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\begin{document}

\title{Harbour porpoises respond to small boats by speeding up and moving away}



\author[1]{Xiuqing Hao}%
\author[2]{Héloïse Hamel}%
\author[2]{Céline Grandjean}%
\author[2]{Ivan Fedutin}%
\author[2]{Magnus Wahlberg}%
\author[3]{Caitlin Frankish}%
\author[1]{Jacob Nabe-Nielsen}%
\affil[1]{Aarhus University}%
\affil[2]{University of Southern Denmark}%
\affil[3]{Aarhus Universitet}%


\vspace{-1em}



  
  \date{May 22, 2024}


\begingroup
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\selectlanguage{english}
\begin{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\selectlanguage{ngerman}°
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.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


Xiuqing Hao\textsuperscript{1,*}, H\selectlanguage{ngerman}éloïse Hamel\textsuperscript{2},
Céline Hagerup Grandjean\textsuperscript{2}, Ivan
Fedutin\textsuperscript{2}, Magnus Wahlberg\textsuperscript{2}, Caitlin
Kim Frankish\textsuperscript{1}, Jacob Nabe-Nielsen\textsuperscript{1}

\textsuperscript{1}Dept. of Ecoscience, Aarhus University, Roskilde,
Denmark

\textsuperscript{2}Dept. of Biology, University of Southern Denmark,
Kerteminde, Denmark

\textbf{Correspondence}

\emph{Xiuqing Hao} :

Email: xiuqing@ecos.au.dk;

Phone: +45 52733636;

Address: Department of Ecoscience, Aarhus University, Frederiksborgvej
399, DK-4000 Roskilde, Denmark

\textbf{Page heading title:} Harbour porpoises' reactions to small boats

\subsection*{Abstract}

{\label{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 (\emph{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; \emph{Phocoena phocoena} ; recreational vessels; underwater
noise

\subsection*{Introduction}

{\label{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 (\emph{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
(\emph{Orcinus orca} ) move in less predictable patterns when disturbed
by vessels (Williams, Trites, \& Bain, 2002) and bottlenose dolphins
(\emph{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 (\emph{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.

\subsection*{Materials and methods}

{\label{materials-and-methods}}

\subsubsection*{Study site and experiment
design}

{\label{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 v2.0\textsuperscript{TM} 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 \selectlanguage{english}[?]2 (Douglas scale), wind speed \textless{}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).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Fig. 1} Area where porpoises were exposed to boat disturbance.
The red rectangle in the top left map indicates the location of the
study area; coloured lines show the boat tracks on different dates
during fieldwork. The country map was extracted using the
``rnaturalearth'' package for R (Massicotte \& South, 2023). Bathymetry
data were obtained from https://emodnet.ec.europa.eu/en/bathymetry.

\textbf{Hosted file}

\verb`image2.emf`  available at \url{https://authorea.com/users/753863/articles/723942-harbour-porpoises-respond-to-small-boats-by-speeding-up-and-moving-away}

\textbf{Fig. 2} Experimental setup.

Each experiment consisted of three phases (see Fig. 2 for experimental
setup): 1) Before exposure (phase 1), the research boat had the engine
turned off \textgreater{}300 m from any porpoise. The distance was
\textgreater{}400 m in the 20 knots experiments to ensure porpoises had
as much time to react to the boat as in the 10 knot experiments. Upon
sighting porpoises, we launched the drone to obtain video footage of
their behaviour for 1.5 min. We only analysed data from the last minute
of this phase for comparison with data collected during the exposure
phase. 2) During exposure (phase 2) the boat started to gradually
increase speed over 30 s while moving towards the drone, passing the
porpoise without changing direction. Due to variations in currents and
waves, the maximum speed (10 or 20 knots) varied by up to 2 knots among
trials. We aimed to pass the target porpoise at a distance of 25 m
(henceforth called the closest point of approach, CPA) and continued
until the boat was \textgreater{}300 m away from the porpoise. If the
porpoise was diving, we estimated its location based on the position of
the drone. When comparing porpoise behaviour ``before'' and ``during''
exposure, we used data from the one minute centred around CPA and from
the last minute of phase 1. 3) After exposure (phase 3), we turned off
the engine and observed the porpoise for another 1.5 minutes with the
drone. To limit the influence of external variables on porpoise
behaviour, we only conducted experiments when there were no moving
vessels \textless{}1 km from the porpoise. Each group of porpoises was
only approached once, and we focused on one animal in each trial. After
each trial we moved more than one kilometre away and waited for minimum
of 30 min before approaching another porpoise to minimize the risk of
exposing the same animal twice. Throughout the experiment period, the
echo sounder of the boat was switched off.

