Fig. 7 Received sound levels (1-ws averages) measured within (a) 1/3 Octave (TOL) centred at 16 kHz and (b) broadband (0.1–150 kHz) from various distances of a boat approaching at 10 or 20 knots; negative distance values mean the boat was approaching the acoustic data logger. The dashed horizontal orange line indicates the threshold of porpoises’ behavioural reactions to noise reported by Wisniewska et al. (2018). Shaded areas indicate distance ranges where most porpoise reactions appeared to take place, as based on raw data.

Discussion

Porpoises started moving faster when approached by small boats at 20 knots, and they had a higher likelihood moving away from the path of the boat when approached at 10 knots. Additionally, porpoises tended to move further away from the boat path (i.e. avoidance distance is longer) when approached at either 10 or 20 knots (Fig. 4). After the boat had passed, the animals quickly slowed down again, and their movements during the minute where the boat was closest did not differ from their behaviour before the experiment started (Figs. 5 and 6). Earlier research has suggested that either the absolute received noise level or rate of increase in received noise level may trigger porpoise responses to vessels (Wisniewska et al., 2018). In our study, noise levels recorded by the acoustic data logger independently of the drone experiments, were the same when the boat moved at 10 and 20 knots when measured at a specific distance. This was the case both for TOL 16 kHz and broadband sound (Fig. 7), suggesting that the differences in porpoise reactions to boats approaching at different speeds is due to the rate of change in noise level, rather than the noise level itself. It also suggests that the porpoises’ reaction to small boats depend on their capacity to predict boat movements, and thereby assess the level of potential danger.
Although there was considerable variation among individuals in observed behaviours, animals generally speeded up (in 20 knots) and moved away from the boat path when the boat was <100–200 m away (Fig. 4a and b). At this distance sound had reached the levels at 100–105 dB at 16 kHz TOL, corresponding to a rapid increase in sound intensity (Fig. 7). Porpoises have been reported to change behaviour at noise levels exceeding 95–96 dB re 1 µPa at the TOL 16 kHz frequency band (Tougaard, Wright, & Madsen, 2015; Wisniewska et al., 2018), which aligns with our observations. However, the observed noise level is below the threshold of 123 dB re 1 μ Pa at 0.25−63 kHz octave bands reported by Dyndo et al. (2015). The reason may be that in the study by Dyndo et al. (2015), porpoises were kept in a net pen, and regularly exposed to specific boat passages over 10 years. Thus, they could not necessarily be assumed to behave naturally prior to disturbance.
We had expected porpoises to turn more abruptly when approached by a boat, which would have resulted in less predictable movements. However, we did not observe changes in turning angles as the boat approached. This contrasts with the observations of Black Sea harbour porpoises in Istanbul Strait, Turkey, which tended to turn more when vessels were nearby but were less likely to turn when the vessel was further away (>400 m; Baş et al. 2017b). We also expected porpoises to dive more when disturbed by a vessel, as has previously been reported for animals in the inner Danish waters (Wisniewska et al., 2018; Frankish et al., 2023), but this was not the case. One possible explanation is that the water was less than 7 m deep, which may not be enough to allow porpoises to avoid boats by diving to the bottom. Additionally, we had expected animals to breathe less often when the boat approached, thus allowing them to dive longer, but we did not observe any change in this behavioural metric. Nevertheless, we observed two instances of porpoises exhibiting porpoising behaviour before the CPA, and we failed to follow seven porpoises during boat approaches as they dove too deep and did not resurface in the same area (10 knots: 4 instances; 20 knots: 3 instances). These observations collectively suggest that boats may represent a significant disturbance to porpoises at close ranges although the strength and type of response is likely context dependent. The reactions of porpoises appear to be contingent on whether they are able to predict the movements of vessels, and as small pleasure boats sometimes move in a very unpredictable manner, they may in reality disturb porpoises more than we report in this study.
