RAIN · Responses of cetAceans to natural and anthropogenIc Noise

Responses of cetAceans to natural and anthropogenIc Noise

The ocean ambient soundscapes, against which cetaceans must detect and decipher signals of interest is dynamic and complex. Where it was previously dominated by spatio-temporally loud signals from natural and biological sources, the post-industrial era has seen rapid influxes in acoustic signals from human-made sources (Hildebrand, 2009), which have been linked to negative effects on cetacean behaviour and physiology, with potential implications for individual fitness that may culminate in population-level consequences (Weilgart, 2007). However, although many natural noise sources can have similar acoustic characteristics to those produced by human-made sources, there is a lack of data on how cetaceans hear, respond to and cope with changes in natural noise within their environment, for which they have been subjected to over evolutionary timescales (Gomes et al. 2021). This ultimately has limited our understanding of how these species are potentially coping with the rapidly changing ocean soundscape of the Anthropocene. RAIN therefore aimed to study the responses of cetaceans to natural noises to better inform and contextualise our understanding of how these species respond to acoustically similar anthropogenic underwater noises such as from shipping. To address this, we used harbour porpoises (Phocoena Phocoena) as a model species to understand how a marine mammal may hear and respond to rain noise as a dominant natural noise source in marine environments, and compared this to their responses to vessel noise. Harbour porpoises were chosen as they are the most common cetacean species in European shelf waters. Although harbour porpoise best hearing (10 to 140 kHz; Kastelein et al. 2002). is considerably above the peak of vessel noise (~125 Hz), exposure to the higher frequency components of vessel noise (up to and beyond 100 kHz; Hermannsen et al., 2014) has been found to illicit behavioural changes (e.g., reduced foraging) in tagged individuals from this species (Wisniewska et al. 2018). These high-frequency components of continuous vessel noise are similar to the noises of rain underwater, which is a principal source of natural abiotic noise, which produces a continuous, broadband (1-50 kHz) and high amplitude signal (Ma et al., 2005). However, to date, the audibility of rain and responses made by cetaceans to this natural noise source underwater has received no attention in the scientific literature. Accordingly, to investigate the aim of RAIN and address these identified knowledge gaps, we initially proposed to answer three overall research objectives: 1. Quantify the absolute contributions of rain and vessel noise to local marine soundscapes experienced by cetaceans using long-term Passive Acoustic Monitoring (PAM). 2. Assess the behavioural and acoustic responses of individual harbour porpoises to rain and vessel noise events using state-of-the-art acoustic tag technology (DTAGs) to determine whether these responses are the same or differ depending on the sound source. 3. Investigate whether rain and/or vessel noise impacts the performance of harbour porpoises during an echolocation discrimination task. Through RAIN we aimed to derive impact via the provision of new knowledge and data on the sounds of rain underwater, and the determine whether this noise source is audible to harbour porpoises and effects their behaviour. Via RAIN we aimed to improve awareness and understanding around how cetaceans respond to natural noise sources to better contextualise how they respond to human-made noise sources. Thus supporting and informing how best to reduce impacts to marine mammals and manage underwater noise in Europe to ensure we are able to sustainably use marine environments to meet blue economy and green transitional needs without compromising biodiversity.

Data: CORDIS, © European Union