Maps, models, and marine vulnerability: Assessing the community distribution of seabirds at-sea

Whether considering the cumulative impact of chronic, small-size oil discharges or accidents associated with marine traffic and offshore oil and gas development, seabirds face a variety of threats in the marine environment. Assessing the vulnerabilities of seabirds to maritime hazards requires an understanding of their species distribution, and a means for combining this information across groups. Using at-sea survey data gathered as part of a regional monitoring program, an efficient framework for integrating multi-species data was developed. Survey data was incorporated within a distance sampling framework to generate bias-corrected seabird densities for an area of over 730,000 km2 size, which were used to construct multiple species distribution models (SDMs). The structural difficulties associated with sampling sparsely distributed individuals that also occur in large, localized concentrations led to the use of three modelling techniques potentially well suited for this type of data: negative binomial, “hurdle”, and random forest methods. Predicted abundances were combined to produce an ensemble forecast, which met or exceeded the accuracy of predictions from the individual models. Multi-species potential sensitivity maps were developed to identify core areas, confirming the general importance of physiographic features such as the shelf break and bathymetry. Distribution was also seasonally influenced, with spring and winter standing out as periods of peak importance. When combined with an oil pollution layer derived from aerial surveillance, vulnerability was highest in the vicinity of major ports (e.g., Halifax and Sydney, Nova Scotia). However, the vulnerability map also indicated lower but widespread levels of oiling risk throughout the shelf, presumably associated with persistently high levels of shipping traffic and ongoing petroleum extraction and exploration. Outside of this region, migratory connectivity is expected to expose seabirds to a wider network of hazards and further underscores the need for the coordinated and routine collection of marine hazard data alongside at-sea distributional data.