New evidence for a South Atlantic stock of humpback whales wintering on the Northwest African continental shelf

Abstract

Seventeen confirmed and four probable sightings of humpback whales, Megaptera novaeangliae, were documented from platform-of-opportunity Research Vessel (R/V) Dr. Fridtjof Nansen in a passing-mode visual survey of continental shelf waters between Conakry and Cap Vert Peninsula (Dakar), 21 October -5 November 2011. None were encountered in the northern stratum Dakar-Agadir, 6 November -15 December 2011. Total visual survey effort was 468 h, 01 min, covering 5335 km. Depending on the exclusion or inclusion of probable records, the sum of group sizes totalled 33 or 43 individuals, respectively. Humpback whale encounter rates between Conakry and Dakar then amounted to 1.74 or 2.27 whales/ 100 km, respectively. Group sizes ranged from 1-6 individuals (mean = 1.94, S.D. = 1.20, n =17; median = 2). Minimally five of 17 groups (29.4%) consisted of adult-calf pairs, with a minimum crude birth rate ranging from 0.060-0.152. All sightings occurred in shallow water, 22-60 m (mean = 35.0 m, S.D. = 10.13, n = 17) but survey effort in deeper, off-shore water was negligible. Sea-surface temperature at sighting locations ranged from 25.5-29.0°C (mean = 27.34, S.D. = 0.96, n = 17). No humpback whales were sighted during a second survey covering Conakry-Tangier-Las Palmas, from 9 May to 22 July 2012 (519 h, 13 min; 6278 km). The Cape Verde Islands comprised, to date, the only known wintering ground located in the North-East Atlantic. This study showed that Senegal, The Gambia, Guinea-Bissau and Guinea are normal range states for M. novaeangliae. A temporal signature, six to seven months out-of-phase with mid-season in the Cape Verde Islands, and neonate presence strongly suggest that the wide Conakry-Dakar continental shelf serves as both wintering and nursery grounds for a South Atlantic stock, in agreement with Bamy et al. (2010). This stock may comprise the northwesternmost component of the large humpback whale assemblage migrating in and out of the northern Gulf of Guinea in austral winter/spring. Further research is required to consolidate insights linking temporal and spatial distribution off western Africa with hemispheric stock identity and migration paths.