Biol. Mar. Mediterr. (2015), 22 (1): 266-270
R. Carlucci, P. Ricci, S. Miccoli Sartori*, G. Cipriano*, A. Cosentino, A. Lionetti, C. Fanizza* Department of Biology, University of Bari, Via Orabona, 4 - 70125 Bari, Italy.
[email protected] *Jonian Dolphin Conservation, Viale Virgilio, 102 - 74121 Taranto, Italy.
CHANGES IN BEHAVIOUR AND GROUP SIZE OF STENELLA COERULEOALBA IN THE GULF OF TARANTO (NORTHERN IONIAN SEA, CENTRAL MEDITERRANEAN SEA) VARIAZIONI NEL COMPORTAMENTO E NELLA DIMENSIONE DEI GRUPPI DI STENELLA COERULEOALBA NEL GOLFO DI TARANTO (MAR IONIO NORD-OCCIDENTALE, MEDITERRANEO CENTRALE) Abstract - The main behaviours affecting the daily variations in the group size of Stenella coeruleoalba in the Gulf of Taranto (northern Ionian Sea, central Mediterranean Sea) were studied discussing its basic social organization in the area. The focal-group method was applied during 254 sightings of striped dolphin occurred in the area from 2009 to 2014. The number of specimens in group size significantly decreased from morning to early afternoon probably due to the shifting of behaviour from resting, with larger aggregation of dolphins groups, to feeding with dispersal of the groups on a larger area. Key-words: behaviour, striped dolphin, Gulf of Taranto, northern Ionian Sea, Mediterranean Sea.
Introduction - This contribution represents a first study on the behaviours of Stenella coeruleoalba (Meyen, 1883) in the Gulf of Taranto (northern Ionian Sea, central Mediterranean Sea). In this basin, the species is observed with a predictable distribution of sightings (Dimatteo et al., 2011; Fanizza et al., 2014), suggesting the presence of a resident population with a high site fidelity and different behavior patterns. In particular, the main behaviours characterizing the daily variations in the group size of the striped dolphin were studied discussing the basic social organization of the species in the area. Generally, the group size in dolphins, as well as in other marine or terrestrial mammals (Sutherland, 1996), was documented to be due to fission-fusion dynamics, where intra-population kin association acts as predominant in aggregating social groups. However, the fragmentation and habitats loss or the alterations in distribution and availability of resources could influence dolphins aggregation through the modification of their social interactions, mostly in coastal areas where marine mammals could be exposed to high levels of anthropogenic threats and no conservation measures were locally enforced, as unfortunately occurred in the investigated basin. Materials and methods - A boat-based focal-group method with instantaneous scan-sampling of the predominant activity (sensu Neumann, 2001) was applied monitoring the behaviour of S. coeruleoalba during 254 sightings carried out in the Gulf of Taranto from 2009 to 2014 (Fig. 1). In particular, observers adopted a responsible behaviour in order to prevent disturbance as well as collisions and possible injuries to the striped dolphins during sightings. In addition, the focal group was scanned with a lag of 3 minutes during session of 15 minutes, recording the predominant behavior simultaneously observed in more than the 50% of the group members (Mann, 2000). Four behaviours were identified according to Shane (1990): (i) the resting with dolphins staying close to the surface, emerging at regular
Changes in behaviour and group size of S. coeruleoalba in the Gulf of Taranto
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intervals and swimming very slowly; (ii) the traveling with dolphins swimming at sustained speed; (iii) the feeding indicating chases or captures of prey items close to the surface; (iv) the socializing with physical interactions among dolphins, ranging from chasing to body contact (except mothers and calves) or copulation (Neumann, 2001). The group size of the striped dolphin was recorded during 3 different time periods of the day (09:00-13:00, 13:01-16:00 and 16:01-18:00). The duration of the time periods was defined aiming to encompass intervals of the day during which a kind of homogeneity in local hydrographic conditions was observed. A cluster analysis was carried out on the log-transformed data using the Bray-Curtis similarity measure and the complete linkage method (software PRIMER 6.1) (Clarke and Gorley, 2006). The percentage of occurrence for each behaviour was calculated within and between clusters. In addition, the non-parametric Kruskal-Wallis test and a post hoc analysis (non-parametric Mann-Whitney pairwise comparison test) were applied using the software PAST 3.05, in order to detail significant differences in the group size related to time-period of the day and behaviour.
Fig. 1 - Map of the study area and sightings of Stenella coeruleoalba occurred in the Gulf of Taranto (northern Ionian Sea, central Mediterranean Sea) from 2009 to 2014. Mappa dell’area di studio e degli avvistamenti di Stenella coeruleoalba avvenuti nel Golfo di Taranto (Ionio nord-occidentale, Mar Mediterraneo centrale) dal 2009 al 2014.
Results - Sightings of S. coeruleoalba were grouped in 4 clusters corresponding to a percentage of similarity of 65% (Fig. 2). In particular, cluster 1 and 2 included respectively 103 and 73 sightings exclusively occurring during morning (09:00 to 13:00) (Tab. 1). In addition, cluster 1 was characterized by the highest value in size group (median value 80 individuals) with striped dolphins mostly in resting (67.0%), socializing (27.2%) or feeding (5.8%) and absence of travelling (Fig. 3). Cluster 2 was characterized by the lowest value in group size (median value 20 individuals)
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with dolphins in feeding (31.5%), resting (24.7%), socializing (21.9%) and travelling (21.9%). A total of 61 sightings, occurring in the early afternoon (13:01 to 16:00), were grouped in cluster 4 characterized by low values in group size (median value 20 individuals), with individuals in socializing (42.6%), travelling (24.6%), feeding (19.7%) and resting (13.1%). Only 17 sightings, occurring during late afternoon (16:01 to 18:00), were grouped in cluster 3 characterized by intermediate values in group size (median value 50 individuals) with dolphins in resting (41.2%), socializing (29.4%), feeding (17.6%) and travelling (13.8%). The main results from the post hoc analysis carried out on the total of sightings indicated as cluster 2 was significantly characterized by feeding (9.1%), associated to a significant reduction in size group (p
