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

Assessing the distribution of marine apex-predators is pivotal to understanding community interactions and defining management goals. However, several challenges arise in both estimates and predictions considering the distinctive and mutable biological/ecological requirements of these species and the influence of human activities. Thus, efforts to study apex-predators' spatial distribution patterns must deal with inherent uncertainty. Relying on different data sources (research programs and social media reports), physiographic and environmental covariates (depth, slope, surface temperature and chlorophyll-a), and specific source-related detection functions, this study selected a Spatial Log-Gaussian Cox Process to model the distribution patterns of an opportunistic apex-predator, the common bottlenose dolphin (Tursiops truncatus), over 14 years (2008-2021) in the Mediterranean Sea (Italy) using a total of 955 encounters. Both depth and slope showed a significant (95% significance) reduction effect in the encounters when deeper and steeper, respectively. Temperature (parabolic) shows a positive effect (90% significance), while chlorophyll-a values did not seem to have a significant effect on encounter intensities within each season. The estimated posterior mean and the coefficient of variation surfaces for the intensity by season showed higher intensity in summer near the Tiber River estuary than other regions. Almost homogeneous predictions were observed in winter, with marginal greater intensities where lower temperatures and higher chlorophyll-a concentration were observed. The relatively low variance was predicted in the more coastal parts of the study area within each season, while higher uncertainty was instead revealed in the southernmost offshore area. This study highlighted the persistent presence of the common bottlenose dolphin in the investigated area both winter and summer, with a coherent distribution within each season, and rare transient occurrences in deeper waters (where uncertainty increases). Thanks to its versatile characteristics, the species seems to well adapt to different seasonal conditions and maintain its distributional range.

Automated summary

Assessing the distribution of marine apex-predators is crucial for understanding community interactions and setting management goals. This study investigated the distribution patterns of the common bottlenose dolphin in the Mediterranean Sea using different data sources and statistical models. The results showed that depth, slope, and temperature significantly influenced the encounters with dolphins, while chlorophyll-a concentration had no significant effect. The study also revealed the persistent presence of dolphins in the study area throughout winter and summer, indicating their adaptability to different seasonal conditions.