Abstract
Digital aerial surveys were carried out twice a month from February 2021 to January 2022 with an extra survey conducted in February 2022. Surveys were conducted in two areas in the southern North Sea, one of which encompassed offshore wind farms. The aim of the surveys was to 1) collect digital images that can later be used to develop software for analysing images; 2) to collect spatio-temporal distribution data on birds and marine mammals to determine differences between wind farm and adjacent areas; and 3) to provide information on how to investigate displacement and habituation of seabirds and marine mammals to wind farms.
A total of 56,580 birds and 1,831 marine mammals were recorded during the 25 surveys between February 2021 and February 2022. Peak numbers were recorded during winter months and lowest figures were recorded from mid-spring to early autumn. Numbers and distributions of six key seabird species (Northern Gannet, Lesser Black-backed Gull, Great Black-backed Gull, Black-legged Kittiwake, Common Guillemot and Razorbill) and a marine mammal (Harbour Porpoise) are described. Monthly density maps do not show any strong tendencies of these species to favour particular areas within the study area. This was confirmed by analyses of densities within and near offshore wind farms and in a reference area. Exceptions were Northern Gannet and Common Guillemot, which showed lower densities within the wind farm areas then just outside and in the reference area respectively.
Densities recorded during the digital aerial surveys and observer-based aerial surveys (MWTL surveys) were comparable for the key species investigated. Species identification and the proportions of individuals aged were similar between methods, but for identification this was slightly higher in the observer-based surveys and for ages this was slightly higher in the digital aerial surveys.
Digital aerial surveys offer several benefits over observer-based aerial surveys, including the higher flight height, enabling data to be collected within offshore wind farms, and a quicker flight speed, allowing more area to be covered per period. On the other hand, digital aerial survey data are currently more time consuming to analyse and as such, costs are higher. Developments in analytical software could decrease the time needed to analyse digital images.