Humpback whale wintering ground on NW African shelf SC/64/SH4

A newly discovered wintering ground of humpback whale on the Northwest African continental shelf exhibits a South Atlantic seasonality signature

Koen Van Waerebeek1,2, Abdoulaye Djiba1,3, Jens-Otto Krakstad 4, Amadeu Almeida5 , Ebou Mass Mbye6

1 COREWAM, Musée de la Mer, Institut Fondamental d'Afrique Noir (IFAN), Ile de Gorée, Senegal. corewam@gmail.com2 Centro Peruano de Estudios Cetológicos (CEPEC), Lima-20, Peru3 Conservateur, Musée de la Mer, IFAN, Université de Dakar Cheikh Anta Diop, Dakar, Senegal4 Institute for Marine Research, Bergen, Norway5 Centro de Investigaçäo Pesqueira Aplicada (CIPA), Bissau, Guinea-Bissau6 Fisheries Department, Banjul, The Gambia.

ABSTRACT Twenty-one sightings, including 17 confirmed and 4 probable records, of humpback whale were documented during a sighting survey from a fisheries research vessel as platform-of-opportunity, making it the most frequently encountered cetacean on the 100km-wide continental shelf between Conakry and Dakar in the period 21 Oct - 5 Nov 2011. No humpback whales were encountered in the northern stratum from Dakar to Agadir, 6 Nov-15 Dec 2011. Guinea-Bissau, The Gambia and Senegal are newly recognized range states for M. novaeangliae; Guinea is re-confirmed after Bamy et al. (2010). Group sizes ranged 1-6 (mean 1.94; SD 1.20, n=17; median, 2). At least 5 groups (29%) consisted of adult/calf pairs, suggesting a nursing ground. All sightings occurred in shallow water with depth range 22-60m (mean 35.0m; SD 10.13, n=17); 22.7 - 61m for probable-humpback whale groups. Survey effort in deeper, more offshore water was negligible. Sea surface temperatures at sighting positions ranged 25.5-29.0°C (mean 27.34; SD 0.96, n=17). Oceanographic survey design with frequent stations did not allow abundance estimation, however the sum of group sizes was 32 individuals, or 41 including probable humpback whales. The Cape Verde Islands are the only known wintering ground in the NE Atlantic. This survey uncovered a distinct wintering ground in continental shelf waters between Guinea and Senegal. Its seasonal signature, ca. 0.5yr out-of-phase with peak occurrence in Cape Verde Islands, in addition to the presence of neonates is consistent with a South Atlantic stock. Seasonality overlaps with that of the large subpopulations assemblage in the Gulf of Guinea and SE Atlantic, of which we suggest it may comprise its northwestern-most component. Surveys in other seasons are necessary to consolidate insights linking temporal and spatial distribution with hemispheric stock identity and migration.

KEYWORDS: HUMPBACK WHALE; WINTERING GROUNDS; SHELF; SENEGAL; THE GAMBIA; GUINEA-BISSAU; GUINEA

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Humpback whale wintering ground on NW African shelf SC/64/SH4

1. IntroductionThe spatial and seasonal distribution of humpback whales Megaptera novaeangliae in low latitudes of the Northeast Atlantic Ocean is poorly documented. Tomilin (1967) suggested a migration route from the Barents Sea to NW Africa and the Cape Verde Islands, where they were hunted by 19th century Yankee whalers in late winter and spring (Townsend 1935). Reeves and Mitchell (1990) argued that in the Cintra Bay region (22°-24°N) along the coast of Rio de Oro (Western Sahara), right whales and not humpback whales were taken by whalers. These authors did not rule out the possibility that humpback whales may have been taken occasionally in continental waters, 'but any such captures would have been rare as there is no reliable evidence for a regular winter concentration of humpback whales in this region'.

Indeed the few vague reports from NW African continental waters remained dubious until two strandings and a sighting were authenticated from Guinea (Bamy et al. 2010). The Cape Verde Islands (CVI) still represent the only recognized humpback whale breeding/wintering grounds in the Northeast Atlantic, with whales appearing in January, peaking after March and migrating out again about May (Lagendijk 1984; Wenzel 1995; Reiner et al. 1996; Hazevoet & Wenzel 2000; Wenzel et al. 2009; Hazevoet et al., 2011). This NE Atlantic stock is severely reduced, numbering around 100 individuals (Northridge 1984; Punt et al. 2006). Here we provide evidence for the common occurrence of humpback whales, including neonates, at the West African continental shelf stretching between Conakry (Guinea) and Dakar (Senegal) in autumn 2011.

