The Indo-pacific finless porpoise (IPFP, Neophocaena phocaenoides) is one of the seven species belonging to the family Phocoenidea. This species inhabits the coastal waters of many Asian countries including China, Korea, Japan, Indonesia, Malaysia and Bangladesh (Reeves et al., 1997; Kasuya 1999) and its status is classified as “Vulnerable” by the IUCN Red List (Wang and Reeves, 2017), making it a high priority for conservation. These animals are highly vulnerable to human impacts, especially fisheries bycatch (Jefferson et al., 2020).

The distribution of finless porpoises around Hainan Island (China) is unknown, but evidences for a resident population have recently emerged (Liu et al., 2018; Caruso et al., 2020a; Liu et al., 2022a). A comprehensive review study revealed 12 stranding events of finless porpoises both on the north and south sides of the island between 1978 and 2016 (Liu et al., 2018). A fisherman survey also revealed that finless porpoises are among the three species more frequently caught as bycatch around Hainan Island (Liu et al., 2016), and a local knowledge survey showed that this species is regularly sighted in all coastal areas of the island (Liu et al., 2019). The survey also revealed the fishermen perception that the abundance and frequency of species sightings have been decreasing in recent years (Liu et al., 2016). Recent recordings of free ranging IPFPs were also obtained during a series of combined visual and acoustics surveys (Caruso et al., 2020a).

The coastal waters around Hainan—that potentially represent good habitat for IPFPs—are in a region of rapid human population growth (between 2001 and 2020 the population grew substantially from 0.796 to 1.012 million people), economic development (coastal construction and touristic business), and increasing offshore human activities (mineral resource exploration and exploitation, and fisheries) (Wang and Wall, 2007; Marcotte et al., 2015). Human activities pose a risk on wild cetacean populations and could determine a general decline in cetacean abundance (Taylor et al., 2007; Schipper et al., 2008). Some of the key factors in determining whether a population is impacted or not by human activities include knowledge of population trends, occurrence and distribution, and the capacity to monitor the population over time.

Monitoring this species in the natural environment is extremely challenging. Finless porpoise, due to its small body size (approximately 1.5 m length), no dorsal fin, and short (1–2 s) surfacing times, are very difficult to spot and count visually (Li et al., 2010). Additionally, variable weather conditions and different levels of experience among the observers can cause additional biases in the detection probabilities during visual surveys. Like other Phocoenidae, finless porpoises use Narrow Band High Frequency (NBHF) echolocation to forage and sense their environment (Li et al., 2010). Their echolocation signals recorded around Hong Kong showed peak frequencies around 142 kHz (with almost no energy below 100 kHz) and a mean pulse duration of 104 µs (Goold and Jefferson 2002). Akamatsu et al. (2001) showed that the acoustic detection is an effective method for field surveys of small cetaceans.

Passive Acoustic Monitoring (PAM) have been extensively used for monitoring wild cetacean populations (Giorli et al., 2016; Caruso et al., 2017; Giorli et al., 2019) and are a cost-effective solution to collect data over large areas (Caruso et al., 2020a). Our study aimed at collecting data to monitor the occurrence of finless porpoises over long time periods in Hainan Island using a large-scale network of passive acoustic sensors moored in very shallow waters (10–16 m water depth).

The IPFP is the most commonly stranded odontocetes in Hainan Island (Lin et al., 2019), and based on local ecological knowledge (LEK), its population seems to be in decline due to overfishing, water pollution, vessel collisions and bycatch on marine mammals (Liu et al., 2016, Liu et al., 2019). Knowing the spatio-temporal distribution and abundance of the species around the island will be essential to efficiently promote conservation actions. During visual surveys of marine mammals in the study area (Liu et al., 2022b), the main species reported was the Indo-Pacific humpback dolphin (Sousa chinenesis), then the Indo-Pacific finless porpoise (Neophocaena phocaenoides), and a single sighting was logged for both the Indo-Pacific bottlenose dolphin (Tursiops aduncus) and the false killer whale (Pseudorca crassidens).

