1. Introduction
The common bottlenose dolphin—
Tursiops truncatus Montagu, 1821—hereafter referred to as bottlenose dolphin, is a cosmopolitan Delphinidae that can be found worldwide in all the oceans. Its distribution is usually confined between 45\1\2 parallels, in tropical and temperate waters of both hemispheres, except for the North Atlantic, where it can reach the 65\1\2 parallel [
1,
2,
3,
4,
5].
This wide distribution of the species is associated with a certain level of morphometric differentiation among the different areas and latitudes, which seems consistent with the presence of a globally distributed offshore ecotype and several inshore ecotypes, scattered along the coasts [
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20].
The inshore bottlenose dolphin communities show a high level of site fidelity, with home range sizes that can vary from tens of km
2 [
21] to hundreds of km
2 [
22]. According to a global synthesis provided by Wells and Scott [
3], the home range of the inshore bottlenose dolphins generally includes areas of about 100–150 km
2, even if wider home ranges have been reported for local geographical units [
23]. It has been hypothesized that the home range sizes and movement patterns of this species depend on habitat differences [
24], food resources [
25] and reproductive resources [
26].
In the context of the Mediterranean Sea, the bottlenose dolphin is considered a regularly present species and the second most sighted cetacean after the striped dolphin,
Stenella coeruleoalba [
27,
28,
29,
30,
31].
According to Notarbartolo di Sciara and Demma [
29], the Mediterranean bottlenose dolphin would be more closely related to the Atlantic inshore ecotype, while Cañadas et al. [
32,
33], who studied this species in the context of the Alboran Sea, suggested a closer link with the offshore Atlantic ecotype. This apparent contradiction could in fact derive from the different ecological habits of the dolphins that inhabit the Alboran Sea (as also emerges from the present study). Gaspari et al. [
34] found that samples from deeper parts of the Mediterranean (e.g., Ionian Sea) were genetically more similar to samples from the Atlantic offshore ecotype and suggested a potential differentiation between offshore and inshore ecotypes also in the Mediterranean context. However, the results from TursioMed, a networking project compiling census data at the Mediterranean level and involving 29 institutes from eight Mediterranean countries, covering 13 years of survey effort from 2004 to 2016 [
35], showed that most sightings of the species occurred within the 200 m isobath marking the border of the continental shelf, while sightings outside this limit have been reported to be quite rare despite the sampling effort. This pattern of distribution seems quite consistent in all sampled study areas, with the already mentioned exception of the Alboran Sea, possibly confirming the inshore habit of the Mediterranean bottlenose dolphin, as originally stated by Notarbartolo Sciara and Demma [
29]. Therefore, it is unclear to what extent the genetic differences found by Gaspari and co-authors, who examined mainly stranded animals, should be correlated to ecological habits (see also [
14]).
According to Moura et al. [
36], the Mediterranean and Black Sea (inshore) populations derive from a globally distributed offshore population, which would explain the presence of an “offshore” genetic inheritance in Mediterranean bottlenose dolphins, regardless of their current ecological habits.
Most studies based on photo identification data are consistent in describing the Mediterranean bottlenose dolphin as a primarily resident species. According to Gnone et al. [
37], who studied the movement patterns of this species in the Pelagos Sanctuary (a specially protected area in the northwest of the Mediterranean basin, see below), the bottlenose dolphins move around their usual area of residence within a maximum distance of 50 kms. The resident behaviour and a local specialization, especially in feeding techniques, seem to produce a segregation between neighbouring dolphins and a clustering of the (meta) population in geographical and demographical units [
37]. According to Carnabuci et al. [
38], the connectivity (or disconnectivity) through the bottlenose dolphin units living in the Pelagos Sanctuary retraces the landscape features and its habitat breakages (see also [
39]). The dolphins living in the Tuscany Archipelagos, for example, and those inhabiting the waters off the northwest coast of Corsica, are separated by a remarkable habitat breakage, delimited by the Corsica’s ‘‘finger’’ (Cape Corso). The east side (Tuscany Archipelagos) is characterized by shallow waters and sandy/muddy ecosystems, while the west side (northwest coast of Corsica) presents a typical narrow rocky platform and a steep slope. The “ecological distance” seems to overcome the geographical distance, producing a clear separation of the two units despite the geographical proximity.
The specialization on the residence habitat and the isolation between geographical units could be responsible for the genetic structure described by Natoli et al. [
40] and, on a finer scale, by Gaspari et al. [
34] and Brotons et al. [
41].
As bottlenose dolphins form resident units scattered over the continental shelf, it is crucial to understand how the physiographic characteristics of the area of residence, which can be very different even in neighbouring areas, can affect the geographical units, shaping their structure.
For this purpose, we analysed six geographical units of bottlenose dolphins living in different areas of the Mediterranean Sea, comparing their home ranges (both as individuals and as a whole unit), their aggregation behaviour (group size), their abundance and density.