On the last day of the fieldwork, we measured underwater noise levels at
varying distances from the boat. A stationary recorder (Sound trap,
OceanInstruments, New Zealand; 576 kHz sampling rate, 16 bits, clipping
level 176 dB re 1 \selectlanguage{greek}µ\selectlanguage{english}Pa p, as determined by relative calibration) was
suspended 2.7 m below the surface and attached to a buoy. This was done
in the same area where exposure experiments were conducted. Following
this preliminary setup, we drove the boat from a distance of
\textgreater{}300 m to pass the recorder at speeds of either 10 or 20
knots. The boat was stopped when it was \textgreater{}300 m away from
the recorder. This process was repeated twice for each boat speed. The
boat's geographical coordinates were recorded using a portable GPS
(Garmin GPSMAP 78s).

\subsubsection*{Data handling}

{\label{data-handling}}

We used the Drone Video Measure tool (Version 1.1.1; Egemose, 2021) to
extract location, swimming state and body length of the target porpoise
(one location per second). Body length was measured from the tip of its
nostrum to the fluke notch. This length was used as an indicator of the
age of the porpoise (following Stepien et al. 2023). Each porpoise was
measured up to three times from different locations (see porpoises' body
length in Table A1 in Appendix A). The swimming state was categorized as
either shallow-dive (porpoise body shape clearly visible; including
breathing animals) or deep-dive (porpoise under the water, body shape
not clearly visible; Fig. 3). To calculate the porpoise's speed and
horizontal turning angles between successive moves, we applied the
''adehabitatLT'' package (Calenge, 2006) to porpoise locations (one per
second). To quantify whether an animal tried to avoid the boat and its
tendency to move away from the boat track, we calculated the distance
between the boat track and the porpoise at two successive boat locations
(i.e., one second apart; Fig. A1 in Appendix A). To ensure that porpoise
tracks were temporally aligned with the boat tracks we compared the
clocks in the boat GPS and in the drone at the point where the boat
became visible in the drone video footage. When needed, we calibrated
the drone clock based on the time difference between the GPS's. The
location measurement accuracy was 2.4\selectlanguage{ngerman}±1.5 m when the drone was 30 m
above the porpoise (Brennecke et al., 2022).

\textbf{Hosted file}

\verb`image3.emf`  available at \url{https://authorea.com/users/753863/articles/723942-harbour-porpoises-respond-to-small-boats-by-speeding-up-and-moving-away}

\textbf{Fig. 3} Examples of porpoise swimming states a. Shallow dive,
non-breathing; b. Deep dive; c. Shallow dive, breathing. Images were
obtained from zoomed-in drone videos.

Data where the porpoise was lost in the video footage for more than five
seconds during boat approaches (i.e. phase 2 before CPA) were excluded
from all analyses. Thus, out of 27 recorded videos, only 17 (8 for boats
moving at 10 knots; 9 for 20 knots) were selected for analysing
variations in porpoise behaviour in relation to the distance to the
boat, but 16 (7 data of 10 knots, as the porpoise was lost in one of the
experiments after boat approach; 9 data of 20 knots) videos were used to
determine if the animals' behaviour during exposure (1 min around CPA)
differed from their pre-exposure behaviour, and to explore how long it
took porpoises to return to their natural behaviour after the
disturbance.

