# Behavioural response of Australian fur seals (Arctocephalus pusillus doriferus) to vessel noise during peak and off-peak human visitation

Jessalyn Taylor, Rebecca Mcintosh, Rachael Gray, Isabelle Charrier

► To cite this version:

Jessalyn Taylor, Rebecca Mcintosh, Rachael Gray, Isabelle Charrier. Behavioural response of Australian fur seals (Arctocephalus pusillus doriferus) to vessel noise during peak and off-peak human visitation. Marine Pollution Bulletin, 2024, 208, pp.116947. 10.1016/j.marpolbul.2024.116947. hal-05356715

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# Behavioural response of Australian fur seals (*Arctocephalus pusillus doriferus*) to vessel noise during peak and off-peak human visitation

Jessalyn J. Taylor<sup>a,*</sup>, Rebecca R. McIntosh<sup>b</sup>, Rachael B. Gray<sup>a</sup>, Isabelle Charrier<sup>c</sup>

## ARTICLE INFO

**Keywords:**

Australian fur seal

Anthropogenic noise

Disturbance

Pinniped

Animal behaviour

## ABSTRACT

Vessel traffic is the greatest contributor to marine anthropogenic noise pollution, and particularly affects species utilising coastal areas. Seal Rocks (Victoria), the largest Australian fur seal breeding colony, has a relatively small vessel exclusion zone during the pugging and breeding season, when vessel traffic is at its peak. It is necessary to understand the impacts of visitation at sensitive marine sites to ensure they are adequately protected from human disturbance. This study assessed the behavioural response of hauled-out Australian fur seals at Seal Rocks to controlled vessel noise exposure during peak and off-peak vessel visitation periods. High level noise exposure (76–80 dB) resulted in increased vigilance and interruption to vital behaviours in both peak and off-peak periods. These behavioural responses were limited to the period of noise exposure but are nevertheless indicative of disturbance. It is important to ensure regulations intended to protect sensitive sites are up-to-date and evidence based.

## 1. Introduction

Noise from anthropogenic activity has become a pervasive and chronic form of pollution, and research into its effects has been identified as a priority area in wildlife conservation (Francis and Barber, 2013; Jerem and Mathews, 2021). Marine vessel traffic is now the dominant noise source in the ocean (Duarte et al., 2021), resulting in disturbance for many marine species, particularly those in coastal habitats where vessel traffic is concentrated (Erbe et al., 2019; Holt et al., 2009; Wisniewska et al., 2018). Such areas usually serve as breeding, spawning, and feeding grounds for a variety of species, and are thus biologically important and sensitive to disturbance (Pine et al., 2016). Pinnipeds (true seals, fur seals, sea lions, and walruses) often forage and aggregate in coastal areas and are exposed to vessel noise both underwater and while hauled out on land or ice to rest, breed, or suckle their young. However, research into the impacts of vessel noise in pinnipeds is limited.

Underwater vessel noise from ship passes has been shown to interrupt at-sea resting dives and initiate an avoidance response in harbour (*Phoca vitulina*) and grey (*Halichoerus grypus*) seals (*Mikkelsen et al.*, 2019). Further, harp seals (*Pagophilus groenlandicus*) significantly reduced vocalisations during the breeding season in the presence of a

trawler (Terhune et al., 1979), indicating potential disruption to mating behaviour. Studies of hauled out pinnipeds have primarily focused on flight initiation distances in response to vessel presence, rather than vessel noise. Results vary between (and within) species, season (breeding versus non-breeding), and location (breeding colony versus haul-out site), however, vessel approaches generally result, at a minimum, in an increase in vigilance (Cowling et al., 2015; Osterrieder et al., 2017; Pavez et al., 2015; Shaughnessy et al., 2008; Speakman et al., 2019; Strong and Morris, 2010), fleeing (Andersen et al., 2012; Henry and Hamill, 2001; Suryan and Harvey, 1999), aggression (Stafford-Bell et al., 2012), and in extreme cases, stampedes (Back et al., 2018). Pinnipeds are myopic (short-sighted) and astigmatic (blurred vision) in air, and several authors suggested that animals were responding to the noise, rather than the sight, of vessels (Back et al., 2018; Cowling et al., 2015; Pavez et al., 2015), however, this was not tested.

To date, only three studies have experimentally assessed the impact of airborne vessel noise on hauled out pinnipeds. When exposed to motor vessel noise, Cape fur seals (*Arctocephalus pusillus pusillus*) showed short-term increases in vigilance and reduced resting and nursing behaviour at two breeding colonies, Pelican Point (Namibia; Martin et al., 2022) and Lambert's Bay (South Africa; Martin et al., 2023). However, Cape fur seals at Cape Town Harbour (South Africa), a haul-

<footnote><sup>a</sup> Sydney School of Veterinary Science, Faculty of Science, The University of Sydney, Camperdown, NSW 2050, Australia</footnote>

<footnote><sup>b</sup> Conservation Department, Phillip Island Nature Parks, Cowes, VIC 3922, Australia</footnote>

<footnote><sup>c</sup> Université Paris-Saclay, Institut des Neurosciences Paris-Saclay, CNRS, 91400 Saclay, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: jessalyn.taylor@sydney.edu.au (J.J. Taylor).</footnote>
out site with high levels of anthropogenic activity, showed no response when subjected to the same noise exposure experiments (Martin et al., 2023). The variation in responses was likely due to differing group structures, arousal levels, and ambient noise levels between the sites (Martin et al., 2023), highlighting the importance of site-specific research to inform management strategies. In the third study, conducted at Kanowna Island (Victoria, Australia), an Australian fur seal (Arctocephalus pusillus doriferus) breeding colony, vessel noise at or above 75 dB sound pressure level (SPL) resulted in short-term increases in aggression and vigilance, and a decrease in rest (Tripovich et al., 2012). In one case, the early phases of a stampede were observed, leading to immediate cessation of the experiment. Changes in vocal behaviour during and after high level noise exposure were consistent with aggressive responses, suggesting that seals perceived motor vessel noise as a threat (Tripovich et al., 2012). This study focused on a single colony in a remote location and, as demonstrated in Cape fur seals, is not necessarily transferable to other sites, particularly those closer to urbanised areas with higher visitation.

