Foraging tactics in Indo-Pacific humpback dolphins (Sousa chinensis)

doi:10.21203/rs.3.rs-4676832/v1

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Foraging tactics in Indo-Pacific humpback dolphins (Sousa chinensis)

https://doi.org/10.21203/rs.3.rs-4676832/v1

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Indo-Pacific humpback dolphins (Sousa chinensis) typically inhabit turbid coastal waters and estuaries, making it hard to observe their behaviour. Therefore, until now, only a few studies have provided descriptions of this species’ behaviour. Studying foraging behaviours is needed to better understand the tactics adopted by the species and inform conservation measures. This study aimed to describe and quantify the foraging behaviours displayed by Indo-Pacific humpback dolphins in three different locations in the northern South China Sea, including Beibu, Zhanjiang, and Pearl River Delta (PRD). Dolphins were observed side breaching, half-side breaching, fish shoal pursuing, mud foraging, foraging around gillnets, seine nets, trawlers, shellfish farms, fish farms, and wind farms. No cooperative foraging was observed and animals were mostly engaged in individual and group random feeding. Differences in foraging behaviours were found among the three studied geographic populations with Zhanjiang dolphins foraging in large and dispersed groups and using fish farm surroundings as a foraging ground; PRD dolphins foraging in small groups with various dispersion modes and engaging in mud foraging and foraging in association with pair trawlers or seine fishing; and Beibu dolphins foraging in small groups with various dispersion modes like those from PRD but unlike them, seldom displaying side or half-side breaches. The observed variations in group size and cohesion, movement patterns and behaviour, or associations with human activities reveal the complexity of this species’ foraging behaviour and differences among populations that may be linked with the characteristics of the environment dolphins live in.

feeding

cetacean

aquaculture

trawler

side breach

The fine-scale behaviour of Indo-Pacific humpback dolphins (Sousa chinensis) has seldom been described and even less quantified. In this study, we present the first description of this species’ foraging behaviours and reveal differences among three geographic populations in terms of their foraging tactics that may be linked to the environment they live in and could reflect a potential transmission among individuals within populations. The variety of observed foraging behaviours highlights the complexity and adaptability of Indo-Pacific humpback dolphin behaviour. The presented results participate to the better understanding of this species’ behaviour, including social aspects (e.g, group size and cohesion) that are of importance for their conservation.

Foraging is an activity all animals engage in and that includes searching for food, detection of a food item, and its capture/consumption. Genetically based decision rules regarding foraging are called foraging strategies while the behaviours that are used to build these strategies are labelled foraging tactics (Heithaus and Dill 2009) Predator species have developed diverse strategies and tactics to access prey and overcome their defenses. For instance, some species engage in group foraging, sometimes involving cooperative tactics that increase the success of involved individuals in catching prey (Alcock 1993; Inman 1987). Marine predators including cetaceans mainly forage beneath the surface making observations of foraging behaviours difficult (Heithaus et al. 2002). Nevertheless, studies have shown that cetaceans exhibit high variety in their foraging strategies and tactics, with species engaging in individual foraging while others often use cooperative techniques (e.g, Bel’kovich et al. 1991; Burgess 2006; Nowacek 2002; Wells and Scott 1999)

While dolphins’ foraging tactics may depend on environmental features (e.g, prey distribution, presence of human activities), some that are only displayed by particular groups, communities, or populations have been considered as cultural elements (see Sargeant and Mann 2009a, b; Wild et al. 2020). The study of cetacean foraging behaviours may therefore provide insights into the environment animals are living in and potential cultural transmission. These elements contribute to the basic knowledge of a species required for its conservation (Caro 1998; Clemmons and Bucholz 1997). For example, various dolphin species have been observed interacting with human activities including gillnet fisheries (e.g, Bearzi et al. 2008; Dawson et al. 2001; Read et al. 2003; Rojas-Bracho et al. 2006), long lines (e.g, Dalla Rosa and Secchi 2007; Rabearisoa et al. 2015), purse seines (e.g, Dias et al. 2022), trawlers (e.g, Gonzalvo et al. 2008), or mariculture structures (e.g, Diaz-Lopez et al. 2005, Díaz López 2009; Methion and Díaz López 2019). These interactions are characterized by an association with, or close proximity to, the net or structure while it is in activity and have been suggested to be associated with increased bycatch risks for dolphins (Du Fresne et al. 2007; Santana Garcon et al. 2018). In some species and populations, these interactions, among other foraging tactics are only displayed by some groups or individuals, therefore suggesting social learning processes (Chilvers and Corkeron 2001; Daura-Jorge et al. 2012; Krützen et al. 2014).

