Introduction

The common bottlenose dolphin, Tursiops truncatus, is a cosmopolitan species, found in tropical and temperate waters around the globe. Its distribution is usually confined within the 45th parallel in both hemispheres, but in the North Atlantic it can reach the 65th parallel1,2,3. This wide distribution is associated with a remarkable morphological variation among populations, but only four subspecies are currently recognised by the Committee on Taxonomy of the Society for Marine Mammalogy4: the nominate common bottlenose dolphin - Tursiops truncatus truncatus (Montagu, 1821); the Black Sea bottlenose dolphin - Tursiops truncatus ponticus Barabash-Nikiforov, 1940; the Lahille’s bottlenose dolphin - Tursiops truncatus gephyreus Lahille, 1908 – distributed along the coastal waters of the South West Atlantic Ocean; the Eastern Tropical Pacific bottlenose dolphin – Tursiops truncatus nuuanu Andrews, 1911. The Indo-Pacific bottlenose dolphin - Tursiops aduncus (Ehrenberg, 1832 [1833]) and Tamanend’s bottlenose dolphin - Tursiops erebennus (Cope, 1865) - are the other two recognised species of the genus Tursiops, distributed in the coastal areas of the Indo-Pacific region and the coastal areas of the eastern United States, respectively.

The common bottlenose dolphin can be found in a variety of habitats, ranging from relatively shallow estuarine systems to deep oceanic waters5. Based on ecological habits and genetic profiles, several authors have identified a pelagic ecotype, differentiated from coastal ecotypes6,7,8,9,10,11,12,13,14. Along the Atlantic coast of North America this separation appeared particularly pronounced ecologically, physiologically and genetically15,16,17,18,19,20,21,22, until the coastal ecotype was finally proposed as a species in its own right, namely the aforementioned Tursiops erebennus19.

In the Mediterranean Sea, the common bottlenose dolphin, Tursiops truncatus truncatus (Montagu, 1821), hereafter “bottlenose dolphin”, is considered a regularly present species and has been reliably reported in virtually all parts of the basin23,24. These dolphins are often regarded as predominantly ‘coastal’ or ‘inshore’, as they are preferentially distributed within the limit of the continental shelf23,24,25,26,27,28,29,30, including the waters around offshore islands and archipelagos24.

According to some authors, the Mediterranean bottlenose dolphin population should be described as a metapopulation, consisting of resident sub-units (often referred to as ‘geographical units’) scattered along the continental shelf and with a certain degree of mutual isolation31,32. The connectivity between these sub-units would seem to retrace the continuity (and discontinuity) of the habitat, possibly as a consequence of local specialisations. This pattern of distribution and organization into sub-units seems fairly consistent, on a smaller scale, with that observed in the Atlantic Ocean (see above) with regard to the coastal ecotype(s) and could possibly reflect a species-specific characteristic of the bottlenose dolphin. Conversely, none of these studies reports the presence of a pelagic ecotype, leaving in parapatry with the coastal one(s), in the context of the Mediterranean Sea, as reported in the Atlantic.

Genetic studies, however, suggest the presence of two bottlenose dolphin ecotypes also in the Mediterranean Sea. Gaspari et al.33 found that samples collected from the eastern Mediterranean, particularly those stranded along the Ionian Sea, had haplotypes similar to those of the North Atlantic pelagic ecotype, suggesting a potential presence of the pelagic ecotype also in the Mediterranean basin. However, as most of the samples came from stranded animals, it was unclear whether those individuals actually had a preference for pelagic habitats. According to Gaspari et al.33 , “microsatellite data suggest that gene flow is stronger from the Ionian pelagic basin to the other coastal basins than in the opposite direction, and also low between different coastal basins. This implies that gene flow between shallow water basins (e.g. Tyrrhenian, Adriatic and Aegean) might be mediated by the pelagic ecotype and explains how coastal populations exhibiting strong site fidelity34,35,36 can still be genetically similar (e.g. Tyrrhenian and Adriatic seas)”.

The term “ecotype” was first proposed by Turesson in 192237,38 to describe “an ecological unit… arising as a result of the genotypical response of an ecospecies to a particular habitat”. The original definition by Turesson was taken up by Gregor39 and refined based on reproductive isolation. According to the latter, an ecotype is a “population distinguished by morphological and physiological characters, most frequently of a quantitative nature; interfertile with other ecotypes and ecospecies, but prevented from exchanging genes by ecological barriers”. Since then, the original definition of ecotype has evolved over time, as new techniques for genetical analysis were made available.

