Biodiversity of cetaceans in coastal waters of Northwest Africa: new insights through platform-of-opportunity visual surveying in 2011-2013

Abstract

We summarize diversity, group size and habitat of cetaceans documented through ship-based visual survey effort (13,694 km; 1163.5 h) off NW Africa (Conakry to Tangier) in spring and fall of 2011-2013. Study area covered mainly continental shelf with some slope waters. Platform-of-opportunity surveying yielded 270 primary sightings of 14 species. Due to passing mode, 35.7% were identifiable only to family/category: unidentified Delphinidae (25.0%), unidentified rorquals (5.88%) and unidentified whales (4.78%). Delphinus delphis predominated with 28.7% of total sightings (32.7%, including probable) and a massive 71.3% (76.5%) of total number of cetaceans observed (n=15,595). Encounter rate was 81.2 common dolphins 100 km-1; mean group size 124.92. Tursiops truncatus, primarily inshore ecotype (median depth, 44.5 m), accounted for 9.56% (11.4%) of sightings and 1.35% (1.83%) of total cetaceans. Megaptera novaeangliae (6.25%) was regularly seen south of Dakar with a Southern Hemisphere seasonality. Other species showed a relatively low % occurrence: Globicephala spp. (2.57), Orcinus orca (1.10), Grampus griseus (0.73), Stenella frontalis (1.84), S. attenuata (0.37), S. clymene (0.37), S. coeruleoalba (0.37), Balaenoptera brydei (1.10), B. musculus (0.37), beaked whale (0.37), probable-B. borealis (0.37). Baleen plates recovered from a bottom-trawl also evidenced B. borealis presence. Seven new range state records included: G. griseus, S. coeruleoalba (Atlantic Morocco), O. orca, B. musculus (The Gambia), G. macrorhynchus, S. frontalis (Guinea-Bissau), S. attenuata (Guinea).