Australian fur seals breed in dense colonies on rocky coastlines or low-lying offshore islands, predominantly in Victorian Bass Strait (Warneke and Shaughnessy, 1985). The panning and breeding season (October–January) is an energetically demanding period for the population, during which they are particularly vulnerable to disturbance. Territorial males fast while guarding their harems for up to 50 days, adopting energy-conserving methods of posturing and vocalisation to attract mates and warn off potential challengers (Stirling and Warneke, 1971; Tripovich et al., 2008; Warneke and Shaughnessy, 1985). Females give birth to a new pup, establish the vocal recognition critical to mother-pup reunion (and therefore pup survival) (Tripovich et al., 2006, 2009), then mate before resuming their intermittent foraging and suckling trips (Arnould and Hindell, 2001). Neonatal pups are left to fast until their mother returns, while simultaneously undergoing a period of rapid growth and physiological development in the ensuing weeks (Spence-Bailey et al., 2007). Many pups and newly weaned yearlings die of starvation during this period, and pups can be vulnerable to injury or death from trampling or conspecific trauma (Kirkwood and Goldsworthy, 2013). Further, body condition of adult females at Kanowna Island has significantly declined over the last two decades (Geeson et al., 2023). While it is not known if the decline in adult female condition is population-wide, live pup abundance has decreased throughout the Australian fur seal population since 2007 (McIntosh et al., 2022), and the species faces multiple threatening processes (Fulham et al., 2020, 2022; Gardner et al., 2022; McIntosh et al., 2015; McLean et al., 2018; Taylor et al., 2021). These effects could be compounded by human disturbance, as frequent diversion of energetic resources away from vital behaviours, such as resting and nursing, can have negative population-level consequences (Lusseau and Bejder, 2007; McHuron et al., 2018).

Seal Rocks (Phillip Island, Victoria), is the largest Australian fur seal breeding colony, representing around one quarter of the total population (McIntosh et al., 2022). Due to its proximity to urban and high recreational areas, Seal Rocks has the potential to receive a high volume of passing vessel traffic and visitation. Vessel visitation is observed to peak in late December and early January, during the austral summer holidays and toward the end of the panning and breeding season, although there have been no official efforts to monitor this. Further, despite a lack of supporting evidence, the Seal Rocks colony has long been assumed to be habituated to vessel visitation compared to other breeding colonies. This is based not on seal behavioural responses but on a human preconception that it has been visited for a long time, so must be habituated (R McIntosh, pers comm). Based on this assumption, Seal Rocks has the shortest minimum approach distances (60 m for allowable vessels, e.g. recreational boats; 260 m for non-allowable vessels, e.g. personal watercraft; 20 m for permitted tour vessels), with no increase in distance during the breeding season, unlike the other designated breeding sites that have greater protection during this time (State of Victoria, 2019). It is important to note that even if a population appears

habituated, this does not imply the absence of negative impacts (Bejder et al., 2009; Francis and Barber, 2013). Recent pup health evaluations at Seal Rocks have highlighted the increased vulnerability and potentially increased stress of neonatal pups, compared to pups at 5–6 months of age (J Taylor, unpub. data). With the downward trend in live pup abundance, assessing the impact of vessel noise and visitation at Seal Rocks is crucial to determine whether the current management strategy is sufficient to protect the population.

The aim of this study was to assess the behavioural response of hauled-out Australian fur seals at Seal Rocks to three levels of airborne motor vessel noise using controlled noise exposure (playback) experiments. In addition, this study aimed to compare the behavioural responses of seals between the peak (breeding) and off-peak (non-breeding) vessel visitation seasons. A difference in behavioural response between noise levels was expected, as well as a difference in response between seasons based on different population demographics and biological context between breeding and non-breeding periods. This research is an important contribution to our understanding of vessel noise impacts at sensitive marine sites and can be used to inform management strategies at Seal Rocks and other Australian fur seal colonies.

## 2. Materials & methods

### 2.1. Study site

The study was conducted on Australian fur seals at Seal Rocks (38°31'34"S, 145°05'59"E), two low-lying islets situated at the entrance to Western Port (Victoria, Australia; Fig. 1). Experiments were conducted over four separate fieldwork seasons: January–February 2022 (peak visitation), May 2022 (off-peak visitation), February–April 2023 (peak visitation), and June–August 2023 (off-peak visitation). Peak season experiments took place at least one month after the peak pupping season to avoid noise exposure to very young pups. All experiments were conducted from one of two 23-ft chartered research vessels on the periphery of the colony, where depth and swell conditions permitted proximity to the rocks (Fig. 1). Areas where seals were high up on rocky platforms and risked injury if they jumped or fell were avoided.

### 2.2. Noise playback

Vessel noise was pre-recorded using a Bluetooth-enabled audio recorder (Roland R-07 High Resolution Audio Recorder, Roland Corporation, Hamamatsu, Japan) held within 2 m of a switched-on outboard motor as it was flushed with water. The frequency band of the recorded engine noise was 0.01–6.0 kHz. Otariid hearing ranges from 0.5 to 32.5 kHz in-air and is most sensitive between 2.0 and 8.0 kHz (Hemila et al., 2006; Moore and Schusterman, 1987; Reichmuth et al., 2013). Noise was then played through a high-powered, wireless waterproof speaker (JBL Boombox 2, Harman International, Stamford, USA) connected to the Bluetooth player. The maximum source level (SL) of the speaker, measured at 1 m away with a sound level meter (Extech 407730 Digital Sound Level Meter, Extech Instruments, Nashua, USA) set to 'A' weighting and fast response, was 100 dB SPL.

