Sousa teuszii (Kükenthal, 1892)

English: Atlantic humpback dolphin, Cameroon dolphin
German: Kamerun-Flußdelphin
Spanish: Delfín jorobado del Atlántico
French: Dauphin du Cameroun

Family Delphinidae

Sousa teuszii © Würtz-Artescienza (see "links")

1. Description

Humpback dolphins are medium sized and robust. Their melon is slightly depressed and slopes gradually to an indistinct junction with the long, narrow beak. The broad flippers are rounded at the tip and the flukes are broad and full, with a deep median caudal notch. The dorsal fin emerges from a hump or ridge of connective tissue on the back. Body length reaches 2.8m and body mass 284 kg. Colour is somewhat variable with slate gray on the back and light gray-whitish below. Some animals have dark spots on the tail stock and near the base of the fin (Jefferson et al. 2008).

Recent morphological as well as genetic research confirms that S. teuszii is in fact a separate species of the genus Sousa (Jeffersson and Van Waerebeek, 2004; Frere et al., 2008).

2. Distribution

Sousa teuszii ranges on the coast of West Africa from Dakhla Bay (23°54'N) in Western Sahara south to Tombua (15°47'S), southern Angola. A total of six contemporary management stocks are provisionally discerned: Dakhla Bay (Western Sahara), Banc d'Arguin (Mauritania), Saloum-Niumi (Senegal), Canal do Gêba-Bijagos (Guinea-Bissau), South Guinea and Angola. Two stocks are historical (now extirpated): Cameroon and Gabon Estuary (Van Waerebeek et al. 2004).

Distribution of Sousa teuszii: coastal waters of tropical West Africa
(Reeves et al. 2008; © IUCN; enlarge map).

The species remains unrecorded in Ghana and neighbouring nations despite apparently suitable coastal habitat (Van Waerebeek et al. 2009).

3. Population size

S. teuszii seems to be particularly common in southern Senegal and northwestern Mauritania (Carwardine,1995). However, there are no rigourous population estimates for any of the regions where the species might exist (Van Waerebeek et al., 2004). From north to south, the Dakhla Bay and the Banc d'Arguin stocks appear to be very small (Van Waerebeek et al., 2004). Rough population estimates for the Saloum delta, Senegal were 100 animals (Ross et al. 1994, Reyes, 1991 and refs. therein). The high number of opportunistic sightings suggests that the still relatively undisturbed waters of Guinea-Bissau, enclosing extensive mangrove forest habitat, may support one of the largest known populations of S. teuszii: the Canal do Gêba-Bijagos stock. The status of the South Guinea, Cameroon and Gaboon Estuaries management stocks is unknown, although the latter two are likely extinct. The Angola stock is presumably very small (Van Waerebeek et al. 2004), but its existence was recently confirmed (Weir, 2007).

4. Biology and Behaviour

Habitat: This species prefers coastal and estuarine waters less than 20m deep and occurs in the surf zone on more open coasts. There are no reports of its presence in offshore waters. The preferred habitat is near sandbanks and mangrove areas, in turbid waters with temperatures ranging between 17°C and 28°C (Maigret, 1982, in Ross et al. 1994). It has been recorded up to 33 miles up the Saloum River and is known to enter the Niger and Bandiala Rivers, and possibly others, though it rarely travels far upstream and usually remains within the tidal range (Carwardine, 1995).

Photo: Jaap van der Toorn @ Jaap's Marine Mammal Pages (see "links").

Schooling: Humpback dolphins form small schools throughout their distribution, ranging from one to about 25 dolphins off West Africa (Ross et al., 1994 and refs. therein).

Reproduction: Breeding has been reported in March and April, but the season may be more protracted (Jefferson et al., 1993).

Food: Schooling fish e.g. mullet (Jefferson et al., 1993). Stomachs contained pomadasyid, clupeid and mugilid fish (Ross et al. 1994 and references therein). There is no evidence for herbivory as suggested by Kükenthal (1892) (Jefferson et al. 1993).

