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Article

First Photo-Identification Study of the Sperm Whale (Physeter macrocephalus) in the Aeolian Archipelago and the Northern Coast of Sicily (Southern Tyrrhenian Sea, Italy)

by
Agata Irene Di Paola
,
Michelle Gelippi
and
Monica Francesca Blasi
*
Filicudi Wildlife Conservation, Stimpagnato Filicudi, 98055 Lipari, Italy
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(3), 147; https://doi.org/10.3390/d17030147
Submission received: 23 December 2024 / Revised: 18 February 2025 / Accepted: 19 February 2025 / Published: 21 February 2025
(This article belongs to the Special Issue Biology, Biodiversity and Conservation of Marine Mammals and Sea Turtles)
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Abstract

:
(1) While photo-identification has been used to study sperm whales worldwide, no long-term photo-id studies are available from the Southern Tyrrhenian Sea for the Mediterranean endangered population. (2) Here, sperm whale occurrence, group size, and the type of encounters were investigated around the Aeolian archipelago (Southern Italy) by photo-ID data collected between 2013 and 2024. Data were obtained through dedicated boat surveys and recreational sailors’ reports. (3) During 58 sighting events, 125 sperm whales were encountered, 60 of whom were photo-identified and cataloged. Of these, two sperm whales were found entangled. Only five individuals were re-sighted in later years or seasons. Encounters were more frequent in autumn, consisting mainly of solitary individuals (53.4%, n = 31) and aggregations of clustered, often not sexed, animals (23.5%, n = 8). Social units of females and calves were also encountered (8.6%, n = 5). (4) The low site fidelity and the group types encountered suggest that dispersed young males and social groups could use the area simultaneously. (4) Future matching of these data with databases from other surveyed areas, acoustic surveys, and enhanced analysis of age classes are crucial to better define the ecological role of the study area for the Mediterranean sperm whale population.

1. Introduction

Photo-identification (photo-ID) is an effective and non-invasive mark-recapture technique used to study sperm whales all over the world because it allows unequivocal identification of individuals through the distinctive trailing edge of their flukes [1,2,3,4]. In the Mediterranean Sea, sperm whales’ photo-ID data are normally coupled with those of pigmentation skin marks present over the animals’ whole bodies [5]. By employing both methods, the number of identified individuals may increase by up to 10% compared to when only fluke notches are investigated. The identification of sperm whales is useful to understand their presence, abundance, distribution, and site fidelity in a defined area.
To date, photo-ID studies of sperm whales have almost completely covered the western part of the Mediterranean Sea (i.e., Strait of Gibraltar, Alboran Sea, Balearic Islands, Gulf of Lyon, Ligurian Sea, and Tyrrhenian Sea) and the Hellenic Trench in the eastern part [5,6,7,8,9,10] (Figure 1).
Based on these investigations, male sperm whales can be encountered in both the western [6,7,10,11] and eastern [8,12] Mediterranean basins. They can be found either as lone individuals or in small groups [8,9,12], called “bachelor groups”, that can comprise immature and, sometimes, mature animals [13,14,15,16]. Concerning females, they are regularly encountered off the Balearic Islands [11,17,18] and in the Hellenic Trench [8]. According to Drouot et al. [11] and Gannier et al. [17], social units of females with calves appear to distribute at latitudes that are below 41°, although more recent data report an increase in the number of sightings at higher latitudes as well [19,20,21].
Generally, the sampling efforts carried out in the Tyrrhenian basin are significantly lower than those applied in other areas, such as the Ligurian Sea and the Algero-Provençal basin [22]. Some data on sperm whales occurring in the Tyrrhenian Sea were already collected visually [11,17,23] and acoustically [22,24,25,26,27], but long-term monitoring efforts and photo-ID results are available only for the Cuma Canyon area, located in the central-eastern part of the Tyrrhenian Sea, and the data are now a decade old [9]. The Mediterranean-isolated population of sperm whales [28,29,30] has been assessed regionally as “Endangered” by the IUCN (International Union for Conservation of Nature) [31]. Entanglements, together with vessel collisions and noise disturbance, represent the most dangerous threats [31]. From 2013 to 2021, nine sperm whales were found stranded (Italian stranding database report available at http://mammiferimarini.unipv.it/, accessed on 5 December 2022) and two others were found entangled but still alive [32] in the southern portion of the Tyrrhenian basin. In this context, regional-scale studies in this still poorly monitored area may provide important information; in particular, the construction of a photo-ID catalogue. This kind of effort could give insights into animals’ abundance, habitat use, and site fidelity, and assess the risks they could face in an area where a declining population trend has already been recorded for other resident cetacean species, including the bottlenose dolphin (Tursiops truncatus) [33,34,35,36,37].
Presented here are the results of the first long-term (from 2013 to 2024) study based on photo-ID and distribution data collected from sperm whales occurring in the southern Tyrrhenian Sea, specifically around the Aeolian archipelago and on the northern coast of Sicily (Italy). Outcomes allowed assessment per year and per season of the occurrence of different sexes/age classes, group sizes, and types of encounters. Additionally, it was possible to analyze photos and the presence of sperm whales observed occasionally by recreational sailors in the closer areas. Results stress the necessity of extensive monitoring campaigns to better understand the role of the southern Tyrrhenian Sea in the life cycle of the Mediterranean sperm whales, which can be regularly encountered there.