Differences in methodology make comparisons between studies difficult, therefore, playback experiments were conducted using the methodology described by Martin et al. (2022). Each experiment consisted of a different focal group (Altmann, 1974), defined as a group of 5–10 seals within a 5 m diameter focal area, and within 10–20 m of the research vessel. One of three noise levels was randomly assigned to each group: low, 64–70 dB SPL; medium, 71–75 dB SPL; or high, 76–80 dB SPL. These levels correspond to the received level (RL) of the focal group (i.e., highest levels at the forefront of the focal group, for example, 70 dB SPL at the low noise level). To ensure the required RL was achieved over multiple broadcast distances (i.e., 10–20 m), volume settings for each noise level were calibrated on land prior to the experiments using the sound level meter. Weather conditions affected sampling opportunities,
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Study site. Left: The region in which the Seal Rocks Australian fur seal breeding colony is located, showing proximity to the mainland, Phillip Island, and Melbourne. Right: Locations of the playback experiments around Seal Rocks in both peak and off-peak seasons. Note, shaded area represents the site at low tide, points that appear to be on the rocks were conducted at high tide.</figcaption></figure>

resulting in six to 21 groups (two to seven per noise level) targeted each experimental season. All playback experiments were video-recorded for further analysis using a Canon EOS 200D II with an 18–55 mm lens (Canon Inc., Tokyo, Japan), mounted on a hand-held three-axis gimbal (AK2000C, Guilin Feyiu Technology Inc., Guilin, China).

Once a focal group was selected, the research vessel was slowly moved into position and recording commenced. Each experiment began with a 10-min pre-playback observation period, immediately followed by the 2-min noise exposure. Speaker volume was set to 100 % and volume on the recorder was adjusted according to the distance and required noise level. Position was recorded using GPS coordinates, and the distance from the speaker to the focal group was determined with a laser rangefinder (Razor HD 4000, Vortex Optics, Barneveld, USA) aimed at the animal in the centre of the group. The noise exposure was then followed by another 10-min observation period (i.e., post-playback period). Climatic conditions (weather, temperature, wind), swell height, and sea state were recorded, in addition to the start and end time of each experiment. All experiments were conducted using live-boating, rather than anchoring (with engine off) because conditions can be highly changeable and the research vessel must be able to respond instantly. Engine use was kept to a minimum and particular care was taken to remain stationary during the 2-min noise exposure.

## 2.3. Behavioural analyses

Each experiment was analysed from video recordings using the methodology described by Martin et al. (2022). Seal behavioural responses to airborne motor vessel noise were assessed using four separate tests: (1) avoidance of the noise source, (2) behavioural response to noise exposure, (3) behaviour change from pre- to during playback, and (4) behaviour change from pre- to post-playback. These four tests were designed to answer the following questions: (1) do seals avoid the site of noise exposure; (2) what is the overall behavioural response of seals to noise exposure; (3) how are behavioural time budgets impacted by the noise exposure; and (4) do seals return to pre-exposure behavioural time budgets when noise exposure ceases? Test 1 involved simply counting the number of seals at the start and end of each experimental phase to

assess seal movement and avoidance of the study area. Tests 2–4 required identification of individuals, who were then followed throughout the various phases at designated sampling intervals, and their behaviours scored according to an ethological scale from 0 to 4 (Test 2) or an ethogram (Tests 3 and 4). Each test is explained below, and a detailed methodology is available as Supplementary material (Table S1). All video recordings were assessed upon return from the field using VLC Media Player (VideoLAN Organisation, Paris, France).

Avoidance of the noise source (Test 1) was assessed by recording seal movement in the focal area throughout the three experimental phases. To facilitate this, the number of seals in the focal group was counted at four time points: the beginning of the pre-playback observation ($t_0$); the end of the pre-playback observation and beginning of noise playback ($t_{10}$); the end of noise playback and beginning of the post-playback observation ($t_{12}$); and at the end of the post-playback observation ($t_{22}$). All animals present in the focal group at each time point were counted, regardless of when they arrived.

The response of seals to noise exposure (Test 2) was assessed by monitoring the behaviour of each seal in the focal group throughout the 2-min noise playback phase. The response of each individual seal present from the beginning of the noise exposure ($t_{10}$) was scored according to the following ethological scale, developed by Martin et al. (2022): (0) no response; (1) eye movement or slight head movement toward noise source; (2) prolonged look toward noise source, change of posture, signs of alertness; (3) slight retreat, defined as moving 1–3 m away; and (4) significant retreat, defined as leaving the focal group. To be included, seals must have been present in the focal group from the beginning of the noise exposure.

Behavioural modification during the noise playback phase (Test 3) was assessed using individual activity budgets. The behaviour of every seal present in the final 2 min of the 10-min pre-playback phase ($t_8-t_{10}$) and the 2-min noise playback phase ($t_{10}-t_{12}$) was point-sampled (Altmann, 1974) every 10 s according to an ethogram developed by Martin et al. (2022; Table 1). Behaviours were treated as mutually exclusive and priority was given to vital behaviours (i.e. nursing) when more than one was displayed. To be included, individuals must have been present in the focal group for the entire 4-min period ($t_8-t_{12}$).
<caption>**Table 1**</caption>

Ethogram of seal behaviour used to assess behavioural modification in the three experimental phases. Modified from Martin et al. (2022).

<table><thead><tr><th>Behaviour</th><th>Description</th></tr></thead><tbody><tr><td>Sleeping</td><td>Lying down with eyes closed.</td></tr><tr><td>Awake</td><td>Lying or sitting in a resting position with eyes open. Includes comfort behaviours such as scratching, grooming, and stretching.</td></tr><tr><td>Vigilant</td><td>Lying with head raised or sitting up and looking attentively toward noise source or conspecific.</td></tr><tr><td>Nursing</td><td>A suckling pup or juvenile and nursing female.</td></tr><tr><td>Agonistic</td><td>Threatening a conspecific with vocalisations (growling, barking), open-mouth displays, or physical aggression (lunging, biting).</td></tr><tr><td>Nuzzling</td><td>Touching the muzzle to any body part of another seal.</td></tr><tr><td>Locomotion</td><td>Walking or running.</td></tr></tbody></table>

Behavioural modification during the post-playback phase (Test 4) was assessed in a similar way to during the playback phase (Test 3). The behaviour of individuals present during the 10-min pre-playback (t₀-t₁₀) and 10-min post-playback (t₁₂-t₂₂) phases was point-sampled every 30 s according to the ethogram in Table 1. To be included, individuals must have been present in the focal group for the entire 22-min experiment. As with Test 3, behaviours were mutually exclusive.

