Unveiling Elasmobranch aggregations in the Mediterranean Sea: behavioral, ecological and spatial association insights

Sharks, rays and skates belong to the sub-class of the elasmobranchs, a taxonomic groupamong the most threatened marine vertebrates worldwide (Dulvy et al., 2014). Like in othermarine regions, elasmobranchs are experiencing a significant decline in population in theMediterranean Sea due to intense anthropogenic pressures, especially unmanaged fisherieswhich can act both directly with target fishing and indirectly via accidental catches (Walls &Dulvy, 2021) and the depletion of their prey resources. Inaddition to this, the survival ofshark and ray species can be threatened by other drivers, including habitat degradation,pollution, climate change and the loss of critical areas such as breeding and nursery grounds(Dulvy et al., 2024). As all these pressures often act simultaneously and on different spatialscales, identifying where and when shark and ray individuals aggregate can reveal importantpatterns of vulnerability and resilience to human threats, thus providing a fundamental basisfor conservation and management actions. Shark and ray aggregations are temporarygatherings of multiple individuals in specific areas and times, often associated with keybiological functions such as reproduction, feeding, parturition, or social interactions(McInturfet al., 2023). These gregarious behaviors may expose grouped individuals toincreased vulnerability when overlapping with human activities, even inside marine protectedareas (Di Lorenzo et al., 2022; Palacios et al., 2023). Despite their biological importance,Mediterranean elasmobranch aggregations are poorly reported, particularly in terms ofstructure, drivers, ecological meanings and vulnerability to anthropogenic stresses. This thesisaims to contribute to filling this gap of knowledge by exploring the spatial and temporaldynamics of shark and ray aggregations in the Mediterranean Sea through a multi-scale andmulti-source approach. A combination of systematic literature review, citizen science data,field observations, and long-term fishery-independent datasets was employed to investigatedifferent aspects of elasmobranch aggregations, ranging from broad-scale patterns to species-specific case studies.Accordingly, this thesis is structured into five chapters, each addressing a specific aspect ofelasmobranch aggregations in the Mediterranean Sea.Chapter 1provides a comprehensive synthesis and classification of the available informationon elasmobranch aggregations across the entire basin. By identifying spatial, seasonal, andecological patterns, this chapter outlines the current state of knowledge on these phenomenaand highlights existing research gaps.Chapter 2 focuses on the critically endangered common eagle ray (Myliobatis aquila),characterizing its aggregation behavior through the integration of published data, social mediarecords, and field observations conducted within a Marine Protected Area in the southernMediterranean.Chapter 3 reports new evidence of a reproductive aggregation site for the common stingray(Dasyatis pastinaca), underlining the ecological and conservation importance of coastalhabitats that serve as critical areas for reproduction and parturition.Chapter 4 explores ecological aspects at a well-known sandbar shark aggregation site aroundLampione Island (Southern Sicily, Italy), contributing to a broader understanding of howshark aggregations may facilitate interspecific interactions (i.e. fish predation), thuspotentially affecting ecological dynamics of communities at local scale.Finally, Chapter 5 investigates the spatial structure of aggregations across demersalelasmobranch species, sexes, and life stages, by analysing thirty years of standardized trawlsurvey data and applying spatial co-occurrence metrics to reveal long-term and multi-speciespatterns.