Indo-Pacific humpback dolphins (Sousa chinensis) are found in the coastal waters of Southeast Asia, including the coast of China. This species is listed as Vulnerable on the International Union for Conservation of Nature’s red list (Jefferson and Smith 2016). Despite the significant amount of research conducted on these dolphins, very little information is available on their fine-scale behaviour, including foraging behaviour (but see Bijukumar and Smrithy 2012; Serres et al. 2023). Until now, the foraging strategies or tactics adopted by Indo-Pacific humpback dolphins have not been systematically described or quantified. This study aimed to characterize the foraging tactics of Indo-Pacific humpback dolphins in three geographic populations in the northern South China Sea through description and quantification of the behaviours displayed by the dolphins.

2.1 Study locations and data collection

Data were collected from May 2021 to May 2024 during boat-based surveys in three geographic populations of Indo-Pacific humpback dolphins in the northern South China Sea: Beibu, Zhanjiang, and Pearl River Delta (PRD). Small (≈ 7m long) and highly maneuverable outboard engine boats were used for the surveys that took place opportunistically depending on the weather and sea-state conditions (Beaufort ≤ 3 and visibility ≥ 1000 m). When dolphins were sighted, the boat approached them at a reduced speed (≤ 6 knots) to allow onboard researchers to observe their behaviour and take photographs. The engines were stopped when reaching a distance that enabled taking good quality photographs (≈ 50m) and the boat could be further manoeuvred to follow dolphins when they moved away (e.g., travelling) or to get closer to particular individuals. In order to prevent disturbance to the animals, particular attention was given to their behaviour to detect any change. Encounters lasted a minimum of 10 minutes and were ended when enough data had been collected or when animals exhibited signs of avoidance.

During encounters with dolphins, a range of data were recorded in a standardized way including data regarding dolphins themselves and their environment (Table 1). Part of the data was recorded using 5-minute scan sampling; the 5-minute interval was adapted depending on the dolphins’ surfacing pattern (animals have to be visible to record most of the data), and part of it was recorded using all occurrence event sampling (Altmann 1974). The dolphins’ behavioural activity was defined using predominant group activity based on previous studies conducted on this and other dolphin species (Neumann 2001; Serres et al. 2023; Shane 1990). Foraging was defined as dolphins not exhibiting clear directional movements, diving often, chasing fish, with fish in the mouth, following active fishing boats, or swimming back and forth along gillnets. Travelling was defined as dolphins moving at a constant speed in the same direction, making substantial progression with a constant orientation. Socializing was defined as some or all group members being engaged in frequent physical contact with one another (except mothers and calves), chasing and escaping bouts, and often displaying surface behaviours (e.g, breaching, leaping) with no obvious forward movement. Milling was defined as dolphins moving in changing directions without clear movement in a particular direction, no surface behaviours, asynchronous breathing, and no apparent physical contact between individuals. Resting was defined as dolphins moving very slowly and/or remaining still at the surface for dozens of seconds.

Unmanned Aerial Vehicle (UAV) flights were conducted during some of the encounters, enabling to obtain a different point of view on the dolphins and get more details about their behaviour. UAV flights were conducted using an Inspire 2 drone (DJI) equipped with an x5S Zenmuse camera. Any disturbance to the animals was avoided through careful observation of the dolphins’ response to the UAV and the ending of the flight if any adverse response was noticed.

