The not so common dolphin
This blog was written by our project partners at the Ischia Whale and Dolphin Project.
For more information about getting involved with whales an dolphins of the Mediterranean please visit their website: www.oceanomaredelphis.org
The adult of the common dolphin reaches 2.5 metres in length and a weight of 130kg. The coloration is variable: its back and sides are black or dark grey, and it has a peculiar hour-glass shape on his sides, yellowish-coloured, a specific dark-grey inverted triangle by the dorsal fin and a black ring around the eye and stretching forward. The common dolphin swims fast making big jumps, managing to dive quite deeply (280 meters) lasting underwater for over 8 minutes. The common dolphin uses both pelagic and coastal habitats, often in association with striped (Stenella coeruleoalba) and bottlenose dolphins (Tursiops truncatus). He forms groups sized 12 to hundreds individuals. It feeds on mesopelagic and epipelagic fish. In fact, the stomach contents of stranded individuals in the Ligurian and Mediterranean Sea confirm that its diet is mainly based on surfacing blue fish, but also on cephalopods and crustaceans.
In the waters of the island of Ischia, the common dolphin has been seen preying mainly Atlantic saury (Scomberesox saurus) and other occasional preys like anchovies (Engraulis encrasicolus) and sardines (Sardina pilchardus).
The common dolphin is one of the species studied by the long-term project “Ischia Dolphin Project”, which has been monitoring the local population since 1997, and on which has been gathering photographic data since 2002. Encounters with this species have happened during all seasons of the year and have concentrated on the coastal heads of the submarine Cuma canyon system until 2013, year in which the last sighting of the known dolphin pod in our study area occurred (additional sightings of single individuals mixed with a group of striped dolphins have been recorded).
Photo-identification analyses have identified 94 individuals. A group of 12 females has been monitored in nine different years during the study period. Data gathered over time show that the waters off Ischia represent for this species an important area for feeding, mating and nursery.
THE DECLINE IN THE MEDITERRANEAN SEA AND CONSERVATION ACTIONS
Over the last decades, the sub-population of common dolphin in the Mediterranean Sea has been dramatically declining, completely disappearing in the Balearic Sea, the Provencal basin, Ligurian and Adriatic Seas. Today, the only areas in which it is relatively abundant are the Alboran Sea (Cañadas&Hammond, 2008) and Malta (Vella 2000; 2005). Isolated groups still occur in the Tyrrhenian and Ionian Seas.
This dramatic decline in abundance has risen severe conservation concerns for the species, which in 2003 was labelled ‘endangered’ and listed in the Red List of endangered animals by IUCN (International Union for Conservation of Nature) according to the A2 Criterion that refers to a 50% decline in abundance in the last three generations, ‘whose cause may not be stopped, understood or reversible”.
Thanks to the results of the ongoing study of Oceanomare Delphis, the island of Ischia has been defined as “Critical Habitat” for the species in the Conservation Plan for Cetaceans by IUCN (Revees et al., 2003) and as “Area of Conservation Importance” in the Conservation Plan of the common dolphin by ACCOBAMS. Moreover, the habitat use maps provided by ODO to the Italian Environment Ministry have allowed the institution of a pelagic zone dedicated to the protection of the critical habitat of this species (the heads of the underwater Cuma canyon system) within the Marine Protected Area of Islands of Ischia, Procida and Vivara “AMP Regno di Nettuno”.
Recently, on the basis of the evidence provided by the decades-long study ‘Ischia Dolphin Project’, the waters of Ischia and Ventotene have been recognized as Important Marine Mammals Area (IMMA) by IUCN, with common dolphin, bottlenose dolphin and fin whale as key species (Marine Mammals Protected Areas Task Force, 2017).
Although the numerous endeavours to protect ‘on paper’ the local population of common dolphin, no real action has been taken for the safeguard of these animals. The photo-id analyses and the ongoing decline in the encounter rate monitored over a span of 16 years (2002-2015) show that the area was once a hotspot for the local population (mainly resident) and that this population is now moving elsewhere.
Several human activities may have an impact on common dolphins in the study area, among which the most significant are the disturbance and decay of the habitat (including the traffic and acoustic pollution) and the phenomenon of overfishing.
The data produced by Oceanomare Delphis offer a strong argument in favour of conservation and management strategies that must be clear and urgent, specific to the population, to be developed and applied locally, always keeping in mind that this species depends on the area for important biological processes.