Stenella frontalis (G. Cuvier, 1829)

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 : 511

20.

Atlantic Spotted Dolphin

Stenella frontalis

French: Dauphin tacheté / German: Zligeldelfin / Spanish: Delfin manchado atlantico

Other common names: Bridled Dolphin, Cuvier’s Porpoise, Spotted Porpoise

Taxonomy. Delphinus frontalis G. Cuvier, 1829 ,

“découvert un aux iles du Cap-Vert” (off Cape Verde Islands, West Africa).

Taxonomy of Stenella is currently in dispute. The genus is likely polyphyletic and will probably be revised in the near future. According to molecular studies, S. frontalis is most closely related to S. coeruleoalba and S. clymene , followed by Delphinus and Tursiops aduncus . These species appear to be more closely related to each other than any of them are to 7. truncatus , S. attenuata , or S. longirostris . If these relationships are confirmed, taxonomic assignment of these species will need to be revised. Similarities in appearance between S. attenuata and S. frontalis are likely due to either homoplasy or pleisiomorphy. There are at least two known geographical forms of S. frontalis , a coastal West Atlantic form and an offshore form, which may be separate subspecies, but the extent of geographical and genetic overlap is currently uncertain. Monotypic.

Distribution. Tropical to temperate coastal and oceanic waters of the Atlantic Ocean, from c.50° N to ¢.25° S.

Descriptive notes. Total length 190-230 cm, weight 110-143 kg. Neonates are 80- 120 cm long. The Atlantic Spotted Dolphin is intermediate in appearance between the Pantropical Spotted Dolphin ( S. attenuata ) and the Indo-Pacific Bottlenose Dolphin ( Sousa chinensis ). The Atlantic Spotted Dolphins has long robust beak, mediumsized falcate dorsal fin with rounded tip, and curved flippers with rounded tips. Color pattern of the Atlantic Spotted Dolphin changes as an individual matures. Immature Atlantic Spotted Dolphins have color pattern like bottlenose dolphin ( Tursiops spp. ), with white belly and throat, pale-gray flanks, and dark-gray cape. There is also a characteristic pale-gray streak or blaze extending upward from dorsal margin of gray flank area below dorsal fin. Spots begin to appear as an individual ages, showing first on back and belly; adults are heavily spotted. Spots on the Atlantic Spotted Dolphin are white and tend to be larger than those on the Pantropical Spotted Dolphin. In some temperate populations of the offshore geographical form of the Atlantic Spotted Dolphin, adults may be unspotted. This form also tends to be slightly smaller. There are 32-42 pairs of teeth in upper jaw and 30-40 pairs in lower jaw.

Habitat. Outer continental shelves and continental slopes, sometimes occurring in shallower water. The offshore form of the Atlantic Spotted Dolphin is found in coastal waters of oceanic islands and deeper oceanic waters 200-2000 m deep. Populations of Atlantic Spotted Dolphins around oceanic islands may prefer shallower waters. Off the Bahamas, they prefer shallow waters to sand flats 6-12 m deep. The coastal form is generally observed within 250-350 km of a coast in shallower waters. In the Gulf of Mexico, they are most abundant in water 20-180 m deep.

Food and Feeding. The Atlantic Spotted Dolphin is an opportunisiic feeder whose diet varies by region. In the Bahamas, benthic feeding appears to be common. The offshore form prefers epipelagic and mesopelagic fish, squid, and occasionally benthic invertebrates. The offshore form has been observed in coordinated feeding, herding schools of fish, often clupeids, against the surface, and it also follows trawlers to eat discarded fish. Foraging dives descend to 40-60 m and last up to seven minutes. Sharks and possibly the Killer Whale ( Orcinus orca ) are natural predators.