2. Materials and Methods

2.1. Study Area

The study area is located in the southern Tyrrhenian Sea and includes the waters from around the Aeolian archipelago (Aeolian Sea) to the northern coast of Sicily (Figure 2).
The Aeolian archipelago is a volcanic area located at the southern limit of the Tyrrhenian basin [38] that is made up of seven islands, with irregular flanks presenting channels, gullies (mostly in the eastern sector of the archipelago—Panarea and Stromboli), and steep slopes [39]. These islands are separated by shallow waters from 50 m to 320 m depth (as between the islands of Vulcano, Lipari, and Salina) and by deeper basins, around 1300–1500 m depth (as between the islands Alicudi-Filicudi, Filicudi-Salina, and Panarea-Stromboli), characterized by submerged volcanic outcrops, seamounts, and canyons [38,39,40]. Steep slopes and peculiar geomorphological structures, together with local hydrobiological and biological features, play an important ecological role in the presence and distribution of top marine predators, including sperm whales [41,42,43,44]. The Aeolian Sea regularly hosts the striped dolphin (Stenella coeruleoalba) and a local but decreasing population of bottlenose dolphins (Tursiops truncatus) [33,34,35,36,37]. In addition, Risso’s dolphin (Grampus griseus), pilot whales (Globicephala melas), and fin whales (Balenoptera physalus) may be occasionally encountered in the area (unpublished data, Monica F. Blasi).

2.2. Monitoring Activity and Data Collection

Sperm whales’ photo-ID and distribution data were collected through dedicated boat surveys within a core study area of 4865 km2 around the Aeolian archipelago [33,34,35,36]. From 2013 to 2024, each year, random boat surveys arbitrarily covered a mean of 626.4 km, which, divided by season, corresponded to a mean effort of 101.3 ± 14.8 km in spring, 413.7 ± 54.4 km in summer, and 111.6 ± 14.8 km in autumn (Table 1). A total of 1440 surveys were carried out, 120 per year, divided into a fixed number of 20 in spring (from 1 March to 31 May), 80 in summer (from 1 June to 30 August), and 20 in autumn (from 1 September to 30 November).
Monitoring surveys were conducted on a 7 m inflatable motorboat, involving one to two experienced observers and four to eight volunteers. All monitoring sessions were performed using standard protocols already implemented in the Aeolian Sea for other cetacean species [33,34,35,36]. The study area was divided into four equally extended sub-areas (NW, NE, SW, and SE). In addition, surveys were equally distributed per sub-area and according to weather conditions [33,34,35,36,37]. No specific permissions were required for these locations and activities under the Italian/regional regulations, and field studies did not involve experimental manipulations of the animals.
Sperm whale data were collected using focal individual/group observation [45,46] and the photo-ID technique [1,47,48]. Animal group size was calculated based on the total number of individuals observed during a whole sighting event in an apparent association, moving in the same direction, and often engaged in the same activity [14,49]. Each time a subgroup segregated from the other members, it was followed, independent of its size and/or performed activity [50]. The sightings were considered different encounters when they occurred on different days or during the same day but more than 60 min apart [9].
Once one or more sperm whales were sighted, the initial position of the sighting was recorded with a Garmin GPS, and the boat started to follow the animal from the rear at a minimum speed level in order to not disturb them. Individuals were then approached to obtain photo-ID data (camera details: Reflex Nikon camera D7000 (Nikon corporation, Tokyo, Japan) equipped with 70–300 mm telephoto). Whenever possible, underwater videos were also collected using GoProHero 7/10 cameras. When two onboard expert operators were present, sighting events were filmed as well to match sequences of video and photo frames of the same individual fluking. This way, it was possible to record simultaneous or sequential fluking and to estimate the relative size of sperm whales when they clustered together close to the surface or approached the boat. Under non-prohibitive weather conditions (Beaufort index < 3), animals were followed until all the sighted members were either photographed or filmed, or until they disappeared from view.
Opportunistic sightings by recreational sailors were added a posteriori to the database, and, whenever possible, a brief interview took place with those individuals. Opportunistic data were collected within a wider area, including the survey area, and also extending from the coastal waters of Termini Imerese (Palermo, Sicily) to those of Cetraro (Cosenza, Calabria) (Figure 2). Additionally, the screening of social media channels where people frequently upload their videos of animal encounters was implemented to optimize the potential contribution of opportunistic sightings. High-resolution videos and photos were then requested for further analysis. Finally, data collection was further promoted with the launch of a focused citizen-science campaign through dedicated social media channels starting in December 2019. Opportunistic data were added to the database only when they occurred in the same timeframe of monitoring efforts (i.e., spring, summer, and autumn).

2.3. Photo-Identification Analysis

To assess the contribution of monitoring campaigns and opportunistic sightings, the mean (±St. dev.) of the photos gained per sighting was calculated first in total and then separately for surveys and opportunistic reports. All the acquired material was classified as “low”, “good”, and “excellent”, based on the distance from the animal to the camera, image sharpness, and the presence of target body parts. All images were then scanned visually, and “good” and “excellent” photos were used to identify individuals from the trailing edge of the fluke, dorsal fin, and other permanent pigmentation marks all around the body. Animals identified and catalogued for the first time were classified as “Newly identified individual”, while individuals previously catalogued and recaptured at least once during the study period were classified as “Recaptured individual”. As proposed by Alessi et al. [5], the marks considered for the analysis were (Figure 3): “Pigmentations”, i.e., white areas (WA) and light gray areas (LA) of a dense homogeneous pigmented portion of the skin, with variable dimensions and irregular shape; “Patches”, i.e., non-circular patches (NCP) or circular patches (CP) dots of white or light gray skin color; “Notches”, i.e., nicks, waves, and scallops of the trailing edge of the fluke [1,5,47,48]. Notches in the dorsal fin were also recorded and classified. Also photographed were “Knobs” (K), i.e., peduncle humps useful mostly for calves [8]; and “Injuries” (IN), i.e., every mark registered as a wound, scar, cut, mutilation, or disease. Finally, when distinguishable, the presence/absence of the dorsal fin callus (C) was registered. It has been used in other studies [21,51,52,53] as a possible female-related mark, even if its presence has been reported both for females and for young males [51].
The photo-ID catalogue contained: (1) a numerical identification code (PHDn), given to every identified individual using multimedia software (Office, ver. 2501) [33,34,35,36,37]; (2) the name given to the animal; (3) spatio/temporal information about the sightings; (4) the individual’s association; and (5) the animal’s photos and video frames. Concerning photos and video frames, they were considered useful when they were of high quality and represented: (a) the dorsal side of the fluke, (b) the ventral side of the fluke, (c) the genital area, (d) the left side of the dorsal fin, (e) the right side of the dorsal fin, (f) the melon, (g) the left side of the body, (h) the right side of the body, (i) knobs, and (j) details of other visually identifiable marks, such as injuries or the presence of parasites. To distinguish between individuals, each animal was compared to the others by the naked eye, matching images of all the body parts collected.

