Resources and population traits modulate the association patterns in the common bottlenose dolphin living nearby the Tiber River estuary (Mediterranean Sea)

Abstract

Sociality and ecological drivers that can influence individual association patterns are infrequently considered in wildlife management, although they are essential aspects affecting animals’ responses to both human-related pressures and conservation strategies. In common bottlenose dolphins (Tursiops truncatus), sex-specific social dynamics and interactions with anthropogenic activities may affect grouping and induce changes in relationships between individuals. Out of a total of 347 individuals, we assessed the level of association among 68 bottlenose dolphins that have been sighted more than five times near the Roman coast (central Mediterranean Sea, Italy). The half-weight index (HWI) of dyadic associations, their network relations, and stability over time were investigated by using the SOCPROG software. Outcomes showed that females were more strongly associated than other individuals, with both preferred constant short-term associations and random long-term associations, possibly resulting in greater success in rearing young. Individuals interacting with the bottom trawl fishery showed weaker and short-term associations. Temporary disruption of individual associations during interaction with fishery and the relatively low number of females with calves participating in depredation seem to denote both the opportunistic nature of interactions with fishing vessels and the offspring-related protection strategy. The results show that the dolphins in this region maintain a complex but flexible social structure that varies with local biological requirements and is resilient to anthropogenic pressures.

Seasonal distribution of an opportunistic apex predator (Tursiops truncatus) in marine coastal habitats of the Western Mediterranean Sea

“…This result is in accordance with the relatively consistent preferences in terms of bottom topography and water depth observed for the common bottlenose dolphin in the Mediterranean Sea (Natoli et al, 2021), although some transient occurrences in deeper waters, where uncertainty in predictions increases, were detected. Coastal waters may be more suitable habitats for common bottlenose dolphin mother-calf pairs than deeper ones, where females with newborns could form stable resident groups as observed in the study area near the Tiber River estuary by both researchers and sea users (Pace et al, 2019;Pace et al, 2021;Pace et al, 2022a;Pedrazzi et al, 2022).…”

“…In addition, the highly opportunistic, versatile, and resilient nature of the common bottlenose dolphin diet and foraging strategies (Gimeńez et al, 2017;Borrell et al, 2021;Carmen et al, 2021;Natoli et al, 2021) enable the species to also adjust its behavior as a consequence of resources' availability. This opportunistic behavior can be also applied to resources related to human activities (e.g., fisheries and aquaculture) (Bonizzoni et al, 2021;Pace et al, 2012;Pace et al, 2022a;Triossi et al, 2013). All these characteristics may facilitate the development of discrete geographical units in the Mediterranean population (Carnabuci et al, 2016;Vassallo et al, 2020), making distribution assessments more difficult and increasing the uncertainty when predicting scenarios.…”

Finite mixtures in capture–recapture surveys for modeling residency patterns in marine wildlife populations

“…One basic assumption of CR experiments that we do not drop is that captures of different individuals are independent. However, it is well known that bottlenose dolphin populations can form structured societies with complex social networks (Pace et al., 2022a). In future studies, we would like to drop the independence assumption and include information on the population's social structure and the consequent statistical dependence in the capture histories.…”

“…This work is motivated by the need to get an abundance estimate of the population of the common bottlenose dolphins inhabiting the area of the Tiber River estuary in the Mediterranean Sea. Boat-based daily surveys have been conducted between 2018 and 2020 under favorable weather conditions to collect photographic and acoustic data of the specimens encountered in the study area during the search (Pace et al, 2022a(Pace et al, , 2022b(Pace et al, , 2022cPapale et al, 2021). The photoidentification technique was used to identify unique individuals over multiple sampling occasions and to build single capture histories.…”