INTRODUCTION
In the eastern tropical Atlantic (ETA) defined here as the waters extending from Mauritania to Angola along the west coast of Africa, the cetacean fauna remains poorly documented, mostly due to the logistical and financial challenges of working in the region and a lack of local scientists. The potential cetacean range states located in the ETA currently include 18 mainland states and the offshore island states and territories of Cape Verde, Ascension (UK), St Helena (UK) and the Democratic Republic of São Tomé and Príncipe. Published reviews of cetacean occurrence in parts of the ETA have been carried out by Jefferson et al. (Reference Jefferson, Curry, Leatherwood and Powell1997) and Weir (Reference Weir2010). Additionally, species checklists and overviews have been produced for some individual range states, such as Mauritania (Robineau & Vely, Reference Robineau and Vely1998), Cape Verde (Hazevoet et al., Reference Hazevoet, Monteiro, López, Varo, Torda, Berrow and Gravanita2010), Guinea (Bamy et al., Reference Bamy, Van Waerebeek, Bah, Dia, Kaba, Keita and Konate2010), Ghana (Van Waerebeek et al., 2009) and Togo (Segniagbeto & Van Waerebeek, Reference Segniagbeto and Van Waerebeek2010). These sources have indicated the presence of at least 34 cetacean species. However, many of those species have been verified in the ETA on only a handful of occasions, and the cetacean fauna inhabiting some ETA range states has barely been described (Weir, Reference Weir2010).
Information on the cetacean species occurring in the waters of each ETA range state is important in understanding particular species' geographical distribution ranges and for implementing conservation measures to manage species. National conservation legislations may apply to the exclusive economic zones (EEZs) of particular states and territories, which rely on an understanding of the species occurring there. In the case of cetaceans, which are highly-mobile and may travel across multiple range states, some international conservation agreements have been developed which rely on the cooperation, participation, and coordination of each of the range states. An example in the ETA is the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia which was adopted in 2008 under the 1979 Convention on the Conservation of Migratory Species of Wild Animals (CMS, or the ‘Bonn Convention'). Clearly, the confirmed list of range states for the species included under such agreements is fundamental to their implementation.
Here, we provide verified species records for two delphinid species, the pygmy killer whale Feresa attenuata and the Fraser's dolphin Lagenodelphis hosei, each of which confirms the occurrence of a new range state for the species. Additionally, these records provide the first verified at-sea sighting for F. attenuata in the ETA region and for L. hosei within the Gulf of Guinea (extending from Cape Palmas in Liberia to Cape Lopez in Gabon; IHO, 1953), providing novel information on group size and habitat in those areas.
MATERIALS AND METHODS
Both cetacean sightings were recorded by dedicated observers during geophysical seismic surveys. The Cote d'Ivoire sighting was recorded by an observer (L.G.) working on the seismic vessel ‘Polarcus Asima'. The Gabon sighting was recorded by two observers on separate seismic vessels, ‘PGS Apollo' (C.W.) and ‘Sanco Spirit' (D.M.), which were operating approximately 6 km distance apart from each other. Cetacean data were collected using standardized methods (see Weir, Reference Weir2007) which, briefly, comprised a single observer scanning 360° around the vessel with the naked eye and with 10 × 42 or 7 × 50 binoculars. Effort and environmental data were logged for each cetacean watch. When cetaceans were sighted, the species, group size, behaviour, position, water depth and associated environmental data were recorded. Photographs were taken whenever possible to validate records, using a Sony Alfa 200 camera with 500 mm lens (L.G.) or Canon SLR cameras with 400 mm lens (D.M. and C.W.).
