Overview

Brief Summary

Biology

The Atlantic humpbacked dolphin is a slow swimming species, which typically moves at about five kilometres per hour, surfacing briefly every minute or so. Typically occurring in groups of four to seven individuals, the Atlantic humpbacked dolphin, unlike many other dolphins, will avoid boats and is rarely seen bow riding (2). Humpbacked dolphins are known to feed on fish, including bream, mullet and herring, and cephalopods. Off the coast of Senegal, the Atlantic humpbacked dolphin has been observed moving inshore with the incoming tide to feed on prey within mangrove channels, and then returning to the ocean as the tide retreats (2). It is thought to use echolocation when foraging; a series of clicks are produced which reflect off objects and help the dolphin locate its prey in the often murky habitat. This dolphin may also emit whistles and screams, vocalisations which may be important in communication with other dolphins (2). Very little is known about reproduction in this dolphin; breeding has been recorded in March and April, and the calves are thought to be about one metre long at birth (2).
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Description

The most distinctive feature of this rather elusive cetacean is its distinctive humped appearance when it breaks the ocean's surface to breathe (2). This is caused by the wide hump or ridge on the dolphin's back, from which the dorsal fin emerges (3). The Atlantic humpbacked dolphin is a robust-bodied marine mammal (2), typically slate grey on the sides and back and light grey on the underside (3). It has a long, narrow, distinct beak (2) (3), broad flippers that are rounded at the tip, and a broad tail fin, deeply notched in the centre (2). It is thought that, like the better known and closely related Indo-Pacific humpbacked dolphin (Sousa chinensis), male Atlantic humpbacked dolphins are larger than females (2) (3).
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Distribution

Atlantic humpbacked dolphins live in tropical coastal waters off western Africa, from central Morocco southward to southern Angola.

Biogeographic Regions: ethiopian ; atlantic ocean (Native )

  • Reeves, R., T. Collins, T. Jefferson, L. Karczmarski, K. Laidre, G. O’Corry-Crowe, L. Rojas-Bracho, E. Secchi, E. Slooten, B. Smith, J. Wang, K. Zhou. 2008. "Sousa teuszii" (On-line). The IUCN Red List of Threatened Species. Accessed July 11, 2011 at http://www.iucnredlist.org/apps/redlist/details/20425/0.
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Range Description

The Atlantic Humpback Dolphin is endemic to the eastern tropical Atlantic, where it is limited to coastal and inshore waters (Ross 2002, Van Waerebeek et al. 2004). It occurs in nearshore waters off tropical to subtropical West Africa, from Western Sahara south to at least southern Angola (Notarbartolo di Sciara et al. 1998, Van Waerebeek et al. 2004).

The map shows where the species may occur based on oceanography. The species has not been recorded for all the states within the hypothetical range as shown on the map. States for which confirmed records of the species exist are included in the list of native range states. States within the hypothetical range but for which no confirmed records exist are included in the Presence Uncertain list.
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East Atlantic coast, from Western Sahara to Angola
  • UNESCO-IOC Register of Marine Organisms
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Range

The Atlantic humpbacked dolphin occurs in the eastern Atlantic Ocean (1), from Western Sahara, south to southern Angola (5).
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Physical Description

Morphology

Atlantic humpbacked dolphins belong to the family Delphinidae. They are gray in color with some lighter speckled markings along the ventral surface. These dolphins are characterized by, and named for, their uniquely elevated and rounded dorsal fin, which is referred to as a "hump-back." Atlantic humpbacked dolphins have a very large melon, rounded flippers, and a long pronounced beak. Adults weigh between 100 and 150 kg and are generally between 2 and 2.5 meters in length. Like most cetaceans, they have homodont dentition (i.e., no differentiation along the tooth row). A distinguishing feature of this species is the number of vertebrae, which is less than that of its sister species, Indo-Pacific humpbacked dolphins. This, along with the number of teeth (26 to 31 pairs), and the species' geographic range, help taxonomists distinguish between Atlantic humpbacked dolphins and Indo-Pacific humpbacked dolphins. Atlantic humpbacked dolphins have a basal metabolic rate of 1200 cm^3 oxygen/hour.

Range mass: 100 to 150 kg.

Range length: 2.0 to 2.5 m.

Range wingspan: 0 (low) mm.

Average basal metabolic rate: 1200 cm3.O2/g/hr.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: sexes alike

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Ecology

Habitat

Sousa teuzii is found mostly in shallow coastal waters, rivers, and estuaries. Although typically found in shallow water, it also occurs in deeper reefs where it apparently seeks refuge from predation by killer whales. When in deeper water, this species swims mostly along the ocean floor. Sousa teuzii typically stays near the shoreline within one or two kilometers of land; it is restricted to warm tropical waters.

