Discover the hunting tactics, migratory patterns and lives led by endangered orcas causing boat attacks
The 35-strong sub-population of orcas in the Strait of Gibraltar has recently risen to attention, following reports of the creatures attacking boat rudders, leaving more than 250 vessels damaged – and three of them sunk – in a spate of incidents over the last three years.
One study suggested that the attacks occurred because one member of the group – an orca known as White Gladis – was left traumatised from an injury sustained by a boat, triggering a behavioural change which other orcas have begun to imitate.
But what do we know about the Strait of Gibraltar subpopulation’s habitat, endangered status and more?
Where are the Strait of Gibraltar orcas found?
The Strait of Gibraltar and the nearby Gulf of Cadiz is the only area in the Mediterranean Sea with a regular presence of orcas (Orcinus orca), killer whales, although they have been occasionally found further east, with one group apparently migrating from Iceland to the coast of Italy in 2019.
Officially known as the ‘Strait of Gibraltar sub-population’, but also referred to by Spanish conservationists as the ‘Iberian sub-population,’ or ‘Iberian orca’ – the group is divided into five pods which gather in the Strait in the early spring, following the migratory patterns of the Atlantic bluefin tuna (Thunnus thynnus), their primary source of food.
They remain in the Strait throughout the summer, moving between the shallow coastal waters and the centre of the Strait, before migrating north along the Atlantic coast of Spain and dispersing into deeper waters during the autumn and returning to the Strait during late winter.
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Is this subpopulation of orcas endangered?
The Northeast Atlantic orcas to which the Strait of Gibraltar subpopulation is related are broadly split into ‘Type 1’ and ‘Type 2’ orcas – where Type 1 are the herring-eating orcas found off the coasts of Iceland, Scotland and Norway, and Type 2 are those which prey on baleen whales and dolphins.
The Strait of Gibraltar orcas are thought to be ‘visiting’ members of the Type 1 Northeast Atlantic orcas; however, studies using photo identification, mitochondrial DNA samples, contaminant loads, stable isotope rates and microsatellite genetic markers have found that the Strait of Gibraltar population is genetically isolated from other northeast Atlantic orcas, including those individuals that are observed sporadically in the nearby Canary Islands.
With a low number of mature individuals, the Iberian subpopulation of orcas was listed as ‘Endangered’ in the Spanish National Catalogue of Endangered Species in 2011, before being assessed as Critically Endangered on the IUCN Red List of Endangered Species in 2019. Four of the orcas have died since the IUCN assessment.
Threats to the orcas
The orcas’ survival depends heavily on the population of their primary prey, the Atlantic bluefin tuna, which was severely overfished to the point that stocks were reduced by 72 per cent in the Eastern Atlantic by 2009. Fishing quotas introduced in 2010 have seen the species recover to the point where it is now listed globally as being of Least Concern, but some regional subpopulations of bluefin remain low.
The Straits of Gibraltar is also an extremely busy shipping channel and commercial fishing ground, placing the orcas in permanent danger from boat traffic including, ironically, whale-watching tours.
Orca hunting strategies
The tuna that the Strait of Gibraltar orcas hunt migrate into the Mediterranean Sea during the spring and migrate back towards the Atlantic during the summer.
During these months, the orcas use a ‘chase-hunting’ strategy – also known as endurance or persistence hunting – during which the orcas follow the tuna until the fish are exhausted, whereby they become easy prey.
In recent years, however, two of the five pods have also been observed stealing fish from the drop lines used by Spanish and Moroccan fisheries.
When analysing numerous testimonies, videos and pictures of the recent orca attacks, one theory which scientists attributed to the attacks was natural pressures identified for orcas, such as prey depletion and interaction with fisheries, as well as theories regarding vessel collisions with the orca or natural curiosity.
How big are the Iberian orcas?
Adult Atlantic orcas reach between 5 to 6.5 metres in length, and can weigh more than 5 tonnes – although they are smaller than Antarctic orcas, which can reach sizes of up to 9 metres. Atlantic orca calves are born between 2-3 metres in length.
The recent orca attacks on boats have illustrated the power that these marine creatures possess. In one video, an orca can be seen ripping a boat rudder off with its teeth off the coast of Gibraltar.
What other marine life lives in the Strait of Gibraltar?
The eastern part of the area in the Strait of Gibraltar is regularly inhabited by six other cetacean species. These include pilot whales, resident common bottlenose dolphins, common dolphins, striped dolphins, and sperm whales. The area is also a migratory corridor for fin whales.
In the western part of the Gulf of Cadiz, common bottle dolphins, common dolphins and Risso’s dolphins can also be found.