Our findings did not entirely support our initial hypothesis that higher boat speeds lead to stronger behavioural reactions, but porpoises reacted differently to boats approaching at 10 knots and 20 knots. For instance, at 10 knots, they were more likely to move away from the boat, but they did not start moving faster. Animals reacted to a boat approaching at 20 knots by swimming faster, but then they did not have higher likelihood to move away from the boat path. Porpoises sometimes accelerated rapidly when the boat was approximately 100 m away, suggesting that these animals possess the ability to assess the level of danger and adapt their avoidance strategies accordingly. The lack of an increase in the probability of moving away from the boat at 20 knots may be that porpoises have too little time to determine the appropriate avoidance direction when the boat was close, leaving them with only the option to speed up to avoid the boat.
Although porpoises responded to approaching boats by speeding up and moving away from the boat path, their behaviour during the minute where the boat was closest did not differ from their pre-disturbance behaviour (Fig. 5). Additionally, after the boat passed at a speed of 20 knots, animals soon started to reduce their speeds, while their speeds never increased much when they were approached at 10 knots. Variations in diving probability and turning angle further did not seem to reflect proximity to the boat (Fig. 6). These results indicate that the direct impact of the boat was brief, and that the behaviour observed for many of the animals during exposure was similar to their – often highly variable – behaviour before the experiment started.
The short-term impacts observed in this study might be due to the use of a single boat in this study, and due to the predictability of its path during our experiments. In reality, porpoises are likely to encounter vessels traveling at different speeds and that make abrupt turns, which would make it more risky for them to decide not to respond to approaching boats. Studies in different locations, such as South Carolina, U.S., and Cardigan Bay, U.K., found that erratic approaches and the presence of multiple vessels had more pronounced negative effects on cetacean behaviour and movement patterns (Mattson, Thomas, & St. Aubin, 2005; Veneruso et al., 2011). Another factor that may influence the porpoises’ behaviour is that our study area is close to a marina with 700 boats; during the summer around 300 boats approach the marina per day (source: https://www.kertemindehavn.dk/kerteminde-marina/). The porpoises are therefore used to boat traffic, which may cause them to respond less to approaching vessels than animals in quieter areas, as has previously been observed in some cetaceans (Stevens, Allen, & Bruck, 2023).
One important take-home message of our study is that animals differ considerably in their response to approaching vessels, as well as in their natural movement patterns (Fig 6, Fig. A3 and Table A1 in Appendix A). While some animals appeared to react to the vessel, others moved faster and turned more prior to exposure. The observed differences in reactions illustrate why it is important to study a random sample of the population, rather than merely report an apparent change in behaviour for a few animals far from a vessel or other disturbances. The observed variability in movement behaviours likely arises from the fact that different animals are involved in behaviours that are more important to them than the approach of a boat. For example, harbour porpoises usually mate between July and August in Danish waters (Sørensen & Kinze, 1994), and two of the animals in our study were chasing each other or attempting to mate, which potentially diverted their attention from the approaching boat.
Animals with calves are likely to attempt to stay together with their calf, and hence to react less to the boat than average animals. In our study there were six mother-calf pairs (Fig. A3; Table A1 in Appendix A), but a visual inspection of their movement patterns (Fig. A3) did not suggest that mothers or calves reacted differently to the approaching boat than other animals. However, as mothers and calves did not always stay closely together during the observation period, it is challenging to conclusively determine whether the boat had a negative impact on the pairs. In addition to mating and nursing behaviour the porpoises may also be engaged in different kinds of foraging behaviours and differ in age and health status, all of which influence their movements and contributes to masking any impact of an approaching boat.