4. Discussion and Conclusions
The analyses performed in this research confirm previous findings on the distribution of the bottlenose dolphin in the context of the Mediterranean Sea: the dolphins are clustered over the continental shelf in discrete geographical units, in agreement with Gnone et al. [
37] and Carnabuci et al. [
38].
The physiographic characteristics of the area of residence, and specifically the size and shape of the continental shelf, seem to affect the home range of the dolphins and, consequently, the size and structure of the geographical units.
Home range and group size
The dolphins living in the Gulf of Lion, an area characterised by a wide continental shelf, are observed to have a much wider individual home range (calculated as the average MCP) compared to the units living in areas where the continental platform is very narrow, like the west coast of Corsica (see
Table 4,
Figure 9). This phenomenon can in part be considered a simple “geometric” consequence of the shape and extension of the continental shelf, as it might be difficult (or impossible) for the dolphins to get a wide home range in a geographical context where the habitat boundaries are very narrow (such as the western coast of Corsica or the Gulf of Ambracia).
A direct consequence of this physical constraint seems to be the average size of the groups, as the dolphins living in those areas characterised by a narrow platform, such as the west coast of Corsica, not only show a smaller MCP, but they are also sighted in smaller groups (see
Table 6,
Figure 11,
Figure 12 and
Figure 13). This seems quite reasonable, since a bigger group needs to exploit a bigger territory to survive [
82] and home range size and group size tend to be correlated [
83,
84].
The average group size is small (4.9 ± 0.12 SE) also in cluster E, which inhabits a stretch of coast (Corsica S–Sardinia NE) where the continental shelf is quite large, if compared, for example, with the west of Corsica (see
Table 4). However, this is not surprising considering the local sampling context. As already mentioned, the dolphins were mainly sighted (80% of all sightings) while feeding around the aquaculture fish cages of “Golfo Aranci”, a peculiar context in which the dolphins are known to form small groups. Diaz Lopez [
85], who studied the bottlenose dolphins of “Golfo Aranci” during their foraging activity around the fish cages, reported an average group size of 3.5 (3.51 ± 0.1 SE). The few other sightings were recorded in the inshore waters between the islands of the Maddalena archipelago and the Strait of Bonifacio. Virtually no effort was carried out in the open waters of the continental platform, so the data available for this unit are representative of both a limited geographical area and a peculiar behavioural context (the opportunistic feeding around the aquaculture fish cages). These results support the idea that bottlenose dolphins could “fuse” into smaller groups when they enter inshore areas and/or engage in specific feeding activities, according to the “fission-fusion” mechanism described by Connor and co-authors [
86].
A correlation between the group size and the “openness” of the habitat has already been reported in cetaceans and is usually considered a social strategy against predators, assuming that a larger group can better face the predation risk [
87]. Our results suggest that other variables should also be taken into consideration.
In the Alboran Sea, the bottlenose dolphins form larger groups (composed by 36.2 individuals on average), but they have relatively small MCPs, according to the analysis implemented with the available data. The average MCP could have been underestimated if the research effort had been invested in an area too small to cover the movements of the dolphins in their complete extension. However, in the geographical unit of the Alboran Sea, the overlap index (see
Table 5) is relatively low (0.9), meaning that the total area sampled is much wider than the average individual MCP, a result that should reassure on the reliability of the results obtained.
In this regard, it should be noted that the bottlenose dolphins living in the Alboran Sea present quite different characteristics compared to the dolphins of the other geographical units analysed. Their distribution is not confined within the continental shelf and, according to Cañadas et al. [
32], these dolphins should be considered more closely related to the Atlantic offshore ecotype, which would also account for the larger size of the groups [
24,
26,
88]. The results are not conclusive, since the distribution of the dolphins in this area could also be affected by the peculiar 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, if it were proven that the dolphins inhabiting the Alboran Sea belong to a different (offshore) ecotype, we could assume that these dolphins are able to exploit food resources in deeper bathymetric habitats, where larger groups may be more successful in their feeding activity [
87,
89].
It should be mentioned that the Alboran Sea presents peculiar oceanographic characteristics if compared to the inside Mediterranean. The primary production is usually much higher, thanks to the inflow of Atlantic waters through the Strait of Gibraltar, which generates permanent anticyclonic gyres in the westernmost portion of the basin [
90]. As so, some authors believe the Alboran Sea should be considered more properly as a portion of the Atlantic Ocean (at least from an oceanographic point of view), while the Mediterranean border should be moved east to the so-called Almeria-Oran front [
90,
91]. Additionally, as regards the cetacean fauna, the Alboran Sea shows quite a unique picture compared to the rest of the Mediterranean basin; the diversity of species is higher in this area, and the short-beaked common dolphin (
Delphinus delphis), which is usually quite rare inside the Mediterranean, is the most sighted species in the shallow waters <400 m [
32,
33]. The presence of the common dolphin could possibly affect the behavioural ecology of the bottlenose dolphin in this sea area, shifting its preferential habitat offshore.
