Endemic shearwaters are increasing in the Mediterranean in relation to factors that are closely related to human activities

The aim of this study was to estimate global population trends of abundance of two endemic migratory seabird species breeding in the Mediterranean Sea, Balearic and Scopoli's shearwaters, from migration counts at the Strait of Gibraltar. Specifically, we assessed how regional environmental conditions (i.e. sea surface temperature, chlorophyll a concentration, NAO index and fish catches), as proxies of climate change, prey availability and human-induced mortality factors, modulate the interannual variation in shearwater numbers. The change in the migratory population size of both shearwater species was estimated by fitting Generalized Additive Models (GAM) to the annual counts against the year of observation. Specifically, we modelled daily counts of migrant shearwaters during the post-breeding season. Contrary to current estimates at breeding colonies, coastal-land based counts of migrating birds provide evidence that Baleric and Scopoli's shearwaters have been recently increasing in the Mediterranean Sea. Our results highlight that demographic patterns in these species are complex and non-linear, suggesting that most of the increases have happened recently and intimately bounded to environmental factors, such as chlorophyll concentration and fisheries, that are closely related to human activities. Counts of migrating birds at strategic coastal points may provide useful estimates of the global population trend, as well as an efficient and rapid assessment of these and other seabird species in the Mediterranean. Keywords: Chlorophyll, Climate change, Fisheries, Seabird, Monitoring