\subsubsection*{Data analysis}

{\label{data-analysis}}

To analyse how porpoises responded to the approaching boat (i.e., phase
2 before CPA, using Dataset 1; Fig. A2 in Appendix A), we built six
models for each boat speed, with either movement speed, change in
distance from boat path (i.e., avoidance distance, Fig. A2), probability
of moving away from the boat path, absolute turning angle, probability
of diving deep or breathing as a response variable. In all models we
used log\textsubscript{10}(distance) as the independent variable, as
sound levels are generally proportional to the logarithm of the distance
to the sound source. Individual ID was included as a random effect, and
an AR1 model was used to account for temporal autocorrelation. To
determine whether porpoises altered their speeds or absolute turning
angles when the boat approached, we used generalised linear mixed
effects models (GLMMs) with each behaviour as a Gamma distributed
response variable (we used a Gamma distribution instead of Gaussian
because variance of residuals was not homogenous after transformation,
and neither of the response variables can be negative). Porpoise speeds
and absolute turning angles were cube root transformed in all
statistical analyses to improve spread of data (i.e. make it more
normally distributed). To identify whether porpoises tended to move
further away from the boat path as the boat approached, we used a linear
mixed effects model (LME) with avoidance distance to the boat path (Fig.
A2) as the response variable. To examine how the probability that
porpoises were avoiding the boat path, deep-diving, or breathing
depended on distance to the boat, we built three GLMMs with each
behaviour as a binary response variable. We used the Newey-West variance
estimator (by adding ``sandwich'' argument) to re-estimate standard
errors and associated significance levels (Newey \& West, 1986), which
accounts for autocorrelation between observations by inflating estimated
standard errors (Lennon, 1999). To estimate the uncertainty of model
predictions, we calculated 95\% confidence intervals (CI) for each
model. We used the Wilson score interval (Wilson, 1927) for
probabilities associated with avoiding the boat, diving deep and
breathing, because it had better coverage probability for binomial
proportion (Brown, Cai, \& DasGupta, 2001).

To explore whether porpoises responded differently to the boat depending
on whether it approached at 10 knots and 20 knots, we used the same
methods and model types as described above for each behaviour, except
that we included boat speed (categorical) and the interaction between
boat speed and log\textsubscript{10}(distance) in the models.

To evaluate whether porpoises changed their behaviours during boat
exposure compared with before exposure (using Dataset 2; Fig. A2), we
constructed GLMMs with experiment phase (before/during exposure;
categorical) as an independent variable and individual ID as a random
effect. We accounted for temporal autocorrelation using the same method
as above. To assess whether porpoises moved faster or turned more
abruptly, we used GLMMs with either porpoise speed or absolute turning
angle (both in their cube root forms with Gamma distribution) as
response variables. To test whether the probability of diving deep or
breathing was higher, we fitted binomial models. We fitted separate
models for the 10 and 20 knot experiments. We used the ``nlme'' package
(Pinheiro et al., 2007) to fit all LME models, and the ``glmmTMB'' and
''glmmAdaptive'' packages (Brooks et al., 2017; Rizopoulos, 2022) to fit
all GLMMs.

To assess how long it took porpoises to resume their pre-disturbance
behaviour, we used generalized additive models (GAMs; using Dataset 3;
Fig. A2) with either porpoise speed, absolute turning angle, or
probability of diving deep as response variables and time relative to
the CPA as predictor (H\textsubscript{0}: the independent variables have
no effect on the response). Individual ID was included as random effect
(bs = ``re''). Models were fitted using a Gamma distribution for speed
or turning angle. For the probability of diving deep we used a binomial
distribution. A k-value of 5 for the smooth term was chosen to limit the
risk of model overfitting. We fitted GAMs using the ``mgcv'' package for
R (Wood, 2012).

We used one-tail tests to compute the statistical significance (i.e., p
\textless{}0.025 is of significance) for models evaluating how porpoises
responded to the approaching boat. Statistical significance was
attributed to a p-value of less than 0.05 across all other models. We
estimated the proportion of variance in the response variables
attributed to the independent variables by computing both marginal
R-Squared (R\textsuperscript{2}\textsubscript{m}: variance explained by
only fixed factors) and conditional R-Squared
(R\textsuperscript{2}\textsubscript{c}: variance explained by both fixed
and random factors). The boat passed the porpoises at an average
distance of 26 m (range: 9\selectlanguage{english}-40 m) during the 10 knots experiment and at
an average distance of 22 m (range: 4\selectlanguage{english}-55 m) during the 20 knots
experiment.