Table 1

Parameters recorded during encounters with Indo-Pacific humpback dolphins in the northern South China Sea.
Category	Parameter	Conditions	Sampling mode
Dolphins	Estimated group size	-Small group (1–5 IDs) -Medium group (6–10 IDs) -Large group (> 10 IDs)	5-min scan sampling + final estimation at the end of the encounter
	Estimated group dispersion	-Tight group (1-20m diameter) -Intermediate group (21-100m diameter) -Dispersed group (> 100m diameter)	5-min scan sampling
	Behavioral activity	-Foraging -Travelling -Socializing -Milling -Resting	5-min scan sampling
	Behavioral events	-Side breach -Half-side breach -Fish shoal herding -Mud foraging	All occurrence event sampling
	Interaction with human activities	-Foraging around gillnets -Foraging behind fish trawlers -Foraging around seine nets	All occurrence event sampling
Environment	Environment type	-Near-shore (< 100 m from shore) -Open water (> 100 m from shore) -Boat channel/harbor	5-min scan sampling
	Water depth	Water depth (m)	Beginning and end of the encounter
	Proximity to shellfish/fish/wind farms	-Close (< 30m from a farm) -Not close (> 30m from a farm)	5-min scan sampling
ID: Individual

2.2 Data analysis

The encounters that did not involve any foraging activity were first removed from the dataset. All observed foraging-related behaviours were categorized and percentages of encounters involving each of the observed behaviours were calculated for each location.

A total of 575 encounters with Indo-Pacific humpback dolphins were monitored between May 2021 and May 2024, among which 367 encounters (63.8%) involved foraging. These foraging encounters were monitored for an average of 27.5 ± 1.0 min. Foraging encounters occurred in waters with an average depth of 7.5 ± 0.2 m, with shallower water depths found in Beibu than in Zhanjiang and PRD (Fig. 2). Most foraging encounters did not involve any other activities in Beibu and PRD, followed by encounters involving travelling and milling while foraging encounters most often involved travelling in Zhanjiang (Fig. 3a). Most foraging encounters occurred in open waters but the location was more diverse in PRD than in Zhanjiang and Beibu where foraging seldom occurred in boat channels or near-shore (Fig. 3b). Most foraging groups were small with various dispersion ranges in Beibu and PRD while they were mostly large and dispersed in Zhanjiang (Fig. 3c, d). During foraging encounters, dolphins were usually apart from each other, either individually or within subgroups of different sizes with individual random orientations and no visible cooperation or coordination of movements.

During foraging, dolphins were usually observed pursuing fish, involving fast chasing bouts, abrupt changes of direction, body position changes (i.e., belly up or on the side), and leaps at the water surface, and some individuals were observed finally catching a fish. A range of foraging-related behaviors was displayed during foraging encounters including side breaching that consists of a dolphin breaching out of the water and landing on the side (the flukes can remain or not in the water during the breach, Fig. 4a). The half-side breach was displayed using the same motion but only half of the dolphin’s body exited the water (Fig. 5). Dolphins were sometimes observed herding fish shoals and catching fish from them (Fig. 4b). During such events, one or more dolphins could pursue the same shoal, but no cooperative behavior was observed. During some foraging encounters, traces of mud were observed on the skin of dolphins (Fig. 4c). During foraging events where dolphins presented mud on the skin and UAV flights were conducted, thick clouds of suspended sediment were often observed on the surface of the water. Round clouds would form one by one, creating a cloud line indicating the route followed by the dolphin and the body of the animal sometimes appearing to move just ahead of the cloud line front. All dolphins present during such a foraging event were observed engaging in mud cloud creation individually. Finally, dolphins were observed feeding around human activities or equipment including trawlers (Fig. 4d), gillnets (Fig. 4e), seine nets, shellfish farms (Fig. 4f), fish farms (Fig. 4g), or wind farms (Fig. 4h). When following trawlers (usually pair trawlers), dolphins would usually remain outside of the net, either behind it or on its sides but on one occasion, an adult individual was witnessed swimming inside the net. Seine net activities consisted into dozens of small wooden boats forming a circle while hitting the floor of their boats and increasing their speed while decreasing the diameter of the circle progressively to catch fish shoals. Dolphin groups were sometimes observed travelling relatively fast and breaching to join these seine netting activities. They would then remain around the boat circle and net and forage actively. Some dolphins were observed staying around until fishermen would finish taking the net out of the water.

Foraging in association with human activities (i.e., around gillnets, trawlers, or seine nets) was occasionally observed (5.8% of foraging encounters). Foraging around gillnets was observed at relatively comparable rates within the three populations (Fig. 6). Side breaching, half-side breaching, mud foraging, and foraging around seine nets were observed most frequently in PRD while foraging next to fish farms was only observed in Zhanjiang.

