TEMPORAL CHANGES IN MIGRATORY SEABIRD ABUNDANCE ON THE PORTUGUESE COAST REVEALED BY A CITIZEN SCIENCE PROJECT

Portuguese waters are a vital migratory corridor for European seabirds. Coast-based counts can monitor local and global populations and study the long-term trends in seabird populations. Using TRIM (Trends & Indices for Monitoring Data) and Generalized Additive Models (GAM) on ten years of a citizen science dataset of seabird counts from seven different sites along the Portuguese coast, we aimed to identify the phenology of the seven most common, migratory, locally non-breeding seabird species and evaluate abundance trends and associations with environmental drivers. The TRIM model estimated that both the Northern Gannet Morus bassanus and Common Scoter Melanitta nigra during the post-breeding period, and also populations of Sandwich Tern Thalasseus sandvicensis, Balearic Shearwater Puffinus mauretanicus, Mediterranean Gull Larus melanocephalus and Great Skua Catharacta skua exhibited negative trends. The Northern Gannet pre- breeding showed a moderate increase and for the Common Scoter pre-breeding and the Razorbill Alca torda the trend was uncertain. From the 14 environmental variables tested, the GAM showed that sea surface temperature (SST), easterly winds and ocean productivity were the variables that most often significantly influenced the species abundance inshore. Five out of seven species showed a significant negative relationship with SST in at least one migratory phase (Northern Gannet post-breeding, Balearic Shearwater, Sandwich Tern pre-breeding, Mediterranean Gull and Great Skua). Detection probability for the Northern Gannet pre-breeding and Common Scoter increased with easterly winds, while the opposite effect was detected for the Sandwich Tern post-breeding. Ocean productivity positively affected the abundance of Common Scoter, Sandwich Tern pre-breeding, Mediterranean Gull and Razorbill, but the opposite effect was detected for the Northern Gannet post-breeding. Awareness of the links between environmental parameters and seabird abundance inshore are essential for better understanding the impact of future environmental changes on the community of seabirds along the Portuguese coast.