RESULTS
Pygmy killer whale (Feresa attenuata)
On 3 November 2011, a group of approximately 10 delphinids was observed by L.G. at a bearing of 130° and a distance of 1.5 km from position 04°43.26′N 003°26.00′W, located around 47 km south of Assini on the coast of Côte d'Ivoire, Gulf of Guinea. Water depth on the vessel's echosounder measured 1801 m. The airguns were not firing at the time of the sighting. Relevant environmental conditions during the sighting comprised good visibility and Beaufort Sea State 3. The animals were observed for 14 minutes, during which they travelled slowly in a westerly direction with occasional breaches. Photographs were taken at the animals' closest approach to the vessel (~1 km) (Figure 1). The animals had relatively small body size (~2.5 m), robust shape and dark grey coloration with a darker narrow dorsal cape that dipped slightly below the dorsal fin. Their dorsal fins were relatively tall, broad-based and slightly falcate. Head shape was rounded with no distinct beak. White pigmentation was seen around the lips and chin of individuals that were observed through binoculars (and photographed; Figure 1) while breaching. Based on these combined features, they were identified as Feresa attenuata.
A breaching Feresa attenuata photographed off Côte d'Ivoire on 3 November 2011.
Fraser's dolphin (Lagenodelphis hosei)
On 17 January 2012, a group of approximately 65 dolphins was observed and photographed by D.M. and C.W. while working on separate platforms. The animals were first detected at 06:38 Coordinated Universal Time (UTC) by D.M., at a distance of 700 m from the first vessel (position 00°14.98′S 008°39.26′E, water depth of 556 m). This location was approximately 42 km north of Cape Lopez in Gabon. Relevant environmental conditions during the sighting comprised good visibility, a 1.5 m swell and Beaufort Sea State 3. The animals passed at speed within 400 m of the vessel, and photographs were taken (Figure 2). The observer (C.W.) on the second vessel (located 7 km south-west of the first ship at 06:38 UTC) detected the same dolphin school at 07:10 UTC (00°15.81′S 008°37.27′E; water depth of 644 m). The closest approach of the dolphins to the second platform was 3 km, and photographs were taken. Neither vessel was firing airguns during this observation. During the sighting the dolphins alternated between travelling rapidly in a tightly-bunched school (creating a lot of white water at the surface) and apparent feeding behaviour. The group changed heading multiple times, and on several occasions the dolphins split up to travel in variable directions, exhibiting fast surface rushes and low fast leaps indicative of chasing prey. Both observers identified the animals as Lagenodelphis hosei based on photographs confirming characteristic features of this species including their short but well-defined beaks, very small and erect dorsal fins with pointed tips and some individuals with dark face masks and a thick dark lateral stripe from the eye to the genitals (Figure 2).
Lagenodelphis hosei photographed off Gabon on 17 January 2012.
DISCUSSION
Both Feresa attenuata and Lagenodelphis hosei are amongst the poorest-documented cetacean species in the ETA (Jefferson et al., Reference Jefferson, Curry, Leatherwood and Powell1997; Weir, Reference Weir2010). Given the relative rarity of both species and the potential for observers to confuse them with other delphinids species, only records for which supporting documentation (i.e. photographs) were available were considered here.
Only three verified records have been published of Feresa attenuata in the region, all involving strandings or captures. Cadenat (Reference Cadenat1958) and Fraser (Reference Fraser1960) described an individual caught off Yenn in Senegal on 12 April 1958. Cadenat (Reference Cadenat1959) reported several further unconfirmed sightings off the coast of Senegal, including an animal harpooned off Dakar that escaped. However, these later records could not be verified. Van Waerebeek et al. (Reference Van Waerebeek, Ofori-Danson and Debrah2009) documented an adult landed as by-catch at Dixcove in Ghana on 31 December 2007. Seven animals stranded at Boavista, Cape Verde, on 10 February 2012, recently confirming a further range state (López-Suárez et al., Reference López-Suárez, Oujo, Acre and Hazevoet2012). Several other mentions of this species in the literature included no supporting information (e.g. Mörzer Bruyns, Reference Mörzer Bruyns1971; Tormosov et al., Reference Tormosov, Budylenko and Sazhinov1980; Jefferson et al., 1997). Consequently, the sighting reported here represents only the fourth verified record of the species in the ETA, adds a further confirmed range state for the species and represents the first documented at-sea sighting for the ETA region.
