Diving pattern and behavioral response to vessel traffic of Indo-Pacific humpback dolphins (Sousa chinensis) in Xiamen, China: implications for conservation

125 Diving pattern and behavioral response to vessel traffic of IndoPacific humpback dolphins (Sousa chinensis) in Xiamen, China: implications for conservation Yi Lu1*, Haojie Zhuo2*, Haizhou Li2*, Xinrong Xu2, Guang Yang1,2 1 Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), No. 1119, Haibin Road, Nansha District, Guangzhou, Guangdong, 511458, China 2 Jiangsu Key Laboratory for Biodiversity and Biotechnology, College of Life Sciences, Nanjing Normal University, 1 Wenyuan Road, Nanjing, Jiangsu, 210023, China Corresponding author: Guang Yang ([email protected]) Copyright: © Yi Lu et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract Marine mammals have evolved sophisticated diving strategies to balance foraging efficiency with the avoidance of disturbances, a challenge that is particularly acute in coastal environments heavily influenced by human activities. The Indo-Pacific humpback dolphin (Sousa chinensis), inhabiting Xiamen Bay, China, is among the species most vulnerable to such pressure. Despite this, knowledge of its diving behavior and sensitivity to vessel traffic remains limited. In this study, a total of 4,408 dives of Indo-Pacific humpback dolphins were recorded, with a mean duration of 20.41 ± 1.77 s. Within the same age group, no significant differences in dive duration were found between traveling and foraging behaviors. However, comparisons across age classes revealed that subadults and adults consistently executed longer dives than juveniles, reflecting an ontogenetic progression in diving capacity. Furthermore, the dolphins demonstrated pronounced negative behavioral responses to speedboats and vessels operating at distances less than 100 m, underscoring marine traffic as a substantial source of disturbance. Based on these findings, it is recommended that vessels maintain a minimum buffer zone of 100 m, comply with a strict 10-knot speed limit, and adopt proactive avoidance practices when transiting through dolphin habitats. Key words: Anthropogenic disturbance, behavior, conservation management, dive, Indo-Pacific humpback dolphin, vessel traffic Introduction Cetaceans are fully aquatic mammals that rely on surfacing to obtain oxygen, a physiological necessity that directly shapes their diving behavior (Kramer 1988). Within marine mammal research, the concept of “diving” has been interpreted in two main ways: some studies restrict its definition to diving intervals alone (Leatherwood and Ljungblad 1979; Mate et al. 1994; Mate et al. 1995), whereas others include both dive duration and depth, thereby encompassing the broader underwater behavior of these species (Martin and Smith 1992; Westgate et al. 1995). Historically, observations of cetacean diving patterns were collected opportunistically during fishing expeditions (Gray and Flower 1882; Heezen 1957). With technological advances such as satellite telemetry and depth recorders, Academic editor: Yu Xu Received: 16 June 2025 Accepted: 27 October 2025 Published: 14 November 2025 ZooBank: https://zoobank.org/ FEC58393-12C1-4839-A8204921400253D5 Citation: Lu Y, Zhuo H, Li H, Xu X, Yang G (2025) Diving pattern and behavioral response to vessel traffic of Indo-Pacific humpback dolphins (Sousa chinensis) in Xiamen, China: implications for conservation. Nature Conservation 60: 125–137. https://doi.org/10.3897/ natureconservation.60.162175 Nature Conservation 60: 125–137 (2025) DOI: 10.3897/natureconservation.60.162175 * These authors contributed equally to this work. 126 Nature Conservation 60: 125–137 (2025), DOI: 10.3897/natureconservation.60.162175 Yi Lu et al.: The behavior of Indo-Pacific humpback dolphins however, researchers have gained the ability to monitor diving profiles and movement patterns accurately (Scott and Chivers 2009; Rasmussen et al. 2013). Body size has long been considered a key factor influencing dive duration, with early studies generally reporting a positive relationship between the two (Noren and Williams 2000; Marino et al. 2006). However, exceptions are evident, particularly among odontocetes (Ridgway et al. 2019). For instance, the pygmy sperm whale Kogia breviceps (average length 2.05 m) can perform dives exceeding 30 min. In comparison, the considerably larger killer whale, Orcinus orca (average length 5.82 m), typically restricts its dives to about 10 min (Ridgway et al. 2019). Diving