In this thesis, multi-annual seabird distribution and abundance were investigated for the first time in WIW using aerial surveys. Each seabird distribution and abundance estimates were presented with the corresponding coefficient of variation (CV) and 95% confidence interval (CI) calculations. MaxEnt software was used to compute seasonal and inter-annual Habitat Suitability models for the Balearic Shearwater and the Northern Gannet. The predicted species distribution was based on occurrence probability in relation to habitat suitability. The use of models with different sets of environmental variables are particularly important in the marine environment because variability in ocean conditions may lead to identifiable species distribution changes (Becker et al. 2017). This modelling approach proved to be effective in determining areas of high and low use by the target species. These data are crucial to policy makers and stakeholders involved in conservation planning, particularly when defining marine protected areas (including SPAs) and threat mitigation measures.
In comparison to vessel platforms, airplane surveys are more cost-effective, use lower effort time and produce a lower responsive effect on seabirds. Data collected in airplane surveys allow estimating absolute and relative abundances and associated errors. Data also allow establishing temporal trends and they can be used to model species distribution. If a double platform is used, airplane surveys reduce the number of animals that are not detected on the line or have evasive responses. However, the availability of suitable platforms for aerial surveys is low, observers must be trained and experienced and the identification of similar species (e.g., Storm-petrels, some Larids and Shearwaters) is difficult. Also, there may be problems in estimating angles to sightings in high-density areas and in collecting additional oceanographic or climatic data. For less common species, there is a risk of high variability in the error associated with sampling, which can hamper the detection of small fluctuations. Overall, line transect aerial surveys proved to be a time and cost-effective method allowing for robust seabird abundance and density estimates across a large study area.
The information collected in this thesis contributed to understand the distribution and abundance of the target species, being necessary for EU member states, such as Portugal and Spain, so that they can meet the European Marine Framework Strategy Directive and the Birds Directive. Furthermore, this information can help decision-makers designate or review proposed marine protection areas. In this study it was also possible to estimate Balearic Shearwater and Northern Gannet PBR in relation to bycatch. Areas of higher bycatch risk were mapped and results emphasised that Balearic Shearwaters bycatch mortality was above the PBR threshold (mostly due to set nets in the Polyvalent fleet). Northern Gannet bycatch mortality was at the PBR limit (mostly due to the Longliner and Polyvalent fleets). These data highlighted the necessity for implementation of mitigation measures to reduce seabird bycatch in fisheries, defined and enforced by European or national governmental agencies, to particularly with focus on the critically endangered Balearic Shearwater.