Citizen Science Illuminates a City-Dwelling Whale: A Report on the Large Aggregation of Narrow-Ridged Finless Porpoises in Tokyo Bay, Japan

Abstract

Tokyo Bay is one of the busiest ocean areas for human activity worldwide, characterized by a high density of maritime traffic and industrial development. This area is also recognized as the habitat of the narrow-ridged finless porpoise, an endangered toothed whale. Although the existence of this species has been recognized, its scientific data, such as population size and geographic distribution, are limited in the region. Therefore, using social media, we initiated a research project to collect sighting information on this species in Tokyo Bay. As a result, 27 valid pieces of information on the finless porpoises were collected from July 2024 to the end of February 2025. The sightings were reported mainly in the waters north of Futtsu Cape, indicating that this species is localized in the northern part of Tokyo Bay. Also, most of the reported sightings were of groups of wo or more animals. Notably, a school of at least 30 animals was recorded, marking the most prominent school ever observed in the region and the third largest in Japan. This study highlights the importance of the northern region of Tokyo Bay as a habitat for this species. The effective integration of citizen science with scientific and quantitative research will enhance our understanding of the life history of this endangered species, thereby supporting conservation efforts.

1. Introduction

Citizen science refers to the active participation of the public in scientific research, primarily through data collection or analyses [1]. The origins of this practice can be traced back to the 18th century, when non-specialists contributed to the study of wildlife, mainly through avian observations and phenological investigations [2]. In recent years, the spread of internet technology, especially social networking services (SNSs), has significantly increased public participation and expanded research coverage. As a result, citizen science has experienced a substantial surge in popularity [3].

The present study focuses on the narrow-ridged finless porpoise (Neophocaena asiaeorientalis) (hereafter, finless porpoise), a toothed whale species (Odontoceti) distributed across eastern Asia, including Japan (Figure 1). Finless porpoises are opportunistic feeders preying on a diverse range of organisms, including schooling fish, cephalopods, and decapods found in both surface and deeper waters [4,5]. Due to their reliance on shallow coastal habitats, they are particularly vulnerable to anthropogenic disturbances [6]. Since 1993, the Japanese government has designated the narrow-ridged finless porpoise as a protected species under the Act on the Protection of Marine Resources [7]. Also, this species is classified as an endangered species in the IUCN Red list [8].

The Japanese population of this species is classified into five distinct subpopulations based on an analysis of skull morphology and mitochondrial DNA polymorphisms: Sendai Bay–Tokyo Bay, Ise–Mikawa Bays, Seto Inland Sea–Hibiki Nada, Omura Bay, and Ariake Sound–Tachibana Bay (Figure 2). Understanding this spatial population’s structure is essential for developing effective management and conservation strategies [9,10,11]. Additionally, some studies suggested that the Sendai Bay–Tokyo Bay subpopulation may comprise more than one genetically distinct group [12,13]. This classification is supported by evidence of discontinuous distribution patterns and genetic differences among the populations [11,13,14,15,16].

The potential existence of two relatively small, localized groups within the Sendai Bay–Tokyo Bay subpopulation increases the necessity for a more comprehensive investigation of their distribution; however, monitoring this species in Tokyo Bay presents significant challenges. While vessel-based surveys are the conventional method for studying whales, finless porpoises are more commonly observed using aircraft and drones instead. These approaches have been employed in various studies (e.g., [17,18,19,20]), because their small body size makes them difficult to detect in high winds and rough waves during vessel-based surveys [21]. Despite some case studies involving aerial surveys in Tokyo Bay conducted by the Ministry of the Environment and the Fisheries Agency of the Government of Japan, the research efforts were concentrated in the southern regions of Tokyo Bay. Notably, the waters north of Futtsu Cape (35°18′46″ N, 139°47′7.8″ E) remain largely unexplored in terms of aerial survey-based sightings [14,22,23]. This limitation is primarily due to the strict regulations imposed by the air traffic control at Haneda Airport (Figure 2). Given these challenges, alternative methods, such as citizen science, could play a vital role in advancing research on the habitat of the finless porpoises in Tokyo Bay.