2. Material and Methods The 56.8m R/V Dr. Fridtjof Nansen carried out oceanographic and fisheries research surveys off NW Africa within the FAO Canary Current Large Marine Ecosystem (CCMLE) framework1, 21 Oct - 15 Dec 2011, a useful platform-of-opportunity for much needed baseline cetacean surveys in the region. Survey effort was conducted entirely in 'passing mode'. Cruise design, track lines, sampling stations and oceanographic research are detailed in Krakstad et al. (2012). Although line transect sampling protocol was not applicable, data collection simulated it for training purposes. Cruise speed was 10knots. During the frequent, circumdiel stations for bottom and mid-water trawling, conductivity-temperature-depth profiling (CTDs), plankton-net hauls among other experiments, the vessel's speed varied from 0-3knots (0-5km/h). Special attention was paid to potential re-sightings during these and a few back-tracking manoeuvres. After plotting tracklines and waypoints, four suspected re-sightings were eliminated from the analysis. O ther cetacean species observed will be discussed elsewhere.

Observers were stationed on the radar deck ca. 24m above sea-level, except in (rare) poor visibility conditions when stationed on the fore-castle deck at 14m height. At cruise speed, observers scanned 180° port to starboard by naked eye and 7x50 binoculars. During stations and low-speed manoeuvering, 360° were scanned as cetaceans could approach from any angle. Sighting data (32 parameters), observer effort and ship activity were recorded on standard dataforms. Graphic evidence included sketches of d iagnostic features and movements relative to the vessel, in addition to digital SLR photos taken with a 70-300mm telephoto lens.

Van Waerebeek surveyed a distance of 2647.53km on transect Conakry-Nouadhibou for 193h 3min (on-effort) from 21Oct–15Nov 2011. Djiba surveyed 2472.72km of transect Nouadhibou-Agadir over 254h 6min, in 7Nov-15Dec 2011. Both observed concurrently on the leg Dakar-Nouadhibou while calibrating methodology. Paul Robinson, ornithologist, implemented a seabird census also from the radar deck between Dakar-Agadir, flagged any (re)sightings the primary observers might have missed. Visibility was good or excellent except on 2nd Nov, when it was moderate due to fog patches. Species identification was made only after confirmation of diagnostic morphological features and distinctive surfacings. Four 'like-humpback whale' sightings were large baleen whales consistent with M. novaeangliae but which could not definitively be confirmed due to great distance.

Two echosounders (Simrad ER60 with operating frequencies of 18, 38, 120 and 200kHz; RD instruments ADCP, operating frequency of 175kHz) and sonar systems (Kongsberg EM710 multibeam echosounder with operating frequencies of 70-100kHz) were active for most of the survey. Output effect was up to 2000W. However any potential bias in encounter rates, due to either attraction or aversion, was

1 www.canarycurrent.org

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Humpback whale wintering ground on NW African shelf SC/64/SH4

thought minimal. Humpback whales appear relatively resilient in the face of high-frequency anthropogenic noises, including seismic air gun noise, presumably because their acoustic sensitivity peaks at lower frequencies (e.g. Gordon et al. 2003; Weir 2008).

Primary survey tracks ran parallel east-west, interconnected by perpendicular 20nm long secondary tracks. Tracks were designed for oceanographic sampling purposes to cover most of the continental shelf and slope (Fig.1). Shallow-water sections of both inshore-bound and offshore-bound legs were conducted in daytime allowing visual survey effort and all sightings occurred on the continental shelf and slope. Offshore oceanographic sampling was conducted mainly at night, thus marine mammal survey exposure to deeper waters was negligible. This scheme helped avoid damage to set-nets in nearshore areas where artisanal fishing was dense.