Range depth: 20 to 65 m.

Habitat Regions: tropical ; saltwater or marine

Aquatic Biomes: reef ; rivers and streams; coastal

Other Habitat Features: estuarine

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Habitat and Ecology

Habitat and Ecology
Atlantic Humpback Dolphins are found primarily in estuarine and shallow (< 20 m) coastal waters with soft sediment bottoms (Van Waerebeek et al. 2004). They have been observed as far as about 50 km up the Saloum River and are known to enter the Niger and Bandiala rivers, but they rarely travel far upstream and usually remain within the range of tidal influence (Van Waerebeek et al. 2004). There is no evidence to suggest the existence of separate freshwater subpopulations. In at least some areas (e.g., Gabon and Mauritania), these dolphins occur in the surf zone just offshore of the breakers (Busnel 1973; Tim Collins pers. comm. to T. Jefferson, 2007). There are no reports from offshore waters. Among the features that have been described as aspects of preferred habitat are proximity to sandbanks, brackish, mangrove-lined estuaries, and turbid waters with temperatures ranging between 17°C and 28°C (Maigret 1980, Ross et al. 1994).

There is little information on the diet of Atlantic humpback dolphins. They appear to feed on nearshore schooling fishes such as mullet (Mugil spp.) and, contrary to some descriptions, are not thought to eat vegetable matter (see Van Waerebeek et al. 2004). Stomach contents have included grunts (Pristipoma jubelini) and bongo fish (Ethmalosa fimbriata) (Van Waerebeek et al. 2004).

Systems
  • Freshwater
  • Marine
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In estuarine and coastal waters
  • UNESCO-IOC Register of Marine Organisms
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Shallow, tropical, coastal waters, less than 25 metres deep, are the preferred habitat of the Atlantic humpbacked dolphin. This includes the often murky waters of bays, river deltas and mangrove channels (2). This dolphin has also been observed entering large rivers, although it rarely travels far upstream, remaining within salty water (5).
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Trophic Strategy

The diet of Sousa teuszii consists mainly of fish, including mullet and sardines. Other important prey items include squid and crustaceans.

Animal Foods: fish; mollusks; aquatic crustaceans

Primary Diet: carnivore (Piscivore )

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Associations

As piscivores, Atlantic humpbacked dolphins likely impact the coastal fish populations of western Africa. In Mauritania, this species maintains an interesting mutualistic relationship with local fisherman. Atlantic humpbacked dolphins respond to signals sent by the fisherman to come into shore. This helps concentrate fish near the shore and allows fisherman to meet economic demands, while decreasing dolphin by-catch. There is no information available regarding parasites of this species.

Mutualist Species:

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Atlantic humpbacked dolphins are preyed upon during all stages of life by killer whales. In order to decrease risk of predation, they often seek cover in reefs and find refuge near shore.

Known Predators:

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Life History and Behavior

Behavior

Atlantic humpbacked dolphins use echolocation to find food and communicate with conspecifics, but have relatively poor eyesight. However, its enlarged melon and high brain to body mass ratio suggest that it is well equipped for communication and perception of its immediate environment. In order to avoid predation from killer whales, Atlantic humpbacked dolphins seek shelter in coral reefs.

Communication Channels: acoustic

Perception Channels: visual ; tactile ; acoustic ; echolocation ; chemical

  • 2009. Distribution, behaviour, and photo-identification of Atlantic humpback dolphins Sousa teuszii off Flamingos, Angola. African Journal of Marine Science, 31: 319-331.
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Life Expectancy

The average lifespan of Atlantic humpbacked dolphins has not been documented, but based on data from other dolphins, is expected to be around 15 to 20 years.

Typical lifespan

Status: wild:
15 to 20 hours.

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Reproduction

Sousa tueszii is polygynous, as a single male mates with multiple females. Calving has been reported from December to February, but may extend into other months. The exact age of sexual maturity is unknown, but most individuals reproduce between the ages of 4 and 8. The closely related Sousa chinensis breeds year round, although calves are typically born during the summer. In other dolphin species, such as Tursiops truncatus (bottlenose dolphins), mating season occurs from March through April, and calves are born between February and May. Gestation lasts for approximately 12 months and young remain close to their mother until they are about 4 or 5 years old. In Tursiops truncatus, most individuals reach sexual maturity between 5 and 12 years of age for females, and between 9 and 13 years of age for males.