For an ecotype to rise from the original population, at least two conditions are needed: (a) the individuals must inhabit different ecological contexts; (b) the individuals must have at least a certain level of (reproductive) isolation from the individuals of the original population. The two conditions, acting together, will potentially produce a non-random distribution of ecologically adaptive variations across the population, manifested through recognisable phenotypic, physiological, and behavioural characteristics40.

Following the above-described concepts, we investigated the presence of distinct ecotypes (pelagic vs. coastal) in the Mediterranean bottlenose dolphin by analysing a large dataset of sightings, covering a substantial portion of the basin. Our analyses, based on the ecological and behavioural habits of dolphins, were aimed at verifying the following preconditions for the genesis of ecotypes: (a) distinct individuals must live in different ecological contexts (specifically, waters within or outside the continental shelf); and (b) individuals inhabiting the two different ecological contexts must have a certain level of isolation. Precondition (a) was investigated through a distribution analysis of sightings and sighting rates in the different bathymetric contexts, while precondition (b) was investigated through social network analyses.

We pooled the data collected by 43 research groups that have been operating in the Mediterranean Sea over a period of 15 years (i.e. from 2004 to 2019). Of these, 40 datasets come from research groups that have uploaded their data on Intercet, a web-based GIS platform for the study of cetaceans and sea turtles in the Mediterranean Sea (https://www.intercet.it, last access 2 August 2023), while three datasets were included at a later stage.

The study areas investigated by the different research groups (see Table 1) included: the Alboran Sea; the Catalan coast; the Gulf of Lion and the Ligurian Sea; the Tyrrhenian Sea; the northern coast of Tunisia, the waters around Sicily and the Sicily Strait; the northern Ionian Sea, both along the Italian and Greek coasts; the Adriatic Sea; the Aegean Sea, along the coast of Türkiye; the Turkish Straits System, consisting of the Marmara Sea, Istanbul, and Çanakkale Straits; the easternmost portion of the Levantine Basin, off the coast of Türkiye, Lebanon, Syria and Israel (Fig. 1).

Table 1 Research groups involved in the study, their study area, collection of photo-identification data (YES/NO), sampling effort (km), number of sightings, and ER (n sightings/km*100) per bathymetric range (0–200 m, 200–500 m, 500–1000 m, 1000–2000 m, > 2000 m). The codes in the first column represent the following research groups: AC (Associació Cetacèa), AdL (Accademia del Leviatano), AR (Archipelagos Institute of Marine Conservation), ARC (Alnilam Research and Conservation), AT (ARPAT - Agenzia Regionale per la Protezione dell’Ambiente Toscano), BB (OEC - Office de l’Environnement de la Corse), BDR (Bottlenose Dolphin Research Institute - BDRI), BR (Association BREACH Méditerranée), CA (Association CARI), CC (CE.TU.S. Cetacean Research Center), CES (CESRAM. - Centro Studi e Ambiente Marino), CG (CNR-IAS), CIM (CIMA Research Foundation), CRC (Centro Ricerca Cetacei), DDP (Delfini del Ponente APS), CDM, DM (Acquario di Genova), EDM (EDMAKTUB Association), EOI (EcoOcéan Institute), GAIA (Gaia Research Institute Onlus), GC (GECEM - Groupe d’Etude des Cétacés de Méditerranée), GP (Golfo Paradiso Whale Watching), IDP (Tethys Research Institute - Ionian Dolphin Project), IM (Morris Kahn Research Station, University of Haifa), JDC (Jonian Dolphin Conservation), KT (Ketos), LA (DMAD - Marine Mammals Research Association), LBA (University of Pisa), LSR (Sapienza University of Rome), MEO (MarEco Osservatorio della Natura), MKB (Menkab), MKI (Morris Kahn Research Station, University of Haifa), MOR (Morigenos – Slovenian Marine Mammal Society), MRS (MeRiS - Mediterraneo Ricerca e Sviluppo APS), MT, MTL (MareTerra - Environmental Research and Conservation), ODO (Oceanomare Delphis Onlus), PE (Pelagos - Provincia di Livorno), SAM (APS Sotto al Mare), SBN (SUBMON), TCR (TCR - Turkish Cetacean Research), TDP (Tunisian Dolphin Project), THA (Thalassa Ricerca e Formazione), TRI (Tethys Research Institute - Cetacean Sanctuary Research), UB (University of Barcelona), UGE (DISTAV - University of Genoa), UNITO (University of Torino - DBIOS). *BDR data were not included in the ER analysis as they were clearly anomalous due to the peculiar sampling context (dolphins were regularly sighted feeding near fish farms). **The total number of photo-identified individuals (4,919) does not correspond to the sum of the individuals photo-identified by the single research group, as the same individual can be identified by several groups.
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Fig. 1
figure 1