Busnel (1973; in Ross, 1994) described a remarkable example of a symbiotic relationship between fishermen and groups of bottlenose dolphins on the Mauritanian coast around Cap Timiris, north of Nouakchott. The fishermen wait for migrating shoals of mullet to appear close to shore, and then apparently summon the dolphins by slapping sticks on the water surface. The dolphins effectively contain the mullet on their seaward edge while feeding, enabling the fishermen to deploy their nets around the fish more easily. Humpback dolphins also take part in the cooperative harvest, though perhaps fortuitously, since the method probably requires a larger number of dolphins than the usual humpback school size.

5. Migration

There are signs of a probable north-south migration, and there is a potential exchange of individuals between known population or subpopulation distribution centres (from north to south): Dakhla Bay (Western Sahara), Banc d'Arguin (Mauritania), Langue de Barbarie (Senegal), Sine Saloum delta (Senegal), NW bank of the Gambia River outer estuary (The Gambia) and Guinea-Bissau archipelago (Van Waerebeek et al. 2000).

Regular cross-border movements between the Saloum Delta (Senegal) and Niumi National Park (The Gambia) technically qualifies S. teuszii as a "migratory species" under the Conservation of Migratory Species (CMS) Convention (Van Waerebeek et al., 2004). They have been recorded in the Saloum Estuary from January to April, with very few observations in subsequent months. However, catch data show that the species was taken north of the estuary from June to August (Reyes, 1991 and ref. therein, confirmed by Ross, 2002). Daily movements observed off Senegal show that humpback dolphins move onshore with the rising tide to feed in the mangrove channels of the Saloum Delta, returning towards the sea with the ebb tide (Maigret, 1981 in Ross et al. 1994).

Maigret (1982, in Reyes, 1991) recorded sightings of this species in the Banc d' Arguin (Mauritania) between May and January, with a peak in August and September.

6. Threats

Direct catch: A few Atlantic humpback dolphins have reportedly been taken along the range. No recent information is available, but direct catches still may occur (Reyes, 1991; Van Waerebeek et al., 2000).

Incidental catch: There are reports of incidental catches in beach seines and shark nets in Senegal. Past and present levels of these captures remain unknown (Reyes, 1991 and refs. therein). The most recent interaction in Senegal was recorded in November 1996, when three animals were found together, each with a piece of netting tied around the tailstock, on a beach of Sangomar Island in the Saloum delta, probably an abandoned take. In Guinea-Bissau, a 190-cm male was by-caught in a fishing trap at Canhabaque Island, Bijagós in March 1989 (van Waerebeek et al. 2000 and refs. therein).

Habitat degradation: In Senegal there has been a permanent reduction of mangrove areas for extension of rice culture and exploitation of forest, especially in the Fathala area. Excessive fishing of prey species may reduce food availability and increase the risk of incidental catch. Pollution may also be a source of habitat destruction, since the species inhabits areas with high population growth subject to agricultural and industrial development (Reyes, 1991 and refs. therein). The possible fracturing of the species' habitat range, resulting in reproductively isolated groups, due to coastal development should be monitored (van Waerebeek et al. 2000).

7. Remarks

Range states (Reeves et al. 2008) :
Angola; Cameroon; Congo; Côte d'Ivoire; Gabon; Gambia; Guinea; Guinea-Bissau; Liberia; Mauritania; Nigeria; Senegal; Western Sahara.

Sousa teuszii is listed in Appendix I&II of CMS. The World Wide Fund for Nature (WWF) considers S. teuszii as one of the most endangered small cetaceans world-wide (WWF, 2009).

Sousa sp. is listed in Appendix I of CITES. The species is categorized as "Vulnerable" by the IUCN (Reeves et al. 2008). This is based on the fact that the current population size is below 10,000 with local populations smaller than 1,000 animals. There is an inferred or suspected continuing decline in population size with ongoing threats (see above).

There is a need to obtain baseline abundance data and establish seasonal patterns of distribution for S. teuszii in northwestern Africa, as well as investigate the level of genetic interchange among different dolphin communities. At least two boat surveys, one each in the rainy and dry season, would need to be conducted in the estuarine and larger creek systems, and the coastal shelf waters of southern Senegal and The Gambia. Similar surveys of inshore and coastal waters are needed in Dakhla Bay and other parts of the Western Saharan and Moroccan coasts known or suspected to have humpback dolphins (Reeves et al 2003).