3. Results3.1. Literature review A comprehensive literature review, including of local journals and unpublished materials, uncovered no authenticated records but a few vague reports of humpback whales at the NW African continental shelf. Aloncle (1964) claimed, without elaborating, that the species was reported along or offshore Morocco's coasts2. A single undocumented observation on 17 Feb 1965 involved an unspecified number of humpback whales among a large multi-species assemblage of pilot whales, bottlenose and other dolphins, between the Moroccan coast and Banc de la Conception (Aloncle 1967; Bayed & Beaubrun 1987).

One humpback whale reportedly stranded in the Baie du Lévrier, northern Mauritania, in February 1954 (Cadenat 1955; Maigret 1981; Robineau & Vély 1998). Unverifiable for lack of photographic or descriptive voucher data, these records nonetheless may be valid. Laptikhovskii (2001) reported minke and fin, but no humpback whales from the Atlant-NIRO survey off Morocco and Mauritania in summer 1998, and off Morocco in the winter 1997-1998. A consensus view emerged that no reliable humpback whale records were available for NW African continental waters south of Nouadhibou, Baie du Lévrier (Duguy 1976; Robineau & Vély 1998; Notarbartolo di Sciara et al. 1998).

More recently, two dedicated cetacean surveys jointly organised by the Governments of Guinea and Japan (Diallo et al. 2002; 2004), covered the same general area as our survey but in winter, respectively from 7-23 Dec 2002 and 23 Jan-10 Feb 2004. No humpback whales were reported. Shore-based monitoring of cetacean strandings and entanglements in Senegal, The Gambia and Guinea-Bissau (Murphy et al. 1997; Van Waerebeek et al. 2000; 2003; Jallow et al. 2005) and an examination of specimens at the IFAN museum (Université de Dakar Cheikh Anta Diop) which curates West Africa's largest cetacean collection, showed no evidence of humpback whale. Some unidentified whales stranded along Senegal's coast might have included that species (Van Waerebeek et al. 2000; 2003). Dupuy (1983) listed an unsupported account by A. Després of several mégaptères leaping off Dakar in July and August 1982. The lack of evidence, the fact that other balaenopterids may also leap, and the odd season may have prevented the account from gaining credibility. Nonetheless, the A. Després claim may have been accurate. Bamy et al. (2010) proposed the presence of Southern Hemisphere (SH) humpback whales in Guinea based on three authenticated records, two strandings and a sighting, in boreal summer.

3.2. DistributionTable 2 lists 17 confirmed humpback whale sightings and 4 probable sightings (like-humpback whale) for continental waters between Conakry, Guinea (N09°27.97',W13°44.92') and Dakar (N14°41.98', W17°27.97'), Cap Vert Peninsula, between 21 Oct and 5 Nov 2011. Three additional whale groups were discarded as re-sightings. The sum of (best estimate) group sizes totals 32 individuals, or 41 if like-humpback whale groups are included. The species was the most frequently encountered cetacean on the Conakry-Dakar shelf. The sightings support the Bamy et al. (2010) hypothesis of an unrecognized stock with a Southern Hemisphere seasonality in coastal waters of Guinea, but also Guinea-Bissau, The Gambia and Senegal.

2 "signalés le long du littoral ou au large des côtes marocaines"

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Humpback whale wintering ground on NW African shelf SC/64/SH4

3.3. Group compositionGroup size (best estimates) ranged 1-6 (mean 1.94; SD 1.20, n=17; median, 2) with 7 individuals the high estimate for the largest group. At least five groups (29%) consisted of adult/calf pairs (Figure 2). A few additional groups sighted at >3km distance may also have included small calves as their detectability is poor at distance, the survey's passing mode being an obstacle. Closest distance estimates ranged 50-7000m (mean 2,703m; SD 2,087; n=17). At least two of the calves were the size of, and behaved as, neonates and parturition is thought to have occurred locally. We suggest the 740km-long and up to 190km wide continental shelf stretching between Dakar and Conakry is a calving/nursing and wintering ground for a South Atlantic population, consistent with the three humpback whale records near Conakry in austral winter/spring (Bamy et al. 2010).

3.4. Ecological parametersAll groups were observed in the shelf area, in shallow water (Figure 1). Echosounder depths at sighting locations ranged from 22-60m (mean 35.0m; SD 10.13, n=17). Similar depths (range 22.7-61m) were associated with the four like-humpback whale sightings. Sea water temperatures, measured at 5m depth, ranged 25.5-29.0 °C (mean 27.34 °C; SD 0.96, n=17).