This is the first study describing the foraging behavior of Indo-Pacific humpback dolphins. In this study, diverse tactics have been observed that were displayed by dolphins more or less frequently depending on the location. The described behaviors and encounter features provide insights into the foraging characteristics of Indo-Pacific humpback dolphins including group size and cohesion, environmental features, and associations with human activities.

4.1 Foraging encounter characteristics

First, dolphins from all locations were often recorded switching from foraging to other activities and back, milling and travelling being the most frequent transitions. This observation was expected given the fact dolphins have to find prey that are usually distributed in patches. Dolphin activity is probably often categorized as ‘travelling’ when they are searching for prey, ‘foraging’ once they found a prey patch, and ‘milling’ or ‘travelling’ again when they attempt to find the next patch. For this reason, an ambiguous behavioral category labelled ‘travel/forage’ has sometimes been used (e.g., Shane 1990). The occurrence of foraging behaviors during other activities may also represent an indication of opportunistic foraging where animals are engaged in an activity and encounter prey without actively searching for it (Nowacek 2002).

Beibu dolphins were found to mostly forage in open but particularly shallow waters (6m deep maximum), Zhanjiang dolphins in open waters of various depths, and PRD dolphins were observed foraging in various water depths and locations, including open waters, near-shore waters, and boat channels. These environmental features probably correspond to the habitat characteristics at each location and choice opportunities for dolphins. In Beibu for example, the high extent of shellfish farms and intense dolphin-watching activities are probably restricting access to near-shore areas (see Serres et al. in Press). On the other side, East-PRD is an area subject to intense human activities, including much shipping traffic and thus associated boat channels and harbors. Dolphins inhabiting this area may be forced to forage in these human-impacted zones with associated risks of collision and related injuries (e.g., Chan and Karczmarski 2019; Kot et al. 2022)

Differences in the dolphin group size and cohesion were observed during foraging encounters among the three geographic populations. Unlike Beibu and PRD dolphins who were mostly observed foraging in small groups with various dispersion ranges, dolphins from Zhanjiang were mainly observed foraging within large and dispersed groups. Several species of cetaceans have been reported to often spread out into widely spaced subgroups (e.g., dusky dolphins Lagenorhynchus obscurus, pilot whales Globicephala spp, Risso’s dolphins Grampus griseus, bottlenose dolphins Tursiops sp, killer whales Orcinus orca). Spreading out may either reduce foraging competition among individuals or increase the probability that prey is detected (Heithaus and Dill 2009). In Zhanjiang, these large groups seem to just be aggregations of animals attracted to the same resource with little interaction among individuals as they appear to pursue prey individually. This observation probably highlights low benefit of defending food resources against competitors in that particular location, potentially linked with high prey availability. It has been proposed that when prey can be defined as “rare and random” (Poisson-distributed), dolphins tend to forage in close coordination whereas when the prey is easy to access, dolphins typically aggregate for other purposes like protection against danger or mating opportunities (Vaughn et al. 2007; Wursig and Pearson 2014). Since the shark-related predation risk has been shown to be slightly higher in Zhanjiang than in other locations, protection against predation could represent an additional factor to explain the large aggregations (Chen et al. in Press; Serres et al. 2021).

The group size, cohesion, and behavior of Indo-Pacific humpback dolphins described in this study suggest that these animals mostly engage in Individual Random Feeding (IRF) and Group Random Feeding (GRF) (Rossi-Santos and Flores 2009). IRF is the foraging activity where dolphins are spaced out and randomly dispersed either alone or in subgroups with random orientations and no visible cooperative or coordinated movements but instead, individuals moving in various directions in an erratic way with frequent accelerations and bursts in the surface. In GRF, dolphins are also moving in different directions and engaging in bursts and accelerations, but in larger more cohesive subgroups with subgroups sometimes merging and splitting and frequent transitions with travelling behavior. Given the observed group cohesion parameters, it can be hypothesized that dolphins from Zhanjiang could tend to mostly engage in IRF whereas dolphins from Beibu and PRD would mostly engage in GRF. However, deeper investigations should be conducted to validate these hypotheses. Because UAV flights were not conducted on all encounters and since the water was usually too turbid to see dolphins once they dived, particular tactics including potential cooperation may have gone unnoticed.

4.2 Observed foraging behaviors

Like reported by Bijukumar and Smrithy (2012) in India, Indo-Pacific humpback dolphins often engaged in prey chasing involving high speed pursuit with body position inversions, abrupt changes of direction, and frequent leaps at the water surface. Cetaceans have been shown to actively herd their prey through taking advantage of natural prey schooling and flight behavior behaviors in order to enhance their ability to capture them (e.g., Gazda et al. 2005). These manipulations may help predators get a hiding prey out, capture an individual prey, or increase the density of prey. In the present study, Indo-Pacific humpback dolphins were observed swimming around and below fish shoals to gather the prey together and eventually accelerating abruptly to catch a fish. Even though up to three dolphins were observed foraging on the same large fish shoal, coordinated efforts involving animals cooperating were not observed during the three years of data collection. It is not an easy task to determine whether groups of foraging cetaceans are cooperating or not, especially from the boat's point of view. However, this result suggests that cooperation during foraging, if existent, is probably not a preponderant tactic in the three studied Indo-Pacific humpback dolphin populations.

During foraging, including prey-herding, breaching may be used by dolphins to lead fish schools to compact (e.g., Acevedo-Gutierrez 1999; Tardin et al. 2011; Vaughn et al. 2008; Würsig 1986; Würsig and Würsig 1980) and the link between foraging activity and side breaching has already been reported in Indo-Pacific humpback dolphins (Serres et al. 2023). This type of percussive behavior may be used to disrupt shoals of fish (Tardin et al. 2014). The difference between side breaching and half-side breaching is the proportion of the dolphin’s body exiting the water. Half-side breaching may require less energy than side breaching and therefore represent an alternative for dolphins to save energy while keeping the targeted impact on the prey. It is interesting to observe that dolphins from Beibu engaged in such percussive behaviors much less than their congeners from Zhanjiang and PRD. One potential explanation for this difference is the presence of intense dolphin-watching activities in Beibu (Wu et al. 2020) Such activities are based on the ability of skippers to find and follow dolphins, and aerial behaviors like side breaches can be easily detectable from a long distance. Beibu dolphins may have associated aerial behaviors with the following dolphin-watching boat disturbance and therefore reduced their display. However, deeper investigations are needed to confirm this hypothesis.

Indo-Pacific humpback dolphins were sometimes observed with mud traces on their skin while foraging, probably revealing that they were searching for or pursuing fish on muddy sea floors. Several cetacean species engage in benthic foraging including killer whales in New Zealand foraging on rays that they pin to the bottom and dig for or bottlenose dolphins in the Bahamas engaging in “crater feeding” where the dolphin dives into the sand almost up to the flippers to catch its prey (Rossbach and Herzing 1997). The observation of mud clouds during several foraging events may indicate that Indo-Pacific humpback dolphins engage in mud-related tactics to catch fish more efficiently. ‘Mud ring feeding’ has been described by Ramos et al. (2021) in Chetumal-Corozal Bay, Mexico with bottlenose dolphins using this strategy to encircle prey and catch them as they escape. In the present study, unlike what is observed during mud ring feeding, not one but most individuals were witnessed individually creating mud clouds, clouds were not forming rings, and dolphins were not observed catching fish that tried to escape in the air. Therefore, it is unlikely that the strategy of Indo-Pacific humpback dolphins was to encircle the fish within the mud rings like observed in Mexico. Conversely, the ‘mud plume feeding’ described in bottlenose dolphins in Florida (Lewis and Schroeder 2003) seems to correspond to the observed Indo-Pacific humpback dolphin behavior. Dolphins may benefit from such a tactic through (1) the concentration of prey in the clouds they use for protection or foraging (higher concentrations of food in the suspended sediment) or (2) the temporary visual and physical barrier clouds represent that confuses and disorientates the prey by preventing collective antipredator responses (Abrahams and Kattenfeld 1997). In the present study, it was impossible to observe the underwater behavior of dolphins because of the high water turbidity, therefore the exact behavioral sequence dolphins engaged in is not clear and ‘mud plume feeding’ remains a hypothesis. It should be noted that this behavior was observed in PRD only, and more specifically in East-PRD. The prevalence of this behavior in this population could be linked to the characteristics of the sea floor of the area, East-PRD seabed being mostly composed of mud/clay (Shi et al. 2013; Xia et al. 2013) while it is mixed in Beibu (sand and clay, Cao et al. 2021) and mostly sandy in Zhanjiang (Han 2015). However, learning processes and cultural transmission within the population could also be responsible for the spreading of this behavior in a restricted area.

Indo-Pacific humpback dolphins were also observed foraging around gillnets or seine nets, and behind trawlers. Dolphins have been observed interacting with gillnets, trawlers, and encircling nets worldwide (Italy, Lauriano et al. 2009; North Carolina, Read et al. 2003; Mexico, Chávez-Martínez et al. 2022) with coastal species such as bottlenose and humpback dolphins being the most commonly observed foraging behind trawlers (Bonizzoni et al. 2022). While foraging around gillnets was observed at all locations, interactions with trawlers and seine nets were most frequent in PRD. In line with these observations, associations with pair trawlers have already been documented in East-PRD (Barros et al. 2004; Jefferson 2000) where only some individuals frequently forage behind pair trawlers while others are never observed engaging in such a tactic (Jefferson 2000). Foraging on gillnets, around seine nets, or following trawlers provides dolphins with a concentrated food source (Díaz López 2006; Fertl and Leatherwood 1997), therefore increasing the success while decreasing the energy expenditure associated with foraging. Trawlers and seine nets are often detectable over kilometres by dolphins thanks to the sounds of the boats’ engines, thus reliably indicating food patches (Norris and Prescott 1961). It remains unknown if Indo-Pacific humpback dolphins feed on the fish outside of the net, stuck in the mesh, or caught in the net, if they are scavenging on discarded animals, or feeding on species that are themselves attracted by the trawler, thus their exact foraging tactic cannot be determined. It should be noted that foraging in association with human activities as observed in this study is linked to increased risks for the dolphins, including risks of entanglement in nets, ingestion of fishing equipment, and exposure to pollutants and noise (Fertl and Leatherwood 1997). Indo-Pacific humpback dolphins are known to suffer from bycatch that can be lethal or leave significant injuries (Serres et al. 2023; Slooten et al. 2013), and such bycatch may sometimes be linked with risky foraging tactics like foraging in association with human activities. In addition, potential effects on dolphins’ distribution, diet, social structure, and behaviour may arise from such foraging tactics (Bonizzoni et al. 2022).

Finally, Indo-Pacific humpback dolphins were occasionally observed foraging near shellfish farms in Beibu and PRD and relatively frequently near fish farms in Zhanjiang. Some dolphin populations or communities have been shown to take advantage of the higher density of prey and often forage in proximity to fish or shellfish farms (e.g, Bearzi et al. 2008; Bedrinana-Romano et al. 2023; Diaz Lopez 2002, 2006, 2012; Diaz Lopez and Shirai 2007; Diaz Lopez and Methion 2017; Harnish et al. 2023; Methion and Diaz Lopez 2019; Pace et al. 2012; Pirrodi et al. 2011). The occurrence of such tactics depends on the presence and density of mariculture activities in the environment of dolphins (Bonizzoni et al. 2014; Ribeiro et al. 2007). Since mariculture was not much developed in Zhanjiang at the time of the study (Serres et al. in Press) and included not only shellfish but also fish farms, dolphins from this location may have benefited from it. However, such tactics may have impacts on predation risk, interindividual competition, bonds between individuals, and resulting social structure (Díaz López and Shirai 2008; Methion and Díaz López 2020). On the other side, foraging next to shellfish farms was only observed in Beibu and PRD. This result was expected since the extent of shellfish farms was much worse in Beibu and West-PRD than in Zhanjiang and animals may therefore have been forced to cohabit with these activities.