## 2.4. Statistical analyses

All statistical analyses were performed in R Studio (Version 4.3.1, R Core Team, 2023). For all analyses, statistical significance was set at *P* < 0.05. Statistical modelling followed the methodology of Martin et al. (2022, 2023), with the exception of one fixed effects variable (i.e. season in the present study), which differed from those used in the other two studies (i.e. stimulus type and study site), and Test 4 (explained in detail below).

A generalised linear mixed effects model (GLMM) with Poisson family and log link, using *glmer* from the *lme4* package (Bates et al., 2015), was used to assess the impact of vessel noise on avoidance of the noise source (Test 1). Variables included as fixed effects were experimental phase (t₀, t₁₀, t₁₂, t₂₂), noise level (low, medium, high), and experimental season (peak, off-peak). The number of individuals counted at each phase was set as the response variable, and focal group ID was included as a random effect. Analysis of deviance (ANODE) was performed using the *Anova* function in the *car* package to obtain *P*-values (Fox and Weisberg, 2019). The timing of the experiments had the potential to affect seal movement and sensitivity to noise playback, therefore a second GLMM was developed to evaluate the effect of time and experimental phase (as fixed effects) on the number of seals present, with focal group ID as the random effect. The ‘time’ variable had two levels: morning (10:00–12:00) and afternoon (12:00–15:00). The GLMMs were checked for overdispersion using the *check_overdispersion* function from the *performance* package (Lüdecke et al., 2021).

A cumulative link mixed model (CLMM) from the *ordinal* package (Christensen, 2023) was used to assess the impact of noise playback on seal behaviour (Test 2), as the response variable was a scale. Prospective fixed effects included in the model were noise level (low, medium, high) and season (peak, off-peak). Focal group ID was included as a random effect. To obtain *P*-values and arrive at the final model, fixed effects were tested by performing a likelihood ratio test comparing a full model, containing all variables, and a reduced model without the variable being tested.

Linear mixed effects models (LMM) from the *lme4* package were developed to assess the impact of noise playback on individual activity budgets from the last 2 min of the pre-playback period and the 2-min playback period (Test 3). Behavioural observations were converted to percentage of activity budget for analysis. For each behaviour variable (Table 1), a full model was developed with season (peak, off-peak), phase (pre, playback), and noise level (low, medium, high) as fixed effects, and individual ID as a random effect. The *Anova* function from the

*car* package was used to perform ANODE and obtain *P*-values for each model. Additional LMMs within each season and noise level combination were developed to investigate the interaction between experimental phase and age class. Phase and age class (adult female, young) were set as fixed effects and individual ID as the random effect for each behaviour variable. Originally, there were five categories of age class: pup, juvenile, subadult male, adult female, and adult male. However, pups and juveniles were observed to engage in very similar behaviours, especially suckling, and were therefore combined into one age class (‘young’). Australian fur seals are known to suckle their offspring for more than one year if their new pup dies, the yearling out-competes the new pup, or if they didn’t give birth to a new pup that season (Hume et al., 2001). Further, too few males and subadult males (total of *n* = 6 and *n* = 7, respectively) were present to satisfy sample size requirements, and they were therefore excluded from this analysis. Residual normality and homoscedasticity were checked via visual inspection of residual plots.

To assess behavioural recovery post-playback (Test 4), the decision was made to compare activity budgets from pre- to during to post-playback, instead of only from pre- to post-playback. Behavioural data during the 2-min playback phase for individuals included in Test 4 was obtained from the Test 3 dataset. Statistical analysis to assess the impact of noise playback on activity budgets from the 10-min pre-playback, 2-min playback, and 10-min post-playback periods was the same as for Test 3 (both for full models and additional models), with the exception that the phases were ‘pre’, ‘playback’, and ‘post’.

To compare significant interactions or fixed effects with more than two levels in the GLMMs (Test 1), CLMM (Test 2), and LMMs (Tests 3 and 4), pairwise analysis of estimated marginal means was conducted using the *emmeans* package (Lenth, 2024), with *Sídák* adjustment for multiple comparisons.

## 3. Results

A total of 52 playback experiments were conducted over the four fieldwork seasons: 17 at the low and high noise levels, and 18 at the medium noise level (Table 2).

In total, the responses of 339 individuals were assessed in the 52 experiments (Table 3). Adult females (*n* = 134) and young animals (*n* = 186: *n* = 76 juveniles, *n* = 110 pups) were the most abundant age classes, whereas adult males (*n* = 6) and subadult males (*n* = 13) were infrequently observed.

### 3.1. Avoidance of the noise source

Avoidance of the noise source (Test 1) was assessed by counting the number of individuals present in the focal group at the beginning and end of each experimental phase (Fig. 2). There was no effect of experimental phase (*P* = 0.390), noise level (*P* = 0.699), or season (*P* = 0.132) on the number of seals present (Table S2). Similarly, there was no interaction between experimental phase and noise level (*P* = 0.915), or experimental phase and season (*P* = 0.774) (Table S2). Further, there was no significant interaction between experimental phase and timing of the experiment (*P* = 0.498) (Table S3).

<caption>**Table 2**</caption>

Summary of controlled noise exposure experiments performed in each of the four fieldwork seasons. Noise level: low, 64–70 dB SPL; medium, 71–75 dB SPL; high, 76–80 dB SPL.

