Spatial and temporal characterization of the exposure of island-associated cetacean populations to whale-watching in Madeira Island (NE Atlantic)

The whale-watching industry can represent a critical anthropogenic stressor for the targeted populations. Identifying and characterizing the individual exposure to wildlife tourism in island-associated cetacean communities is especially relevant in small, remote oceanic regions relying on tourism. This study characterized the spatial and temporal variation of the whale-watching pressure on common bottlenose dolphins (Tursiops truncatus) and short-finned pilot whales (Globicephala macrorhynchus) in Madeira Island and identified the most exposed portion of the population, making use of diverse long-term datasets (i.e. sighting data, photographic data and vessels’ tracks) obtained year-round from platforms of opportunity (whale-watching operators) and proposing a new descriptive index which estimates the cumulative exposure rates. Core areas of encounters with the whale-watching vessels were identified off the capital city of Funchal. A daily average of 6.5 (±2.2 SD) vessels were concurring in this area, with a significantly higher number recorded between May and October and a peak of 14 vessels in August. From a total of 367 identified bottlenose dolphins and 536 pilot whales, a minimum of 6% and 10%, respectively, were identified as island-associated individuals. Cumulative exposure rates for these individuals could reach levels over two times higher than the general trend for both species. A higher proportion of island-associated bottlenose dolphins were recorded in the winter season. In contrast, island-associated pilot whales were predominant (over 50%) or in a high proportion (over 40%) year-round. The present study highlights the importance of using data from platforms of opportunity to investigate anthropogenic stressors, as tourism-based cetacean observation. It also provides insights into the whale-watching pressure on island-associated individuals occurring in oceanic areas and calls attention to the need of monitoring the fitness and well-being of these wildlife populations in Madeira Archipelago, an outermost European region.