Cetaceans are known to present variations in foraging tactics (Heithaus and Dill 2009). Within a species, different populations or communities can exhibit different foraging tactics linked with differences in group structure, social behaviour, predation risk, and habitat characteristics (Bearzi et al. 1999; Bel’kovich et al. 1991; Shane 1990; Shane et al. 1986; Würsig and Pearson 2014) with the prey distribution potentially playing a central role (Acevedo-Gutierrez and Parker 2000). For example, foraging behaviors restricted to certain groups, communities, or matrilines have been documented in several bottlenose dolphin populations (e.g., Chilvers and Corkeron 2001; Mann and Sargeant 2003). In the present study, the human activities experienced by dolphins in each population were different with pair trawling being common in PRD and fish farms being mostly present in Zhanjiang for instance. Dolphins may take advantage of the activities that are present in their habitat (e.g., higher prey density next to fish farms or around trawlers). Regarding other factors potentially impacting the differences in foraging tactics observed among populations, the predation risk has seldom been investigated in Indo-Pacific humpback dolphin populations but it has recently been suggested to be higher in Zhanjiang than in Beibu (Serres et al. 2021) where a particular community seems to be more subject to shark attacks than others (Chen et al. in Press). On the other side, prey availability remains uninvestigated at the studied locations, making it hard to propose hypotheses linked to this factor. Finally, the potential social learning processes and spreading of cultural elements have never been studied in this species. Social learning is a process that involves the social transmission of a novel behavior among peers and between generations (De Waal, 2001) and is an essential element to characterize culture in animals (Laland and Hoppitt 2003), this behavior being shared by some but not necessarily all individuals of the same species. The presented observations could reflect such social learning of specific foraging behaviors within populations. We suggest that predation risk, prey density and distribution, and social learning should be further investigated to better understand the factors underlining the differences in foraging tactics highlighted in the present study.

The present study provided insights into the foraging behavior displayed by Indo-Pacific humpback dolphins in three geographic populations of the northern South China Sea. The results showed that Indo-Pacific humpback dolphins do not seem to engage in cooperative foraging tactics. However, the absence of cooperation within this study does not necessarily mean that such a tactic does not exist in the species. Each population presented its own characteristics in terms of foraging, probably related to the environment they live in. Dolphins from Zhanjiang usually foraged in large and dispersed groups, potentially indicating that prey is easily accessible, and seemed to use fish farm surroundings as a foraging ground. Oppositely, dolphins from PRD foraged in small groups with various dispersion modes and were relatively often observed engaging in mud foraging and foraging in association with pair trawlers or seine fishing. Finally, dolphins from Beibu foraged in small groups with various dispersion modes like those from PRD but, unlike them, seldom displayed side or half-side breaches. The observed behaviors provide cues about the potential differences in terms of human activities, predation risk, prey distribution, and competition in each of the populations. In addition, these differences reveal potential learning or transmission processes occurring within Indo-Pacific humpback dolphin groups and populations. Differences in foraging strategies and tactics can be linked to differences in social structures (Ansmann et al. 2012; Chilvers and Corkeron 2001; Díaz López and Shirai 2008; Methion and Díaz López 2020), which in turn have the potential to affect population fitness (East et al. 2010), reproductive success (Gromov 2021), genetic structure (Altmann et al. 1996), behaviour learning processes (Allen 2019; Whiten 2021), and pathogen spreading (Sah et al. 2018). Therefore, the present study should be followed by further investigations to better understand and protect each Indo-Pacific humpback dolphin populations.

Authors have no competing interests to declare.

Ethical approval:

This research involved the observation of free-ranging dolphins. All applicable international, national, and/or institutional guidelines for the use of animals were followed and particular attention was given to avoid any disturbance to the dolphins. The data collection was conducted under the Ethical Statement “IDSSE-SYLL-MMMBL-01” delivered from the Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences.

Acknowledgments

We would like to thank all the staff and students at the Marine Mammal and Marine Bioacoustics Laboratory for their logistical support. This research was financially supported by the National Natural Science Foundation of China (Grant numbers 42225604 and 32150410369), the CAS President's International Fellowship Initiative (PIFI) for postdoctoral researchers, and the National Key RD Program of China [2022YFF1301603]. This work was performed under the Ethical Statement “IDSSE-SYLL-MMMBL-01”.

Data availability

The dataset used for the present study are available at https://data.mendeley.com/datasets/bp633zssmx/1

Compliance with Ethical Standards

Conflict of interest:

The authors have no conflict of interest to declare.

Ethical approval:

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Foraging tactics in Indo-Pacific humpback dolphins (Sousa chinensis)

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1 Introduction

2 Material and Methods

2.1 Study locations and data collection

2.2 Data analysis

3 Results

4 Discussion

4.1 Foraging encounter characteristics

4.2 Observed foraging behaviors

Conclusion

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Acknowledgments

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References

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