dusky Dolphin is likely promiscuous and dependent on sperm competition. There is some evidence for limited male dispersal among groups in coastal populations. Longevity is thought to be 35-36 years.

Activity patterns. The Dusky Dolphin is a very acrobatic species known for high leaps and bow riding. They are generally approachable and curious about boats. Most dives appear to be short, averaging c.21 seconds, but foraging dives may be longer (more than 90 seconds). Foraging dives are also generally shallow but may be up to 130 m. Off Kaikoura, New Zealand, resting behavior is most common in the middle of the day, foraging takes place primarily at night, and socializing occurs in the morning and afternoon, especially after feeding, when socio-sexual activity is common. Dusky Dolphins tend to move into more near-shore waters for resting. The Killer Whale ( Orcinus orca ) is a known predator of the Dusky Dolphin, and moving into very shallow near-shore waters may be a strategy to avoid predation. Groups with young offspring also are observed more commonly in near-shore waters, again likely for safety. Dusky Dolphin echolocation clicks have a bimodal frequency spectrum (40-50 kHz and 80-100 kHz) similar to other delphinids.

Movements, Home range and Social organization. The Dusky Dolphin is a gregarious species. Most groups consist of 20-500 individuals, but groups of more than 1000 individuals have been observed. Smaller groups (under 20 individuals) are more common during winter. Associations are mostly fluid, but there may be some stable subgroup structure. Groups of Dusky Dolphins off New Zealand tend to be more closely knit than elsewhere where subgroups are more spaced out within a group. Subgroups in most populations will separate during resting behavior. The Dusky Dolphin is often seen in association with the Long-beaked Common Dolphin ( Delphinus capensis ) and various seabirds (cormorants, boobies, and pelicans) in feeding aggregations. They also will travel with Short-beaked Common Dolphins (Delphinusdelphis) oft New Zealand and Southern Right-whale Dolphins ( Lissodelphis peronii ) oft Namibia. The latter association may result in occasional hybridization. Off Kaikoura, nursery groups consist mostly of mother—offspring pairs, with young all about the same age. These groups are found most often in near-shore shallow waters less than 20 m deep to avoid predation by sharks and Killer Whales. There are known seasonal inshore—offshore movements in some areas, such as off Argentina and New Zealand. For example, the Dusky Dolphin is known to move from Kaikoura into the Marlborough Sounds (¢.200 km) to feed during winter. They can also move long distances; the longest known linear distance traveled by the Dusky Dolphin is 780 km.

Status and Conservation. CITES Appendix II. Classified as Data Deficient on The IUCN Red List. There is limited abundance data currently available for the Dusky Dolphin, and Fitzroy’s Dusky Dolphin has not been assessed independently. Off Patagonia, there are ¢.7252 individuals and off Argentina, c.6628 individuals. Nevertheless, the Dusky Dolphin is one of the most abundant small cetaceans on the Patagonian Shelf and off coastal Peru. It is currently not known if oceanic island populations are isolated. Without abundance, trend, or population structure data, the risk status of the Dusky Dolphin cannot be adequately assessed. Off Peru, until the 1990s, there used to be extensive small cetacean fisheries, and in 1991-1993, ¢.7000 Dusky Dolphins were killed annually—an unsustainable catch rate. Dolphin hunting was banned in Peru in 1996, and this, in combination with population depletion, has likely contributed to a diminished catch rate. Some are still caught for use as bait in long-line and gillnet fisheries due to a lack of law enforcement; current catch rates unknown. The Dusky Dolphin is susceptible to incidental catch in Patagonian trawl fisheries. In the 1980s, 400-600 dolphins were killed per year, mostly females. This catch rate declined in the 1990s, but several hundred Dusky Dolphins are still killed yearly off southern Argentina. This may have had a severe effect on the population. In New Zealand, there has been some incidental entanglement in recreational gillnets, but catch rates have diminished since the 1980s. Offshore aquaculture may be a growing problem in Marlborough Sound. Dolphin watching and swimming have also become a major tourist industry since late 1980s in New Zealand. Harmful effects on Dusky Dolphins do not appear to be substantial, but over the long term, they have exhibited less resting behavior in the vicinity of tourism vessels, which suggests a lowered tolerance to vessel traffic. In other species, such as bottlenose dolphins (7 Tursiops spp. ), chronic exposure to disturbance is related to habitat displacement, increased levels of stress hormones, and reduced reproductive success, so there is the potential for a chronic impact to Dusky Dolphins. Some tour companies have voluntarily refrained from offering tours during midday when Dusky Dolphins rest; this restriction became mandatory in 2010. Tourism is also increasing off Patagonia, Argentina.

Bibliography. Alonso et al. (1998), Brownell & Cipriano (1999), Cassens et al. (2005), Coscarella et al. ((2003), Dans et al. (1997), Gill et al. (2000), Hammond et al. (2008f), Jefferson et al. (2008), LeDuc et al. ((1999), Lundquist (2012), Markowitz et al. (2003), May-Collado & Agnarsson (2006), McKinnon (1994), Philippi ((1893), Read et al. (1988), Van Waerebeek & Read (1994), Van Waerebeek & Wirsig (2009), Vaughn et al. (2007)), Weir, Duprey & Wirsig (2008), Wursig et al. (2007), Yazdi (2002).

Figures

On following pages: 3. Peale’s Dolphin (Lagenorhynchus australis); 4. Hourglass Dolphin (Lagenorhynchus crucigen; 5. Pacific White-sided Dolphin (Lagenorhynchus obliquidens); 6. Dusky Dolphin (Lagenorhynchus obscurus); 7. Irrawaddy Dolphin (Orcaella brevirostris); 8. Australian Snubfin Dolphin (Orcaella heinsohni).

Subspecies and Distribution. L.o.obscurusGray,1828—coastalwatersofSAfrica(Namibia,SouthAfrica),PrinceEdwardandAmsterdamIs. L. o. fitzroyi Waterhouse, 1838 — coastal waters of S South America as far N as Peru in the Pacific Ocean and N Argentina in the Atlantic Ocean. Dusky Dolphins of a still undescribed form (New Zealand Dusky Dolphins) are present near New Zealand, Campbell, Auckland, and Chatham Is, and an uncertain subspecies has also been observed infrequently off the Falkland Is, Gough I, S Australia, and Tasmania.