Killer whales in the Strait of Gibraltar
Killer whales have been described to occur in the Strait of Gibraltar 2000 years ago. The species' occurrence in this area seems to be related to the presence of large bluefin tuna that migrate through the Strait. In spring, tuna cross the Strait and enter the Mediterranean Sea to reproduce. During the summer, tuna return to Atlantic waters through the Strait.
A total of 39 individuals were identified in 2007 in two areas of the Strait of Gibraltar. In spring they occur in the Bay of Barbate, and in summer in the central waters of the Strait, often in association with Spanish and Moroccan fisheries.
Killer whales in the Strait of Gibraltar are structured in five social groups. Three of them interact with fisheries during the summer, while the other two groups were never seen associated with fishing boats.
During spring, killer whales use the so-called 'endurance-exhaustion' technique to chase bluefin tuna. The groups spread in different units and in silence they listen for bluefin tuna groups. When they find a group, they coordinate themselves and chase the fish for about 30 minutes. After this time the fish is exhausted and killer whales can easily catch and eat them.
During the summer, killer whales are seen in the central area of the Strait. When bluefin tuna migrate out of the Mediterranean, Spanish and Moroccan fishermen use longlines to fish them. Killer whales take advantage of the tuna that are hooked.
Known threats relate primarily to interactions with fisheries. Until the late 80s, killer whales were deliberately killed by Spanish fishermen. To date, direct killings are unlikely to occur in Spain, but little is known about interactions in Moroccan waters.
The subpopulation living in the Strait of Gibraltar qualifies for Critically Endangered in the IUCN Red List.
For more information on killer whales in the Strait of Gibraltar see:
Cañadas A., de Stephanis R. 2006. Killer whale, or Orca Orcinus orca (Strait of Gibraltar subpopulation). Pp. 34-38 in Reeves R.R., Notarbartolo di Sciara G. (compilers and editors). The status and distribution of cetaceans in the Black Sea and Mediterranean Sea. IUCN Centre for Mediterranean Cooperation, Malaga, Spain. (whole report: 2,752 Kb)
De Stephanis R., Cornulier T., Verborgh P., Salazar Sierra J., Pérez Gimeno N., Guinet C. 2008a. Summer spatial distribution of cetaceans in the Strait of Gibraltar in relation to the oceanographic context. Marine Ecology Progress Series 353:272-288. (473 Kb)
Guinet C., Domenici P., De Stephanis R., Barrett-Lennard L., Ford J.K.B., Verborgh P. 2007. Killer whale predation on bluefin tuna: exploring the hypothesis of the endurance-exhaustion technique. Marine Ecology Progress Series 347:111-119. (236 Kb)