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: