Social Ecology of Bottlenose Dolphins in the Kvarnerić (Northern Adriatic Sea)

Abstract

A bottlenose dolphin community was studied from small inflatable craft from 1987 to 1994 in a relatively large area (about 800 km2) east of the islands of Loˇsinj and Cres, northern Adriatic Sea. A total of 106 individuals were photoidentified based on natural permanent marks on their dorsal fins. Most of the dolphins were resighted on a regular basis, indicating a high level of year‐round site fidelity, although their range was evidently greater than the chosen study area. Dolphin density was highly variable and considerably lower than for most well‐known bottlenose dolphin communities. Groups averaged seven individuals, with a mode of two. Groups entirely composed of adults were the smallest, groups with calves the largest. Group fluidity was high, seasonal and yearly changes in mean group size being also considerable. Summer was the peak calving season, with a striking variation in the number of births on alternate years. Poor evidence of shark predation was found. The social organization of this dolphin community seemed to be highly flexible, possibly as an adaptation to cope with environmental changes as well as with a limited and variable availability of prey.

Group size and composition of Tursiops truncatus (Cetacea: Delphinidae), in a coastal insular habitat off southeastern Brazil

“…This report agrees with the results obtained in this study. Although group size varies according to the biogeographical region, the average sizes recorded in the present study are typical of T. truncatus groups in coastal waters (e. g. BEARZI et al, 1997;WELLER, 1999;MAZE-FOLEY;WÜRSIG, 2002;LUSSEAU et al, 2003) despite the inherent differences among the residency patterns in a restricted area.…”

“…The variation in group size observed in this study especially after 2008 may be a consequence of one or more of the following factors: (a) the flexible and highly adaptable behavior of the species (SHANE et al, 1986;BEARZI et al, 2008); (b) the fluid nature of a fission-fusion society that undergoes frequent changes in group size, composition and social relationships (see BEARZI et al, 1997;CONNOR et al, 2000 and references therein); (c) the irregular distribution of prey items in coastal habitats (WELLS et al, 1987;WELLER, 1999); and (d) the decrease in biological diversity within the archipelago due to human-induced stresses (RANGEL et al, 2007;ECOMAR/SEAP, 2009). During the censuses, the observed groups often formed small ephemeral subgroups whose composition changed frequently (i.e., the groups showed the fission-fusion behavior) (LODI, 2009).…”

“…This trend may be directly or indirectly linked with factors (c) and (d) cited above. Shane et al (1986), Lacerda (1987), andBearzi et al (1997; reported that the lowest densities of common bottlenose dolphins were correlated with habitat quality determined by a loss of biodiversity and/or degradation of the marine environment. The proximity of the site to shore results a greater influence of land-based activities, including sewage discharge (CARREIRA;WAGENER, 1998).…”

“…The individuals sighted were classified into four age categories based on their behavior and size estimates (BEARZI et al, 1997;SMUTS, 1998;CONNOR et al, 2000;MANN et al, 2000;QUINTANA-RIZZO;WELLS, 2001;CAMPBELL et al, 2002): (a) neonate -< 1/3 of adult size, presence of fetal folds, immature swimming, stereotyped surface respiration, frequent and non-coordinated head-butting against the water; (b) calf -1/2 of adult size, typical dislocation behind the dorsal fin of the presumed mother; (c) juvenile -2/3 of adult size; and (d) adultapproximately 3 m in length.…”

“…In contrast, the species occurs throughout the year in other coastal habitats despite seasonal variation in abundance. For example, this common pattern of occurrence is found in the Adriatic Sea (BEARZI et al, 1997;FORTUNA, 2006), the United States (SHANE, 2004), Venezuela (OVIEDO;SILVA, 2005), and Santa Catarina Island, Brazil (FLORES;FONTOURA, 2006;WEDEKIN et al, 2008).…”