capacity is also influenced by physiological development. In juvenile cetaceans, blood oxygen storage and diving capacity are not fully developed but gradually improve with age and body size (Noren and Williams 2000; Noren et al. 2002). Bottlenose dolphins (Tursiops truncatus), for instance, achieve near-adult levels of blood oxygen reserves by approximately 3 years of age (Noren et al. 2002). Beyond foraging or physiological constraints, diving behavior also functions as a response mechanism to environmental pressures. In the presence of vessel traffic, bottlenose dolphins have been observed to increase swimming speeds, alter directional movements, synchronize breathing, and significantly prolong dive durations, reflecting an adaptive response to disturbance (Janik and Thompson 1996; Nowacek et al. 2001; Hastie et al. 2003; Lusseau 2003). The Indo-Pacific humpback dolphin (Sousa chinensis) is distributed across the southeastern coast of China, throughout Southeast Asia, and along the coastal margins of the Bay of Bengal, extending to the coast of Odisha, India (Jefferson and Rosenbaum 2014). Several populations, particularly those in Xiamen Bay, the eastern Taiwan Strait, and the waters around Hong Kong, face intense anthropogenic threats (Jefferson 2000; Ng and Leung 2003; Chen et al. 2008; Dungan et al. 2011; Jefferson and Smith 2016). These regions are characterized by rapid industrial development, frequent vessel collisions, and heavy marine traffic (Jefferson 2000; Ng and Leung 2003; Araújo et al. 2014; Chen et al. 2018). Xiamen Bay, in particular, represents a critical habitat for S. chinensis (Chen et al. 2008; Chen et al. 2011). However, it is also subject to intense human activity, including large-scale cargo transportation, extensive fishing operations, and routine channel dredging, resulting in persistently high levels of vessel traffic (Wang et al. 2015; Chen et al. 2018). Although cetacean diving behavior and responses to vessel disturbance have been documented elsewhere, studies focusing on S. chinensis have primarily been restricted to the waters of Hong Kong (Ng and Leung 2003). Thus, the present study aimed to examine the diving patterns of S. chinensis and assess their behavioral responses to vessel traffic in Xiamen Bay. These findings will contribute to a more comprehensive understanding of the species’ behavioral ecology and provide crucial insights to inform conservation and management strategies for the local population. Materials and methods Study area and data collection Xiamen waters represent a dynamic environment shaped by the interaction of river discharge and tidal exchange. The Beixi and Xixi Rivers, two principal tributaries of the Jiulongjiang River, deliver substantial volumes of freshwater and sediment 127 Nature Conservation 60: 125–137 (2025), DOI: 10.3897/natureconservation.60.162175 Yi Lu et al.: The behavior of Indo-Pacific humpback dolphins into the bay, while the currents of the Taiwan Strait govern tidal motion. The region experiences an irregular semi-diurnal tidal cycle, with an average tidal range of 4.08 m (Zuo et al. 2014). The sediment composition in the area is diverse, encompassing eight distinct types, although it is predominantly characterized by the accumulation of fine particulate matter (Zuo et al. 2016). Xiamen Port Channel is among the busiest maritime hubs in China, accommodating over 1,000 vessel transits daily. The western sector of the bay is home to more than 10 terminals that regularly service ferries and cargo ships. Ferry activity is particularly concentrated along the route connecting the mainland with Gulangyu Island, where vessel frequency averages approximately five ships every 20 minutes (Lu et al. 2024). Boat-based surveys were conducted between 2017 and 2019 across four regions: Xiamen Bay, Zhangzhou, Quanzhou, and Kinmen (Fig. 1a). Weather permitting, the survey vessel navigated along predetermined transect lines designed to encompass as much of the Indo-Pacific humpback dolphin habitat as possible (Fig. 1a). During each survey, a minimum of two experienced observers were stationed at the bow of the vessel. Using either the naked eye or 10×50 mm binoculars, each observer scanned a 100° field of view in opposite directions. To minimize fatigue, observer roles were rotated every 30 min, and scheduled rest breaks were implemented every 3 hours. Upon dolphin encounters, individuals were photographed using a digital camera (Canon EOS 1Dx Mark II with a 28–300 mm zoom