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).