Mating System: polygynous

Little information exists regarding the reproductive behavior of Sousa tueszii. Calving has been reported from December to February, but may extend into other months. The exact age of sexual maturity is unknown, but most individuals reproduce between the ages of 4 and 8. Sousa tueszii has an average of one offspring per cycle, which weighs between 9 and 11 kg. Weaning has been reported in individuals as young as 24 months but usually is completed by 48 months. In the closely related Sousa chinensis, males often court females by somersaulting, chasing them in circles, and waving their flippers. In other dolphin species, such as Tursiops truncatus, males aggressively engage females during mating season and use a social hierarchy system based on size to determine which individuals mate.

Breeding interval: Sousa tueszii breeds once yearly.

Breeding season: Sousa tueszii breeds during April or May.

Average number of offspring: 1.

Average gestation period: 12 months.

Range weaning age: 24 (low) months.

Average weaning age: 48 months.

Range time to independence: 4 to 5 years.

Range age at sexual or reproductive maturity (female): 4 to 7 years.

Range age at sexual or reproductive maturity (male): 4 to 7 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous

There is no information available regarding parental care in Sousa teuszii. In other dolphin species, such as Tursiops truncatus and the closely related Sousa chinensus, gestation lasts for approximately 12 months. In these species, calves become completely independent when they are approximately 4 to 5 years old. Until the calf reaches sexual maturity, it remains close to the mother.

Parental Investment: pre-hatching/birth (Provisioning: Female, Protecting: Female)

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Conservation

Conservation Status

Sousa teuszii is classified as vulnerable on the IUCN's Red List of Threatened Species. Major threats include hunting/whaling, entanglement in fishing nets, habitat destruction, and pollution. This species is listed under Appendix I of CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora) and appears to be especially vulnerable to population decrease due to its small and fragmented range and its narrow ecological niche.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

IUCN Red List of Threatened Species: vulnerable

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IUCN Red List Assessment


Red List Category
VU
Vulnerable

Red List Criteria
C2a(i)

Version
3.1

Year Assessed
2012

Assessor/s
Reeves, R., Collins, T., Jefferson, T.A., Karkzmarski, L., Laidre, K., O’Corry-Crowe, G., Rojas-Bracho, L., Secchi, E., Slooten, E., Smith, B.D., Wang, J.Y. & Zhou, K.

Reviewer/s
Brownell Jr., R.L. & Cooke, J.

Contributor/s

Justification
This species is considered Vulnerable C2a(i) because of an inferred or suspected continuing decline, where the total number of mature individuals is considerably fewer than 10,000, and each of the defined subpopulations is estimated to contain fewer than 1,000. In a comprehensive assessment of available information, Van Waerebeek et al. (2004) identified eight possible “management stocks” but acknowledged that not all were likely to prove to be biologically distinct populations (subpopulations). It is inferred from the available information summarized above that the total population (all ages) is only a few thousand, and almost certainly fewer than 10,000. Of those, only about half would be mature (Taylor et al. 2007). Although there have been no quantitative studies of trends, the ongoing threats – particularly bycatch in fisheries, which almost certainly has been increasing in recent decades – support the suspicion of a continuing decline in population size.

History
  • 1996
    Data Deficient
  • 1994
    Insufficiently Known
    (Groombridge 1994)
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Status

Classified as Vulnerable (VU) on the IUCN Red List (1) and listed on Appendix I of CITES (4).
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Population

Population
Little information on population size is available, as this is one of the least-known delphinids. Although there has been no assessment in most areas of their overall range, the population of Atlantic Humpback Dolphins appears fragmented, with subpopulations separated by areas of low or zero density (Van Waerebeek et al. 2004). Eight “management stocks” have been provisionally identified. Six of them are identified on the basis of “sightings, or other contemporary records, clustered around a confirmed habitat” (Van Waerebeek et al. 2004) – Dahkla Bay, Banc d’Arguin, Saloum-Niumi, Canal do Gêba-Bijagos, South Guinea, and Angola. The other two – Cameroon Estuary and Gabon – are based on “historical evidence.” The continued presence of Humpback Dolphins in the Estuaire de Gabon was recently confirmed (Tim Collins pers. comm. to R. Reeves, 8 January 2008). A ninth management stock is suspected to exist off western Togo (Van Waerebeek et al. 2004).

