Long-finned pilot whales in the Mediterranean Sea
Species: Globicephala melas (Traill, 1809).
Long-finned pilot whale, globicefalo, Grindwal, globicéphale noir, calderón común.
Long-finned pilot whales are oceanic dolphins that can also be found in the Mediterranean Sea. They share a genus with the short-finned pilot whale (Globicephala macrorhynchus) with whom they also share a lot of similarities. The main difference between both species is that long-finned pilot whales have longer pectoral fins which can make up to 18-27% of their body length (Still et al., 2019) while there are also some differences in skull shape, and the number of teeth (Olson, 2009).
Long-finned pilot whales have large and robust bodies with a thick tailstock and a wide dorsal fin that is located in a forward position on their body. Their name ‘Globicephala’ comes from the Greek words globus (globe) and kephale (head) which refers to the bulbous head that is distinctive for this species. They can be glossy black or dark grey-brown with a narrow white underbelly and a faint gray‘saddle’ patch behind the dorsal fin. In some cases, they also show a pale grey area on the melon and a grey stripe from the eye up to the dorsal fin. Calves are paler than adults.
The long-finned pilot whale exhibits an important sexual dimorphism, which means that there are apparent differences between females and males from the same species. In this case, males are longer than females reaching a length of up to 7.5 meters and a weight of 3000 kg, while females can reach 6.5m and weight up to 1.300 kg (Still et al., 2019). Males also have a more pronounced melon and a much larger dorsal fin (Olson, 2009).
Long-finned pilot whales can be found in temperate and sub-polar waters of both hemispheres. In the North Atlantic, they are commonly found in waters along the continental shelf slope at a depth of 1000 meters. In the Mediterranean Sea, they are almost exclusively found in the western Basin with some records of them around Malta (Verborgh et al., 2016). The highest abundance is found in the Alboran Sea while sightings in the Tyrrhenian Sea during the last decades are rather rare with an observed decline since the 1980s. Based on genetic evaluations there are two separate populations, one near the Strait of Gibraltar and a Mediterranean population.
AS: Alborán Sea; ADS: Adriatic Sea; BS: Balearic Sea, G: Greece, GL: Gulf of Lion, GoV: Gulf of Vera, LB: Levantine basin; LS: Ligurian Sea; M: Malta, PB: Provençal Basin; SoG: Strait of Gibraltar; SWB: south-western Mediterranean Basin; T: Tunisia; TS: Tyrrhenian Sea (Verborgh et al., 2016).
Long-finned pilot whales are very social forming big schools or pods that can range from 2 to 40 individuals (Stillet al.,2019) while even bigger aggregations have been observed near the Alborán Sea (Cañadas and Sagarminaga, 2000; Verborghet al., 2016). In the Western Mediterranean, the long-finned pilot whales overlap their habitat with Risso’s dolphins (Grampus griseus) and sperm whales (Physeter macrocephalus). Even so, they are not a direct competition when it comes to food as they feed on a lower trophic level (Praca and Gannier, 2008; Verborghet al., 2016) than the previously mentioned. In fact, Risso’s dolphins hunt on the upper part of the continental slope, sperm whale on the lower part while pilot whales occur in deeper regions. Therefore, competition for resources is unlikely to occur amongst them (Verborghet al., 2016). Long-finned pilot whales are primarily nocturnal feeders and are therefore typically found just traveling near the surface during the daytime.
They were a common species in the 1970s in the north-western Mediterranean Basin (Vallon et al., 1977; Verborgh et al., 2016) but rapidly declined in the 80s and 90s due to their by-catch in driftnets (Di Natale, 1995). These driftnets may also have caused the disappearance of this species from the Tyrrhenian Sea. The subpopulation of the Mediterranean Sea is currently listed as Data Deficient (DD) on the International Union for the Conservation of Nature (IUCN) Red List. The lack of data available about them makes it difficult to assess their conservation status properly. Even though there is no total population estimation, it is likely that the number of mature individuals is less than 2500 (Verborgh et al., 2016).
Some of the threats that affect long-finned pilot whales in the Mediterranean Sea are their incidental catch in fishing gear, marine traffic, contaminants, pollution, and underwater noise. As we mentioned before, the reduction of the long-finned pilot whale population in the Mediterranean Sea is attributed to the by-catch of this species in the driftnet fishery. In 2002 the European Union decided to prohibit the use of driftnets that caused an estimate of 111 to 132 pilot whales between 1990 and 1991 in the Tyrrhenian Sea (Di Natale, 1995; Verborgh et al., 2016). Even though this ban is in place, illegal driftnets are still being used in some areas of the Mediterranean Sea, continuing to pose a danger to this species and other cetaceans. This is also the case for the Tyrrhenian Sea where two sperm whales (Physeter macrocephalus) were found tangled up in driftnets last summer off the Island of Ponza, where a driftnetting fleet was monitored over time (Miragliuolo et al., 2002; Cornax et. al., 2006). Also, other cases of fishing gear interference have been recorded such as by-catch in long-line fleets especially near Spain and Morocco.