The Mediterranean basin, in which the study areas investigated by the different research groups are located. The five bathymetric ranges (0–200 m, 200–500 m, 500–1000 m, 1000–2000 m, > 2000 m) used in the analyses are shown. The figure was created using ArcGIS Desktop 10.5 (http://www.esri.com) by one of the authors (Michela Bellingeri).

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Results

The 43 research groups involved in the study covered a total of 957,749 km of survey effort from 2004 to 2019 (Table 1). The effort distribution was not even across the whole Mediterranean Sea, as it was concentrated mainly in the western basin and in the northern portion of the eastern basin (Fig. 2). Data from the southern portion of the Mediterranean were scarce in the western basin and nearly absent in the eastern basin. The survey effort resulted in 6,726 bottlenose dolphin sightings, with an additional 1,663 sightings obtained during surveys with no effort information provided, resulting in a total of 8,389 sightings. Of these, 5,131 sightings (61%) included photographic data for individual identification (Fig. 3).

Fig. 2
figure 2

The survey effort produced by the research partners (2004–2019, 929,472 km in total). In orange are the tracks of partners whose protocol included the collection of photo-identification data, in blue are the tracks of the partners whose protocol did not include photo-identification data. The figure was created using ArcGIS Desktop 10.5 (http://www.esri.com) by one of the authors (Michela Bellingeri).

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Fig. 3
figure 3

Distribution of bottlenose dolphin sightings (n = 8,389): sightings with photo-identification data (n = 5,131) are shown in orange, sightings without photo-identification data (n = 3,258) are shown in blue. The figure was created using ArcGIS Desktop 10.5 (http://www.esri.com) by one of the authors (Michela Bellingeri).

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Encounter rate

The encounter rates (ER = sightings/km*100), calculated for each set of data provided by the partners involved in the study (where sampling effort was available) in the five bathymetric ranges (0–200 m, 200–500 m, 500–1000 m, 1000–2000 m, > 2000 m), are displayed in Table 1, while the total ER, over the same bathymetric ranges, is shown in Table 2 and Fig. 4. The overall pattern shows a higher ER in the continental shelf domain (0–200 m), with few exceptions, e.g. in the Alboran Sea (data from ARC in Table 1), western Corsica (data from GC in Table 1) and in some area of the Adriatic and Ionian Seas (data from GAIA in Table 1). The ER of the aggregated data was about five times higher over the continental shelf (0–200 m) than in the neighbouring bathymetric range (200–500 m) and halved in each subsequent bathymetric range (Table 2; Fig. 4). According to the Chi-square test performed, differences in ER between bathymetric ranges were highly significant in all possible combinations (χ > 6.635; p < 0.01), except for the two deepest ranges (1000–2000 m and > 2000 m), between which no significant difference was found (χ < 3.841; p > 0.05).

Table 2 Sampling effort (km), number of sightings and encounter rate (ER = sightings/km*100) of bottlenose dolphin and striped dolphin in the different bathymetric ranges (m). The same data were used to elaborate the graph in Fig. 4.
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Fig. 4
figure 4

Bottlenose dolphin and striped dolphin encounter rate (ER = sightings/km*100) in the different bathymetric ranges (Tt: Tursiops truncatus; Sc: Stenella coeruleoalba). The bars represent the ER, while the dots represent the effort in km. The original values are shown in Table 2.

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In the striped dolphin (Stenella coeruleoalba), which we used as a “control species” to test the effectiveness of the sampling effort in the different bathymetric ranges (see Methods), the ER has an opposite pattern (Table 2; Fig. 4), reaching its maximum value in the 1000–2000 m range (differences in the striped dolphin ER were highly significant in all possible combinations: χ > 6.635; p < 0.01).

The ER map (Fig. 5) shows a general pattern that is consistent with these results: the bottlenose dolphin is present in almost all the continental shelf waters sampled, as well as over sea shoals and around the islands, and in most of the areas investigated the probability of encountering this species (measured as ER) appears to decrease abruptly beyond the 200 m isobath.