To investigate how received noise levels were related to distance to the
research boat, we used MATLAB (version 2022b) to analyse the recorded
data. Noise levels (in dB re 1 \selectlanguage{greek}μ\selectlanguage{english}Pa rms, 1 s average) were calculated at
full bandwidth (0.1-150 kHz) and at the 1/3 octave (TOL) 16 kHz
frequency band. To investigate how noise levels changed over time for
the two boat speeds, we calculated noise increments per 10 seconds for
both frequency bands.

\subsubsection*{Ethical note}

{\label{ethical-note}}

The research protocol was approved by the Danish Environmental
Protection Agency and by the University of Southern Denmark's Animal
Ethics Committee for non-license requiring experiments, under the
authority of the Danish Animal Ethics Inspectorate (DVO approval number:
2022/07). The potential harm to porpoise individuals was very limited,
as animals only had a risk of being disturbed when the boat was moving,
i.e., typically for 4--7 min. During before- and after-exposure
observations, the boat was stationary with the engine turned off. We did
not use the echo sounder at any time. To minimise the risk of exposing
the same animal twice we waited \textgreater{}0.5 hours and then moved
to a new different area \textgreater{}1 km away after each experiment.
Porpoises resumed their natural behaviours shortly after exposure.

\subsection*{Results}

{\label{results}}

\subsubsection*{Porpoises' behavioural response to an approaching
boat}

{\label{porpoises-behavioural-response-to-an-approaching-boat}}

A small motorboat approaching at 20 knots caused animals to swim faster
(z =-3.05, p =0.002, R\textsuperscript{2}\textsubscript{m} =0.20),
although the swimming speed varied considerably among individuals
(R\textsuperscript{2}\textsubscript{c} =0.93; Fig. 4a). No significant
change was observed at 10 knots (z =-0.57, p =0.568; Fig. 4a). Porpoises
tended to move further away from the boat track when approached by boats
at 10 or 20 knots, and there was only little variation among individuals
(10 knots: t =-4.57, p \textless{}0.001,
R\textsuperscript{2}\textsubscript{m} =0.12,
R\textsuperscript{2}\textsubscript{c} =0.19; 20 knots: t =-2.28, p
=0.023, R\textsuperscript{2}\textsubscript{m} =0.05,
R\textsuperscript{2}\textsubscript{c} =0.05; Fig. 4b). However, the
probability of moving away from the boat track depended on the boat
speed (p =0.02, interaction term between log\textsubscript{10}(distance)
and boat speed). Specifically, porpoises were more inclined to move away
when approached at a speed of 10 knots (z =-2.37, p =0.002,
R\textsuperscript{2}\textsubscript{m} =0.13,
R\textsuperscript{2}\textsubscript{c} =0.28; Fig. 4c). Although most
animals started moving away from the boat track when the boat was
100--200 m away, some animals did not move away till the boat was very
close (Fig. A3 in Appendix A). Turning angles did not increase as the
boat approached (z =-1.06, p =0.289 for 10 knots; z =0.11, p =0.908 for
20 knots), and neither did the probability of using deep dives (z
=-2.20, p =0.027 for 10 knots; z =-1.21, p =0.226 for 20 knots).
Additionally, porpoises did not breathe less often (z =1.88, p =0.060
for 10 knots; z =1.28, p =0.200 for 20 knots). Model residuals for
porpoises' speed, distance that moving away from the boat track and
absolute turning angle indicated our modelling approaches were
appropriate (Fig. A4 in Appendix A).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Fig. 4} Observations (dots) and model estimates (lines) of
porpoise behavioural responses to an approaching boat (averaged across
animals; data from phase 2 before the closest point of approach, i.e.
CPA). X-axis values were back-transformed from their logarithms. In a
and d, values on the y-axis were back-transformed from their cube root;
in b, c and d, bars show observed frequencies of a specific behaviour at
each distance range. Shaded areas show 95\% confidence intervals. *
denotes significant models (p \textless{}0.025).