Other species of cetaceans have been reported to respond to vessels in ways that resemble those observed in this study. For example, bottlenose dolphins (S. M. Nowacek, Wells, & Solow, 2001; Marley et al., 2017), and killer whales (Williams, Trites, & Bain, 2002; Williams et al., 2009) exhibit altered movement patterns in response to vessel disturbances. However, killer whales, in contrast to porpoises in our study, exhibited larger turning angles between successive dives in the vicinity of boats, which is something we did not observe for porpoises. This may partly be due to the shorter time interval between consecutive moves in our study, which automatically reduces the occurrence of sharp turns. Humpback whales have been observed to dive more frequently in the presence of whale-watching vessels and to move away when the vessel was within 100 m (Stamation et al., 2010). While previous research regarding bottlenose dolphins (Papale, Azzolin, & Giacoma, 2012; Baş, Amaha Öztürk, & Öztürk, 2015; Baş, Christiansen, Öztürk, Öztürk, Erdoǧan, et al., 2017) found that animals reacted more negatively to faster vessels, our observations indicate that porpoises responded distinctively to the approaching boat at different speeds. However, it is uncertain whether faster vessels have led to increased energy expenditure in the animals here.
Our findings that the impact of boats is brief for harbour porpoises corresponds to what has previously been reported in other species of cetaceans. For instance, in Yaldad Bay, Chile, Chilean dolphins (Cephalorhynchus eutropia ) rapidly resumed their natural behaviour after encountering boats, possibly as an energy conservation measure (Ribeiro, Viddi, & Freitas, 2005). However, notably, individuals engaged in foraging took longer to return to their natural behaviours than those that did not forage. A similar trend was observed in Indo-Pacific bottlenose dolphins in areas with high vessel traffic, where short-term responses to boats were even less pronounced (Bejder et al., 2006). This diversity in responses highlights that cetaceans differ in their sensitivity to vessel disturbances, and that they adopt different strategies to avoid them. This emphasizes the need for context-specific impact assessments.
The widespread presence of recreational boats exposes cetaceans to high levels of disturbance. In Danish waters, where our study was conducted, recreational boats are often found in areas that are important harbour porpoise habitats (Hao & Nabe-Nielsen, 2023). In such habitats even seemingly minor avoidance responses to individual boats may influence the porpoises’ foraging behaviour and energy budgets due to repeated exposure. In areas with limited food resources, missed foraging opportunities could lead to energy deficits and reduced reproductive rates (Lusseau, 2004). In addition to vessel disturbances, porpoises are affected by bycatch, chemical pollutants, and climate change (MacLeod et al., 2007; Pierce et al., 2008; Nabe-Nielsen et al., 2014), and amplifying cumulative impacts could ultimately alter population dynamics (Nabe-Nielsen et al., 2018; Gallagher et al., 2021). This emphasizes the need for holistic assessments of the combined impacts of different stressors, and to do this, it is important to study the impacts of each stressor in isolation. This requires an experimental setup, like the one we have used here, where we provide the first direct results on how harbour porpoises react to approaching vessels in Danish waters.

Data availability

The raw data used for the analysis in this study, including porpoise locations, swimming states, boat locations and recorded boat noise levels, is accessible on Dryad (https://doi.org/10.5061/dryad.q83bk3jq8). The corresponding R scripts used for conducting the analysis and calculating porpoise’s avoidance behaviour from the boat track are also available at the same location.

Acknowledgements

Xiuqing Hao’s PhD study is funded by China Scholarship Council and Department of Ecoscience, Aarhus University. The drone was employed with permissions from Trafik-, Bygge- og Boligstyren (Danish transport-, construction- and housing authority; permit number: 5032864 and 5411169).

Author contributions

All authors contributed to writing the manuscript. Xiuqing Hao, Héloïse Hamel, Magnus Wahlberg, Jacob Nabe-Nielsen and Caitlin Kim Frankish contributed to the study conception and design. Material preparation, data collection and analysis were performed by Xiuqing Hao, Héloïse Hamel, Céline Hagerup Grandjean, Ivan Fedutin, and Magnus Wahlberg. All authors read and approved the final manuscript.

Conflict of interest

The authors have no relevant financial or non-financial interests to disclose.

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Appendix A: