Correspondence
Xiuqing Hao :
Email: xiuqing@ecos.au.dk;
Phone: +45 52733636;
Address: Department of Ecoscience, Aarhus University, Frederiksborgvej 399, DK-4000 Roskilde, Denmark
Page heading title: Harbour porpoises’ reactions to small boats

Abstract

Recreational boats are common in many coastal waters, yet their effects on cetaceans and other sensitive marine species remain poorly understood. To address this knowledge gap, we used drone videos to quantify how harbour porpoises (Phocoena phocoena ) responded to a small motorboat approaching at different speeds (10 or 20 knots). The experiment was carried out in shallow waters near Funen, Denmark (55.51° N, 10.79° E) between July and September 2022. Porpoises moved further away from the boat path during approaches at both boat speeds. In addition, porpoises swam faster when approached at 20 knots but not when approached at 10 knots, and they had a higher likelihood of moving away from the boat path when approached at 10 knots but not at 20 knots. Importantly, the received sound level did not depend on how fast the boat approached, suggesting that differences in porpoise responses were related to the speed of the boat’s approach rather than to sound itself. The porpoises’ behaviour during the minute where the boat was closest did not differ from their behaviour before boat exposure, indicating that the direct impact of small vessels on porpoise behaviour was most likely small. Nevertheless, repeated exposure to noise from small vessels could influence porpoises’ foraging efforts and cause them to relocate from disturbed areas. The approach used in this study increases our understanding of recreational boats’ impact on harbour porpoises and can be used to inform efficient mitigation measures to help conservation efforts.
KEYWORDS: behavioural response; boat disturbance; drone footage; motorboat; Phocoena phocoena ; recreational vessels; underwater noise

Introduction

As small boats become more prevalent in coastal waters worldwide they increasingly interfere with wildlife (Davenport & Davenport, 2006; Hermannsen et al., 2019; Carreño & Lloret, 2021). In particular, species that use sound for foraging, navigating, and communicating, such as the harbour porpoise (Phocoena phocoena ), are continuously at risk of being disturbed. Vessel traffic is known to affect porpoise behaviour (Dyndo et al., 2015; Wisniewska et al., 2018; Frankish et al., 2023), and can potentially influence the animals’ foraging success, fitness and population dynamics (Oakley, Williams, & Thomas, 2017; Wisniewska et al., 2018; Lusseau, Kindt-Larsen, & van Beest, 2023). However, as opposed to large vessels, studies investigating how animals react to small boat disturbances or how long their responses last are particularly lacking. Considering the overlap of small boat traffic with harbour porpoise habitats and the overlap between the frequency range of boat noise and porpoise hearing (Hermannsen et al., 2019; Hao & Nabe-Nielsen, 2023), such knowledge is important for improving the conservation of porpoises and other cetaceans.
Cetaceans have been reported to exhibit different types of behavioural responses to approaching vessels, including changes in speed, altered diving behaviour and spatial avoidance (Janik & Thompson, 1996). Orcas (Orcinus orca ) move in less predictable patterns when disturbed by vessels (Williams, Trites, & Bain, 2002) and bottlenose dolphins (Tursiops truncatus ) sometimes increase their inter-breath interval, speed and alter their surfacing behaviour in response to approaching boats (Lemon et al., 2006; Nowacek et al., 2001). Indo-Pacific bottlenose dolphins (Tursiops aduncus ) exhibit short-term erratic movements when approached by boats (Bejder et al., 2006) while harbour porpoises have been observed to porpoise more often, move away, dive to the bottom, and to display interrupted foraging when exposed to vessel noise (Dyndo et al., 2015; Wisniewska et al., 2018; Frankish et al., 2023). However, to assess the potential health impact of such behavioural changes it is important to quantify how the animals’ behaviour changes when exposed to boat disturbance. This is challenging as it is difficult to assess the exact distance between animals and boats as well as observe changes in animal behaviour from a distance.
Over the past few years, the development of increasingly advanced drones (unmanned aerial systems) has made it easier to observe cetacean behaviours remotely and non-invasively (Álvarez-González et al., 2023; Nowacek et al., 2016; Rees et al., 2018; Sprogis et al., 2020). Compared to traditional observations from boats or from land, drones have the advantage that they can hover over an animal while continuously collecting high-quality data (Koh & Wich, 2012; Rees et al., 2018; Morimura & Mori, 2019). They also make it possible to quantify detailed behavioural changes and how these are related to features in the environment, including the distance to a boat (Koh & Wich, 2012; Chabot & Bird, 2015). With these advantages, drones hold great potential for enhancing our understanding of how anthropogenic disturbances affect marine animals like harbour porpoises.
In this study we used a drone to quantify behavioural changes in harbour porpoises as they were approached by a small boat at a constant speed (either 10 or 20 knots). Given the reactions described above, we hypothesized that porpoises would respond to this disturbance by speeding up, by moving away from the boat’s path (distance moved away and probability of doing so), turning more abruptly, diving deep and breathing less often. We also investigated whether they responded more strongly to boats that moved fast than to slow boats, considering animals might get more scared towards rapid changes in approaching distance or noise levels. Furthermore, we compared porpoise behaviour when the boat was nearby with their natural behaviour (i.e., prior to boat approach) and explored how rapidly porpoises resumed their natural behaviour to assess if small boats are likely to have long-term effects on porpoises. We measured the sound level at different distances to the boat to determine if porpoise responses were mostly related to the sound level or to the speed at which the boat approached. As porpoises are strictly protected in European waters (Council Directive 92/43/EEC, 1992), studies of how animals react to small boats, like the present study, are important for informing management.

Materials and methods

Study site and experiment design

To investigate how harbour porpoises responded to approaching boats, we conducted an experiment using a research boat while monitoring porpoise movements with a DJI Phantom 4 Pro v2.0TM drone with a mounted camera recording in 4K resolution (4096 × 2160 pixels) and up to 60 frames per second. The camera was equipped with polarizing filters to avoid sun glare in the video footage. The experiment was carried out in Romsø Sound, located by the eastern coast of Funen, Denmark (55.51° N, 10.79° E; Fig. 1), which is recognized as an important habitat for harbour porpoises (Sveegaard et al., 2011). The experiment took place between 11th July – 10th September 2022, with a total of 20 days spent in the field collecting data (see Fig. 1 for dates when videos were recorded). We conducted the experiments on days with favourable weather conditions, i.e., sea state ≤2 (Douglas scale), wind speed <10 m/s, and without rain. It was done at water depths between 1–7 m to ensure a clear view of the porpoises in the drone footage. The research boat used in the experiment was a 5.5 m Pioner Multi III, powered by an 80 hp outboard engine. Boat tracks were collected using a portable GPS (Garmin GPSMAP 78s). Previous studies have suggested that drones flying at low altitudes (10–23 m) have minimal impact on cetacean behaviours (Ramos et al., 2018; Fettermann et al., 2019; Aubin et al., 2023), and when flown above 5 m, they have negligible effects on underwater noise levels (Christiansen et al., 2016). In our experiment, we maintained the drone’s flight height between 10–30 m to minimize its impact on porpoises while keeping track of the animals when they were diving deep. We did not observe obvious reactions from porpoises to the drone. Our selection of 10 and 20 knots as experimental speeds was based on the observed travelling speed for motorboats equipped with outboard engines (without sails) in Danish waters; 10 knots corresponds to mean travel speed while 20 knots corresponds to fast moving vessels (Hao & Nabe-Nielsen, 2023).