Marine top predators such as marine mammals are key elements of the ecosystem, which are normally used as indicators of a specific area environmental state. Understanding both the distribution of predators and the spatially explicit cumulative impacts of multiple stressors on resident populations is fundamental. Both in Azores and Madeira long-term monitoring of cetaceans has been held. Sperm whales (Physeter macrocephalus) use the Azores as a foraging and calving ground especially by matriarchal groups that return there every year, staying from weeks to months foraging in the area, but also by mature males (Silva et al. 2014). Bottlenose dolphins (Tursiops truncatus) are considered resident in both archipelagos (Silva et al. 2009; Dinis et al. 2016), in Madeira there are also resident groups of short-finned pilot whales (Globicephala macrorhynchus) (Alves et al. 2013).

Human activities can create significant impacts that could affect marine mammals. Most of the products that we use daily are disposables or contain chemicals, which can find their way out to the sea becoming marine litter or contaminants. In Madeira resident cetacean species are distributed within a corridor highly used by large ships (mainly cargo and cruise ship) (Cunha et al. 2017). In Azores, background noise was found to potentially affect temporally long-range communications in baleen whales (Romagosa et al. 2017). Although whale watching activities are regulated by regional laws, behavioural changes were recorded indicating some degree of disturbance, but the biological relevance of these behavioural changes is unknown. Cumulative exposure to chronic stressors could scale up to induce physiological changes and have effects at the population level. Indeed, stress can lower immune response and make the animals more vulnerable to infectious pathogens which can in turn affect their survival and/or reproductive rates.