Free-ranging Finless Porpoises Research Articles

Variations in echolocation click characteristics of finless porpoise in response to day/night and absence/presence of vessel noise.

Small odontocetes produce echolocation clicks to feed and navigate, making it an essential function for their survival. Recently, the effect of vessel noise on small odontocetes behavior has attracted attention owing to increase in vessel activities; however, the effects of the surrounding environmental factor, vessel noise, and day/night on echolocation click characteristics have not been well studied. Here, we examined the effects of vessel noise and day/night on variations in echolocation clicks and click trains parameters. Passive acoustic monitoring of on-axis echolocation clicks produced by free-ranging finless porpoises (Neophocaena asiaeorientalis sunameri) was performed at two sites in Japan, Seto Inland Sea and Mikawa Bay, in June-September 2021 and March-August 2022, using A-tag and SoundTrap 300HF. Generalized Linear Model was used to elucidate the effect of vessel noise, day/night, and surrounding environmental factors (water temperature, synthetic flow velocity, and noise level) on echolocation click and click train parameters. Echolocation click and click train parameters were strongly affected by day/night, whereas the absence/presence vessel noise did not exhibit statistically significant influence. Particularly, -3 dB bandwidth was wider, click duration was shorter, and inter-click intervals in a train were shorter at night, which may facilitate information processing at night, thereby compensating for the lack of visual information. The interaction between day/night and the absence/presence of vessel noise affected the source level of finless porpoises, with higher levels observed in the absence of vessel noise during the daytime compared to other conditions at the site with low vessel traffic. Overall, these results suggest that echolocation clicks by finless porpoise were likely to fluctuate to adapt with surrounding complex environmental conditions, especially day/night.

A Drone Study of Sociality in the Finless Porpoise Neophocaena asiaeorientalis in the Ariake Sound, Japan

The social structure of animal populations is a fundamental component of their biology, influencing gene flow, habitat use, competition and co-operation around resources, and communication. However, ecological and social relationships can be challenging to describe in most marine mammals, who spend the majority of their lives underwater. The finless porpoise (Neophocaena asiaeorientalis) is one such cetacean species with a largely unknown social structure. Recent advances in drone technology enable more systematic surveys, photogrammetry, and photo-identification for diverse animal species. The present study aimed to validate new survey methods and provide a preliminary description of the spatiotemporal distribution of free-ranging finless porpoises in the coastal open-sea area of Ariake Sound, Japan. A vertical take-off and landing (VTOL) drone equipped with an action camera yielded GPS location datasets through line and area surveys, covering a total sea area of 120 km2. The results suggest highly flexible and varied aggregation sizes in finless porpoises. Distance analysis across individuals and aggregations revealed a cohesive tendency among groups, compared to solitaries and in pairs. Therefore, the present VTOL drone surveys both elucidated some social aspects of the study population and confirmed the efficacy of these standardized research protocols involving automated, programmed, and repeatable flight missions.

Population-level laterality in foraging finless porpoises

Laterality has been reported in many vertebrates, and asymmetrical cerebral hemisphere function has been hypothesized to cause a left-bias in social behavior and a right-bias in feeding behavior. In this paper, we provide the first report of behavioral laterality in free-ranging finless porpoises, which seems to support the aforementioned hypothesis. We observed the turning behavior of finless porpoises in Omura Bay, Japan, using land-based and unmanned aerial system observations. We found a strong tendency in finless porpoises to turn counterclockwise with their right side down when pursuing and catching fish at the surface of the water. Our results suggest that this population of finless porpoises shows consistent right-biased laterality. Right-biased laterality has been observed in various foraging cetaceans and is usually explained by the dominance of the right eye-left cerebral hemisphere in prey recognition; however, right-biased laterality in foraging cetaceans may have multiple causes.

Acoustic time synchronization among tags on porpoises to observe their social relationships

Observing and monitoring underwater social interactions of cetaceans is challenging. Because cetaceans spend most of their time underwater, it is important to monitor their underwater behavior individually. The finless porpoise is small and has no available natural identification marks that causes little knowledge of its sociality. Here we used acoustic datalogger to synchronize individual depth profile among individuals within a second. Acoustic and behavior tags were deployed on six free-ranging finless porpoises simultaneously and released in open water. Echolocation sounds were used as the trigger signal to synchronize the clock of all logging systems. Synchronous dives characterized by similar time-depth profile were used as an index of association. Two pairs tended to participate in long periods of synchronized diving more frequently than 13 other possible pairs, indicating that these four porpoises chose their social partners. Initiator and follower could be identified by precisely time synchronized data. The adult males tended to follow the immature female and juvenile male, respectively. However, the role of an initiator often changed within the pair during synchronized diving, and their body movements appeared to be non-agonistic.The time-synchronized bio-logging method was useful for observation of the social relationships of free-ranging aquatic animals.