Table 1. Incidental sightings of unidentified baleen whales off Cap Vert Peninsula, Senegal, consistent with M. novaeangliae. Voucher material is lacking. Comments in Cadenat (1957) translated from French.

Date Location Comments Source02 August 1955

east of Ile de Gorée, Dakar

A whale with a 'small dorsal fin' swimming near Gorée Island was sighted by islanders.

Cadenat (1957)

21 August 1955

east of Ile de Goree, Dakar

2 whales with 'short and rounded' dorsal fin, (estimated 10-15m, in comparison to fishers canoes) moved very slowly, and were observed for 1hr

Cadenat (1957)

2 October 1955

Near Ile des Madeleines, Cap Vert Peninsula

Longline fishers saw whale surface with 'small dorsal fin' very close to their boat

Cadenat (1957)

September 2000

West of Mermoz, ca. 1km from shore, Cap Vert Peninsula

2-3 whales seemingly resting at surface. In 4min, some 5-6 blows were observed. Dorsal fin and lower back visible before whale entered long slow roll. Larger whale had a prominent dorsal fin.

Tim Dodman, pers. comm. to KVW

4. Discussion

4.1. Distribution Confirmed (C) and probable (P) sightings added three new African range states for M. novaeangliae, Senegal (4C,1P), The Gambia (1C,1P) and Guinea-Bissau (6C). Common presence was demonstrated (6C,1P) in Guinea for which only one prior sighting and two stranding records have been documented (Bamy et al. 2010). Despite equivalent search effort, no humpback whales were encountered in Nov-Dec 2011 in shelf waters off Mauritania, Western Sahara and Morocco. The northernmost sighting (C20111105-1) comprised two adults at N14°15.559',W17°07.125' south of Dakar, Cap Vert Peninsula, on 5 November 2011 (Table 2).

The survey protocol with frequent oceanographic stations did not allow abundance estimation. It seems surprising that such a significant population was not described before. Also the question may be raised whether its distribution might have shifted due to climate change. However, indications are that this area has been utilized for at least three decades. Five sighting records, probably of humpback whales (Table 1), and leaping whales off Dakar in July-August 1982 (Dupuy 1983) may not have entered mainstream literature for lack of evidence and a perceived incongruency of encountering humpback whales in the tropical NE Atlantic during boreal summer. The possibility of SH stock presence (Bamy et al. 2010) off Guinea was echoed by Hazevoet et al. (2011) suggesting also possible SH whales in the Cape Verde seas and off Guinea-Bissau due to seasonality considerations. Unidentified whales reported by Gambians some 10km west of Sanyang Point (N13°16.8', W16°44.43') (Van Waerebeek et al. 2000) may also have

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Humpback whale wintering ground on NW African shelf SC/64/SH4

been this species. With few local experts, species can long evade scientific detection following misidentifications or non-reporting. Humpback whales in Togo, for instance, remained undocumented despite small-scale commercial whale-watching for years (Segniagbeto & Van Waerebeek 2010).

4.2. Stock identity and seasonality The earliest humpback whale group was sighted on 21 Oct 2011, the latest on 5 Nov 2011, consistent with South Atlantic stock seasonality. The observation of five adult/calf pairs adds to the SH argument. Two dedicated ship-board cetacean surveys, one 7-23 Dec 2002 and another 23 Jan-10 Feb 2004 (Diallo et al. 2002; 2004) covered a similar area between Conakry and Dakar and did not encounter humpback whales. That finding is also consistent with our premise as in that period any SH humpback whales would have recently migrated south towards feeding grounds in the Southern Ocean. In the northern Gulf of Guinea (Ghana, Togo, Benin) humpback whales arrive in coastal waters by late August and most have left by late November (Van Waerebeek et al., 2001, 2003; 2009; Segniagbeto & Van Waerebeek 2010), also broadly overlapping with periodicity off Gabon and Angola (Rosenbaum & Collins 2006). Bamy et al. (2011) recently surveyed coastal waters around the Tristao Islands (10.997°N-10.387°N, 15.216°W-14.840°W) in northern Guinea, 26 July– 1 Aug 2011, and five cetacean species were registered, but no humpback whales. Considering that a calf was reported stranded in 2008, in an unknown month, additional surveys are planned there.