The abundance of finless porpoises in Tokyo Bay is presumed to be considerably low. Although opportunistic sightings and accidental strandings have been recorded in various Japanese databases (e.g., Marine Mammal Stranding Database of the National Museum of Nature and Science; Stranding Record of the Institute of Cetacean Research), reports on this species in Tokyo Bay remain scarce (Table S1), particularly in comparison to in other habitats across Japan. This scarcity suggests that the local subpopulation is relatively small in this area. Several vessel-based sighting surveys have been conducted in Tokyo Bay to assess the presence of the finless porpoise, underscoring concerns over its small population size [15,16]. The authors of the present study also carried out vessel-based sighting surveys, recording only two groups—one consisting of three individuals and the other of five—over a cumulative survey period of 30 days between 2018 and 2024 (survey effort was 140 h, covering 1781 km in total). The findings indicate the daily probability of encountering this species is less than 10%, with an average sighting rate of 0.1 groups per 100 km. This is exceptionally low compared to other known habitats (Nakamura, unpublished) [22].

To address this knowledge gap, we initiated a research project to gather sighting information on this species in Tokyo Bay through public participation. Since July 2024, we have solicited reports, including photographs and videos, primarily via SNSs. By February 2025, 41 sightings had been documented. Although the project remains ongoing, preliminary findings have already provided valuable insights into the species distribution patterns within just a few months of its initiation. Here, we present these initial results as a preliminary report.

2. Materials and Methods

We created a website for reporting sightings of finless porpoises on the official homepage and X’s Laboratory of Cetacean Biology account, which includes both real-time sightings and retrospective reports of past encounters. We also created a poster and leaflets that featured a QR code for easy access to the reporting site. This poster was placed on the Tokyo University of Marine Science and Technology (TUMSAT)’s bulletin boards. Additionally, through the Rakusuikai (alumni association of TUMSAT), we requested that shipping companies operating in Tokyo Bay and affiliated organizations such as the Japan Marine Recreation Association distributed posters and leaflets—this effort aimed to engage the individuals involved in marine activities and encourage the widespread sharing of sighting information.

The survey form was hosted on Microsoft Forms and included the following questions: (1) sighting date and time; (2) location (specific latitude and longitude, if known, or the area number from the six designated areas shown in Figure 1); (3) presumed species (or, if unidentified, description of morphological characteristics such as body size, coloration, and the presence/absence of the dorsal fin); (4) estimated number of animals; (5) availability of photographs or videos; and (6) contact details of the informant. When the reporter provided photographs or videos, the authors identified the species and estimated the number of animals.

3. Results and Discussion

3.1. Group Size

Of the 41 recorded sightings, 27 were confirmed as narrow-ridged finless porpoises (Table S2). The observed group sizes were categorized as follows: four solitary animals (14.8%); fifteen groups of two to five animals (55.6%); three groups of six to ten animals (11.1%); two groups of eleven to twenty animals (7.4%); and three groups of twenty-one to forty animals (11.1%) (Figure 3). Finless porpoises are typically described as solitary animals, with an average group size of no more than two individuals and a reported maximum of ten [13,15,24]. Previous investigations in the Seto Inland Sea [25], from Sendai Bay to the Boso Peninsula [13,14], and in Ise Bay [24] indicated that 35–60% of observed individuals were solitary. On the other hand, in the present study, the proportion of solitary animals was only 14.8%. Two possible factors may explain this result. The first is that the sighting probability varies depending on the group size. It is well established that cetaceans in groups are more easily detected than solitary individuals [26]. Due to their small body size, the absence of the dorsal fin, and inconspicuous blow, single individuals could be easily overlooked. The second possibility is that the social behavior of this species in Tokyo Bay differs from that of other habitats. The findings suggest that the subpopulation within Tokyo Bay exhibits a stronger tendency toward group living than other subpopulations.

3.2. Distribution Areas

Most of all sightings were reported within Areas 1 through 4, corresponding to the waters north of the line extending between Futtsu Cape and Kannonzaki. Notably, about half of all sightings were accounted for in Area 2 (the northeastern portion of Tokyo Bay) (

Table 1. The number of groups and individuals in the marine area divisions. A number of individuals were based on the minimum count. Most reports were limited to Areas 1 to 4, north of the line between Futtsu Cape and Kannonzaki.

	Area 1	Area 2	Area 3	Area 4	Area 5	Area 6
Number of groups	6	14	3	3	1	0
Number of individuals	37	83	7	60	5	0

). Previous studies have similarly indicated that finless porpoises in Tokyo Bay are primarily distributed in the northern waters beyond this line [15,16]. Our findings further suggest that the species may occupy a relatively localized habitat rather than being widely distributed throughout the bay. This pattern highlights that the northern part of Tokyo Bay may serve as a critical habitat for finless porpoises.