\subsubsection*{Porpoise behaviour before and during
exposure}

{\label{porpoise-behaviour-before-and-during-exposure}}

The porpoises' behaviour during the minute where the boat was closest
did not differ significantly from their behaviour before the experiment
started (their speed, absolute turning angles, and the probability of
diving or of breathing, were the same; p \textgreater{}0.05 for all
variables and both boat speeds; Fig. 5).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Fig. 5} Porpoise behaviour before (phase 1) and during (phase 2)
boat exposure, measured over a period of one minute. The values on the
y-axis of a1, a2, b1, and b2 were transformed back from their cube root
form.

\par\null

\subsubsection*{Time before resuming pre-disturbance
behaviour}

{\label{time-before-resuming-pre-disturbance-behaviour}}

Porpoise speeds varied in the course of the experiments (10 knots: p
\textless{}0.001, R\textsuperscript{2} =0.083; 20 knots: p
\textless{}0.001, R\textsuperscript{2} =0.17). This was particularly
evident in the 20 knots experiments, where animals tended to move faster
when the boat approached, then slowing down \textless{}50 seconds after
the boat had passed. When the boat approached at 10 knots, this response
trend seemed not as clear (Fig. 6). However, many animals moved as fast
when the boat was not in motion as they did when the boat was nearby.
Additionally, porpoises were more likely to dive deep as the boat
approached at 10 knots, decreasing rapidly after it passed (p
\textless{}0.001, R\textsuperscript{2} =0.11). But no similar trend was
observed at 20 knots (p =0.09). Although the results presented above did
not suggest that porpoises turned more steeply or that they were more
likely to dive deep when approached by boats, the GAM analyses indicated
that the porpoises' horizontal movements (10 knots: p =0.003,
R\textsuperscript{2} =0.029; 20 knots: p \textless{}0.001,
R\textsuperscript{2} =0.028) changed significantly in the course of the
experiments. It is, however, worth noting that time relative to CPA
explained a very small proportion of the variation in this behaviour.
The probability of breathing remained unchanged throughout the
experiments for both speeds (p \textgreater{}0.05 for both boat speeds).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Fig. 6} Observations (dots) and model estimates (smooth curves)
of variations in porpoise behaviour during the exposure experiments
(phase 1, 2 and 3). Shaded areas along smooth curves show 95\%
confidence intervals. The vertical purple lines represent the closest
point of approach (CPA). The shaded light brown areas indicate the
periods where the boat was in motion (i.e. phase 2). * denotes
statistically significant models (p \textless{}0.05).

\subsubsection*{Sound received from an approaching
boat}

{\label{sound-received-from-an-approaching-boat}}

As the boat moved towards the CPA at 10 knots, the TOL 16 kHz sound
level increased from approximately 90 to 115 dB while the broadband
sound level (0.1 to 150 kHz) increased from 110 to 135 dB. The increase
in sound level was similar at 10 knots (TOL 16k: 27.2--31.6 dB;
Broadband: 26.6--29.4 dB) and 20 knots (TOL 16k: 26.1--29.4 dB;
Broadband: 26.3--30.3 dB) for both sound frequency bands (Fig. 7). The
broadband sound level was 17 dB higher than the TOL 16 kHz band level at
CPA. At a speed of 10 knots, the mean absolute change in noise level per
10 seconds was 3 dB for broadband levels and 5.1 dB for 16 kHz TOL.
Conversely, at 20 knots, the corresponding changes were 5.1 dB for
broadband levels and 5.3 dB for the 16 kHz TOL. In the 10-knot scenario,
the most rapid change in sound levels occurred during the 10 seconds
around CPA, where changes in sounds reached 14.3 dB and 17.9 dB for
broadband and 16 kHz TOL, respectively. In the 20-knot recordings, these
changes were even more pronounced; 22.0 dB per 10 s for broadband and
27.4 dB per 10 s for 16 kHz TOL.