Do porpoises choose their associates? A new method for analyzing social relationships among cetaceans.

BackgroundObserving and monitoring the underwater social interactions of cetaceans is challenging. Therefore, previous cetacean studies have monitored these interactions by surface observations. However, because cetaceans spend most of their time underwater, it is important that their underwater behavior is also continuously monitored to better understand their social relationships and social structure. The finless porpoise is small and has no dorsal fin. It is difficult to observe this species in the wild, and little is known of its sociality.Methodology/Principal FindingsThe swim depths of 6 free-ranging finless porpoises were simultaneously recorded using a time-synchronized bio-logging system. Synchronous diving was used as an index of association. Two pairs, #27 (an immature female estimated to be 3.5 years old) and #32 (an adult male), #28 (a juvenile male estimated to be 2 years old) and #29 (an adult male), tended to participate in long periods of synchronized diving more frequently than 13 other possible pairs, indicating that the 4 porpoises chose their social partners. The adult males (#32, #29) tended to follow the immature female (#27) and juvenile male (#28), respectively. However, during synchronized diving, the role of an initiator often changed within the pair, and their body movements appeared to be non-agonistic, e.g., rubbing of bodies against one another instead of that on one-side, as observed with chasing and escaping behaviors.Conclusions/SignificanceThe present study employed a time-synchronized bio-logging method to observe the social relationships of free-ranging aquatic animals based on swimming depth. The results suggest that certain individuals form associations even if they are not a mother and calf pair. Long synchronized dives occurred when particular members were reunited, and this suggests that the synchronized dives were not a by-product of opportunistic aggregation.

Distinguishing between two species of finless porpoises (Neophocaena phocaenoides and N. asiaeorientalis) in areas of sympatry

Abstract The existence of at least two biological species of finless porpoises (Neophocaena phocaenoides and N. asiaeorientalis) was accepted only recently. Given the vulnerability of finless porpoises to human activities throughout their distribution, it is crucial that the two species be distinguished during field studies to better understand their biology and conservation status. Because finless porpoises are notoriously difficult to detect and to photograph at sea owing to their intrinsic cryptic nature (i.e., small size, no dorsal fin, brief surface period, avoidance of boats), differentiating between the two species in situ has often been neglected and attempts have usually been met with skepticism. Moreover, a large area (Taiwan Strait and adjacent waters) exists where the two species are sympatric, which further complicates the task of species identification. In this study, we demonstrated that the two species can be differentiated at sea by direct observation and examination of photographs. By comparing photographs of living and dead specimens of both species, we describe useful diagnostic field characters for species identification of free-ranging finless porpoises. Differentiating these species through direct observation was possible but not trivial and the most confident and convincing identifications were those supported by quality images.

Scanning sonar of rolling porpoises during prey capture dives

Dolphins and porpoises have excellent biosonar ability, which they use for navigation, ranging and foraging. However, the role of biosonar in free-ranging small cetaceans has not been fully investigated. The biosonar behaviour and body movements of 15 free-ranging finless porpoises (Neophocaena phocaenoides) were observed using electronic tags attached to the animals. The porpoises often rotated their bodies more than 60 deg., on average, around the body axis in a dive bout. This behaviour occupied 31% of the dive duration during 186 h of effective observation time. Rolling dives were associated with extensive searching effort, and 23% of the rolling dive time was phonated, almost twice the phonation ratio of upright dives. Porpoises used short inter-click interval sonar 4.3 times more frequently during rolling dives than during upright dives. Sudden speed drops, which indicated that an individual turned around, occurred 4.5 times more frequently during rolling dives than during upright dives. Together, these data suggest that the porpoises searched extensively for targets and rolled their bodies to enlarge the search area by changing the narrow beam axis of the biosonar. Once a possible target was detected, porpoises frequently produced short-range sonar sounds. Continuous searching for prey and frequent capture trials appeared to occur during rolling dives of finless porpoises. In contrast, head movements ranging +/-2 cm, which can also change the beam axis, were regularly observed during both dives. Head movements might assist in instant assessment of the arbitrary direction by changing the beam axis rather than prey searching and pursuit.

