Spatial Modelling of Aerial Survey Data Reveals an Important European Storm-Petrel Hotspot and Its Underlying Drivers...

Determining the distribution and population size of marine species is crucial for conservation and management. However, for many species, the abundance and at sea distribution are poorly known because of their large geographic ranges, high mobility and cryptic breeding habits. This is especially true for small pelagic seabirds such as the European storm-petrel. Large-scale observer-based aerial surveys were conducted over four summers in the North-East Atlantic extending 200 nautical miles from the coast of Ireland. Species distribution models were produced using generalised additive models with a combination of static and dynamic environmental variables to assess the impact of survey altitude on storm-petrel detectability, and to model their abundance and distribution. Reduced storm-petrel detectability was identified at higher survey altitudes and rougher seas, and an at-sea abundance of 154,044 (95% CI: 94,347-452,299) individuals was estimated. Our results reveal fine-scale variation in the spatial distribution of storm-petrels and highlight the unsuitability of foraging radius distribution models for such species. Storm-petrels were found to avoid coastal areas, which we speculate is linked to the avoidance of large coastal avian predators during the day. Although the continental shelf edge was highlighted as a significant feature in the distribution of this pelagic species, a more prominent hotspot was identified in neritic areas, 20-40 km off the south and south-west coasts of Ireland in a region highly influenced by shelf fronts, coastal currents, upwellings and eddies in the summer months. The identified hotspot has global significance since Ireland holds more than 20% of the entire European storm-petrel breeding population.