Fig. 5
figure 5

Bottlenose dolphin encounter rates (ER = sightings/km*100). Each sampling cell (20 × 20 km) is coloured according to the corresponding ER value (see the legend). The bathymetric line represents the 200 m isobath. The figure was created using ArcGIS Desktop 10.5 (http://www.esri.com) by one of the authors (Michela Bellingeri).

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Network analysis

As mentioned above, over 8,389 sightings, 5,131 (61%) included photo-identification data. Through the analysis of the photographic data, 4,919 individuals were identified. Of these, 1,350 were sighted at least four times and therefore included in the network analysis (see Methods). The network diagram shows the network divided into 18 clusters (Fig. 6), with each cluster identified by a different colour.

Fig. 6
figure 6

Network diagram of photo-identified bottlenose dolphins with at least four photographic captures (1,350 individuals) using the Spring embedding lay out. The colours identify the different clusters according to a Girvan-Newman analysis based on the betweenness of the bonds.

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In Fig. 7, the sighting positions of the individuals included in the network analysis are displayed on map, using the same cluster-colours. Each colour appears to identify a different geographical unit, with its own area of residence: (1) Alboran Sea; (2) Gulf of Lion (France); (3) Liguria-Tuscany (Italy); (4) Tuscany archipelago (Italy); (5) Corsica S (France)-Sardinia NE (France); (6) Corsica W; (7) Lazio-Campania (Italy); (8) Sardinia NW (Italy); (9) Lampedusa (Italy); (10) Sicily SW (Italy); 11) Istanbul Strait (Türkiye); 12) Gulf of Ambracia (Greece); 13) Antalya (Türkiye); 14) Gulf of Catania (Italy); 15) Northern Adriatic Sea; 16) Cap de Creus (Spain); 17) Bizerte (Tunisia); 18) Israel. The names of the distribution area have been added “a posteriori” as labels to the network shown in Fig. 6, for a better understanding of the diagram. The maximum distance travelled by the same individual (a bottlenose dolphin moving from the Alboran Sea to the Gulf of Lion) is 1,026.20 km, while the average of the maximum distance travelled is 89.92 km (n = 1,350; median = 45.22 km; standard deviation = 108.60 km; standard error = 0.28 km).

Fig. 7
figure 7

Sighting positions of bottlenose dolphins included in the network analysis (Fig. 6). The colours indicate the previously identified clusters (Fig. 6). The bathymetric line in light grey represents the 200 m isobath. The figure was created using ArcGIS Desktop 10.5 (http://www.esri.com) by one of the authors (Michela Bellingeri).

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The network analysis (Fig. 6) was then recoloured according to the sighting position of the same individuals in relation to the continental shelf boundary (200 m isobath). Different colours were used to identify three categories of dolphins (Fig. 8): blue for individuals sighted only within the 200 m isobath, green for those sighted both within and outside the 200 m isobath, and yellow for dolphins observed only outside the 200 m isobath. Most individuals within the network (1,040 out of 1,350), are coloured blue (≈ 77%), 303 are coloured green (≈ 22%) and only four individuals are coloured yellow (≈ 0.3%). However, in cluster 1 (corresponding to the area of the Alboran Sea) almost all individuals (89 out of 93) are coloured green, while four individuals are coloured yellow, whereas in cluster 6 (corresponding to the area of western Corsica) most individuals are coloured green (54 out of 66).

Fig. 8
figure 8

Spring embedding visualization showing the 18 main clusters identified (see also Figs. 6 and 7). Each aggregation of points indicates a different cluster of individuals and the respective area of distribution (1,350 individuals in total). Blue points represent the individuals only observed within the 200 m isobath marking the boundary of the continental shelf (1,040), green points represent the individuals observed both inside and outside the shelf (303), yellow points identify the individuals observed only outside the shelf (4).

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As the results obtained for the Alboran Sea and western Corsica are peculiar compared to the rest of the Mediterranean, we re-performed the previous analyses focusing on these two clusters.