Importantly, porpoises exhibited obvious reactions, including speeding
up and moving away from the boat, when the approaching boat was within
the range of 100--200 m. This coincided with a rapid rise in sound
levels that started occurring from around 200 m with noise levels at
100--105 dB at 16 kHz TOL (Fig. 4 and 7). After the boat had passed, the
sound levels rapidly decreased (Fig. 7).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{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.

\subsection*{Discussion}

{\label{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 \textless{}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 \selectlanguage{greek}µ\selectlanguage{english}Pa 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 \selectlanguage{greek}μ \selectlanguage{english}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
(\textgreater{}400 m; Ba\selectlanguage{polish}ş \selectlanguage{english}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).

\par\null

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 (S\selectlanguage{ngerman}ørensen \& 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; Ba\selectlanguage{polish}ş, \selectlanguage{english}Amaha
\selectlanguage{ngerman}Öztürk, \& Öztürk, 2015; Ba\selectlanguage{polish}ş, \selectlanguage{english}Christiansen, \selectlanguage{ngerman}Öztürk, Öztürk, Erdo\selectlanguage{romanian}ǧan, 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
(\emph{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.

\subsection*{Data availability}

{\label{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.

\subsection*{Acknowledgements}

{\label{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).

\subsection*{Author contributions}

{\label{author-contributions}}

All authors contributed to writing the manuscript. Xiuqing Hao, H\selectlanguage{ngerman}éloïse
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.

\subsection*{Conflict of interest}

{\label{conflict-of-interest}}

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

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\subsection*{Appendix A:}

{\label{appendix-a}}

\textbf{Hosted file}

\verb`image8.emf`  available at \url{https://authorea.com/users/753863/articles/723942-harbour-porpoises-respond-to-small-boats-by-speeding-up-and-moving-away}

\textbf{Fig. A1} Calculation of porpoise's reaction to an approaching
boat at time t1. If avoidance distance \textgreater{}0, the porpoise is
avoiding the boat. If assuming porpoises remain neutral to boat
disturbance, the avoidance distance from the boat track would be around
0, and the probability of moving away from the boat track is close to
0.5.

\textbf{Hosted file}

\verb`image9.emf`  available at \url{https://authorea.com/users/753863/articles/723942-harbour-porpoises-respond-to-small-boats-by-speeding-up-and-moving-away}

\textbf{Fig. A2} Summary of the datasets used in the analysis for
different models (T\textsubscript{0} indicates the time when the boat
starts moving; T\textsubscript{CPA} represents the time at closest point
of approach; T\textsubscript{1} indicates the time when the boat stops
moving)\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

\par\null

\textbf{Fig. A3} Individual porpoise's behaviour when a boat approached
from when it started moving (at per second interval) to before the
closest point of approach (CPA) at 10/20 knots. Porpoise's relative
speed is the difference between speed at T \textsubscript{t+1} and
T\textsubscript{t}. M\&C indicates mother-calf pair was involved in the
corresponding experiment.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\textbf{Fig. A4} Model residuals for porpoises' behaviours in relation
to their distance to the boat. Speed and absolute turning angles were
transformed to their cube root form with a Gamma distribution; Avoidance
distance was modelled with a Gaussian distribution.\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
\begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Boat speed (knots)\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
ID\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Length (m, mean)\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
SD\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Group size\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Identity\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
10\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
10\_1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.36\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.02\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
10\_2\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.82\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.07\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
2\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
mother\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
10\_3\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.48\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.03\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
10\_4\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.83\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.03\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
2\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
mother\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
10\_5\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.46\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.03\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
10\_7\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
2.21\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.06\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
3\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
mother\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
10\_8\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.33\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.02\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20\_1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.55\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.03\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20\_2\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.61\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.04\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
mother\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20\_4\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.06\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.00\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
2\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
calf\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20\_5\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.53\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.02\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20\_6\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.56\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.03\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20\_7\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.78\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.08\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20\_8\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.80\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.01\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20\_9\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.57\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.05\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
2\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
mother\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table A1} Summary of porpoise metadata. Porpoise age can be
inferred from Stepien et al. (2023) according to body length. Group size
was determined as the number of porpoises at the beginning of each
experiment.

\subsection*{}

{\label{section}}

\selectlanguage{english}
\FloatBarrier
\end{document}

