Bottlenose dolphins share fish farm areas while maintaining sexual segregation: Investigating group memberships through spatially and temporally explicit parameters

Abstract

Group membership is a key attribute of animal societies and central to the study of social structure in several taxa. However, social structure analyses are sensitive to the way data are collected and associations defined. In this study, a time–space method was used to investigate the social structure of common bottlenose dolphins Tursiops truncatus observed and photographed across 7 years in the semi‐enclosed Gulf of Corinth, Greece. Instead of adopting traditional group definitions, individuals were considered as being members of the same group if photographed within a specific time and space window. This approach can be applied post hoc across studies and can offer advantages under challenging sampling conditions (e.g. when dealing with groups spread over vast areas or when group membership is otherwise hard to assess). Dolphins were mostly found around coastal cage aquaculture facilities farming European sea bass Dicentrarchus labrax and gilthead seabream Sparus aurata. Dolphins formed clusters largely or entirely composed of individuals of the same sex, suggestive of sex‐based homophily. Habitat partitioning was not detected: there was substantial spatial overlap among dolphin clusters, with all individuals using a relatively small area in the northern portion of the Gulf, where most of the productive fish farms were located. Associations between females were stronger than those between males, and daughters tended to stay in the group of their mothers. Sex‐based social clustering may allow females and calves to limit interactions with potentially aggressive males, while individuals of both sexes benefit from prey concentrated around fish farms. Adaptation to foraging around farms can result in trade‐offs between the costs and benefits of nourishment and social interaction. This may have both positive or negative effects on the animals that should be considered in the context of ensuring their favourable conservation status.

Social and community structure of striped dolphins in a semienclosed Mediterranean embayment

“…The Gulf of Corinth is inhabited by common bottlenose dolphins Tursiops truncatus (hereafter "bottlenose dolphins"), striped dolphins, common dolphins, striped  common dolphin hybrids, and a single Risso's dolphin Grampus griseus (Antoniou et al, 2018;Bearzi et al, 2016;Frantzis & Herzing, 2002). Consistent research effort from 2009 through 2017 resulted in baseline information on the abundance, movements, habitat preferences and conservation status of these odontocete species within the Gulf (Bearzi et al, 2011(Bearzi et al, , 2016Bonizzoni et al, 2019Bonizzoni et al, , 2023Santostasi et al, 2016). parallel to their route, while avoiding sudden changes of speed and direction as recommended by Würsig and Jefferson (1990).…”

“…Following the method described by Bonizzoni et al (2023), associations relied on the time and position metadata of dorsal fin photographs. Individuals were considered associated if they were photographed at least once within a given time and space interval of one another, and not associated otherwise.…”

“…The large size and scattered nature of groups in the Gulf of Corinth made the assessment of group membership particularly challenging (see Section 2.5), also considering that large groups could contain multiple smaller groups (Syme et al, 2022). The spatio-temporal criterion used to define associations helped disentangle the social pattern of this population, under a sampling scenario that would not allow for the application of more traditional approaches (Bonizzoni et al, 2023).…”

“…This study provides preliminary insight on the social and community structure of striped dolphins. It takes advantage of three main factors: (1) access to a community composed of a discrete number of resident individuals (approximately 1,300), of which 595-836 are considered mature (Bearzi et al, 2016(Bearzi et al, , 2022; (2) predictable occurrence of striped dolphins in waters relatively close to shore, within a semienclosed Gulf with little or no emigration/ immigration (Bearzi et al, 2016); and (3) reliance on a recently published time-space method to define and investigate associations using the position and time metadata of dorsal fin photographs of identified individuals (Bonizzoni et al, 2023). Based on this method, individuals are considered as members of the same group if photographed within a specific time and space window.…”

“…Based on this method, individuals are considered as members of the same group if photographed within a specific time and space window. This offers advantages under challenging sampling conditions, and when “groups” may be difficult to define objectively, e.g., because they are spread over vast areas (Bonizzoni et al, 2023). This method is particularly well suited to study the striped dolphin population in the Gulf of Corinth, often composed of large and widely scattered groups (Bearzi et al, 2016), making associations and group membership difficult to assess objectively through traditional visual approaches (Syme et al 2022; Whitehead, 2008a).…”