Capitoline Dolphins: Residency Patterns and Abundance Estimate of Tursiops truncatus at the Tiber River Estuary (Mediterranean Sea)

Abstract

Periodic assessments of population status and trends to detect natural influences and human effects on coastal dolphin are often limited by lack of baseline information. Here, we investigated for the first time the site-fidelity patterns and estimated the population size of bottlenose dolphins (Tursiops truncatus) at the Tiber River estuary (central Mediterranean, Tyrrhenian Sea, Rome, Italy) between 2017 and 2020. We used photo-identification data and site-fidelity metrics to study the tendency of dolphins to remain in, or return to, the study area, and capture-recapture models to estimate the population abundance. In all, 347 unique individuals were identified. The hierarchical cluster analysis highlighted 3 clusters, labeled resident (individuals encountered at least five times, in three different months, over three distinct years; n = 42), part-time (individuals encountered at least on two occasions in a month, in at least two different years; n = 73), and transient (individuals encountered on more than one occasion, in more than 1 month, none of them in more than 1 year; n = 232), each characterized by site-fidelity metrics. Open POPAN modeling estimated a population size of 529 individuals (95% CI: 456-614), showing that the Capitoline (Roman) coastal area and nearby regions surrounding the Tiber River estuary represent an important, suitable habitat for bottlenose dolphins, despite their proximity to one of the major urban centers in the world (the city of Rome). Given the high number of individuals in the area and the presence of resident individuals with strong site fidelity, we suggest that conservation plans should not be focused only close to the Tiber River mouths but extended to cover a broader scale of area.

Keywords: Mediterranean Sea; Tiber River; Tursiops truncatus; abundance; bottlenose dolphin; capture–recapture; site fidelity.

Figure 1

Study area in Central Tyrrhenian Sea: black lines represent the tracks made in the period 2017–2020, black dots represent single-points mooring (SPM), and the red square identifies Secche di Tor Paterno (marine protected area, MPA).

Figure 2

Results of the cluster analysis: (a) optimal number of clusters; (b) cluster dendrogram; (c) distributions of total individual identifications by cluster.

Figure 3

Estimated abundance by each sampling occasion.

Figure 4

Estimated capture probability as a function of the effort (total monthly surveyed days).