8. Sources

· Carwardine M (1995) Whales, Dolphins and Porpoises. Dorling Kindersley, London, UK, 257 pp.
· Frere CH, Hale PT, Porter L, Cockcroft VG, Dalebout ML (2008) Phylogenetic analysis of mtDNA sequences suggests revision of humpback dolphin (Sousa spp.) taxonomy is needed. Mar Freshw Res 59: 259-268.
· Jefferson TA, Leatherwood S, Webber MA (1993) FAO Species identification guide. Marine mammals of the world. UNEP/FAO, Rome, 320 pp.
· Jefferson TA, Van Waerebeek K (2004) Geographic variation in skull morphology of humpback dolphins (Sousa spp.). Aquat Mamm 30: 3-17
· Jefferson TA, Webber MA Pitman RL (2008) Marine mammals of the world. Elsevier, Amsterdam, 573 pp.
· Reeves RR, Smith BD, Crespo EA, Notarbartolo di Sciara G (compilers) (2003)
· Dolphins, whales and porpoises: 2002-2010 Conservation Action Plan for the world's cetaceans. IUCN/SSC Cetacean Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK. ix + 139pp.
· Reeves RR, Dalebout ML, Jefferson TA, Karczmarski L, Laidre K, O'Corry-Crowe G, Rojas-Bracho L, Secchi ER, Slooten E, Smith BD, Wang JY, Zhou K (2008) Sousa teuszii. In: IUCN 2009. IUCN Red List of Threatened Species. Version 2009.2. <www.iucnredlist.org>.
· Reyes JC (1991) The conservation of small cetaceans: a review. Report prepared for the Secretariat of the Convention on the Conservation of Migratory Species of Wild Animals. UNEP/CMS Secretariat, Bonn.
· Ross GJB (2002) Humpback dolphins - Sousa chinensis, S. plumbea, and S. teuszii. In: Encyclopedia of marine mammals (Perrin WF, Würsig B, Thewissen JGM, eds.) Academic Press, San Diego, pp. 585-589.
· Ross GJB, Heinsohn GE, Cockroft VG (1994) Humpback dolphins - Sousa chinensis (Osbeck, 1765), Sousa plumbea (G. Cuvier, 1829) and Sousa teuszii (Kükenthal, 1892). In: Handbook of marine mammals (Ridgway SH, Harrison SR, eds.) Vol. 5: The first book of dolphins. Academic Press, London, pp. 23-42.
· Van Waerebeek K, Ndiaye E, Djiba A, Diallo M, Murphy P, Jallow A, Camara A, Ndiaye P, Tous P (2000) A survey of the conservation status of cetaceans in Senegal, the Gambia and Guinea-Bissau. UNEP/CMS Secretariat, Bonn, Germany,. 80 pp. PDF-copy of full report: (www.wcmc.org.uk/cms/reports/WAFCET/ WAFCET2/pdf/Wafcet2_Report.pdf)
· Van Waerebeek K, Barnett L, Camara A, Cham A, Diallo M, Djiba A, Jallow A, Ndiaye E, , Samba Ould Bilal AO, Bamy IL (2004) Distribution, status, and biology of the Atlantic humpback dolphin, Sousa teuszii (Kükenthal, 1892). Aquat Mamm 30: 56-83.
· Van Waerebeek K, Ofori-Danson PK, Debrah J (2009) The cetaceans of Ghana, a validated faunal checklist. West Afr J Appl Ecol 15: 61-89
· Weir C (2007) Occurrence and distribution of cetaceans off northern Angola, 2004/05. J Cetacean Res Manag 9: 225-239
· WWF (2009) Small cetaceans, the forgotten whales. (Elliott W, Sohl H, Bugener V). Whaling Report.Indd 34

© Boris Culik (2010) Odontocetes. The toothed whales: "Souza teuszii". UNEP/CMS Secretariat, Bonn, Germany. http://www.cms.int/reports/small_cetaceans/index.htm
© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.