<table><thead><tr><th rowspan="2">Fieldwork season</th><th rowspan="2">Dates</th><th rowspan="2">No. days</th><th colspan="3">No. experiments</th></tr><tr><th>Low</th><th>Medium</th><th>High</th></tr></thead><tbody><tr><td>Peak</td><td>January–February 2022</td><td>4</td><td>7</td><td>7</td><td>7</td></tr><tr><td>Off-peak</td><td>May 2022</td><td>3</td><td>5</td><td>5</td><td>5</td></tr><tr><td>Peak</td><td>February–April 2023</td><td>3</td><td>2</td><td>2</td><td>2</td></tr><tr><td>Off-peak</td><td>June–August 2023</td><td>2</td><td>3</td><td>4</td><td>3</td></tr></tbody></table>
<table><caption>**Table 3**
Number of individuals in each age class per season and noise level. Noise level: low, 64–70 dB SPL; medium, 71–75 dB SPL; high, 76–80 dB SPL.</caption><thead><tr><th rowspan="2">Age class</th><th colspan="3">Peak</th><th colspan="3">Off-peak</th></tr><tr><th>Low</th><th>Medium</th><th>High</th><th>Low</th><th>Medium</th><th>High</th></tr></thead><tbody><tr><td>Adult female</td><td>36</td><td>32</td><td>26</td><td>10</td><td>21</td><td>9</td></tr><tr><td>Adult male</td><td>0</td><td>0</td><td>1</td><td>1</td><td>2</td><td>2</td></tr><tr><td>Subadult male</td><td>1</td><td>1</td><td>1</td><td>6</td><td>3</td><td>1</td></tr><tr><td>Young</td><td>20</td><td>31</td><td>33</td><td>37</td><td>31</td><td>34</td></tr></tbody></table>

## 3.2. *Behavioural response to noise playback*

An ethological scale from 0 to 4 was used to assess the overall behavioural response to noise exposure (Test 2) of 339 individuals. There was no significant effect of season on the behavioural response of individuals (P = 0.538), however, there was a significant effect of noise level (P = 0.002) (Table S4). Pairwise comparison showed a significant difference between the response of seals exposed to the high noise level, compared to those exposed to the low (P = 0.005) and medium (P = 0.003) noise levels (Fig. 3, Table S5). There was no significant difference in behavioural responses between the low and medium noise levels (P = 1.000). The high noise level was more likely to elicit strong vigilance and retreat responses (i.e., levels 2–4 on the ethological scale), although

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Seal movement throughout the playback experiments (Test 1; NS = not significant). Top: Number of seals present in the focal group during experiments conducted in the peak visitation season. Bottom: Number of seals present in the focal group during experiments conducted in the off-peak visitation season.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. Predicted probabilities and 95 % CI of seal behavioural response to noise playback at each noise level (Test 2). Response scale: (0) no response; (1) eye or slight head movement toward noise source; (2) prolonged look toward noise source, change of posture, signs of alertness; (3) slight retreat; and (4) significant retreat.</figcaption></figure>

both types of retreat were rare. The low and medium noise levels were more likely to elicit no response or slight vigilance (i.e., levels 0–1 on the ethological scale).

### 3.3. Behavioural modification during noise playback

Behavioural modification during noise exposure (Test 3) was assessed by comparing the activity budgets of 256 individuals from the last 2 min of the pre-playback period and the two-minute playback period. This analysis was performed on 83 adult females and 65 young in the peak seasons, and 38 adult females and 70 young in the off-peak seasons.

In both the peak (Fig. 4) and off-peak (Fig. 5) seasons, the most frequently observed behaviours were sleeping, awake, and nursing. Nuzzling was excluded from the analysis as it made up <1 % of observed behaviour in both seasons. Analysis of deviance and pairwise comparisons for all Test 3 models are available as Supplementary material (Tables S6–S8). In the peak visitation season, there was a significant decrease in sleeping from 24.3 % to 15.4 % (P = 0.025) and increase in vigilance from 1.1 % to 9.5 % (P < 0.001) during the playback phase at the high noise level only. In the off-peak visitation season, there was a significant decrease in sleeping from 35.6 % to 15.4 % (P < 0.001), an increase in awake from 36.9 % to 58.8 % (P < 0.001), a decrease in nursing from 22.7 % to 13.6 % (P = 0.010), and an increase in vigilance from 0.8 % to 8.8 % (P < 0.001) during the playback phase at the high noise level only. There were no significant changes in agonistic behaviour or locomotion at any noise level or in any season (P > 0.05 for all). No significant changes in any behaviours were seen at the low or medium noise levels (P > 0.05 for all). In addition, no significant interactions were observed between phase and age class for any of the behaviours tested (P > 0.05 for all).

### 3.4. Behavioural modification after noise playback

Behavioural modification post-exposure (Test 4) was assessed by

<figure><img src="image_6.png" /><figcaption>Fig. 4. Comparison of activity budgets between the last 2 min of the pre-playback phase to the 2 min of playback (Test 3) during the peak visitation (breeding) season. Low: n<sub>fem</sub> = 29, n<sub>yang</sub> = 17; Medium: n<sub>fem</sub> = 28, n<sub>yang</sub> = 23; High: n<sub>fem</sub> = 26, n<sub>yang</sub> = 25. ^Scale altered due to low occurrence. \*Statistically significant (P < 0.05).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Comparison of activity budgets between the last 2 min of the pre-playback phase to the 2 min of playback (Test 3) during the off-peak visitation (non-breeding) season. Low: n<sub>fem</sub> = 10, n<sub>yng</sub> = 23; Medium: n<sub>fem</sub> = 20, n<sub>yng</sub> = 22; High: n<sub>fem</sub> = 8, n<sub>yng</sub> = 25. ^Scale altered due to low occurrence. \*Statistically significant (P < 0.05).</figcaption></figure>

comparing the pre-, during, and post-exposure activity budgets of 178 individuals present for the entire 22-minute experimental period. This analysis was performed on 59 adult females and 46 young in the peak seasons, and 27 adult females and 44 young in the off-peak seasons.

