The diversity of cetacean species in the Cape Verde Islands is comparable to that of other archipelagos of the Macaronesia region. Despite the low research effort dedicated to studying the occurrence and distribution of whales and dolphins in Cape Verde, up to 24 species have been described so far in the waters of this archipelago (5 baleen whales and 19 toothed whales). Most of the records have come from both opportunistic sightings at sea and the finding of specimens washed ashore.
To date, Cape Verde the only known humpback whale (Megaptera novaeangliae) breeding ground in the North-eastern Atlantic Ocean. The west coast of Boa Vista (from Ponta do Sol to Lacação) has been recognized as the most important mating and nursery habitat of this species in the archipelago. Current research aims to estimate population size and to describe the genetic relationships with other humpback whale populations in the North Atlantic (e.g. in the West Indies).
Historic data indicate that Cape Verde was an important whaling area. Humpback whales and sperm whales (Physeter macrocephalus) were the major target species taken by USA whaling ships. The over exploration of the whale species along the XIX and early XX centuries led to the collapse of the main stocks in the North Atlantic Ocean and the start of whaling activities in other regions, chiefly in the Southern Hemisphere.
As recently as the mid-1980s, there was no certainty that humpbacks whales were still using the waters of Cape Verde as a breeding area. Periodic cetacean surveys conducted in the archipelago between February and April in the 1990’s and early 2000’s confirmed that the species still occurred in Cape Verde, preferably in the coastal waters off the easternmost islands (Sal, Boa Vista and Maio).
The onset of whale watching activities in Boa Vista in the late 2000s brought renewed attention to this species. Whale-watch tour boats have been used as platforms to conduct some research activities such as studying on the temporal and spatial distribution of the species, estimating relative abundance, identifying important whale areas and getting some insights into the acoustic behaviour of this large whale.
Most humpback whale populations have recovered worldwide since commercial hunting was banned in 1966. Currently, the species is listed as Least Concern by the International Union for Conservation of Nature (IUCN). However, because of its relative small population size (likely less than 300 individuals) and genetic isolation (Cape Verde humpback whales do not interbreed with those in the West Indies), effective conservation measures must be urgently implemented to preserve this population.
A cetacean is regarded to be stranded when occurs, alive or dead, ashore. If the animal is alive, it will likely need medical assistance or help to return to the sea. Briefly, there exist two kinds of stranding events: a) Single (only a single specimen involved) and b) Mass (when two or more individuals, besides a mother and her offspring, beach at the same place and at the same time).
Cetacean may strand in response to several different causes or a combination thereof: infectious and parasitic diseases, trauma (boat strikes and entanglement in fishing gear), acoustic pollution of the marine environment (natural or man-made noises), toxic alga blooms (bio-toxins), weather or oceanographic conditions that influence the distribution and availability of food resources (global warming), magnetic field anomalies, navigational error and ingestion of wastes or toxic substances.
Boa Vista is the island with a highest number of cetacean stranding events recorded since 2000. The relative frequency of cetacean strandings in Boa Vista could be related to its geographical position and large shelf; the occurrence of near shore reefs and shallow waters that may act as lethal traps for pelagic species; the influence of the sea currents and the existence of geomagnetic field anomalies that may affect the orientation capabilities of some large whales and dolphins. We cannot rule out that either some of the stranding events in Boa Vista and other islands could be of anthropogenic origin: interaction with fisheries (by-catch), collision with boats, naval activities (use of active sonar) or seismic surveys.
Since 2000 more than 20 cetacean strandings have been reported in Boa Vista, involving 10 different species, of which 4 were new records for the archipelago: the Minke whale (Balaenoptera acutorostrata), the Dwarf sperm whale (Kogia sima), the False killer whale (Pseudorca crassidens), and the Pygmy killer whale (Feresa attenuata). The Melon-headed whale (Peponocephala electra) and the Short-finned pilot whale (Globicephala Macrorynchus) are the two species with more stranding records in Boa Vista.