lens, Japan). Simultaneously, to ensure reliable behavioral classification, a video camera (Sony FS5K, Japan) was employed to record dolphin activity for subsequent laboratory analysis. In this study, diving duration was defined as the time elapsed from the end of one surfacing event to the start of the next (Fig. 1b). Diving behavior was monitored through continuous sampling, with dive durations measured using a handheld stopwatch. For large groups, it was impossible to accurately track multiple individuals simultaneously (especially more than 10 individuals). A strategy involving sequential processing and video-based supplementation was employed. Observations of a focal individual were terminated if the animal was lost from view or if the tracking period exceeded 30 minutes, after which the survey vessel returned to the transect line to resume data collection. Figure 1. a. Study area in Xiamen Bay and its adjacent waters; b. Schematic diagram of the diving duration; c. Three age classes of Indo-Pacific humpback dolphins. 128 Nature Conservation 60: 125–137 (2025), DOI: 10.3897/natureconservation.60.162175 Yi Lu et al.: The behavior of Indo-Pacific humpback dolphins Age classification Individuals were assigned to age classes based on body size and skin coloration, both of which undergo progressive changes with growth. In general, dolphin coloration develops through three broad stages. Newborns are typically dark gray to nearly black. Within a few months, their pigmentation begins to lighten, black spots gradually appear, and the skin takes on a whitish tone. Eventually, mature individuals are predominantly white, with relatively few black markings. While previous studies have proposed six detailed categories—unspotted calves, unspotted juveniles, mottled, speckled, spotted adults, and unspotted adults (Jefferson and Leatherwood 1997; Jefferson et al. 2012)—the present study employed a simplified scheme comprising three age classes: juvenile, subadult, and adult (Fig. 1c). This approach was adopted to minimize ambiguity and reduce overlap between intermediate categories. Behavioral response Building on previous studies, dolphin behaviors were categorized into two primary classes for analysis: traveling and foraging (Parsons 1998; Van Parijs and Corkeron 2001; Steiner 2011) (Table 1). Although further behavioral states were observed, only these two categories were included in the analyses. Because individual dolphins within the same group may respond differently to similar environmental stimuli, such as the presence of vessels (Shane 1990), behavioral data were recorded at the individual rather than the group level (Ng and Leung 2003) (Table 1). All behavioral responses were documented in real time during field surveys. To minimize disturbance, the survey vessel maintained a minimum distance of 100 m from the dolphins whenever possible. The study considered three common vessel types operating in Xiamen Bay: speedboats, fishing vessels, and cargo ships. When a vessel approached within 500 m of a dolphin, it was assumed that the animal’s behavior could be influenced, and its response was recorded accordingly. The distance between the dolphins and the boat was estimated by referring to buoys and other floating objects near the dolphins. When no reference objects were available, these estimates were conductTable 1. Descriptions of behaviors observed and responses to vessels in Indo-Pacific humpback dolphins. Behavior Description Traveling Regular movement in one direction. Dolphins came together to breathe at regular intervals (sometimes long, sometimes short). Foraging Dolphins came to breathe separately at irregular intervals, with some long dives. Fish chasing, hitting the water surface, and changes in the general direction are often observed. Synchronous breathing Two or more dolphins surfaced and entered the water at the same time. Positive Actively approaching the vessel or boat-chasing afterwards. Negative Actively moving away from the vessel, i.e., boat-avoidance or fleeing away. Neutral No observable change in behavior; continue to perform ongoing activities. 