Peak presence for CVI insular stock (March) is 6-7 months out-of-phase with the NW African continental stock documented here. However, four unusual sightings off the CVI between 12 June and 15 August could also have a SH origin (Hazevoet et al., 2011). Wenzel et al. (2009) argued there is a major disparity between the numbers of humpback whales seen in the CVI and in the NE Atlantic feeding grounds, hence most NE Atlantic whales should be going somewhere else to winter, not only to the CVI. The vast Guinea-Senegal continental shelf, apart from hosting a SH population, could also host some of the 'missing' NE Atlantic humpback whales and future surveys in late boreal winter and in spring (February-May) should test this hypothesis. Slijper et al. (1964) reported several observations south of the equator in the boreal winter and concluded that these whales probably belonged to a NH stock.

Various aspects of humpback whale distribution off West Africa remain elusive. With partial temporal coverage (October-February) ample scope exists for future surveys before we can confidently delineate spatial and temporal distribution in the CCMLE study region, and off western Africa in general.

Acknowledgements Fellow scientists and the crew, in particular Paul Robinson, offered an excellent collaboration on R/V Dr. Fridtjof Nansen. Without the support of Aboubacar Sidibe and Birane Sambe (FAO/CCMLE, Dakar) this survey would not have materialized. Alan Baker provided constructive comments on an earlier draft, Tim Dodman described a sighting in Senegal and Claire Bass, Mika Samaba Diop, Rudy Herman and Mika Odido, supported with funding applications. We express our gratitude to the above-mentioned people. The survey was conducted as part of the EAF-Nansen project (www.eaf-nansen.org), a partnership with FAO, Norad (Norwegian Agency for Development Cooperation) and the Institute of Marine Research (Bergen) in collaboration with CCLME (Canary Current Large Marine Ecosystem project). CCLME and IMR are gratefully acknowledged for all vessel-related expenses and logistics. AD thanks Fondation Internationale du Banc d'Arguin (FIBA) for aid with travel and equipment. KVW acknowledges travel support from WSPA-International. Data collection, analysis and reporting was backed by UNESCO (consultancy contract N° 4500160357).

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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. 80pp.

Van Waerebeek K., Tchibozo S., Montcho J., Nobime G., Sohou Z., Sohouhoue P. & Dossou C. 2001. The Bight of Benin, a North Atlantic breeding ground of a Southern Hemisphere humpback whale population, likely related to Gabon and Angola substocks. Document SC/53/IA21, IWC Scientific Committee, London, July 2001. 8pp.

Van Waerebeek, K., Barnett, L., Camara, A., Cham, A., Diallo, M., Djiba, A., Jallow, A.O., Ndiaye, E., Samba Ould Bilal, A.O. & Bamy, I. L. 2003. Conservation of cetaceans in The Gambia and Senegal 1999-2001, and status of the Atlantic humpback dolphin. WAFCET-2 Report. UNEP/CMS, Bonn, Germany. 55 pp.

Van Waerebeek, K., Ofori-Danson, P.K. & Debrah, J. 2009. The cetaceans of Ghana: a validated checklist. West African Journal of Applied Ecology 15: 61-90.

Weir, C.R. 2008. Overt responses of humpback whales (Megaptera novaeangliae), sperm whales (Physeter macrocephalus), and Atlantic spotted dolphins (Stenella frontalis) to seismic exploration off Angola. Aquatic Mammals 34 (1): 71-83.

Wenzel, F.W. 1995. Humpback whale project in the Cape Verde Islands. Sonar 13: 22-23.Wenzel, F.W., Allen, J., Berrow, S., Hazevoet, C.J., Jann, B., Seton, R.E., Steiner, L., Stevick, P., López-

Suárez, P. & Whooley, P. 2009. Current knowledge on the distribution and relative abundance of humpback whales (Megaptera novaeangliae) off the Cape Verde Islands, Eastern North Atlantic. Aquatic Mammals 35 (4): 502-510.

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Figure 1. Sightings of humpback whales (17 confirmed, 4 probable records) off Senegal, The Gambia, Guinea-Bissau and Guinea, made from the R/V Dr. Fridtjof Nansen in the period 21 Oct – 15 Nov 2011. No humpback whales were encountered north of Dakar, although survey effort extended northwards to Agadir, Morocco.

