Species: Stenella coeruleoalba (Meyen 1833)
The striped dolphin reaches an average length of 2.2meters and a weight of 80-100 kg. The male is slightly bigger than the female. The species has a long and tapered beak, small but pointed teeth; the body is slender and it has a torpedoes shape, the dorsal fin is small and falcate, the pectoral fins tapered.
Even the livery of the striped dolphin, like that of the common dolphin, is indicative of the recognition of the species. The coloration is variable, brown or dark blue-gray back, white belly. A narrow dark band runs along the sides from a spot around the eye and ending spreading around the anal opening; another thinner strip departs from the eye and ends at the base of the pectoral fin; a third strip, depart from the dorsal fin forward. The striped dolphin is a very fast swimmer, often with huge jumps out of the water.
The striped dolphin is a cosmopolitan species, distributed in temperate and tropical waters around the world.
The species lives in groups that can vary from twenty to a hundred individuals. In the Mediterranean, where it seems that it reproduces during the summer, it is the most common species. It feeds on bluefish and squid, revealing feeding opportunism.
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The Mediterranean population of striped dolphins is subject to several threats that, cumulatively, have reduced its size and slowed its recovery.
Between 1990 and 1992, a morbillivirus outbreak killed thousands of individuals, reducing the size of the groups to less than one-third of previous levels. Pollution (PCBs and other organochlorines) has been identified as a factor causing immunosuppression and sterility. Other important threats are represented by bycatches in fishing gears and by the decrease of resources (prey) due to intense fishing exploitation.
The local Population
Every year, large groups of striped dolphins can be observed off the island of Ischia, in the area of the Cuma Canyon, composed of 100 or more individuals.
In our study area, striped dolphins are often sighted in association with common dolphins.
For both species feeding, socialization, and reproduction behaviors are observed. Newborns are sighted in July, August, and September.
Our data showed a relevant overlap in distribution between the common and the striped dolphins suggesting the possible existence of antagonism between the species in habitat use.
Stenella is the most frequently sighted species during our observations, and the most abundant in the study area. Sightings occur throughout the year.
Stenella is distributed throughout the study area, both in coastal waters and further offshore in pelagic waters. However, the species tends to concentrate in the north-west and south-west areas of the island of Ischia, corresponding to the canyons of Cuma and Punta Imperatore.
Striped dolphins produce high-frequency whistles extending from a few Hz to more than 20 kHz, often lasting less than a minute. Striped dolphins’ clicks have energy peaks of 50 to 150 kHz.
Other vocalizations of the species are highly varied but poorly studied.
Preliminary analyses of whistles (n whistles 209, n samples 5696) recorded in the study area show mean duration values of 0.35 s, range 0.12-0.9, DS ±0.2 and mean frequency values of 9262 Hz, range 4823-18561 Hz, DS ±2218.