129 Nature Conservation 60: 125–137 (2025), DOI: 10.3897/natureconservation.60.162175 Yi Lu et al.: The behavior of Indo-Pacific humpback dolphins ed by observers using the naked eye. In such cases, to prevent additional interference, the survey vessel either remained stationary or switched off its engines during observation. Once the dolphin had clearly moved away from the research vessel, the engine was restarted to resume tracking. Statistical analyses All results are expressed as mean ± standard error (SE) and were analyzed using SPSS software (version 26). Before conducting statistical tests, data were assessed for normality and homogeneity of variance. Separate chi-square tests of independence were conducted to assess the associations between dolphin behavioral responses and (1) vessel type and (2) vessel distance category. Statistical significance was set at P < 0.05 for all analyses. Results Survey summary Between 2017 and 2019, a total of 181 survey days were completed, generating 52,092 minutes of effective observation effort. During this period, Indo-Pacific humpback dolphins were tracked for 5,319 minutes, yielding 4,408 recorded dives. Furthermore, 3,469 individual dives and 939 synchronous breathing events were recorded. Of these, 893 dives occurred during traveling behavior, while 1,843 were associated with foraging behavior (Table 2). Table 2. Summary of Indo-Pacific humpback dolphin dive durations. NMean ± SE (s) P value One individual 3469 20.36 ± 0.49 0.707 Synchronous breathing 939 20.56 ± 0.91 Traveling 893 24.96 ± 1.12 0.315 Foraging 1843 21.31 ± 0.16 Juvenile in traveling 113 15.92 ± 1.14 0.363 Juvenile in foraging 56 15.85 ± 1.34 Subadult in traveling 265 24.48 ± 1.96 0.046 Subadult in foraging 301 19.89 ± 1.19 Adult in traveling 476 21.75 ± 1.44 0.082 Adult in foraging 1475 21.28 ± 0.71 Juvenile 171 15.43 ± 0.85 0.073Subadult 751 20.54 ± 0.96 Adult 2547 20.64 ± 0.62 Juvenile in traveling 113 15.92 ± 1.14 0.015Subadult in traveling 265 24.48 ± 1.96 Adult in traveling 476 21.75 ± 1.44 Juvenile in foraging 56 15.85 ± 1.34 0.654Subadult in foraging 301 19.89 ± 1.19 Adult in foraging 1475 21.28 ± 0.71 N, number; SE, standard error. 130 Nature Conservation 60: 125–137 (2025), DOI: 10.3897/natureconservation.60.162175 Yi Lu et al.: The behavior of Indo-Pacific humpback dolphins With respect to age classification, 171 dives were attributed to juveniles, 751 to subadults, and 2,547 to adults (Table 2). When behavioral categories were further analyzed within age groups, juveniles showed 113 traveling events and 56 foraging events; subadults recorded 265 traveling events and 301 foraging events; and adults performed 476 traveling events alongside 1,475 foraging events (Table 2). Diving pattern Across all 4,408 recorded dives, the mean dive duration was 20.41 ± 1.77 s, with the majority of dives lasting less than 20 s. For both individual and synchronous diving events, 75.14% and 75.83% of dives, respectively, were under 20 s (Fig. 2a). The mean durations for individual and synchronous dives were 20.36 ± 0.49 s and 20.56 ± 0.91 s, showing no significant difference between the two (Mann–Whitney U test, Z = −0.376, P = 0.707) (Fig. 3a; Table 2). When examined by behavioral category, mean dive durations were 24.96 ± 1.12 s during traveling and 21.31 ± 0.61 s during foraging. This difference was not statistically significant (Mann–Whitney U test, Z = −1.004, P = 0.315) (Fig. 3b; Table 2). However, a large proportion of dives remained short, with 68.76% of traveling dives and 72.76% of foraging dives lasting less than 20 s (Fig. 2b). Figure 2. Histogram showing the percentage distribution of dive durations: a. Different dive modes; b. Different behavioral categories; c. Juvenile behavior; d. Subadult behavior; e. Adult behavior; f. Different age classes regardless of behavior. 131 Nature Conservation 60: 125–137 (2025), DOI: 10.3897/natureconservation.60.162175 Yi Lu et al.: The behavior of Indo-Pacific humpback dolphins Analysis of dive durations by age group revealed that juveniles primarily performed short dives. Specifically, 74.34% of traveling dives and 76.78% of foraging dives were shorter than 20 s (Fig. 2c). The mean durations were 15.92 ± 1.14 s for traveling and 15.85 ± 1.34 s for foraging, with no significant difference between the two behavioral categories (Mann–Whitney U test, Z = −0.910, P = 0.363) (Table 2). Among subadults, 66.04% of traveling dives and 70.10% of foraging dives lasted less than 20 s (Fig. 2d). Mean durations were 24.48 ± 1.96 s during traveling and 19.89 ± 1.19 s during foraging, with dive duration during foraging significantly shorter than during traveling (t-test, t = 1.998, P = 0.046 < 0.05) (Table 2). For adults, 74.58% of traveling dives and 73.69% of foraging dives were under 20 s (Fig. 2e). Mean dive durations were 21.75 ± 1.44 s for traveling and 21.28 ± 0.71 s for foraging, with no