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|Atlantic Humpback Dolphin - Sousa teuszi|
|Tweet Topic Started: Jan 9 2012, 11:50 AM (1,771 Views)|
|Taipan||Jan 9 2012, 11:50 AM Post #1|
Atlantic Humpback Dolphin - Sousa teuszi
Species: Sousa teuszi
Sousa taxonomy is based largely on small sample sizes for most populations. The several different nominal species are distinguished on several primary characters. Characters include tooth counts; number of vertebrae; form of the dorsal fin base and especially colour patterns (G. Ross, pers. comm.). G. Ross suggests that all Sousa from S. Africa to China and Australia are one species, and probably S. teuszii is also conspecific. However, "we will have a better idea when we can include more definitive genetic work" (G. Ross, pers. comm.). For the purpose of this review, I followed Rice (1998) who separates the Genus Sousa into three species.
Sousa teuszii ranges on the coast of West Africa from Dakhla (23°54'N) in Western Sahara south to the Arquipélago dos Bijagós (11°13'N) in Guinea-Bissau, and also in Nigeria and Cameroon (published assertions that it ranges to Angola are purely conjectural; Rice, 1998). One skull was found on a beach at Solifor Point (13°20'N,16°49'W), The Gambia (van Waerebeek, 2000).
According to Carwardine (1995) this species seems to be particularly common in southern Senegal and northwestern Mauritania.
Distribution of Sousa teuszii: coastal waters of tropical West Africa
(mod. from Jefferson et al. 1993; Rice, 1998; © CMS / GROMS).
3. Population size
Little information on population size is available. Rough population estimates for the Saloum delta, Senegal were 100 animals (Ross et al. 1994, Reyes, 1991 and references therein). In the Rio Grande de Buba (11°32'N, 15°25'W), a fjord-like sea arm rather than a river, a community of at least 20 Atlantic hump-backed dolphins are known to reside, also upstream to the confluence of the Rio Sahol (11°39'N, 15°1O'W) (van Waerebeek et al. 2000).
There are no further detailed population estimates for any of the regions where the species exists, although it is always reported as common (Reyes, 1991).
4. Biology and Behaviour
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 Niger and Bandiala rivers, and possibly others, though rarely travels far upstream and usually remains within the tidal range (Carwardine, 1995).
Humpback dolphins form small schools throughout their distribution, ranging from one to about 25 dolphins off West Africa (Ross et al. 1994 and references therein)
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 fishes (Ross et al. 1994 and references therein). There is no evidence for herbivory as suggested by Kükenthal (1892) for S. teuszii. (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.
There are signs of a probable north-south migration for this species and there is a potential exchange of individuals between known population or subpopulation distribution centres (from north to south): Dakhla Bay, Banc d'Arguin, Langue de Barbarie, Sine Saloum delta, NW bank of the Gambia River outer estuary and Guinea-Bissau archipelago (van Waerebeek et el. 2000).
Atlantic hump- backed dolphins 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). On the other hand Maigret (1982, in Reyes, 1991) recorded sightings of this species in the Banc d' Arguin between May and January, with a peak in August and September.
Off Senegal, 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). According to Ross (2002), Senegalese animals may also shift northward in the summer.
A few Atlantic hump-backed 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 ref. 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 ref. therein).
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 ref. 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).
The Atlantic hump-backed dolphin remains one of the less known small cetaceans, and since it inhabits coastal waters near populated areas it is likely that the level of threat is more extensive than reflected in this account.
No national legislation protecting this species has been identified, but several conservation areas may protect the habitat, in particular the national park of Banc d'Arguin in Mauritania and the Saloum National Park in Senegal (Reyes, 1991 and ref. therein).
Furthermore, the symbiotic relationship between fishermen on the Mauritanian coast between Noudadibou and Nouakchott and dolphins may result in protection of that population, since it is absolutely forbidden to harm the dolphins (Reyes, 1991 and ref. therein).
Sousa sp. is listed in Appendix I&II of CITES . The species is categorized as "Data Deficient" by the IUCN. Evidence of migration along the northwest coast of Africa were the reason for listing under Appendix II of CMS.
Range States are Benin, Cameroon, Gambia, Ghana, Guinea, Guinea-Bissau, Cote d'Ivoire, Liberia, Mauritania, Nigeria, Senegal, Sierra Leone and Togo.
More studies are needed to establish the actual range of the species, on the basis of which more range states south of Cameroon could be included. In addition research on biology, taxonomy, abundance, fishery interactions and human effects on habitat should be addressed to give a better idea of the status of the species (Reyes, 1991 and ref. therein). See detailed recommendations on S. chinensis.
|Taipan||Jan 11 2012, 08:43 PM Post #2|
Photos showing the Humpback of this dolphin species :
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