2.4. Sex Determination

When possible, “Female” (F) and “Male” (M) individuals were defined directly by the analyzing their genital area, either recorded on underwater videos or photo frames collected both from boat surveys and from opportunistic reports. If such records were not available, sex was estimated/provisionally determined based on the sperm whale’s relative dimensions and social behavioral differences [13,14,15]. The body size of sperm whales in the Mediterranean population is considered to be smaller than that of oceanic ones [54]. Specifically, males appear to leave their family social unit when they reach around 8 to 9 m, and physical and behavioral sexual maturity is thought to be reached when they are about 13 m long [7,55,56]. Regarding females, there is evidence of individuals giving birth when around 9/10 m long [8]. Consequently, if an estimated 8–11 m long sperm whale was observed in strict contact with a calf for the whole duration of the encounter, it was classified as “Female”. The category “Estimated Male” (EM) was used when an 8–12 m long sperm whale was sighted and recaptured alone, or in groups of two or three animals with similar dimensions, all of which were defined as “Males” (either adult, immature, or estimated ones).

2.5. Age Determination

When possible, the age class of sexed individuals was estimated either according to relative dimensions between individuals both for sightings documented by reporting or by survey activity, or by comparing animal body length to boat dimensions (7 m boat) [21]. Small individuals of an estimated size from 4 to 6 m, in close contact with an animal about twice its body length, were classified as “Calves” [21]. “Immature” were males or estimated males with a body length from 6 m to 12 m and females with a body length from 6 m to 8 m, while “Adults” were males of estimated size > 12 m and females with estimated size > 8 m. Finally, age class/sex assessments resulted in six categories [9]: “Calves” (C), “Immature females” (IF), “Adult females” (AF), “Immature males” (IM), “Estimated males” (EM), and “Adult males” (AM). Sperm whales whose sex and age class were not determined were defined as Unknown (U).

2.6. Group Size, Type of Encounters, and Sighting Locations

Group size evaluated during the sighting events was confirmed by analyzing video and photos acquired and the mean group size (±St. Er.) of sighting events was calculated per season and per year of research effort. Sperm whales’ sociality, which is considered to be complex and based on sex, defines different types of animal aggregations [13,14,15,55]. In this study, based on previous data [8,9,57], five categories were used: “Solitary Individuals” (SI), i.e., single individuals (no other sperm whale detected visually or acoustically during its sighting) [8,9,57]; “Social Units” (SU), i.e., aggregations of at least four individuals that included “Calves” [8,9]; “Bachelor groups” (BG), i.e., aggregations of similarly sized/aged whales, comprising “Male” and/or “Estimated Male”; “Clusters” (CL), i.e., all encounters different from SI, SU, and BG, as several animals, often of unknown sex, display coordinated movements within 100 m of one another [1,9,15]. Clusters were further subclassified based on the number of individuals found together: Small Clusters (SCL) consisting of two to four individuals; Large Clusters (LCL) of five or more animals; “Unknown” (UK), when group size and age class/sex of members were not clearly determined. The GPS location of the first sighting of every encounter was mapped (QGis 3.4 software) to study animals’ distribution in relation to bathymetry.

