Marine Windfarms and Wildlife – Selecting the Most Effective Digital Aerial Survey Design

Environmental impact assessments based on inadequate survey data pose a significant planning and permitting risk for offshore windfarms. Potential displacement effects on waterbirds and marine mammals have traditionally been... [ view full abstract ]

Environmental impact assessments based on inadequate survey data pose a significant planning and permitting risk for offshore windfarms. Potential displacement effects on waterbirds and marine mammals have traditionally been based on a small number of visual line-transect surveys using ships or low-flying aircraft. These methods rely on a statistical correction to account for birds and mammals that are missed by the human observers. In addition, the presence of the survey platform is known to impact the distribution of birds, with some species being deterred by low flying aircraft and survey vessels, and others being attracted to vessels. Thus, aerial digital surveying techniques using aircraft flying at significantly higher (safer) altitudes are a key tool in EIA of offshore windfarms. Permanent digital record also overcomes the need for observers to estimate bird numbers and allows for data to be quality assured.
While there is general agreement on the methodological advantages of digital aerial over observer-based surveys techniques, there remains an urgent need to establish statistically sound (and cost-effective) sampling designs that generate interpretable geospatial data. Here, we demonstrate how a grid-based survey design using high resolution still imagery makes best use of digital aerial technology for seabird population studies and impact assessments. We carried out simulations based on sub-samples of gapless vertical imagery collected in areas with high numbers of waterbirds to assess the levels of confidence of population estimates resulting from contrasting sampling designs (grid design versus continuous transect design). We found that the statistical power of the transect sampling was generally lower than that of the grid approach. The grid design generated larger numbers of independent samples, the standard error remained lower and precision was higher.
Digital aerial surveys deliver high-quality and auditable data on marine wildlife for monitoring all phases of offshore and nearshore windfarm developments. Linear-transects require a greater percentage coverage than grid design to achieve an equivalent level of data quality and confidence in population estimates. Therefore, in many cases grid-based methods should be favored over continuous transects in future EIA of offshore wind farms, especially in cases where species show high degrees of ecological patchiness.