Patterns of spatial changes on demersal species in the Gulf of Cadiz and northern Alboran Sea

The Gulf of Cadiz (GoC) in the Atlantic Ocean and the northern Alboran Sea (AS) in the Mediterranean Sea are part of an extremely important oceanographic complex in terms of water exchange between the Atlantic and the Mediterranean through the Strait of Gibraltar. Besides its significant role as an ecological transition system, it is unknown whether regional and local envi ronmental drivers affect similarly species inhabiting these two adjacent and connected ecosystems. This study analyses the spatial shifts of representative demersal species and their response to the environmental and demographic drivers in these two regions, using information from two trawl surveys carried out in the GoC and in the AS from 1994 to 2015. Species distribution trends were observed in both basins. However, they were more evident in the GoC, where six out of eleven species presented temporal trends, than in the AS, where only two species displayed significant distribution changes. In both basins, these species showed geographic displacements towards the Strait of Gibraltar. Also, a high percentage of species presented significant differences in mean depth distribution on the two sides of the Strait of Gibraltar. Our study shows a general heterogeneity and independence in the drivers influencing species distributions in the two areas. GoC was highly influenced by both large- and regional-scale climate factors, which affected several species, while in AS the diversity of drivers was greater, with species density being the most common. This study provides a scientific basis for improving ecosystem-based management measures in these important transitional ecosystems.0,56