3. Results

3.1. Photo-Identification Analysis

During a total of 58 encounters, 125 sperm whales were sighted, of which 60 were photo-identified and included in the catalog. A total of 12,933 photos and video frames were collected, 98.7% (n = 12,765) of which came from survey activities and 1.3% (n = 168) from reports of opportunistic sightings. Full videos totaled 347. Even though the number of opportunistic sightings reported (n = 37) was higher than the number of encounters coming from survey activities (n = 21), during survey activities a mean of 797.4 (±1000.1) photos/frames per sighting were collected, compared to a much lower mean number of 7.4 (±17.5) obtained by recreational sailors (Table 2). Of those 12,933 photos, only 285 fulfilled the required high-quality standards, representing different body regions (i.e., 3% the genital area; 6% the right side of the body; 6% the left side of the body; 8% the melon; 9% injuries in different body parts; 12% the dorsal side of the fluke; 12% the right side of the dorsal fin; 13% the left side of the dorsal fin; 13% knobs; 18% the ventral side of the fluke).
Of the 60 cataloged individuals, 50 were identified through their fluke edge (N) and 10 via their white areas (WA). Five of the 60 animals were re-encountered on different days, months, or years (Table 3). Four of the five “Recaptured Individuals” were recaptured only once, while the other animal was re-sighted four times over the years (PHD24).
-
PHD23: first sighted in a BG during the survey activity, and images of the fluke and the flank showing a triangle-shaped WA were acquired. A few days later, on 7 August 2018, it was recaptured with PHD24 by a tourist who filmed the flank of this animal moving away from the boat with the GoPro.
-
PHD24: first identified on 7 August 2018 through the opportunistic video obtained with a GoPro, allowing the observation of its genital area, defining it as a male. This encounter type was then defined as a bachelor group of two individuals. During all the recaptures, PHD24 was sighted as a solitary individual and recaptured through the fluke edge and pigmentation marks on the fluke. These last marks allowed a clearer recognition since the fluke edge showed small notches only.
-
PHD30: first captured by an opportunistic report in a small cluster of two individuals and then recaptured as a solitary individual through the trailing edge of the fluke. No information was available to determine the sex/age class of the animals.
-
PHD44: found entangled in a driftnet [32], it was identified through white pigmentation areas plus the same recognizable driftnet around its fluke. PHD44 was classified as an “Estimated Male”, it was named “Furia”, and was first sighted in July 2020 with a section of driftnet wrapped around its fluke [32]. The attempt at disentanglement by the Italian Coast Guard and private diving center operators failed, and the animal was resighted in October 2020, with the net still around the fluke [32].
-
PHD54: of unknown sex, was first sighted in June 2020 in a social unit and then recaptured in October 2020 again in a social unit. During its last sighting, the other members of the social unit were not identified; thus, making it impossible to clarify if it was joining the same social unit in June or a different one.
As visible in Table 3, no females were apparently recaptured.
However, given that only 40.5% (n = 17) of the members of social units were identified, the possibility that females with calves did not return to the study area cannot be excluded. In Figure 4, the cumulative curve of identification shows an increase of individuals over the years, indicating that the number of sperm whales occurring in the study area is a long way from being established.
Among all sperm whales presenting “injuries”, PHD21, a male, had a large wound close to the mouth, on the left side of the head. Video of its sighting came from recreational sailors who encountered the sperm whale near an illegal fishing aggregation device (FAD) used to catch dolphinfish (Coryphaena hippurus). Another animal, PHD03, showed a notable recovering scar around the dorsal fin. PHD33 and PHD56 had deep holes of unknown origin in the dorsal part of the head between the blowhole and the dorsal fin (Figure 3). PHD12, PHD13, PHD14, and PHD49, all classified as adult females, presented a visible dorsal fin callus (Figure 3). Three individuals, whose sex was not determined (PHD31, PHD37, and PHD39), all sighted in the same large cluster, also had small dorsal fin calluses. In this case, the presence of the callus led us to infer that this large cluster was probably a social unit.

3.2. Sex and Age Class Determination

The sex/age class was determined for about a third (n = 40) of the sighted animals, while the rest (68.0%, n = 85) remained unknown (Table 4).
All age class categories were encountered, and sex was defined for 33 animals (26.4% of total animals). Specifically, 13 of the sighted individuals were “Males” (nine “Immature Males” and four “Adults”), 10 were “Estimated Males”, and 10 were “Females”. Of those females, an “Immature Female” (PHD06) was identified by observing its genital area. She was sighted in a small cluster with three other individuals of similar dimensions, of which two were confirmed to be “Immature Males” after the observation of their genital areas through underwater videos. The other nine animals were defined as “Adult Females” based on estimates of their relative body dimensions and behavioral observations (e.g., an > 8 m whale, with a calf for the entire period of the sighting). No large bulls (body length >15 m, [9]) were encountered. Regarding yearly data, all sex categories were registered only in 2020, and all age classes were encountered in 2016 and 2020. Social units formed by “Females” and “Calves” were observed more in summer (n = 4) than in autumn (n = 1) in 2013, 2016, and 2020, highlighting the possible periodic presence of females with calves, with periods of three or four years from previous encounters. “Estimated Males” and “Males” were sighted mainly in summer (n = 12) and autumn (n = 8); one “Male” and one “Estimated Male” were observed in spring. The percentage of individuals per sex/age class category observed in each season is shown in Figure 5, considering that the total number of individuals encountered in autumn is 32, in summer is 83, and in spring is 10.

