Community structure and dynamics of sharks and their relatives in the Adriatic sea
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Document type: Dissertation
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| Keywords |
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| Author keywords |
Sharks; Chondrichthyans; Species distribution modelling (SDM) |
| Abstract |
Chondrichthyans—sharks, rays, and chimaeras—represent one of the most evolutionarily distinct and ecologically diverse groups of vertebrates. However, overfishing poses a severe threat to this group, with one-third of global chondrichthyan species at risk of extinction. The Mediterranean Sea is a critical hotspot for this risk. In my research, I address the urgent need for comprehensive knowledge about chondrichthyan species distribution and their interaction with fisheries in the Adriatic Sea—a heavily exploited basin within the Mediterranean. By analyzing long-term fishery-independent monitoring data and employing state-of-the-art distribution modeling techniques, I uncover key insights. The chondrichthyan community exhibits strong depth and spatial sorting. Some viviparous species are abundant in the shallower northern Adriatic, while there is a west-to-east gradient of increasing abundance and species richness. Over time, dominant chondrichthyan species have increased in abundance, yet at different rates, except for the threatened spurdog, which continues to decline. Consequently, the community now displays a lower community-weighted age at first maturity and fraction of viviparous species compared to the 1990s. To estimate the spatial overlap between bottom trawling and chondrichthyans, I introduce a novel approach that combines Vessel Monitoring System data (representing fishing effort) with geostatistical species distribution models. By employing diverse overlap metrics that capture different levels of organization—from individual species to the entire community—I estimate substantial spatial overlap between intensive trawling, areas of high species richness, and the presence of threatened chondrichthyans (i.e. assessed as Vulnerable, Endangered and Critically Endangered by the IUCN Red List). These areas of high overlap are primarily concentrated in the northern and central offshore sectors of the western Adriatic, highlighting specific areas of conservation concern. In summary, this thesis contributes innovative statistical frameworks and ecological insights and may help in guiding conservation efforts to sustain chondrichthyan populations in the Adriatic Sea and beyond. |
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