by Aimee O'Hara
Vessel presence in coastal marine environments is on the rise, and there is growing concern as to the potential impacts the noise emitted from these vessels could have on cetaceans such as dolphins. Vessels generate underwater noise due to mechanical vibrations originating in the engine or hull, however most of the noise is created as a result of cavitation, a process in which air bubbles form and collapse along the edge of the fast-moving propeller blades of the vessel [1]. An issue with vessel noise presence is that cavitation noise is often very broadband, as a result of this, it overlaps with the frequency ranges of many cetacean sounds [2]. This overlap of anthropogenic noise with the acoustic frequencies that are biologically important to cetaceans is of growing concern to scientists [3].
Researchers have found that dolphins will have a varying response to vessel noise, ranging from no change at all to disruptions to feeding behaviour, amount of time spent socialising, and in some cases, total displacement of individuals from the affected area. This is linked to the varying dosages of exposure experienced by individuals, with higher dosages leading to stronger responses. This relationship is known as a ‘dose-response’ relationship, in which the probability of the individual behaviourally responding (avoiding the source), increases with dose (higher levels of noise) [4]. A wide range of responses to such disturbances have been documented including longer diving times, increased swimming speeds, decreased time spent socialising and changes in vocal behaviour [5,6,7,8].
Some vessel disturbances may have long-term effects such as impacts on population viability and reproductive success, while others may have short-term behavioural effects [5]. The cumulative impacts of repeated short-term effects or of a combination of short-term and long-term effects is difficult to evaluate and remains unknown. Perhaps a less obvious aspect of dolphin behaviour to observe is changes in the acoustic behaviour of these individuals. Dolphins use a variety of vocal modes for communication. Perhaps most notably so are signature whistles. A prominent function of signature whistles is to provide information on the location of the individual within a group, facilitating group cohesion [6]. It has been found that in the instance of vessel approaches, the whistle production rate has been reported to have increased [9].
Acoustic technologies have been developed to increase our understanding of cetacean responses to disturbances, notably so the creation of a digital acoustic tag (DTAG) [10] which has allowed for researchers to measure the locomotory behaviour of cetaceans in response to disturbances, as well as assess the environmental noise levels and vocal responses of these animals using the inbuilt hydrophones. An example of this research is work conducted by Aimee Kate Darias-O’Hara at the University of St. Andrews, in which the behavioural implications of recreational vessel noise on mother-calf pairs of bottlenose dolphins was assessed using DTAG data [11].
With new technologies on the rise every year in the field, this has made assessing the implications of vessel noise increasingly easier. Wildlife technology has played an imperative role in our understanding of dolphin response behaviours to vessel disturbances. Without this technology, scientists face a great amount of uncertainty as to the potential impacts our ever-changing marine environments could have on marine mammal species. Both vessel presence and ocean noise has been, and continues to be, rising. As a result, it is imperative that we increase our understanding of the implications of these increased noise levels on our marine megafauna, in order to implement appropriate mitigation measures to conserve and protect these species.
Thanks to the Sarasota Dolphin Research Programme (SDRP) for providing the DTAG data for Aimee's work.
Image credit: Photo by the Chicago Zoological Society's Sarasota Dolphin Research Program, taken under National Marine Fisheries Service Scientific Research Permit No. 20455.
50% Complete
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.