3.3. Group Size, Type of Encounters, and Sighting Locations

The group size was confirmed for 51 sightings (87.9%). The non-parametric Kruskal–Wallis test was used to find significant differences in the mean group size (±St.Er.) among years and seasons. The normality of data were assessed by Shapiro–Wilks’s test (p > 0.05), and the assumption of homogeneity of variance was rejected by Levene’s test (p > 0.05). The mean group size of total encounters was 2.1 (±0.3), with a maximum group size value of 11 animals and a minimum of one. Concerning seasonal differences (Figure 6a), the mean group size was higher in summer (mean: 2.8 ± 0.6), when social units and clusters were mostly encountered, than in spring (mean: 1.1 ± 0.20) and autumn (mean: 1.9 ± 0.5) (Kruskal–Wallis test: H(2) = 3.695; p = 0.07465) (Figure 6a). No significant differences were found in the mean group size among years (Figure 6b). The mean group size for social units was 8.40 (± 0.81), with a minimum group size of 6 individuals (on 3 August 2013) and a maximum of 11 (on 7 July 2020) (Figure 6). Small clusters had a mean group size of 2.3 (± 0.3), and they were sighted in summer (mean: 2.0 ± 0.4), in spring (two of two individuals), and in autumn (two of four animals). Only one large cluster was observed, on 29 June 2020, and it consisted of a group of eleven specimens. Bachelor groups were encountered twice (on 4 and 7 August 2018), and both times were composed of only two individuals. For 13.1% (n = 7) of total encounters, it was not possible to define the group size because the recreational sailors who collected the data were unable to establish it; thus, it was classified as “unknown”.
The types of encounters differed mostly in summer, when all the types of groups were registered (Figure 7). In total, eight encounters occurred in spring, 31 in summer, and 19 in autumn. Solitary individuals (51.7%, n = 30) were encountered in each monitored season (n = 5 in spring, n = 14 in summer, and n = 11 in autumn) (Figure 7). The encounter rate calculated on the mean km of research effort per season was slightly higher in autumn than in spring and summer.
The sighting of social units (n = 5) represented 8.6% of the total encounters, and most of them (n = 4) occurred in summer (August 2013; August 2016; June 2020; July 2020; October 2020; three of them spotted from reports and two from surveys). Bachelor groups corresponded to 3.4% (n = 2), while clusters accounted for 20.7% (n = 12) of the total encounters. As regards clusters, eight of them were sighted in summer, two in spring, and two in autumn. Two of the clusters were formed by four individuals. The first, observed in November 2015, was composed of two immature males, one immature female, and one unknown specimen, all observed resting together at the surface and then performing synchronized fluking. The second, observed in June 2020, was formed by three individuals resting not far from PHD29, “Spike”, entrapped in the illegal gillnet [32]. The largest cluster of all was sighted on 29 June 2020 near the southern coast of Filicudi Island. Of the 11 members, none was sexed, but three of them presented a dorsal fin callus, suggesting the presence of females. Photo IDs were taken for 10 of those sperm whales, and they were all classified as “Newly Identified Individuals”. The specimens were observed floating close to each other, performing side fluking and surface rolling; breaching was also observed (for a description of behavioral categories, see [9,14].
Finally, the first sighting locations of 55 of 58 encounters are reported in Figure 2, specified by group type. Most of the sightings (58.6%; n = 33) occurred at water depths between 200 m and 1000 m, followed by encounters at depths between 1000 m and 2000 m deep (20.7%; n = 12) and those in shallower waters, from 0 m to 200 m (15.5%; n = 10). Social units were sighted mainly in deep waters (2 of them at water depths between 200 m and −1000 m and the other three between 1000 m and 2000 m), while solitary individuals were mostly encountered in shallower waters (the 12.9%, n = 9, at 0–200 m; the 50.0%, n = 17, at 200–1000 m; the 16.6%, n = 5, at 1000–2000 m). Clusters were encountered in both shallow and deep waters, with higher frequency over water depths between 200 and 1000 m. Finally, one bachelor group was observed over water depths between 0–200 m and one between 1000–2000 m. The frequency of each type of encounter per bathymetric range was tested, and no association between the two factors was found (Fisher’s Exact Test for Count Data: p = 0.1249).

4. Discussion

4.1. Photo-Identification Analysis

Based on historical data, sperm whales were quite abundant in the Tyrrhenian Sea [58]. More recent studies, however, reported a generally low rate of sightings when compared to other Mediterranean areas, such as the Sardinian, Ligurian, or Ionian Sea [23], and a low acoustic and visual detection during the summer months [11,17]. In 2018, within the ACCOBAMS monitoring program (ASI, ACCOBAMS Survey Initiative) [26], a density of 1.71 to 1.88 individuals/1000 km2 was estimated alongside an abundance that can vary from 83 to 91 animals, while Lewis et al. [25] reported a mean density of 0.71 individuals/1000 km2 and an abundance of 149 individuals. Here, the first catalog of identified sperm whales for the southern Tyrrhenian Sea is presented, based on data collected during a long-term monitoring effort carried out from 2013 to 2024. Outcomes suggest a regular presence of sperm whales in the waters around the Aeolian archipelago and the continental slopes of the northern coast of Sicily during spring, summer, and autumn. A total of 125 individuals were encountered, 60 were identified, and five were recaptured during the study period. Consistent with [5], the use of pigmentation marks increased the likelihood of recapture of individuals, with one individual marked and recaptured through white areas alone. For two other recaptured individuals, the identification was easily confirmed by white areas on the fluke together with small notches.
“Citizen science” has been used successfully in monitoring wildlife and cetacean populations, despite some uncertainty and the need for careful validation [59,60,61,62,63,64]. Here, reporting of opportunistic sightings allowed the monitoring of a wider area than that covered by dedicated surveys alone. This supports the utility of this tool as an important methodology in conservation studies, such as cetacean monitoring activities [62]. Data collected by recreational sailors represented a sizeable part of our general dataset (63.8%, n = 37 encounters), and most sightings were confirmed with high-quality data provided during dedicated interviews. A large number of individuals recorded by sailors, however, were not identified (n = 44, 63.7% of the 69 individuals sighted from reports), because photos and video frames were not taken from the right angle and perspective or did not capture relevant body parts. On the other hand, during monitoring sessions, only 23.6% (n = 13) of the encountered sperm whales were not identified. Based on these findings, it appears advisable in the future to continue to work with the sailors who cross the Tyrrhenian Sea, especially during the tourist season. The planning of continuous citizen science monitoring campaigns, however, should involve the implementation of tools for real-time reporting and contain clear instructions on how to report sightings and what data to collect.
The abundance and distribution of sperm whales can vary depending on the topography of the sea bottom, prey distribution, primary productivity, and SST fronts [18,41,42,43,44,65,66]. Animal abundance and presence are also influenced by reproduction. The peak in sperm whale ovarian activity in the northern hemisphere is supposed to occur between June and July, as supported by the observation of males’ excursions to the breeding area of the Balearic Islands during this timeframe [7,55]. Based on all this, male animals staying in the study area for extended periods (e.g., months) or coming back over the years could be an indication of seasonally or periodically favorable foraging conditions. On 29 June 2020, a large cluster of eleven individuals was sighted displaying social behavior and a social unit of nine sperm whales. According to one interpretation, these aggregations and their timing could suggest mating purposes, but according to another, age class and sex were not defined for the large cluster’s specimens; thus, it was not possible to speculate in detail based on the data. In that specific year, moreover, the highest number of sightings for the entire monitoring period was recorded, with a total of 19 sightings compared to an average of just 2.1 encounters per year. Even considering only the number of sightings from monitoring sessions, the number of encounters was markedly higher in 2020 than in the other years. Surveys need to be carried out over a longer study period, considering all the factors that may influence the presence of sperm whales in the area, to understand this peak in animal sightings. Given that the research effort has remained similar in all the years (i.e., 80 surveys in summer, 20 in autumn, and 20 in spring per year), it cannot be excluded that the significant decrease of maritime traffic during the COVID-19 period played an important role in this peak of sightings.
Here, it was not possible to assess site fidelity due to the high number of individuals encountered but not identified (n = 57), especially those forming social units (i.e., 40.5% of animals not identified). For this reason, even if only five individuals were recaptured, and they did not include females, more data are needed to investigate more deeply whether the site fidelity of females occurs in the area. For example, “Donatello” (PHD54) was seen for the first time in June 2020, in a social unit of nine animals, including a calf, and it was the only sperm whale identified. In October of the same year, “Donatello” was recaptured again in a social unit of eight animals, including a calf, and at that time all individuals were identified (except for the calf). Because of the lack of identification of the specimens of the first social units, it cannot be determined if “Donatello” changed social units or if it remained in the same unit for a prolonged time. In other areas of the Tyrrhenian Sea, such as Cuma Canyon [9] and the Hellenic Trench [8], social units have dynamic structures, and member composition may change; the presence of individuals occasionally joining a social unit, defined as “visitors”, was observed as well [8]. Future studies are needed to provide insights into social unit dynamics in the study area.