In both the Alboran Sea and western Corsica, the ER reaches a maximum in the second depth range (200–500 m) and decreases in subsequent ranges (Fig. 9). This pattern looks very different from that observed using aggregated data (Fig. 4). In Corsica the ER in the first bathymetric range (0–200 m) was significantly higher (χ > 6.635; p < 0.01) than in the fourth (1000–2000 m) and fifth (> 2000 m), while the ER in the second bathymetric range (200–500 m) was significantly higher (χ > 6.635; p < 0.01) than in the third (500–1000 m), fourth (1000–2000 m) and fifth (> 2000 m); in all other possible combinations, no significant differences were found (χ < 3.841; p > 0.05). In the Alboran Sea, the ER in the first (0–200 m) and second (200–5000 m) bathymetric ranges was significantly higher (χ > 6.635; p < 0.01) than in the third (500–1000 m) and fourth (1000–2000 m) (the bathymetric range > 2000 m was excluded from the analysis as the sampling effort was very low); in all other possible combinations, no significant differences were found ((χ < 3.841; p > 0.05).

Fig. 9
figure 9

Encounter rates (ER = sightings/km*100) recorded in the Alboran Sea (AS) and western Corsica (WC) in the different bathymetric ranges (m). The bars represent the ER while the points represent the sampling effort (km).

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The ER analyses suggest that the bottlenose dolphins living in the two areas have a similar behaviour in relation to the bathymetric profile, but if we consider the distance from the continental shelf boundary (200 m isobath), together with the water depth, results are rather different (Fig. 10): in western Corsica, bottlenose dolphins only move a few kilometres from the shelf, where, due to the steep slope, they immediately find deep waters; in the Alboran Sea, on the other hand, dolphins move up to 22 km beyond the shelf limit.

Fig. 10
figure 10

Distribution of bottlenose dolphin sightings in relation to water depth and distance from the continental shelf (200 m isobath) in the Alboran Sea and western Corsica (the distance from the 200 m isobath to the inshore is considered with a negative sign).

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We finally reperformed the social network analysis to identify possible sub-clusters that could suggest the presence of different ecotypes living in parapatry. Cluster 1 (Alboran Sea) appeared homogeneous in the network analysis (Q < 0.3; Fig. 11), while two main sub-clusters were identified in cluster 6 (western Corsica) (Q = 0.33; Fig. 12A). Based on spatial distribution of these sub-clusters (Fig. 12B), it appears that they correspond to two partially overlapping geographical sub-units, both located on the continental shelf: one to the north, the other further south (Fig. 12B).

Fig. 11
figure 11

Network analysis of cluster 1 (Alboran Sea). The Girvan-Newman analysis does not identify any sub-clusters (Q < 0.3).

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Fig. 12
figure 12

Network analysis of cluster 6 (western Corsica), with two main sub-clusters identified (Q = 0.33) by colours blue and red (A). Distribution map of the same sub-clusters (B). The 200 m and 2000 m isobaths are highlighted.

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Discussion and conclusions

Our work, based on a large, aggregated dataset, including both survey and photo-identification effort, constitutes the most extensive study ever developed on the Mediterranean population of bottlenose dolphin, allowing us to investigate the ecology of this species over a wide portion of the basin. In order to investigate the potential presence of a pelagic vs. coastal ecotype, we looked for the coexistence of at least two conditions: (1) the co-presence of bottlenose dolphins in both shelf (0–200 m) and pelagic (> 200 m) waters; (2) a certain degree of isolation between individuals distributed in the two bathymetric domains.

The results showed a significantly higher presence of bottlenose dolphins in the waters of the continental shelf (Tables 1 and 2; Figs. 4 and 5), with the ER dropping abruptly beyond the 200 m isobath. Conversely, the ER of the striped dolphin (the control species) is minimum in the continental shelf range and increases in offshore waters, peaking in the deepest portion of the slope (1000–2000 m). It should be noted that, although the total effort in offshore waters > 200 m (508,432 km) was slightly greater than that produced on the continental shelf (449,317 km), the effort was not equally distributed in the western and eastern basins and in the latter the offshore effort was rather low (see Fig. 2).

However, bottlenose dolphins were not absent from pelagic waters and in the Alboran Sea and western Corsica the ER was even higher in the second bathymetric range (200–500 m), although the difference was not statistically significant. In the Adriatic and Ionian Seas, the ER reaches its maximum level in the 500–1000 m range, possibly due to the particular bathymetric profile of the seabed, characterised by a steep slope and deep waters very close to the coast, as indicated by the fact that most sightings have been recorded in the coastal waters bordering the continental shelf, while sightings in offshore waters were rare (see Table 1; Figs. 2 and 3).