3.3. A Report on a Large Aggregation of More Than 30 Finless Porpoises

Among the reported sightings, one case involved the largest aggregation of finless porpoises ever recorded in Tokyo Bay. On 26 September 2024, a paramotor pilot reported a sighting of a large group of finless porpoises in waters about 500 m north of the Futtsu Cape. The authors examined the smartphone photographs provided by the observer. While at least 30 individuals were identifiable in the images (Figure 4), additional individuals were probably either submerged or beyond the frame. Thus, the total number of finless porpoises in the aggregation is estimated to be approximately 40. This group included multiple pairs of individuals with different body sizes, likely representing mother–calf pairs (e.g., Figure 4: 7 and 8, 9 and 10, 19 and 20, and 22 and 23).

Furthermore, video footage captured a large school of fish. Multiple finless porpoises were swimming around the school, scattering the fish (Figure 5). Based on these observations, it is likely that the finless porpoises were engaged in foraging behavior, preying on these fish, which were identified as dotted gizzard shad (Konosirus punctatus) according to the observer.

This aggregation is presumed to be a temporary group-forming behavior. Although the observer, a paramotor instructor, has flown over the same waters daily for over eight years, he stated that this was the first time he had encountered finless porpoises. Subsequently, on 12 October 2024, the same observer reported a group of approximately 10 individuals in the same area, followed by a sighting of about 20 individuals on 13 October. However, as of the end of December, no further sightings have been recorded at this location. These findings suggest that finless porpoises in Tokyo Bay are not evenly distributed, but instead migrate within the northern part of the bay, likely in response to prey availability.

As noted earlier, finless porpoises are typically solitary, with an average group size of fewer than two individuals [15]. However, cetaceans, including finless porpoises, are known to form temporary aggregations in response to favorable foraging conditions or specific environmental factors [15]. For instance, exceptionally large groups of 117 and 82 individuals have been reported in Tachibana Bay [27].

Previously, a total of 32 cases of finless porpoise occurrences (13 strandings and 19 live sightings) have been documented in Tokyo Bay [16,28,29]. The largest previously recorded group in Tokyo Bay was a sighting of 20 individuals in May 1934, approximately 1.5 miles off Iwase, Futtsu City. Historical records indicate that “These animals entered the bay in large numbers while pursuing schools of sardine” (Marine Mammal Stranding Database of the National Museum of Nature and Science). The sighting of over 30 individuals notably exceeds this historical record, marking it as the most extensive documented aggregation of finless porpoises in Tokyo Bay and the third largest recorded in Japan.

4. The Potential of Citizen Science for Research on Finless Porpoises in Tokyo Bay

This study suggests that the northern part of Tokyo Bay is an essential habitat for finless porpoises; however, this highly urbanized water area presents considerable challenges to research efforts, mainly due to logistical restrictions (Figure S1). Under these circumstances, citizen science offers excellent potential as a tool for monitoring wildlife, providing an alternative to traditional methods. In fact, the present study has recorded 27 significant sightings within just over eight months, shedding light on the current status of finless porpoises in Tokyo Bay. These results highlight the ability of citizen science to quickly and widely collect distribution data, even in areas where human activities are prevalent. Additionally, as Miya et al. [30] has noted, citizen science can help raise public awareness of the natural environments in local contexts, thereby offering broader benefits to the surrounding communities.

On the other hand, citizen science has certain limitations. One key challenge is that the experience and accuracy of observers can vary, making it difficult to ensure consistent data collection. Another limitation is the inability to control the survey effort, as the timing and frequency of citizen observations are often inconsistent. This variability may introduce biases, making citizen science less suitable for quantitative studies such as estimating population size.

Our citizen science survey supports the possibility that the finless porpoise subpopulation in Tokyo Bay does not actively exchange with populations outside the bay. All but four sightings indicated group-living behavior, suggesting a potential difference in social organization compared to other subpopulations. Most of the sightings were recorded in the inner part of the bay, further pointing out that this subpopulation may be isolated. However, these results may be biased, indicating a need for combining methods, including quantitative sighting surveys, to improve conservation efforts for finless porpoises in Tokyo Bay. Long-term data collection across multiple years is expected to describe seasonal changes in finless porpoises’ spatial distribution. Given that the subpopulation in Tokyo Bay is considered small, the effective use of citizen science will contribute to a better understanding of their distribution, abundance, and life history.