There have been nine previous confirmed records of Lagenodelphis hosei in the ETA, and one further probable record. The first verified record was a skull found on the beach of Sangomar Island in Senegal during 1997 (Van Waerebeek et al., Reference Van Waerebeek, Ndiaye, Djiba, Diallo, Murphy, Jallow, Camara, Ndiaye and Tous2000). A by-catch monitoring programme in Ghana documented three by-catch events (involving four animals) during 2000 (Weir et al., Reference Weir, Debrah, Ofori-Danson, Pierpoint and Van Waerebeek2008; Van Waerebeek et al., Reference Van Waerebeek, Ofori-Danson and Debrah2009). More recently, Weir et al. (Reference Weir, Debrah, Ofori-Danson, Pierpoint and Van Waerebeek2008) presented details of a probable sighting of this species off Nigeria during 2004, and two confirmed sightings offshore Angola in 2007 and 2008. A third photographically-verified sighting for Angola (see map in Weir, Reference Weir2011a) comprised 80 animals recorded on 19 May 2009. Torda et al. (Reference Torda, López Suárez and López Jurado2010) recently reported the first documented records of L. hosei in Cape Verde, comprising a pod of 20 animals photographed south-west of São Nicolau on 31 August 2003 and a carcass found on the southern coast of Boavista on 10 March 2006. The sighting reported here adds Gabon to Senegal, Ghana, Angola and Cape Verde (and probably Nigeria) as confirmed ETA range states for L. hosei. This fifth confirmed at-sea sighting of the species in the ETA adds to knowledge on the ecology of L. hosei in the region. The group size recorded off Gabon fell within the documented range of 20 (Torda et al., Reference Torda, López Suárez and López Jurado2010) to 120 (Weir et al., Reference Weir, Debrah, Ofori-Danson, Pierpoint and Van Waerebeek2008) animals. However, both the water depth of the Gabon sighting (556 m) and the recent Cape Verde sighting (~500 m; Torda et al., Reference Torda, López Suárez and López Jurado2010) were shallower than the previously-recorded depths in the ETA which ranged from 1371 to 1874 m (Weir et al., Reference Weir, Debrah, Ofori-Danson, Pierpoint and Van Waerebeek2008). This confirms that L. hosei is found along the upper slope as well as in deeper areas.
The low number of records of these two species in the ETA to date may have several potential explanations. Previous to the last decade there was a dearth of cetacean survey work in the region, and the ETA remains under-sampled compared to other oceanographic regions. Additionally, much of the cetacean survey work that has occurred in the ETA has been limited to strandings, by-catch monitoring at fishing harbours and boat surveys within coastal waters, and is consequently less likely to record species that primarily inhabit offshore, deep waters such as Feresa attenuata and Lagenodelphis hosei. However, there are also indications that sightings of these two species might be genuinely scarce in the ETA region relative to other delphinid species. For example, several years of extensive cetacean surveys in suitable deep-water habitat between the Gulf of Guinea and Angola did not conclusively record F. attenuata and logged only three definite sightings of L. hosei (Weir, Reference Weir2011a, Reference Weirb). Similarly, despite the relatively large amounts of offshore survey work carried out off Gabon in recent years (de Boer, Reference de Boer2010; Weir, Reference Weir2011a, Reference Weirb) neither of these species had previously been recorded. It is expected that as survey coverage in appropriate deep-water habitat in the ETA increases in future years, the occurrence of both Feresa attenuata and Lagenodelphis hosei will be verified in additional range states.
Given that range states can change in their delimited spatial extent over time, and that pelagic cetacean species may be wide-ranging and transboundary in distribution, multi-state coordinated conservation agreements will be needed in order to manage these species within the wider ETA.
ACKNOWLEDGEMENTS
We are grateful to Total E&P and to Ophir Energy for their permission to publish sightings recorded during seismic surveys off Côte d'Ivoire and Gabon respectively. Thanks to the crews of the ‘Polarcus Asima', ‘PGS Apollo' and ‘Sanco Spirit' for their hospitality during survey work.