These results would seem to fulfil at least the first condition for the formation of two ecotypes, namely the presence of individuals distributed in both shelf and offshore waters. However, the network analysis, integrated with the distribution of dolphins in relation to the continental shelf border (Fig. 8), does not suggest the presence of purely pelagic individuals in any of the examined areas. Indeed, only four individuals out of 1,350 (all sighted in the Alboran Sea) were sighted exclusively in pelagic waters (> 200 m); all others were sighted only in shelf waters (1,040 individuals) or in both domains (303 individuals). This result may be partly due to the fact that in some regions, such as the northern Adriatic Sea or the Gulf of Ambracia (Greece), the sampling areas lie entirely on the continental shelf. However, the prevalence of ‘blue dolphins’ (those sighted only within the 200 m isobath, see Fig. 8) can be observed in all sampled areas (with the aforementioned exceptions of the Alboran Sea and western Corsica) and this is consistent with the ER analysis (Figs. 4 and 5). Furthermore, ‘blue dolphins’ and ‘green dolphins’ (those sighted both within and beyond the 200 m isobath) appear to be connected in the network, with no clear separation (except in the Alboran Sea, where all individuals are green or yellow, see Fig. 8).

In-depth analyses of the data from the Alboran Sea show that bottlenose dolphins living in this area (93 individuals considered for the network analysis) are distributed in both shelf and pelagic waters but are grouped in a single homogeneous cluster, with no sub-clusters within it (Fig. 11). On the contrary, the dolphins sighted around Corsica (66 individuals considered for the network analysis), are divided into two distinct sub-clusters, which however are both resident over the continental shelf (Fig. 12) and show limited movements beyond the 200 m isobath in terms of distance (Fig. 10). The encroachment of these dolphins into deep waters is most likely due to the conformation of the shelf itself, which has an indented edge and deep waters close to its boundary (see Figs. 10 and 12).

Overall, our results, do not support the hypothesis of the presence of two ecotypes (pelagic vs. coastal), living in parapatry, in any of the areas analysed. As a general pattern, Mediterranean bottlenose dolphins seem to inhabit the shelf macro-habitat, where they cluster in distinct geographical units, tracing habitat features, in agreement with other smaller-scale studies31,32. This pattern may partly be driven by behavioural specialisation, running through generations like a cultural tradition, which would allow bottlenose dolphins to better exploit available local resources41,42. This specialization may also include opportunistic exploitation of fishing gears, such as gill nets and trawls, and aquaculture cages28,43,44,45,46,47,48,49, resulting in a kind of “cultural evolution” driven by exposure to anthropogenic activities.

The specialisation on the habitat of residence, together with the distribution limited to the continental shelf, seem to determine a certain degree of isolation between different geographical units. It would therefore seem reasonable to ask whether these phenomena (isolation and specialisation), acting together, may favour the development of distinct ‘coastal’ ecotypes even on a Mediterranean scale. However, it is known that some individuals, sometimes referred to as ‘long-distance travellers’25,31,32, make much longer than average movements, travelling between different areas of residence. These observations seem to support the hypothesis that geographical units are not demographically closed, but rather close local units with sufficient genetic exchange across their borders to avoid the development of true ecotypes. Targeted genetic studies would be needed to clarify this issue. Bottlenose dolphins living in the Alboran Sea, however, seem to deviate from the general pattern, as their distribution ranges over both coastal and pelagic waters, as already pointed out by Canadas et al.50. This pattern of distribution could also be affected by the bathymetric profile of the Alboran Sea, characterised by a gentle slope and wide offshore shallow water shoals (such as the Alboran Ridge), which could attract the bottlenose dolphins in the open sea. However, the bottlenose dolphins inhabiting the Alboran Sea tend to form larger groups than those living in the inner Mediterranean32, a characteristic that makes them more similar to the Atlantic pelagic ecotype51,52,53, and their acoustic signals (whistles) would also be more similar to those emitted by Atlantic pelagic bottlenose dolphins54,55,56. These results suggest that bottlenose dolphins in the Alboran Sea may actually belong to a different ecotype. Again, specific genetic analyses would be needed to investigate this hypothesis.

