| Summary: | Chondrichthyans (sharks, rays and chimaeras) play a key role in marine ecosystems, since they are top predators. Because of their life-history traits (slow growth, late maturity and low fecundity), they are especially vulnerable to fishing. In the Mediterranean, which hosts 7% of living chondrichthyans, a notable change in marine ecosystems has occurred due to anthropogenic impacts, with overfishing as one of the most important. For this reason, and because these species can be an important by-catch of fisheries, the International Union for Conservation of Nature (IUCN) considers this region a key hotspot of extinction risk for chondrichthyans. This Thesis contributes to improve the scientific knowledge of chondrichthyans on the circalittoral and bathyal soft bottoms along the western Mediterranean. Methods traditionally used to study population dynamics and biological communities, jointly with molecular tools, were applied to assess the past and present status of these vulnerable species. Data and samples were obtained during the MEDITS bottom trawl research surveys, developed annually in spring-summer since 1994 throughout the four geographic sub-areas (GSAs) established by the General Fisheries Commission for the Mediterranean (GFCM) in the study area: Northern Alboran Sea (GSA01) and Alboran Island (GSA02) considered jointly as GSA01, Balearic Islands (GSA05) and Northern Spain (GSA06). A total of 33 species of demersal chondrichthyans, belonging to 13 families were identified, of which 26, 28, and 19 species were caught in GSA01, GSA05 and GSA06, respectively. Two different assemblages were detected in each GSA, corresponding to the continental shelf and slope. The highest values of diversity, abundance, and biomass were recorded on the continental shelf of GSA05 and the slope of GSA01, while GSA06 showed the lowest values at both depth strata. These results can be explained by differences in trawl fishing exploitation and types of bottom between GSAs. Biological parameters such as optimum depth, length at ...
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