In both the peak (Fig. 6) and off-peak (Fig. 7) seasons, the most frequently observed behaviours were sleeping, awake, and nursing. Nuzzling and locomotion were excluded from the analysis, as they each made up <1 % of observed behaviour in both seasons. Analysis of deviance and pairwise comparisons for all Test 4 models are available as Supplementary material (Tables S9–S11). In the peak season, there was a significant increase in vigilance from 1.4 % pre- to 7.9 % during playback (P < 0.001), followed by a significant decrease down to 0.1 % post-playback (P < 0.001), at the high noise level. There was also one instance of a significant interaction between phase and age class in the peak season (P = 0.035). While there was no significant difference in the mean occurrence of agonistic behaviour overall, after medium level noise exposure adult females showed a decrease from 2.9 % pre- to 0.8 % during playback, followed by an increase to 1.7 % post-playback, whereas young showed an increase from 0.3 % pre-, to 1.3 % during, to 2.6 % post-playback.

In the off-peak season, there was a significant decrease in sleeping from 38.0 % pre- to 18.8 % during playback (P = 0.003), followed by a significant increase to 35.4 % (P = 0.017); a significant increase in vigilance from 0.9 % pre- to 9.3 % during playback (P < 0.001), followed by a significant decrease down to 1.7 % post-playback (P < 0.001); and a significant increase in agonistic behaviour from 0.3 % during to 3.0 % post-playback at the high noise level only. There were no significant interactions between phase and age class observed during the peak season (P > 0.05 for all). No significant changes in behaviour were seen at the low or medium noise levels (P > 0.05 for all).

No significant changes in awake or nursing between experimental phases were observed in either season (P > 0.05 for all). There were also no significant differences in any behaviour between the pre- and post-playback phases in either season (P > 0.05 for all).

# 4. Discussion

This study assessed the behavioural responses of Australian fur seals to airborne motor vessel noise at the Seal Rocks breeding colony in the context of peak and off-peak human visitation. Results of controlled noise exposure experiments show that noise above 75 dB is likely to cause short-term disruptions to nursing and an increase in energy-consuming behaviours. Contrary to what was expected, the difference in behavioural response between seasons was minimal. In addition, high level noise exposure was more likely to cause seals to move away from the focal area but did not result in avoidance of the study area overall. Extreme responses, such as the start of a stampede, were never observed during any of the experiments. While these results indicate only short-term responses, they are nevertheless indicative of disturbance caused by vessel noise. As well as noise from vessels, the exposure frequency experienced by the colony should be considered.

Based on the number of seals in the focal group at each stage of the experiment (Test 1), noise exposure did not result in avoidance of the focal area in either the peak or off-peak seasons, or in the morning or afternoon. This is similar to the findings of Martin et al. (2023), who found no effect of experimental phase or timing on the number of Cape fur seals at either the Lambert's Bay breeding colony or Cape Town Harbour haul-out site, but contrasts with the significant decrease in number of Cape fur seals present during noise exposure at the Pelican Point breeding colony (Martin et al., 2022). In the present study,
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Comparison of activity budgets between the 10-minute pre-playback, 2-minute playback, and 10-minute post-playback observation phases (Test 4) during the peak visitation (breeding) season. Low: n<sub>fem</sub> = 17, n<sub>yang</sub> = 11; Medium: n<sub>fem</sub> = 21, n<sub>yang</sub> = 19; High: n<sub>fem</sub> = 21, n<sub>yang</sub> = 16. ^Scale altered due to low occurrence.
*Statistically significant (P < 0.05) difference between pre- and during playback. +Statistically significant (P < 0.05) difference between during and post-playback.
XSignificant interaction between phase and age class.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Comparison of activity budgets between the 10-minute pre-playback, 2-minute playback, and 10-minute post-playback observation phases (Test 4) during the off-peak visitation (non-breeding) season. Low: n<sub>fem</sub> = 8, n<sub>yang</sub> = 13; Medium: n<sub>fem</sub> = 11, n<sub>yang</sub> = 12; High: n<sub>fem</sub> = 8, n<sub>yang</sub> = 19. ^Scale altered due to low occurrence.
*Statistically significant (P < 0.05) difference between pre- and during playback. +Statistically significant (P < 0.05) difference between during and post-playback.</figcaption></figure>
affected individuals that left the focal area during noise exposure could have been replaced by other individuals moving through or sitting within the focal group during head counts, which could have confounded results. While this indicates that the presence of the boat or noise exposure did not prevent seals from using the focal area, it perhaps underestimates the number of individuals affected by the noise exposure; a limitation that is specific to this aspect of the analysis as the other three analyses involved examining individual behavioural responses in detail.

Responses of individual seals to noise exposure (Test 2) were variable, but generally showed that vessel noise above 75 dB (i.e., high level) was significantly more likely to result in vigilance and retreat responses (levels 2-4 on the ethological scale), although both retreat responses (slight or significant) were rare. There was no significant difference in responses to noise in the 64-70 dB or 71-75 dB range (low and medium levels, respectively), which were significantly more likely to result in a glance toward the noise source or no visible response. These results were consistent between peak and off-peak seasons and are similar to responses observed in Australian fur seals at Kanowna Island (Tripovich et al., 2012) and Cape fur seals at Pelican Point (Martin et al., 2022). Ambient noise levels were opportunistically recorded with the sound level meter from the research vessel during observation periods, ranging from 61 to 75 dB (median = 71 dB) in the peak visitation season and 60-83 dB (median = 66 dB) in the off-peak visitation season (including vocalising seals and seabirds, noise from wind, swell against the rocks, and the research vessel), which could partly explain the lack of response to vessel noise around this level. In addition to ambient noise, behavioural responses to a stimulus are determined by intrinsic factors, such as an individual's age, current behavioural state, arousal level, health status, and past experiences with anthropogenic noise, as well as other extrinsic factors, such as group structure and position in the group (Boren et al., 2002; Ellison et al., 2012; Erbe et al., 2019; Zenth et al., 2021). For example, an awake seal at the front of the focal group could have a stronger response than a sleeping seal at the back of the group. Although we were unable to account for such factors, these results indicate behavioural disturbance in response to high level vessel noise exposure.