4.2. Sex and Age Class Determination

Generally, male sperm whales show high densities in foraging areas only while moving through their whole habitat, and females appear to be keener to reside in smaller areas [67]. A long-term monitoring program carried out in Cuma Canyon (central Tyrrhenian Sea) [9] revealed the presence of both males, alone or in bachelor groups, and of females, in social units with calves and juveniles. Here, results appear to be consistent with Cuma Canyon data. However, the recapture rates of sperm whales appear to be lower in the southern Tyrrhenian Sea (about 8.3%, from a total of 60 identified animals) than in the central part of the basin (55%, from a total of 60 identified animals, data inferred from [9]). The peculiar position of the Aeolian archipelago, between the western and eastern basins of the Mediterranean Sea, could justify the low recapture rate. The study area could indeed be an important stayover during the long migration of maturing sperm whales, as suggested by the higher number of sighted immatures rather than adult males. To date, genetic studies do support the presence of a unique eastern-western population [68], and photo-ID evidence [57] suggests the movements of animals between these basins. Future long-term studies are necessary to confirm this or other hypotheses. The scarcity of adult males in the study area needs further verification. Male sperm whales are known to live their pubertal phase in bachelor schools when sized between 8.7 and 10.3 m. They then reach sexual maturity when sized between 11 and 12 m and behavioral maturity when they become nearly 13 m long [7,11,13]. For example, Drouot-Dulau & Gannier [7] evaluated a 12.3 m long male observed off the Balearic Islands, not behaviorally mature yet, and another male, 12.9 m long, as an adult because it was in association with social units. Then, Frantzis et al. [8] reported an exceptional observation of a 9.8 m long male attempting to mate within a social unit. Here, all individuals (but two) classified as males were estimated to be less than 12 m long; thus, they were considered immatures. Based on all the previous considerations, male age class estimates would need to be confirmed by other analyses, such as the calculation of IPI (inter-pulse-interval) [56,69,70] to certainly define whether adult males visit the area or not. Additionally, acoustic analyses can help to avoid the underestimation of the presence of sperm whales in the Aeolian Sea due to biases associated with visual census.

4.3. Group Size, Type of Encounters, and Sighting Locations

Similarly, to what was previously documented in Cuma Canyon [66], social units were generally found in deeper waters than males here. Moreover, the number of social units observed in this study is similar to that reported for Cuma Canyon [9], although social unit size was closer to that estimated in the Hellenic Trench [8] (i.e., Aeolian archipelago: 8.40 ± 1.42; Hellenic Trench: 8.4 ± 2.93; Cuma Canyon: 6.66 ± 0.55). The Hellenic Trench is one of the two well-known breeding grounds of sperm whales in the Mediterranean basin, where social units are reported to be two times bigger than male aggregations and three times more frequent than solitary individuals [8]. Here, social units were less frequently encountered than in the Hellenic Trench; however, their presence over the years, particularly in summer and autumn suggests suitable conditions for the use of the southern Tyrrhenian as a breeding ground [55,67]. The presence of social units in 2013, 2016, and 2020 may indicate a possible periodic presence in the area, and long-term monitoring efforts are needed to investigate this result in greater depth. Nevertheless, no females were recaptured during the study period, and no up-to-date evidence of site fidelity is available for the study area, different from other well-known breeding grounds inside and outside the Mediterranean Sea [3,9,14].The splitting and mixing of social units’ members is well-documented in other parts of the species’ range distribution [71,72]. Frantzis, Alexiadou, & Gkikopoulou [8] reported that two or three members of a particular social unit can temporarily segregate from the rest of the group. Based on this, individuals observed in small clusters could have belonged either to a social unit or to a bachelor group. However, the spatial distribution of small clusters reflected that of solitary individuals, who were mostly identified as males, which was consistent with results from other areas [9,57]. It could therefore be inferred that these small clusters represented bachelor groups. One small cluster, however, was composed of an immature female, two immature males, and one unknown. Consequently, this small cluster looked more like a mixed group, or “mixed school”, defined by Clarke R. [73] as a rare group of immature males and immature females. Continuous monitoring campaigns, together with matches with other datasets, could clarify the social meaning of these clustering individuals. Long-term studies, coupled with animals’ sex and age class determination, could help to define the social structure of sperm whales visiting the Aeolian Sea, an area that could hypothetically play an important role as a small-scale stable breeding area. An example could be the large cluster of eleven individuals, whose members were not sexed and performed different social behaviors. Without further research efforts, it will remain unknown if females could be part of such large clusters and if animals found in the study area are also using the contiguous Cuma Canyon area. This information would be crucial not only in the understanding of sperm whales migration routes in the southern Tyrrhenian Sea but also in improving conservation measures.
It is crucial to emphasize that four individuals were found entangled in driftnets during the study period. In 2020, two of them were found alive, and disentanglement attempts took place, with different results [32]. Two other individuals were found dead along the Aeolian coasts, one in 2012 and one in 2022, with clear skin lesions related to driftnet interactions. This confirms that bycatch events dangerously threaten the sperm whale population in this area, where still no conservation measures are implemented for cetacean populations. The study area is subject to seasonal mass tourism, leisure boating, and intense commercial fishing activity [35,37,74]. In particular, fishing takes place by drift netting, an activity that has been well documented in the whole Tyrrhenian Sea [32,74,75,76]. Interactions between cetaceans and fishery activities began to show critical issues both for the local fishing community and the cetacean population [32,35,37]. The Aeolian Sea deserves attention to preserve the biodiversity of its marine environment and to foster a sustainable coexistence between human activities and wildlife. This slow and challenging process should induce the creation of a Marine Protected Area [77].