Size and density
In relation to the size of the geographical units identified, this seems to be correlated to the size of the area of occupancy (the complex multi-polygon of the cluster as a whole) which, in turn, is correlated to the size of the continental platform (
Figure 10,
Table 8). The bottlenose dolphin unit of the Gulf of Lion, for example, inhabiting a wide territory with no geographical or habitat breakages, was estimated to be larger than the others (see
Table 7). However, when considering the density of the dolphins in their respective areas of occupancy, we found significant differences. In this regard, it should be noted that the density was estimated considering the cluster multi-polygon as an approximation of the area of residence of the related geographical unit. If the effort implemented over space was not sufficient to cover the movements of the dolphins in their complete extension, the density could be overestimated, so we should use some caution when comparing different geographical units. However, we got the maximum density (0.372, see
Table 8) in the geographical units residing in the Gulf of Ambracia. Due to the peculiar physiography of this semi-closed gulf and the resident behaviour of the dolphins, this result should be considered quite reliable. In fact, the bottlenose dolphins living in the Gulf of Ambracia are considered a subpopulation, spending their entire life in the waters of the gulf [
54].
It seems as though the different aggregation in bigger or smaller groups, adopted by the dolphins in the different geographical contexts, could be functional to exploit the area of residence (with its habitat constraint) to get the maximum density according to the local resources. Given this general pattern, the productivity and food resources available in the different areas of residence can obviously contribute to the density of dolphins in the same areas, as already described in both marine and land mammals [
92,
93].
While the size and shape of the continental shelf are stable physiographic features that can be easily measured and used as predictive parameters, the productivity and exploitability of the area of residence might change over time, according to local ecological variables, including anthropic pressures (such as the fishing effort and levy), but also according to the capability of the resident bottlenose dolphins to exploit the resources.
Behavioural plasticity is a key feature of the bottlenose dolphin and is probably transmitted from one generation to the next one as a local cultural tradition, allowing the dolphins to improve their ability to exploit the residency area and to adapt to changes. As part of this plastic behaviour, bottlenose dolphins can learn to obtain food from trawlers, gillnets and aquaculture fish cages, and these opportunistic strategies can become an integral part of their feeding habits [
30,
54,
77,
94,
95,
96,
97,
98,
99,
100,
101,
102,
103,
104,
105,
106].
Bottlenose dolphins can take advantage of the human activities either directly, for example, taking the fish entangled in gillnets or farmed fish [
107,
108,
109], but also indirectly in the form of modified habitats that could be favourable for feeding (e.g., increase input of nutrients from aquaculture activities [
99,
110]). We know that human activities can have negative consequences on dolphin conservation (overfishing, bycatch, habitat degradation, noise pollution, etc.), but it can be difficult to measure the balance between negative and “positive” effects from the point of view of the bottlenose dolphins. Furthermore, the ability of bottlenose dolphins to exploit resources can change over space and time, depending on the human activities carried out locally and the behavioural adaptation of the resident dolphins, so it may be challenging to compare the productivity of the different areas in terms of food resources actually available to dolphins.
Conclusive remarks
The results obtained in this collective research effort highlight the importance of data sharing to better understand the ecology of cetaceans in different contexts, to grasp local peculiarities, but also to understand general patterns at a higher level.
The Mediterranean bottlenose dolphin population has been recently reassessed as Least Concern by the International Union for Conservation of Nature [
111], with the exception of the subpopulation inhabiting the Gulf of Ambracia, Greece, which has been assessed as Critically Endangered [
112].
The Mediterranean bottlenose dolphin population is also listed in the Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), in the Appendix II of the Washington Convention on International Trade in Endangered Species (CITES), in the Appendix II of the Convention on the Conservation of European Wildlife and Natural Habitats (Bern Convention), in Appendix II of the Protocol to the Barcelona Convention on Specially Protected Areas of Mediterranean Importance (SPAMI). Finally, the bottlenose dolphin is included in the Annexes II and IV of the EU Habitats Directive (Council Directive 92/43/EEC) as a species of “community interest whose conservation requires both designation of special areas and strict protection” [
113].
Our findings can help to better characterise the different geographical and management units that constitute the Mediterranean bottlenose dolphin metapopulation in relation to the physiographic characteristics of their area of residence. This can contribute to develop proper management and conservation measures for the bottlenose dolphin in the context of the Mediterranean Sea and beyond.
At the same time, further insights are needed to complete the picture. For example, we should consider that most of the data analysed were collected in daylight time and in the favourable conditions (i.e., the calm sea and long daylight time) given by the “good season” (late spring, summertime, early autumn). We cannot exclude that the behavioural patterns of the bottlenose dolphins, in relation to the continental platform, could change during nigh time or in winter. Further studies, possibly including other techniques such as passive acoustic, could help to monitor the behaviour of the dolphins in these other contexts.
Furthermore, the role of so-called “long travellers” needs to be better clarified. These dolphins, despite being a clear minority, could represent a means of continuity between the different geographical units, and their role in this regard could be more important than is understood.