significant difference detected between the two (Mann–Whitney U test, Z = −1.739, P = 0.082) (Table 2). When behavior was not taken into account, 76.61% of juvenile dives, 71.11% of subadult dives, and 76.24% of adult dives lasted less than 20 s (Fig. 2f). Mean dive durations for the three age groups were 15.43 ± 0.85 s for juveniles, 20.54 ± 0.95 s for subadults, and 20.64 ± 0.62 s for adults. These differences were not statistically significant (Kruskal–Wallis test, H = 5.221, P = 0.073) (Fig. 3c). When analyzed by both behavior and age, juveniles showed significantly shorter dive durations than subadults during travel (Kruskal–Wallis test, H = 8.426, P = 0.015) (Table 2). For foraging dives, however, no significant differences were detected among age groups (Kruskal–Wallis test, H = 0.848, P = 0.654) (Table 2). Overall, an interactive effect of age and behavior on dive duration was observed when both variables were considered simultaneously (Fig. 3d). Figure 3. a. Differences in dive duration between individual and synchronous breathing; b. Differences in dive duration between traveling and foraging behaviors; c. Differences in dive durations among age classes; d. Interaction effects between age and behavior on dive duration. 132 Nature Conservation 60: 125–137 (2025), DOI: 10.3897/natureconservation.60.162175 Yi Lu et al.: The behavior of Indo-Pacific humpback dolphins Response to vessel traffic During the study, 197 instances of dolphin responses to vessels were documented. Chi-square analysis revealed a significant association between dolphin responses and vessel type (χ2 = 17.684, P = 0.001). In most encounters, dolphins displayed neutral responses, maintaining their ongoing behavior despite the presence of the vessel. Speedboats elicited the highest rate of negative responses (68.18%) among all vessel types and typically involved abrupt directional changes or deep-diving behavior. Conversely, positive responses were rare across all vessel types (Fig. 4a). Dolphin responses also varied significantly with vessel distance (χ2 = 15.263, P = 0.004). Positive responses occurred only when vessels approached within 200 m, while no negative responses were observed at distances greater than 200 m (Fig. 4b). Figure 4. a. Dolphin responses to different vessel types; b. Dolphin responses at varying distances from vessels. 133 Nature Conservation 60: 125–137 (2025), DOI: 10.3897/natureconservation.60.162175 Yi Lu et al.: The behavior of Indo-Pacific humpback dolphins Acknowledgements The authors are very grateful to Min Xu for her helpful suggestions on data analysis. We also thank the fishermen and the members who participated in the vessel-based field survey. Special thanks to the reviewers for their valuable comments that helped improve the manuscript. Additional information Conflict of interest The authors have declared that no competing interests exist. Ethical statement No ethical statement was reported. Use of AI No use of AI was reported. Funding This work was supported by the Youth Fund of the National Natural Science Foundation of China (grant number 32400411) and the National Key Research and Development Program of China (grant number 2022YFF1301600). Author contributions Study conception and design were performed by Yi Lu and Guang Yang. Data collection and analysis were performed by Yi Lu, Haojie Zhou, Haizhou Li, and Xinrong Xu. The first draft of the manuscript was written by Yi Lu and Guang Yang, and all authors commented on previous versions of the manuscript. All authors read and approved the final version. Data availability The datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request. References Araújo CC, Wang JY, Hung SK, White BN, Brito D (2014) Viability of the Critically Endangered eastern Taiwan Strait population of Indo-Pacific humpback dolphins Sousa chinensis. Endangered Species Research 24: 263–271. https://doi.org/10.3354/ esr00605 Barbara JM (1999) Behavioral development in wild bottlenose dolphin newborns (Tursiops sp.). Behaviour 136: 529–566. https://doi.org/10.1163/156853999501469 Baylis AM, Page B, Peters K, McIntosh R, McKenzie J, Goldsworthy S (2005) The ontogeny of diving behaviour in New Zealand fur seal pups (Arctocephalus forsteri). Canadian Journal of Zoology 83: 1149–1161. https://doi.org/10.1139/z05-097 Bowen W, Boness D, Iverson SJ (1999) Diving behaviour of lactating harbour seals and their pups during maternal foraging trips. Canadian Journal of Zoology 77: 978–988. https://doi.org/10.1139/z99-065