Gaspari et al.33 found that bottlenose dolphins stranded along the coast of the Ionian Sea featured haplotypes similar to those found in the Atlantic pelagic ecotype. As pointed out by Gaspari et al.33, expansion into new habitats may contribute to the sharing of haplotypes across distant regions. Some of the genetic traits found by Gaspari and co-authors may be a heritage of the immigration from the Atlantic57, which would no longer coincide with the ecology of “modern” Mediterranean bottlenose dolphins. The Atlantic pelagic ecotype, which potentially colonized the Mediterranean Sea via the Strait of Gibraltar (together with the coastal Atlantic ecotype) in one or more events after the Messinian salinity crisis58,59, may have lost its original pelagic habits due to the ecological pressure of the newly occupied areas, while retaining original genetic traits.

A genetic study conducted by Moore60 on free ranging bottlenose dolphins sighted and photo-identified off the south-western coast of Sicily could support this hypothesis. According to the author, the genetic markers of these dolphins (eight individuals) were more similar to the Atlantic pelagic ecotype than to other Mediterranean individuals, despite the area occupied being within the continental shelf. According to Papale et al.61, the behavioural traits of these same dolphins, namely the fluid association pattern and low site fidelity, would also be more akin to the Atlantic pelagic ecotype.

The possibility that distinct ecotypes or even young species may “recast” as a consequence of changed ecological conditions has already been described in other aquatic species, e.g. in freshwater fishes62,63,64,65, suggesting that ecological barriers may be reversible40.

However, it is possible to formulate at least one alternative hypothesis. As already mentioned, the areas investigated, although quite extensive and showing consistent results, do not cover the entire Mediterranean Sea and in the eastern basin the sampling effort in pelagic waters was rather poor. Therefore, the possibility that a pelagic bottlenose dolphin ecotype may be found in some areas, not covered by our research network, cannot be ruled-out. In their study on bottlenose dolphins in the Northern Ionian Sea, Cipriano et al.66, identified some dolphins that seemed to prefer pelagic waters. These were only 8 individuals, in a gulf enclosed between two long peninsulas, but it is noteworthy that the area lies at the northern edge of the Ionian Sea, where the samples analysed by Gaspari et al.33 were collected. Analogous sightings of bottlenose dolphin in pelagic waters were also reported by Awbery et al.67 in the eastern Mediterranean Sea of Türkiye and, on a much larger scale, during the Aerial Survey Initiative (ASI) of ACCOBAMS (Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and Contiguous Atlantic Area)68. Unfortunately, due to inherent limitations of aerial surveys methodology, no photo-identification data were available, so it remains unknown whether these sightings represent pelagic ecotype animals or whether they are “coastal” individuals occasionally moving into pelagic waters. As already mentioned, although most bottlenose dolphins show strong site fidelity25,35,36,48,69,70,71,72 and the connectivity between various local geographical units is limited31,32,73 movements between them do occur. For example, Genov et al.74 documented a long-distance movement of an individual, which was initially resident in the southern Tyrrhenian Sea, subsequently photographed in the northern Adriatic Sea, and finally photographed in the western Ligurian Sea. Moreover, using genetic markers, Gaspari et al.75 reported connectivity between the Northern Adriatic Sea and the Gulf of Ambracia in western Greece: two animals sampled and photo-identified in the Gulf of Trieste were identified as having genetically pure Ambracian ancestry. Bottlenose dolphins may also be encouraged to move into pelagic waters to visit shoals and seamounts, which may provide feeding opportunities, as reported in the eastern Mediterranean of Türkiye, between the Finike-Anaximander Seamonths mountains and the coast76and in the Tyrrhenian Sea, on the Albano Mountain (Pace, personal observations29;).

To test the above-described hypotheses, the sampled area should be enlarged, including new study areas, and genetic analysis should be performed on a larger number of photo-identified individuals, especially in pelagic habitats. Further efforts in this direction are urgently needed, as a truly pelagic bottlenose dolphin ecotype, if present, may be relatively rare and thus more vulnerable to a potential decline that may go unnoticed due to the widespread presence of ‘coastal’ bottlenose dolphins.

Methods

The data used for the present study include: (i) sampling tracks in the respective study area; (ii) sighting position (latitude and longitude) of the target species (T. truncatus); (iii) photographic data for individual identification of the sighted dolphins (when available). Data were collected using different research platforms (rigid-hull inflatable boats, sailboats and motorsailers, whale watching boats, and ferries) and the sampling effort was produced mainly from May to September, in favourable weather conditions (sea state < 4 on the Douglas scale), following both predetermined line transects and non-systematic tracks.

The sampling tracks and the sighting positions were mapped using the software ArcGIS Desktop 10.5 (ESRI). Subsequently, the common dataset was used to perform the following analyses (see below for details): encounter rate analysis for different bathymetric ranges, photoidentification analysis, network analysis and cluster identification, mapping of geographical units and analysis of sighting positions in relation to the continental shelf boundary.