Breeding. Female Atlantic Spotted Dolphins are promiscuous breeders. Males tend to mate within their own social subgroup or with females from closely associated subgroups. Sexual maturity occurs at 8-15 years of age in females, and males seem to be at least 18 years old before they can mate successfully, although they likely mature earlier. This may reflect male-male competition. The birthing interval is 1-5 years and averages three years. Weaning can happen aslate asfive years, and longevity is at least 23 years.

Activity patterns. Atlantic Spotted Dolphins usually dive to depths of less than 10 m, but they may dive up to 40-60 m and stay submerged for up to seven minutes. They are also known to bow-ride readily and breach acrobatically.

Movements, Home range and Social organization. Groups of Atlantic Spotted Dolphins usually consist of less than 50 individuals. For the coastal form, group sizes of 5-15 are common, but aggregations of more than 100 individuals may occur while traveling. In the Bahamas, associations appear to be fluid, with small-scale levels of clustering. Groups may be segregated by sex and age classes. For example, adults with young offspring are usually separated from younger adults. The Atlantic Spotted Dolphin has been known to occasionally forage and travel with the Common Bottlenose Dolphin ( 7 truncatus ) in the Bahamas, and aggressive interactions between them have been observed. Off the Azores, the Short-beaked Common Dolphin ( Delphinus delphis ) and large tuna species ( Thunnus thynnus and T. albacares) may participate in coordinated herding of schooling fish against the surface. Little known about migration patterns or home ranges of Atlantic Spotted Dolphins. There appears to be some seasonal movement off the southern USA. Atlantic Spotted Dolphins tend to be more abundant in near-shore waters in November—May.

Status and Conservation. CITES Appendix II. Classified as Data Deficient on The [IUCN Red List. There are currently no estimates of total abundance or population trends for the Atlantic Spotted Dolphin. In the Gulf of Mexico, there are 30,947 individuals, but that may be an underestimate due to limited survey methods. The population off southern Brazil may be isolated genetically. There is some direct catch of Atlantic Spotted Dolphins in small Caribbean fisheries and possibly off the Azores and West Africa. Incidental catch had been documented throughout the distribution of the Atlantic Spotted Dolphin, including off Brazil, the Caribbean, Venezuela, eastern USA, and Mauritania, and possibly in tuna purse seines off the West African coast. Nevertheless, there are currently no reliable estimates of catch rates. The Atlantic Spotted Dolphin is known to follow trawlers to eat discarded catch at night in Mexican waters, but it does not seem to approach nets closely, so chance of incidental catch is likely low. Some individuals are captured incidentally in gillnet fisheries off Brazil and Venezuela. In Venezuela, incidentally caught dolphins are used as bait in other fisheries and for human consumption. How much of this catch is actually intentional is uncertain. Bioaccumulation of organochlorine pollutants also may be an issue for populations off Brazil. Levels of PCBs and PBDEs sampled from incidentally captured dolphins off Brazil are comparable to those in species from highly industrialized areas. This is likely related to inshore—offshore movements that periodically expose Atlantic Spotted Dolphins to coastal industrial regions of Brazil.

Bibliography. Adams & Rosel (2006), Clua & Grosvalet (2001), Elliser & Herzing (2012), Green (2008), Green et al. (2011), Griffin & Griffin (2004), Hammond et al. (2008m), Herzing et al. (2003), Jefferson et al. (2008), LeDuc et al. (1999), Leonel et al. (2012), Moreno et al. (2005), Perrin (2009b), Perrin, Caldwell & Caldwell (1994).

Figures

17. Pantropical Spotted Dolphin (Stenella attenuata), 18. Clymene Dolphin (Stenella clymene), 19. Striped Dolphin (Stenella coeruleoalba), 20. Atlantic Spotted Dolphin (Stenella frontalis), 21. Spinner Dolphin (Stenella longirostris), 22. Rough-toothed Dolphin (Steno bredanensus)

Distribution. Tropical to temperate coastal and oceanic waters of the Atlantic Ocean, from c.50° N to ¢.25° S.