Species: Physeter macrocephalus (Linnaeus 1758)
The sperm whale is the largest of the Odontocetes, with an average length of 18 meters and a weight of 50 tons. The female is smaller (about 11 m).
Characteristic of the species is the profile of the enormous head. The colour is uniformly dark grey, while along the outside of the upper and lower jaw, the skin is often white. The blowhole is located above the head, shifted to the left and the blow is low, messy and directed obliquely forward with an inclination of about 45 °. The dive is reaching up to 2000 meters. Before diving, the sperm whales breath at the surface for 10-20 minutes, then arches their back pulling out the big fluke (fluke out).
With its pelagic habits, sperm whales approach the coast only where there is a steep slope. The species is cosmopolitan.
The sperm whale feeds on mesopelagic cephalopods, tuna, barracuda and demersal species such as cod, hake and medium-size and large weeverfish.
In 2012, the Mediterranean population of sperm whale (Physeter macrocephalus) was listed as threatened on the IUCN Red List due to the population’s numerical decline and the small number of adult individuals.
Sperm whale numbers in the Mediterranean are alarming, with the population reduced to a few hundred individuals.
Threats include bycatch (particularly drift nets, still widely used in the central and southern Mediterranean, both legally and illegally) and collisions. In addition, the population is disturbed by heavy maritime traffic. It is likely that the combination of these factors has led to a decline (of unknown magnitude) over the past half century, and without effective management to mitigate ongoing threats, the population decline is likely to continue.
The local Population
The presence of sperm whales in the study area has been monitored since 1991. Since 2004, due to the development of the acoustic detection system, the number of sightings has increased significantly.
Sperm whale residence and movements in the study area have been studied regularly through photo-identification data collected over an 18-year period (2003-2020).
A total of 96 individuals were photo-identified based on natural marks present on their flukes. The rate of recruitment of sperm whale individuals in the photo-identification catalog varies irregularly over the years, suggesting that the waters of Ischia represent only part of the home range of the population and that the area is exploited by different groups, and that the number of specimens to be cataloged is still large.
The data confirm the presence of groups of immature males, supporting the hypothesis that this area is relevant to all age classes of the species, with groups composed of females and immatures, solitary males, and groups of young males (bachelors).
Finally, it is relevant that a stable association between young males has been documented over time. This type of association is poorly known and previously it had not been possible to document stable associations among immature males. The association we have documented over the years suggests that the social structure of groups of non-reproductive males may reflect that of females, in terms of complexity and long-term relationships among individuals.
Thanks to the data collected by our organization on this species, the Pontine Archipelago of Campania has been recognized as an Important Marine Mammal Area (IMMA) by the IUCN Marine MammalProtectedAreas Task Force.
Sperm whales have been observed mainly in the deep central head of the Cuma submarine canyon and southwest of Ischia. Some sightings have also been made in the Magnaghi and Dohrn canyons (southeast, and east of Ischia, respectively).
The distribution of sperm whales in the area has a strong association with the submarine canyons, in an area about 20-40 km in diameter.
Sperm whales emit signals called clicks, which are broadband in frequency and directional, with energy between 5 and 25 kHz. These sounds are used for echolocation and communication. Clicks can be very powerful up to 223 dB re 1µPa / 1m, the most intense biological source ever recorded.
Normally the sperm whale emits regular sequences of clicks (usual clicks) with which it explores the environment in search of potential prey.
Sometimes, the clicks change the rhythm, accelerating to an interval between 5 and 100 milliseconds from one click to another. In this case, the vocalization is called a creak and can be emitted either at depth or at the surface. During the dive phase, the acceleration of clicks is interpreted as an attempt by the sperm whale to focus on identified prey and is therefore used as an index of feeding activity.
Creaks emitted at the surface seem to have a shorter duration and a more constant interval between clicks; depending on the authors, they have been defined as coda-creaks or chirrups. They have been described as social sounds; however, it is still unclear whether used for echolocation or communication.
Certainly, the most interesting vocalization of this species is the so-called codas, a stereotyped pattern of clicks whose function could be to communicate through the patterns and rhythms of click sequences. They seem to be signals used within groups rather than to communicate between one group and another, in fact, they have a very low emission intensity.
The cultural transmission has been described for codas: some social units share an acoustic repertoire (dialect) that allows them to be classified into clans. Clans are sympatric (they share the same territory); in the oceans, clans can be composed of thousands of whales and be distributed over thousands of kilometers. The acoustic repertoire of codas is culturally transmitted within the smallest module of the sperm whale social structure, the social unit or family group composed of adult females and immature individuals of both sexes. Several patterns of codas have been recorded in the oceans.
Some types of codas seem to be common to different areas and for a long time, it was believed that the model (3+1) was the only type existing in the Mediterranean, although, on a few occasions, other codas were recorded. In the study area, the codas model 3+1 is well represented, however, the presence of several different models suggests a more varied repertoire.