Marine monitors: Land-based citizen science observations show AIS data underrepresents coastal vessel traffic co-occurring with marine mammals

Marine mammals are vulnerable to a variety of impacts from vessels, including underwater noise disturbance and injury from collision. To quantify this risk, AIS (Automatic Identification System) vessel tracking data is often used as a proxy of vessel presence. However, many vessels do not appear within AIS datasets, meaning evaluating impacts using AIS alone will likely underestimate the potential for effects to occur. To understand the scale of underestimation and the types of vessels co-occurring with marine mammals, 3-yrs of land-based surveys recorded all vessels (sighted within ∼10 km of shore) that were observed concurrently with whales, dolphins and seals (sighted within ∼3 km of shore). Surveys were conducted from multiple sites within five Scottish Marine Regions. Observations of responses to vessels were also recorded opportunistically. AIS data accurately reflected coastal vessel traffic co-occurring with marine mammals during 30 % of the surveyed period. 59 % of vessels seen were not broadcasting AIS, with seasonal and spatial variation in AIS transmission rates, with lowest AIS transmission rates in summer (when 38 % of co-occurring vessels were broadcasting AIS). Non-AIS vessels were more frequently observed travelling at speeds that may pose an elevated risk to marine mammals, and were also more frequently recorded to elicit a response. 45 % of responses to vessels involved non-AIS powered vessels, and 33 % were in relation to human-powered vessels (e.g. kayaks). The results show that the majority of vessels that co-occur with marine mammals are non-AIS, and as such AIS data alone is insufficient to represent vessel-related impacts.