Species: Delphinus delphis (Linnaeus 1758)
The common dolphin reaches 2.5 meters in length and a weight of 75 kg. The coloration is variable: his back and sides are black or blackish, his belly and chest are of cream color, with a range of shades of yellow, gray and white along the sides and belly.
A typical dark inverted triangle is recognizable more or less by the dorsal fin and a black ring stretching forward around the eye.
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 stripe and bottlenose dolphins.
It feeds on mesopelagic epipelagic fish. In fact, the stomach contents of stranded individuals in the Ligurian and the Mediterranean Sea confirm that its diet is mainly based on surfacing bluefish, but also on cephalopods and crustaceans.
In 2003 the Mediterranean common dolphin ‘subpopulation’ was listed as endangered in the IUCN Red List of Threatened Animals, based on criterion A2, which refers to a 50% decline in abundance over the last three generations, the causes of which “may not have ceased or may not be understood or may not be reversible”.
The local Population
The first records of the presence of the common dolphins in the Gulf of Naples date back to the IX X century, during which the Royal Museum of Zoology of the University of Naples acquired several specimens of the species, 12 of which are still preserved.
The local population of common dolphins of Ischia has been monitored by Oceanomare Delphis since 1997; encounters with the animals have occurred in all seasons of the year and have been concentrated in the coastal heads of the submarine canyon system of Cuma.
Photo-identification analyses identified a total of 94 individuals. A core of 12 identified females was seen together in nine different years during the study period.
Data collected over time show that the waters around the island of Ischia represent a feeding, breeding, and important nursery area for this local population, offering favorable conditions for giving birth and raising young.
Thanks to the studies undertaken by Oceanomare Delphis, the island of Ischia has been defined as a “critical habitat” for the species in the IUCN Cetacean Conservation Plan (Revees et al., 2003) and “Area of Conservation Importance” in the ACCOBAMS Common Dolphin Conservation Plan.
Moreover, the habitat use maps produced for the Italian Ministry of Environment allowed the establishment of a pelagic area dedicated to the protection of the critical habitat of the species (the coastal heads of the submarine canyon system of Cuma) within the Marine Protected Area “Regno di Nettuno”.
Finally, the waters of Ischia and Ventotene have been recognized as an Important Marine Mammals Area (IMMA) by the IUCN, with common dolphin, bottlenose dolphin, and minke whale as key species (Marine MammalsProtectedAreas Task Force, 2017).
Despite several attempts to protect “on paper” the local common dolphin population, no concrete actions have ever been taken to safeguard these animals that, in recent years, have declined until they almost disappeared.
In fact, the analysis of photo-identification data and the continuous decline in encounter rates monitored over a period of 16 years (2002-2015) testify that the area has been for years a hotspot for the local population (mainly resident) and that now this population is dying or moving to other locations.
Several human activities at sea can impact the common dolphin in the study area, among the most significant being habitat disturbance and degradation (including traffic and noise pollution) and overfishing of food resources.
The data presented by Oceanomare Delphis provide a strong argument for explicit and urgent population-specific conservation and management strategies to be developed and applied locally for common dolphins, considering that they depend on the area for important biological processes.
Like other odontocetes, the common dolphin is a highly vocal species capable of generating:
- Clicks, short duration, broadband signals used for echolocation and navigation, ranging from 0.2 to 150 kHz and lasting 50-150 microseconds.
- Bursts, series of rapidly produced clicks, are perceived as tonal sounds that are emitted for both echolocation and communication.
- Whistles, frequency modulated, long duration, tonal calls generally 3-24 kHz, lasting 0.5-0.9 s, used for communication.
- Other sounds other than whistles and clicks called buzz, such as bark, chirp, yelp, squeal.
Most of the available knowledge on acoustic signals of common dolphins in the Mediterranean Sea is related to the structural characteristics of whistles, their emission pattern associated with various behavioral states, and their geographic variation within the basin.