AIS data underrepresents vessel traffic around coastal Scotland

Automatic Identification System (AIS) data is often used as a proxy to quantify vessel densities and estimate their associated impacts (e.g. emissions, underwater noise, likelihood of collision with marine megafauna), yet it is increasingly acknowledged that AIS data does not fully capture all vessels that may be present within a given area. Therefore, impacts that are evaluated using only AIS-based vessel data (e.g. counts) are likely underestimating the volume of vessel traffic, and thus the potential and scale that effects may occur. The extent of this underestimation is unclear due to the lack of data on the volume and distribution of vessels that are not transmitting AIS. To investigate this, > 1800 hours of land-based and at-sea visual surveys were conducted across nine Scottish Marine Regions between 2019 and 2024, collecting data on coastal vessel activity (<10 km of shore). These data were compared with corresponding AIS data to quantify AIS vs non-AIS traffic. Non-AIS vessels were present during 67 % of the total time surveyed. Of the vessels recorded, only 43 % were broadcasting AIS. AIS transmission rates were varied between seasons (range = 38–55 %), regions (range = 20–58 %), and by vessel type (range = 0–95 %). Given AIS data is increasingly being used to quantify vessel activity and predict associated impacts, it is vital that further consideration is given to the volume of vessel traffic absent from these datasets and predictive efforts. Underestimation of actual vessel traffic present, and the potential associated impacts may lead to inadequate policies, management or mitigation efforts.