Behavioural modification of seals during noise exposure (pre-playback versus playback; Test 3) was consistent across age classes, but varied slightly between seasons, and was significantly affected by the high noise level. The significant reduction in sleeping and increase in vigilance in both seasons, closely resembles the behavioural modifications of Australian fur seals at Kanowna Island (Tripovich et al., 2012) and Cape fur seals at Lambert's Bay (Martin et al., 2023). In addition, the decrease in nursing and increase in wakefulness observed during the off-peak season is similar to changes in behaviour observed in Cape fur seals at Pelican Point (Martin et al., 2022). The difference in changes to nursing behaviour between seasons could reflect a difference in underlying behavioural motivations and available energetic resources between the two periods. For example, harbour seals were less responsive and more reluctant to flee approaching boats during the breeding season, likely due to the presence of neonatal pups (Andersen et al., 2012). Further, vessel visitation is significantly lower in the off-peak season (J Taylor, unpub. data), therefore the experimental vessel noise exposure could have been an unexpected disturbance for the seals. Despite the slight differences, the observed behavioural modifications in both seasons are nevertheless indicative of short-term behavioural disturbance. Long-term anthropogenic disturbance simulations show that when female foraging behaviour is disrupted, resulting in interruptions to nutritional intake and energetic costs for both females and young animals, this can have negative effects on population dynamics, even when disturbance is of a short duration (McHuron et al., 2018; New et al., 2014). More research is required to understand the energetic costs of the observed interruptions to resting and nursing during high level noise playback in the present study. While there is potential for reduced offspring recruitment and female survival (i.e. through higher energetic

costs of not suckling or resting), the frequency of potential disturbance events (particularly in the off-peak season) is likely to be low.

Behavioural modifications following noise exposure (pre-playback, versus playback, versus post-playback; Test 4) revealed no significant differences in any behaviour between the pre- and post-playback phases. There was a decrease in sleeping and increase in vigilance from pre- to during high level noise exposure, however, these behaviours returned to pre-playback levels post-exposure. Importantly, there was no evidence of a lasting effect of noise exposure on nursing. While there was a significant increase in agonistic behaviour from during to post-exposure at the high noise level, this was very small (i.e. 0.3 % to 3.0 %). These results are similar to those reported for Australian fur seals at Kanowna Island, who displayed decreased alertness and increased rest post-exposure (Tripovich et al., 2012), and to Cape fur seals, who primarily showed no significant changes between pre- and post-playback at any of the three sites tested (Martin et al., 2022, 2023). Importantly, in the present study and in both Martin et al. (2022, 2023) studies, behaviours were treated as mutually exclusive, however, this may not have been the case. For example, a female could be nursing a pup (or juvenile) and be simultaneously vigilant, but the behaviour would be marked as nursing to prioritise detection of disruptions to vital behaviours, resulting in an underestimate of vigilance at all phases of the experiment. The lack of a long-term effect on nursing and return to baseline sleeping and vigilance behaviours post-exposure in both seasons indicates that seals recovered from the noise exposure and re-initiated vital behaviours.

When comparing activity budgets between the pre-, during, and post-playback phases, there was also one instance of disparate responses between age class. In the peak season, adult females and young animals showed differences in agonistic behaviour during and after medium level noise exposure, whereby adult females significantly reduced agonistic behaviour and young animals significantly increased agonistic behaviour. The driving factor behind this is unclear and the low level of occurrence of agonistic behaviour (<3 %) needs to be considered. It is possible that the 30 s sampling regime was not sensitive enough to pick up short-duration behavioural events such as agonistic interactions and other low-occurrence behaviours such as vigilance, locomotion and nuzzling during the 10-min pre- and post-playback observation periods.

In addition to those already highlighted, there are several limitations inherent in the current study, which could be underestimating the impact of vessel noise and visitation at Seal Rocks. Firstly, the 2-min noise playback used in this study is likely shorter than what seals would be exposed to under 'real' conditions, the impacts of which should be further investigated. For example, the local tour boat sits at the site for at least 20 min per visit under a tourism permit from the state government, and visiting recreational vessels usually remain at Seal Rocks for at least 5 min (pers. obs.). Secondly, in the case of activity budgets, these only include animals that were present for the entire 4- or 22-minute periods, and are potentially based on the least sensitive animals, a similar finding to that in Cape fur seals by Martin et al. (2022). Thirdly, animals could be less likely to respond or move away during early lactation (i.e. adult females remaining with their offspring), or if they are sick or injured to avoid discomfort or save energy. However, there is likely to be simultaneous physiological changes, such as a stress response, occurring that are not manifested in behavioural changes (Bejder et al., 2009; Francis and Barber, 2013). Detection of a stress response directly related to noise exposure would require capturing individual animals and collecting blood samples before and after noise exposure. This is neither possible nor practical in free-ranging populations, not only due to the difficulty involved in attaining a large enough sample size, but also because capture and handling affects stress levels. Such studies can be carried out in captivity (e.g. Romano et al., 2004), but results are not necessarily transferrable from captive to wild animals. However, an opportunistic study by Rolland et al. (2012) of faecal stress hormone metabolites revealed a decrease in stress in the absence of shipping noise in right whales (Eubalaena glacialis). Monitoring of seasonal stress levels in the population could provide useful
context for additional impacts of vessel noise and visitation, as animals that are already stressed likely have less resilience to human disturbance.