Rough “estimates” for the Saloum River delta, Senegal, were 100 animals, and there were thought to be at least several hundred in Guinea Bissau several decades ago (Ross et al. 1994, Reyes 1991). A small group of at least 20 dolphins resides in the Rio Grande de Buba (a fjord-like sea arm rather than a river) and upstream to the confluence of the Rio Sahol (Van Waerebeek et al. 2004). Although the species often has been reported as common (Reyes 1991, Culik 2004), there are no other numerical data on abundance. Considering the relatively small numbers observed, and even taking account of the many areas of the species’ range where there has been little or no assessment, the total population probably numbers only a few thousand.

Population Trend
Decreasing
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Threats

Major Threats
Incidental mortality in fishing nets and lines is known from at least Morocco, Mauritania, Senegal and Guinea Bissau (Van Waerebeek et al. 2004). Although such mortality has not been assessed properly anywhere in the species’ range, it probably occurs in most or all areas and is considered the most serious immediate threat to the species.

Some Atlantic Humpback Dolphins are probably taken directly for food by local people (Van Waerebeek et al. 2004). The fishing communities of Joal and Fadiouth in Senegal have a tradition of hunting cetaceans, and others in the Petite Côte were known to hunt dolphins until at least 1996 (Van Waerebeek et al. 2004). Past and present levels of these captures, and their potential impacts on subpopulations, remain unknown (Reyes 1991). The most recently documented interaction in Senegal was in November 1996, when three dolphins were found together, each with a piece of netting tied around the tailstock on a beach of Sangomar Island in the Saloum delta (Van Waerebeek et al. 2004).

Habitat destruction, boat strikes, and environmental contamination are additional potential threats, although generally little is known about them. In Senegal there has been a permanent reduction of mangrove areas to facilitate the extension of rice culture and exploitation of forests, especially in the Fathala area. Habitat destruction and degradation may be significant factors affecting the species’ status given its nearshore distribution and the high human population densities, associated with agricultural and industrial development, in some areas. These problems will contribute to fragmentation of the dolphin population. Offshore oil exploration and development are underway in at least Gabon and Angola. Excessive fishing of neritic fish stocks in parts of West Africa also may have reduced food availability for these dolphins (Van Waerebeek et al. 2004).
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As an inhabitant of coastal waters, the Atlantic humpbacked dolphin is highly vulnerable to the effects of human activities (2). Fisheries impact the dolphin, firstly, by causing incidental mortality when the dolphin becomes entangled in fishing gear (6). This is currently considered to be the greatest immediate threat to this species (1), and may be the reason behind the rarity of this species in the coastal waters of Senegal and Gambia, and in Dakhla Bay in Western Sahara, two areas in which the Atlantic humpbacked dolphin was once common (6). Secondly, fisheries may impact this dolphin by reducing the availability of prey (6). In certain areas, the Atlantic humpbacked dolphin is also captured intentionally for food by local people (5), although the numbers that are caught, and what impact this has on the population, is not known (1). In addition to the threat of fisheries, the Atlantic humpbacked dolphin is potentially being affected by boat strikes and habitat degradation. For example, mangroves are being converted to rice cultivation in some areas, destroying an important foraging habitat of this dolphin (1) (5). A high human population density within its range, and the associated development of agriculture and industry, will undoubtedly continue to have an impact on the shallow coastal waters in which the Atlantic humpbacked dolphin is found (1) (5).
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Management

Conservation Actions

Conservation Actions
The species is listed in Appendix I of CITES and Appendix II of CMS.

These dolphins are a high priority for research and conservation because of their restricted and apparently fragmented range, narrow ecological niche, apparently low numbers, and continuing threats (IWC 2003, Reeves et al. 2003, Van Waerebeek et al. 2004).
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Conservation

The Atlantic humpbacked dolphin is listed on Appendix II of the Convention on Migratory Species (CMS). This indicates that this is a species that would benefit from international co-operation, with regards to its conservation, and the Convention encourages the relevant countries to implement suitable conservation measures (7). Considering its specific habitat preferences, estimated low abundance and the threats it faces, the Atlantic humpbacked dolphin is likely to be in great need of further research and conservation measures (6).
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Relevance to Humans and Ecosystems

Benefits

Although there are no known adverse effects of Sousa teuszii on humans, it is thought that this species competes with local fisherman for fish off the west coast of Africa.

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Atlantic humpbacked dolphins benefit fisherman in Mauritania by schooling fish into shore. Local villages intentionally hunt this species for food.

Positive Impacts: food

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Risks

IUCN Red List Category

Vulnerable (VU)
  • IUCN (2008) Cetacean update of the 2008 IUCN Red List of Threatened Species.
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