5. Conclusions

This long-term study reveals, for the first time, regular encounters of sperm whales in the southern Tyrrhenian Sea. The observation of males recaptured over days or months and the record of a male that returned to the area over the years suggest that good foraging conditions may occur at least periodically. At the same time, the presence of social units with calves were quite conspicuous in the area and, further results could show whether female site fidelity may occur in this area of the Mediterranean Sea, suggesting the site could be a suitable breeding ground and explaining its role in the movements of females and calves. Furthermore, the peak in the number of encounters registered in one year (2020) should be investigated with continuous and long-term monitoring of the area. Despite the lack of acoustic detections, the establishment of the first catalog of the Aeolian Sea is an important first step in allowing matching with other catalogs all around the Mediterranean Sea. This can become an important tool that would provide more information about migrations and would contribute to the improvement of conservation measures in an area where the population is still seriously threatened.

Author Contributions

Conceptualization, M.F.B.; methodology, M.F.B. and A.I.D.P.; software, A.I.D.P.; validation, M.F.B. and A.I.D.P.; formal analysis, A.I.D.P.; investigation, M.F.B.; resources, M.F.B.; data curation, M.F.B. and A.I.D.P.; writing—original draft preparation, A.I.D.P.; writing—review and editing, M.F.B. and M.G.; visualization, M.F.B.; supervision, M.F.B.; project administration, M.F.B.; funding acquisition, M.F.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially funded as part of the conservation actions of the European Life Project LIFE Delfi (LIFE18 NAT/IT/000942) (https://lifedelfi.eu/, accessed on 18 February 2025), of which Filicudi Wildlife Conservation is a partner.

Institutional Review Board Statement

This study did not require ethical approval.

Data Availability Statement

All relevant photoidentification data are included in this paper. Additional information may be requested from the corresponding author by email.