IPFPs are mainly present in the western area of Hainan Island and specifically in the Dongfang region (Figure 1), according to different monitoring methodologies. The acoustic data analyzed in this study confirmed the information obtained by the questionnaire-based survey of local fishermen (Lin et al., 2019). Four sightings were also reported from 2014 to 2019 and all of them in coastal waters south of Dongfang (close to sites #4, #5 and #6) (Liu et al., 2022b). Moreover, Liu et al. (2016) reported during the fishermen interviews that a substantial proportion of informants in this area have eaten or sold marine mammal meat and the bycatch events were also higher in western coastal waters of the island. In general, the monitoring effort was not uniform among the study area (Liu et al., 2022b), and the PAM stations in the northernmost region (sites #7, #8, #9 and #10) acquired less data during the first 6 months of the study (Figure 2). Additionally, the low number of acoustic detections are most likely a result of the limited detection range of the finless porpoise echolocation signals (Figure 3).

Moreover, the highest number of clicks from Indo-Pacific humpback dolphins (IPHDs), in association with communication and feeding sounds, were also reported in the south area of Dongfang (Caruso et al., 2020b). No files were found in which clicks of IPHDs and IPFPs were recorded together. The site with most IPFP detections (site #5) was found to have a high number of sounds from soniferous fishes, the least number of anthropogenic noises and in general the lowest average of broadband sound pressure levels (Xu et al., 2020; Caruso et al., 2020b). Therefore, as described for IPHDs (Caruso et al., 2020b), the distribution of IPFPs seems to be influenced by low levels of ambient noise and anthropogenic activities, and abundance of possible prey. The variation of soundscape resulted an important feature influencing the distribution of both these near-shore and small-size cetaceans and future research will have to investigate their habitat overlap and if there is any competition for the use of the available resources.

IPFP occurs year-round also in waters of southern and eastern Hong Kong. Goold and Jefferson (2002) recorded the acoustic signals of this species for the first time and described the clicks to be narrowband and high-frequency ultrasonic pulses (Goold and Jefferson 2002). In this study, we confirmed that the echolocation clicks from the IPFP have no energy below 100 kHz (Figure 4A), and all the short pulses with Q_RMS bigger than 6 and peak frequency higher than 110 kHz are echolocation clicks of IPFPs (Caruso et al., 2020a). Akamatsu et al. (2001) detected thousands of ultrasonic pulses emitted by Yangtze finless porpoise (N. asiaeorientalis), with dominant frequencies higher than 100 kHz, a local peak at 140 kHz and local minimum at 125 kHz, and an average value of the maximum SPLs corresponding with visual detection at 100 m (137.4 dB SPLp–p re 1 µPa). Regarding the detection range of the acoustic sensors used in this study (Figure 3), the values are similar to the results found for the Yangtze finless porpoise (reliably detected within 275 m) (Akamatsu et al., 2001). The differences detected in this study could depend on the data acquisition system, the analysis of off-axis clicks, the acoustic behavior of porpoises in the marine environment (compared to a freshwater river), the different levels of ambient noise and, of course, the different species investigated.

The use of PAM has expanded our survey ability in the study area, acquiring data simultaneously at ten locations and advancing field observations. In the waters around the Hainan Island, marine mammals are facing several threats (e.g., overfishing, environmental pollution, habitat degradation) and currently expansions of coastal development seem to be one of the main concerns (Caruso et al., 2020a). Lin et al. (2019) showed that regulating coastal fisheries in the Pearl River Delta region is an effective management tool in securing the long-term persistence of the finless porpoises (Lin et al., 2019). Therefore, the increase of human activities could potentially affect Hainan’s population and a conservation framework for coastal cetaceans around the island is urgently needed, considering the possible establishment of Marine Protected Areas (MPAs) in locations where the animals occur more frequently and in relatively high abundance. In particular, mitigation actions should be focused on a reduction of the effects of fishing activities and coastal development, for a species that is particularly vulnerable to both these threats. Moreover, it is very important to increase community-based education initiatives to raise public awareness about marine mammal conservation, and to improve the enforcement of wildlife protection laws around the Hainan province. Regional community-based conservation actions could be promoted through environmental education, ecosystem management and a sustainable development based on the strict collaboration between governance, scientists, fishermen, tourism and local communities.