Figure 2. Adult humpback whale with calf in the foreground (C20111029-3) encountered onthe shelf (37.2m depth) of Guinea-Bissau on 29 October 2011 (Photo K.Van Waerebeek).

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New wintering ground of humpback whale off NW Africa Van Waerebeek et al.

Table 2. Sighting records of humpback whales off Guinea (G), Guinea-Bissau (GB), The Gambia (TG) and Senegal (S) made during 2011 CCMLE survey. Records

in bold were concluded to be re-sightings of the preceding record and were not further considered in the analysis. Records with waypoint numbers 082, 083, 095, 138, and 141 are probable records (like-humpback whale); the 17 others are confirmed.

9

Observers Vessel mode

Date SST °C EEZ Reaction to vessel

082 KVW cruise 21/10/11 15:12 3000 ON 61 26.7 5 9 1.235 14 35.005 G neutral

083 KVW cruise 21/10/11 15:19 3000 ON 61 26.7 5 9 2.798 14 33.176 G neutral

084 KVW cruise 21/10/11 15:33 3000 ON 61 26.7 5 9 4.311 14 30.911 G neutral

086 KVW cruise 21/10/11 17:25 800 ON 43 26.7 6 9 13.737 14 21.441 G neutral

087 KVW cruise 22/10/11 08:06 50 ON 43 25.5 1 9 0.471 15 09.915 G avoidance

088 KVW cruise 22/10/11 09:29 2200 ON 47 26.2 2 9 15.846 14 48.490 G neutral

094 KVW cruise 23/10/11 09:44 400 ON 60 27.4 2 9 17.937 15 16.020 G neutral

095 KVW cruise 23/10/11 11:54 6000 ON 40 27.5 2 9 28.224 15 04.577 G neutral

096 KVW sampling 23/10/11 12:59 3500 ON 42 27.5 2 9 38.724 14 52.385 G neutral

097 KVW cruise 23/10/11 16:55 3500 ON 27.7 27.6 1 9 48.725 14 40.103 G neutral

114 KVW sampling 28/10/11 16:00 800 ON 28.9 26.5 2 10 48.674 16 34.722 GB neutral

115 KVW sam pling 28/10/11 16:47 800 ON 33.2 27 2 10 47.995 16 34.454 GB avoidance

117 KVW cruise 29/10/11 11:40 1600 ON 24.6 27.4 2 11 16.773 17 03.023 GB neutral

118 crew , KVW sampling 29/10/11 12:07 1500 OFF 37.2 27.5 1 11 15.867 17 05.208 GB neutral

123 KVW cruise 30/10/11 09:40 4500 ON 44.7 28.2 1 11 33.738 17 06.493 GB neutral

125 KVW sampling 30/10/11 11:20 4500 ON 26.8 26.1 2 11 38.077 16 57.800 GB avoidance

126 KVW cruise 30/10/11 12:52 5000 ON 25.8 26.7 1 11 38.336 16 58.477 GB neutral

129 KVW sampling 31/10/11 15:10 7000 ON 29 28.9 2 12 33.883 17 21.308 S neutral

130 captain, KVW cruise 31/10/11 17:30 2500 ON 31.4 29 3 12 33.102 17 25.455 S neutral

132 KVW cruise 01/11/11 13:02 4500 ON 31.4 28.2 1 12 53.616 17 10.820 S neutral

137 KVW sampling 02/11/11 11:41 5000 ON 31 27.9 1 13 33.879 17 04.638 TG neutral

138 KVW cruise 02/11/11 13:54 3500 ON 22.7 28.4 na 13 33.87 17 01.405 TG neutral

140 KVW,Djiba cruise 05/11/11 12:27 800 ON 22 27.5 2 14 15.559 17 07.125 S neutral

141 KVW,Djiba sampling 05/11/11 17:07 4500 ON 50.4 27.2 2 14 28.119 17 18.734 S neutral

GPS w aypoint

Time (GMT) 1st cue

closest distance (m)

Effort ON/ OFF

Water depth (m)

Group size Best estimate

Lat dd North

Lat mm North

Long dd

West

Long mm West