Acknowledgments

We would like to thank all the volunteers and students of Filicudi Wildlife Conservation who assisted during the monitoring sessions, supporting the team in the collection and analysis of scientific data. We would also like to thank EON Italy for the support in the monitoring sessions of CAPODEOLO project. We also thank all the people who collaborated with this study through the citizen science campaign “SEGNALACI UNA CODA”, sending reports and images of sperm whale sightings in the area, and all the boaters who kindly accepted our interviews.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Mediterranean Sea with western and eastern basins where photo-ID investigations have been carried out. A detail of the study area in the southern Tyrrhenian Sea with the Aeolian archipelago is shown too.
Figure 1. Mediterranean Sea with western and eastern basins where photo-ID investigations have been carried out. A detail of the study area in the southern Tyrrhenian Sea with the Aeolian archipelago is shown too.
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Figure 2. Representation of the study area, which includes the waters around the Aeolian archipelago (survey area in the dashed rectangle) and coastal waters from Termini Imerese (Sicily, Italy) to Cetraro (Calabria, Italy) (38°2′15.576″ N–13°46′4.332″ E; 39°32′26.52″ N–15°51′40.14″ E) settled as limits of the area to include opportunistic reporting in the dataset. The different symbols represent different encounter types (as defined in Section 2.6), and their coordinates indicate the first sighting location of each encounter (55 of 58 encounters).
Figure 2. Representation of the study area, which includes the waters around the Aeolian archipelago (survey area in the dashed rectangle) and coastal waters from Termini Imerese (Sicily, Italy) to Cetraro (Calabria, Italy) (38°2′15.576″ N–13°46′4.332″ E; 39°32′26.52″ N–15°51′40.14″ E) settled as limits of the area to include opportunistic reporting in the dataset. The different symbols represent different encounter types (as defined in Section 2.6), and their coordinates indicate the first sighting location of each encounter (55 of 58 encounters).
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Figure 3. Skin marks observed in the animals. WA = white area; N = notch; LA = light area; NCP = non-circular patch; K = knobs; CP = circular patch; IN = injuries; C = callus. [1,5,8,47].
Figure 3. Skin marks observed in the animals. WA = white area; N = notch; LA = light area; NCP = non-circular patch; K = knobs; CP = circular patch; IN = injuries; C = callus. [1,5,8,47].
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Figure 4. Cumulative curves: (a) cumulative number of identifications by the total number of identifications, and (b) cumulative number of identifications per year, from 2013 to 2024.
Figure 4. Cumulative curves: (a) cumulative number of identifications by the total number of identifications, and (b) cumulative number of identifications per year, from 2013 to 2024.
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Figure 5. (a) Percentage of sperm whales classified in the different sex categories per season. (b) Percentage of sperm whales classified in the different age class categories per season (the total of individuals encountered per season is 10 in spring, 83 in summer, and 32 in autumn).
Figure 5. (a) Percentage of sperm whales classified in the different sex categories per season. (b) Percentage of sperm whales classified in the different age class categories per season (the total of individuals encountered per season is 10 in spring, 83 in summer, and 32 in autumn).
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Figure 6. Mean group size per year (a) and per season (b) of research effort.
Figure 6. Mean group size per year (a) and per season (b) of research effort.
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Figure 7. Percentage of different types of encounters (solitary individual (SI), bachelor group (BG), small clusters (SCL), large cluster (LCL), social unit (SU), and unknown (UK)) per season, considering a total of eight encounters in spring, 31 in summer, and 19 in autumn (a). Encounter rate per season on mean km of efforts (b).
Figure 7. Percentage of different types of encounters (solitary individual (SI), bachelor group (BG), small clusters (SCL), large cluster (LCL), social unit (SU), and unknown (UK)) per season, considering a total of eight encounters in spring, 31 in summer, and 19 in autumn (a). Encounter rate per season on mean km of efforts (b).
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Table 1. Research effort in km per year and per season. The number of surveys per year was fixed at 20 surveys during spring, 80 during summer, and 20 during autumn.
Table 1. Research effort in km per year and per season. The number of surveys per year was fixed at 20 surveys during spring, 80 during summer, and 20 during autumn.
201320142015201620172018201920202021202220232024
Spring90908010010080115120120120100100
Summer310350380430430380440480460500405400
Autumn1009090110120120135120120130105100
Total effort (km)500530550640650580690720700750610600
Table 2. Number of encounters, sighted individuals, newly identified individuals, and number of recaptures, by survey activity and by reports.
Table 2. Number of encounters, sighted individuals, newly identified individuals, and number of recaptures, by survey activity and by reports.
N of EncountersN of Sighted IndividualsN of Newly Identified IndividualsN of Recaptures
By survey activity2156394
By reports3769214
Total58125608
Table 3. Dates of sightings of the five cataloged individuals recaptured in the study area. (S) indicates sightings/recaptures occurring during survey activity, and (R) those documented by reports.
Table 3. Dates of sightings of the five cataloged individuals recaptured in the study area. (S) indicates sightings/recaptures occurring during survey activity, and (R) those documented by reports.
Individual CodeSex *Date of First SightingRecapture 1Recapture 2Recapture 3Recapture 4
PHD23EM4 August 2018 (S)7 August 2018 (R)
PHD24M7 August 2018 (R)13 September 2020(S)4 May 2022 (R)22 August 2023 (S)27 August 2023 (R)
PHD30U28 June 2020 (R)15 July 2020 (R)
PHD44EM18 July 2020 (S)5 October 2020 (S)
PHD54U29 June 2020 (R)11 October 2020 (S)
* Sex was determined as described in Section 2.4 with EM = estimated male, M = male, F = female, and U = unknown.
Table 4. Percentage (%) and number (n) of sighted animals in each sex and age class per year and in total: C = calves; IF = immature female; AF = adult female; IM = immature male; AM = adult male; EM = estimated male; U = unknown.
Table 4. Percentage (%) and number (n) of sighted animals in each sex and age class per year and in total: C = calves; IF = immature female; AF = adult female; IM = immature male; AM = adult male; EM = estimated male; U = unknown.
YearCIFAFIMAMEMUTotal (n)
201333.0% (2)033.0% (2)00033.0% (2)6
201400050.0% (1)0050.0% (1)2
2015020.0% (1)040.0% (2)020.0% (1)20.0% (1)5
201620.0% (2)040.0% (4)00040.0% (4)10
201700028.6% (2)0071.4% (5)7
201800050% (2)050.0% (2)04
201900000000
20204.9% (3)04.9% (3)3.3% (2)1.6% (1)11.5% (7)73.8% (45)61
202100000000
2022000025.0% (1)075.0% (3)4
2023000025.0% (2)075.0% (6)8
2024000000100.0% (18)18
Total5.6% (7)0.8% (1)7.2% (9)7.2% (9)3.2% (4)8% (10)68% (85)125
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Paola, A.I.D.; Gelippi, M.; Blasi, M.F. First Photo-Identification Study of the Sperm Whale (Physeter macrocephalus) in the Aeolian Archipelago and the Northern Coast of Sicily (Southern Tyrrhenian Sea, Italy). Diversity 2025, 17, 147. https://doi.org/10.3390/d17030147

AMA Style

Paola AID, Gelippi M, Blasi MF. First Photo-Identification Study of the Sperm Whale (Physeter macrocephalus) in the Aeolian Archipelago and the Northern Coast of Sicily (Southern Tyrrhenian Sea, Italy). Diversity. 2025; 17(3):147. https://doi.org/10.3390/d17030147

Chicago/Turabian Style

Paola, Agata Irene Di, Michelle Gelippi, and Monica Francesca Blasi. 2025. "First Photo-Identification Study of the Sperm Whale (Physeter macrocephalus) in the Aeolian Archipelago and the Northern Coast of Sicily (Southern Tyrrhenian Sea, Italy)" Diversity 17, no. 3: 147. https://doi.org/10.3390/d17030147

APA Style

Paola, A. I. D., Gelippi, M., & Blasi, M. F. (2025). First Photo-Identification Study of the Sperm Whale (Physeter macrocephalus) in the Aeolian Archipelago and the Northern Coast of Sicily (Southern Tyrrhenian Sea, Italy). Diversity, 17(3), 147. https://doi.org/10.3390/d17030147

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