A passive acoustic monitoring method applied to observation and group size estimation of finless porpoises

The present study aimed at determining the detection capabilities of an acoustic observation system to recognize porpoises under local riverine conditions and compare the results with sighting observations. Arrays of three to five acoustic data loggers were stationed across the main channel of the Tian-e-zhou Oxbow of China's Yangtze River at intervals of 100-150 m to record sonar signals of free-ranging finless porpoises (Neophocaena phocaenoides). Acoustic observations, concurrent with visual observations, were conducted at two occasions on 20-22 October 2003 and 17-19 October 2004. During a total of 42 h of observation, 316 finless porpoises were sighted and 7041 sonar signals were recorded by loggers. The acoustic data loggers recorded ultrasonic signals of porpoises clearly, and detected the presence of porpoises with a correct detection level of 77.6% and a false alarm level of 5.8% within an effective distance of 150 m. Results indicated that the stationed passive acoustic observation method was effective in detecting the presence of porpoises and showed potential in estimating the group size. A positive linear correlation between the number of recorded signals and the group size of sighted porpoises was indicated, although it is faced with some uncertainty and requires further investigation.

New Stereo Acoustic Data Logger for Free-ranging Dolphins and Porpoises

To observe the bio-sonar behavior of dolphins and porpoises, a miniature stereo acoustic data logger was developed to record the echolocation clicks of small cetaceans. The 'A-tag' device is small enough to be attached to a dolphin or porpoise. A-tag can record the sonar pulse intensity, precise inter-click-intervals, and time difference between sounds arriving at two different hydrophones. The A-tag works for up to 60 hours continuously and allows observation of the sonar target range of free-ranging odontocetes. The time of arrival at the two hydrophones on the tag allows vocalizations from nearby individuals to be identified. A less invasive tagging technique using a suction cup was also developed. A mean attachment time of 15 hours was obtained on free-ranging finless porpoises in a freshwater system in China. The A-tag proved to be a useful tool for investigating the underwater echolocation behavior of odontocetes.

Off-axis sonar beam pattern of free-ranging finless porpoises measured by a stereo pulse event data logger

The off-axis sonar beam patterns of eight free-ranging finless porpoises were measured using attached data logger systems. The transmitted sound pressure level at each beam angle was calculated from the animal's body angle, the water surface echo level, and the swimming depth. The beam pattern of the off-axis signals between 45 degrees and 115 degrees (where 0 degrees corresponds to the on-axis direction) was nearly constant. The sound pressure level of the off-axis signals reached 162 dB re 1 microPa peak-to-peak. The surface echo level received at the animal was over 140 dB, much higher than the auditory threshold level of small odontocetes. Finless porpoises are estimated to be able to receive the surface echoes of off-axis signals even at 50-m depth. Shallow water systems (less than 50-m depth) are the dominant habitat of both oceanic and freshwater populations of this species. Surface echoes may provide porpoises not only with diving depth information but also with information about surface direction and location of obstacles (including prey items) outside the on-axis sector of the sonar beam.

Biosonar behaviour of free-ranging porpoises

Detecting objects in their paths is a fundamental perceptional function of moving organisms. Potential risks and rewards, such as prey, predators, conspecifics or non-biological obstacles, must be detected so that an animal can modify its behaviour accordingly. However, to date few studies have considered how animals in the wild focus their attention. Dolphins and porpoises are known to actively use sonar or echolocation. A newly developed miniature data logger attached to a porpoise allows for individual recording of acoustical search efforts and inspection distance based on echolocation. In this study, we analysed the biosonar behaviour of eight free-ranging finless porpoises (Neophocaena phocaenoides) and demonstrated that these animals inspect the area ahead of them before swimming silently into it. The porpoises inspected distances up to 77 m, whereas their swimming distance without using sonar was less than 20 m. The inspection distance was long enough to ensure a wide safety margin before facing real risks or rewards. Once a potential prey item was detected, porpoises adjusted their inspection distance from the remote target throughout their approach.