Encounter rate

The encounter rate (ER), calculated as the number of sightings over the sampling effort in kilometres (ER = sightings/km*100), was used to measure the sighting frequency of the target species in standard weather conditions (sea state < 4 on Douglas scale). For each set of data, the ER was first calculated in the main bathymetric domains: the continental shelf (0–200 m); the continental slope, which was subdivided into three intervals (200–500 m, 500–1000 m, 1000–2000 m); the pelagic waters > 2000 m deep.

To test the effectiveness of the sampling effort in the different bathymetric ranges, we used the striped dolphin as a ‘control species’, pairing the ER of this species with that of the bottlenose dolphin, as in25. Unlike the bottlenose dolphin, the striped dolphin is in fact known to have predominantly pelagic habits, and its distribution appears complementary to that of the bottlenose dolphin25,30. Subsequently, the ER was mapped using a grid of 20 × 20 km cells, as in77. To mitigate possible ER bias due to low effort, only the cells with a sampling effort greater than the diagonal of the same cells (≥ 28 km) were selected, as in30. When considering cells intersected by the continental shelf boundary (200 m isobath), only the portion of the cell falling within the relevant depth range was considered in the calculation of the ER. The diagonal for the cut-off, in this case, was calculated using the diagonal of the square with the same area as the portion of the cell of interest. We used a contingency table to compare the ER in each bathymetric range, calculated the respective chi-square value and applied the Bonferroni correction to determine the correct significance level.

Analysis of photo-identification data and matching process

The Intercet platform requires that the photographic data uploaded to the common database meet predefined standards, based on both the quality of the image and the distinctiveness of the individual. Photographic quality is based on focus, lighting, angle of the fin in relation to the photographer, contrast between dorsal fin and background, and the visibility of the fin78,79. The distinctiveness is based on the presence of notches, deformities, unusual fin shapes, scars and discoloration79. Notches, deformities, and unusual fin shapes are considered permanent marks80 and are used as primary distinctive elements for the photo-identification of individuals, while scars and discolorations are used in association with primary distinctive elements25.

Each Intercet partner developed its own photo-identification catalogue independently, but the quality of the photographic data was validated by the Intercet platform operator (Fondazione Acquario di Genova), which verified compliance with the common standards. Finally, the same Intercet operator performed a cross-matching to find possible correspondences between catalogues. This last process was carried out with the support of finFindR (https://github.com/haimeh/finFindR), a software for computer-assisted identification of dolphins through natural markings on the dorsal fins81. The semi-automated matching process was finally validated by an experienced researcher and, in case of doubt, a second person was asked to confirm the match.

Network analysis

Based on photo-identification data and matching results, a network analysis was performed in SOCPROG 2.982, selecting all individuals photographically captured at least four times, as in31. Two dolphins were assumed to be associated if they were sighted in the same group at least once. The clusters within the network were identified through a Girvan-Newman analysis83,84,85,86, based on edge betweenness measurements. The significance of the clustering found was weighted through the modularity index Q, which varies between 0 (community structure no better than in that of a random network) and 1 (strong community structure) and can be considered meaningful if it falls in the range 0.3–0.787. The network structure was then visualized using the stochastic Spring embedding algorithm88,89 in NetDraw90.

Mapping the geographical units

Once the clusters were identified, the sighting positions of the individuals selected for the network analysis were mapped, using the same cluster-colours to identify the distribution area of the individuals belonging to the different clusters. For each dolphin selected for the network analysis, we also calculated the maximum distance between sighting points and performed basic statistics.

To analyse the spatial behaviour of the dolphins in relation to the bathymetric domains, the Spring embedding layout, showing the network structure, was then recoloured according to the sighting points of the same individuals with respect to the continental shelf (200 m isobath): blue for the dolphins sighted only within the continental shelf, green for those sighted both within and outside the shelf, and yellow for the ones sighted only outside the shelf.

Based on the results, as the geographical units of the Alboran Sea and western Corsica showed a different pattern (see the Results section), we re-performed the ER analysis per bathymetric domain, focusing on these two units and supplementing it with a scatter plot analysis of the distance from the 200 m isobath vs. the water depth. Finally, we performed a new network analysis of the dolphins belonging to the same clusters/geographical units (Alboran Sea and western Corsica).