It is also likely that seals responded to the presence and movement of the research vessel. Australian fur seals at Kanowna Island responded to boat approach at 75 m (Back et al., 2018), long-nosed fur seals (Arctocephalus forsteri) at 20 m (Cowling et al., 2015), grey seals at 150 m (Strong and Morris, 2010), and harbour seals as far out as 830 m, although this was possibly in response to the auditory, rather than visual, stimulus (Andersen et al., 2012). The use of a boat was unavoidable, primarily because a team cannot land on Seal Rocks without causing large-scale disturbance and pushing seals into the water, but also because the areas closest to the water, where seals would normally be exposed to vessel noise, are not safely accessible by researchers. The use of live-boating was also unavoidable due to safety requirements, and while engine use was kept to a minimum, on occasion, the vessel would suddenly move closer due to swell surge. The pre-playback observation period was implemented to condition the seals to the visual stimulus of the research vessel prior to noise exposure and partly addresses this concern. However, although there were no extreme responses (e.g. the start of a stampede) observed during the experiments, on at least three occasions, approaches to potential focal groups were aborted because the seals appeared too flighty (i.e. beginning the cascade response that precedes a stampede) and the level of disturbance would have been unacceptable. Therefore, the focal groups that were tested were already the most tolerant of vessel proximity.

Finally, while these results show that exposure to vessel noise above 75 dB can result in short-term behavioural disturbance, the vessel speed and/or proximity that creates such noise levels are unknown and would vary by vessel type, size, and power. As suggested by Tripovich et al. (2012), this could be determined using sound propagation tests, either from controlled approach or observational studies, in future. There is also no official record of the daily frequency of vessel visitation at Seal Rocks because Automatic Identification Systems (AIS) and Vessel Monitoring Systems (VMS) are not mandatory in small recreational vessels (Australian Government, 2012). While this study has established behavioural responses to vessel noise, monitoring of visitation at Seal Rocks will assist in determining the number of potential disturbance events during peak and off-peak seasons and facilitate better understanding of cumulative impacts of vessel noise.

Even with these limitations in mind, as well as the apparent short duration of the behavioural changes and lack of extreme responses shown by Australian fur seals at Seal Rocks, these results nevertheless indicate behavioural disturbance in response to high level vessel noise exposure. Animal responses to anthropogenic disturbance often equate to anti-predator responses (Frid and Dill, 2002). While these effects are generally non-lethal and individual tolerance to disturbance can increase with exposure, the cumulative impact of repeated disturbance should be considered, especially in the context of multiple threats acting on the population. Further, the observed behavioural responses in the present study are similar to those reported by Tripovich et al. (2012) at Kanowna Island, a colony that is believed to be more naïve to vessel visitation than Seal Rocks (Back et al., 2018). Therefore, concerns over visitation at Seal Rocks should not be dismissed based on the presumption that the seals are habituated.

In light of this, the disparity in minimum approach distances between the colonies is of particular concern. At Kanowna, the minimum approach distances are 200 m for private vessels and 100 m for tour vessels from November to January (inclusive), and 50 m for all vessels from February to October (State of Victoria, 2019). By contrast, at Seal Rocks the minimum approach distances are 60 m (i.e. the colony boundary) for private vessels and 20 m for permitted tour vessels year-round (State of Victoria, 2019). The minimum approach distance for small personal watercraft (PWC; e.g. jet-ski) at all colonies is 200 m from the colony boundary (Victorian Government, 2019). Particularly in the case of private vessels and PWCs, these regulations are frequently

violated at Seal Rocks, especially during the peak visitation season (J Taylor, unpub. data). Furthermore, there are no regulations regarding the time a vessel can remain onsite or how many vessels can visit concurrently. While differences in behavioural responses between peak and off-peak visitation seasons were minimal, pups are most vulnerable during the peak visitation (breeding) season (J Taylor, unpub. data). While further research is required to fully determine the impact of vessel visitation at Seal Rocks, these results can contribute to future evaluations of vessel approach limits. In the interim, better education and enforcement of existing approach limits is recommended.

## 5. Conclusions

This study is one of very few to evaluate the impacts of airborne vessel noise in pinnipeds and the first to examine seasonal differences in behavioural responses. By conducting conceptual replication using an established methodology, this study will facilitate comparisons between colony sites and species. Based on results of the present study, exposure to boat engine noise above 75 dB from vessels typical of the area results in short-term behavioural disturbance at Seal Rocks. However, if such responses are frequent, this could result in population-level consequences and has implications for management of the site. The results presented in this study can contribute to future evaluations of vessel approach limits at Seal Rocks. Monitoring to establish the frequency of potential disturbance events and better enforcement of current vessel approach limits are therefore recommended. Human activity is changing the environment faster than many species can adapt (if they can adapt). To be effective, regulations intended to protect wildlife should be evidence-based and frequently reviewed.

### CRediT authorship contribution statement

Jessalyn J. Taylor: Writing – original draft, Visualization, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis. Rebecca R. McIntosh: Writing – review & editing, Supervision, Project administration, Funding acquisition. Rachael B. Gray: Writing – review & editing, Supervision, Project administration, Funding acquisition. Isabelle Charrier: Writing – review & editing, Supervision, Resources, Methodology, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgements

We acknowledge the Bunurong of the Eastern Kulin nation, upon whose land and sea Country this research took place, and pay our respects to Elders past, present, and emerging.

The authors would like to thank KinaDiving, Dive Phillip Island, and all field volunteers for their assistance with carrying out the playback experiments, Phillip Island Marine for assistance with recording boat engine noise, Dr. Mathilde Martin for advice and assistance with the initial data analysis, and Harley Schinagl (TerraLab) for generating maps for publication.

The authors would also like to thank the two anonymous reviewers, whose insightful comments and suggestions greatly improved the quality of the manuscript.
## Ethical statement

All work was conducted under Wildlife Research Permit 10010062 from the Victorian Department of Environment, Land, Water, and Planning (DELWP). This research was approved by Phillip Island Nature Parks Animal Ethics Committee (Permit number 3.2020).

## Funding sources

This research was funded by the Foundation for Australia's Most Endangered Species (FAME); the Holsworth Wildlife Research Endowment & Ecological Society of Australia; Phillip Island Nature Parks; and the CNRS (IRP Social Communication Network in Marine Mammals). J. T. was funded by a PhD stipend and Postgraduate Research Support Scheme from the University of Sydney.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.marpolbul.2024.116947.

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