As long-finned pilot whales normally live far from the coast, their threat of colliding with marine traffic is a little more reduced than coastal species. Nevertheless, in the region near the Strait of Gibraltar and the Alboran Sea, they are exposed to high traffic of merchant ships and ferries (de Stephanies et al., 2005). In 2006 a collision with one of these ferry ships was described and other animals bearing marks were associated with this kind of collisions. Besides the danger of collision, the high traffic of vessels also causes high noise levels that can affect cetaceans and forced pilot whales to increase their vocalizations in the Mediterranean Sea (Rendel and Gordon, 1999; Verborgh et al., 2016).
Further, cetaceans are top predators in the marine food web and can, therefore, be used as ocean and biodiversity health indicators (Azzelino et al., 2014; Verborgh et al., 2016). Pollution levels detected in the animals can give us an idea about the pollution state of the sea (Barón et al., 2015; Verborgh et al., 2016). The Mediterranean Sea has a rather high contamination level and in fact, the levels of contaminants found in long-finned pilot whales in the Mediterranean Sea were above the threshold considered harmful to the immune and reproductive system in aquatic mammals (Jepson et al., 2005; Kannan et al., 2000; Verborgh et al., 2016). This high contamination in the animals increases their risk of being affected by diseases and the contaminants are also passed down to their offspring. Also, the effect of plastic has been noticed on marine mammals as the Mediterranean Sea has a higher proportion of large plastics debris than typically found in other oceans. Plastic can be ingested by marine mammals and has been recorded as the cause of death for other cetaceans.
Even though long-finned pilot whales are not common in the Tyrrhenian Sea, they have been sighted during monitoring effort that started in 1980 around the waters of Ventotene (Pontino Archipielago) in the Gulf of Naples (Italy) (Notarbartolo Di Sciara et al., 1990). From 1995 Oceanomare Delphis Onlus has documented the presence of a free-ranging pod off the south-western coast of the Island of Ventotene. This was part of the Ischia Dolphin Project, a long term study of cetaceans that started in 1991 in this region. In 1995 the pod was composed of 6 individuals: three adult males (Cagliostro, Santiago, and Enea), one adult female (Señora), one juvenile female (Emma) and one immature male (Pan).
The pod was headed by the largest male, the “pilot” Cagliostro, as observed in other social groups of this species. In 1996 one of the adult males (Enea) was sighted for the last time, while in 1999 a newborn was recorded (Mussi et al., 2000). Direct observations documented that Cagliostro, the “pilot”, was taking constant care of the young Pan (5-6 years-old), isolating him from the rest of the pod. On the other hand, the two females and the smaller male of the group were taking care of the newborn, in a way that it was impossible to approach the juvenile closer.
The opportunity to perform direct and continuous “in situ” observation on a wild pod of long-finned whales off Ventotene Island, confirmed the time stability of social groups in this species. The social structures of pilot whale pods are similar to those of “resident” killer whales. The pods are highly stable and the members have close matrilineal relationships. The evident interaction between the pilot and the youngest member of the pod brought new insight on the role of adults, dominant males in the complex social structure of Globicephala melas.
The limited number of individuals of the pod allowed to assign calls to whales to demonstrate that whales produced mostly their own signature call (Mussi et al., 2003). Signature remains stable over long periods and its frequency versus time “contour” shows a high degree of stereotypy. Male individual’s calls were found to be predominant and in particular two members of the pod, the youngest one (Pan) and the pilot, seemed to be strongly linked by the number of whistles emitted since 1995, announcing a particular relationship between the whales that was revealed only in 1999 with the separation of the pod.
At last was possible to document the development of a signature whistle in a juvenile male of pilot whale since it was 1-2 years old, starting from a simple whistle to arrive a defined call with an introductory and a terminal structure well recognizable.
In the last years, long-finned pilot whales have been sighted less and less. Nevertheless, the monitoring effort of Oceanomare Delphis Onlus is still active providing data on the absence of the species in the Pontino Archipelago.
As we can see, there are a lot of anthropogenic factors affecting long-finned pilot whales in the Mediterranean Sea. This is a problem when it comes to a rather uncommon species in this Sea. In fact, little is known about their abundance in this area which makes it difficult to assess the conservation status of the Mediterranean subpopulation.
Monitoring efforts in the Mediterranean Sea to study the population status of long-finned pilot whales are hence of utmost importance for the conservation of the species. Even though sightings in the Tyrrhenian Sea are rather rare, they continue to be one of the target species of the monitoring efforts in the Ischia Dolphin Project.