The Ocean Oasis project – Habitat Utilization by Risso’s Dolphins (Grampus griseus)

Chieh-Hsi Hu|Cetacean conservation researcher2024.03.21
Fig. 1. The second most common cetacean species in Hualien waters, Risso’s dolphin
The International Union for Conservation of Nature initiated the "Important Marine Mammal Areas" (IMMAs) program in 2016, aimed at integrating data on the distribution, migratory routes, and threats to endangered marine mammals across international borders (Fig. 2).
 
Fig. 2. IMMAs worldwide from 2016 to 2022 (Image source: IUCN)

To apply IMMAs, one crucial indicator is to understand the significance of the inshore Hualien habitat for cetaceans in terms of resting, feeding, and breeding areas. Using this indicator, we focused on the most frequently encountered cetaceans inshore of Hualien: Risso's dolphins (Grampus griseus) (Fig. 1) and spinner dolphins (Stenella longirostris). We aimed to understand where these dolphins engage in breeding, resting, feeding, socializing, and other activities via 20 surveys conducted throughout the four seasons per year. Additionally, we investigated whether behavioural patterns were influenced by whale-watching and fishing activities. Following a trial survey in April 2021, we officially commenced a three-year investigation, continually collecting data at sea. In this article, we look forward to sharing the new understanding made over the past three years with our readers, focusing on the activities of Risso's dolphins.

Surveying without causing disturbance


Fig. 3. Map of the survey areas in the Northern (N1), Central (N2), and Southern (N3) regions of this project

The scope of the habitat use survey is delineated by the southern boundary at Yanliao (鹽寮) and the northern boundary at Heping River Estuary (和平溪口). Three key survey areas exist between these two areas: Northern (N1), Central (N2), and Southern (N3) (Fig. 3). This study aimed to observe the population characteristics and behavioral patterns of the most commonly encountered spinner dolphins and Risso's dolphins inshore of Hualien, Taiwan. Each survey typically spans approximately 8–10 h. As of January 19, 2024, 60 surveys (from 2021 to 2024) were conducted in the Ocean Oasis Habitat Use Survey, which lasted 476 h. 


Fig. 4. Focal animal sampling, recording dolphin behavior at 5-min intervals

The method employed for habitat use surveys is “Focal Animal Sampling,” when encountering Risso's dolphins in the designated survey areas, we focus on a stable primary group without disrupting their behavior. We stayed with the same group for as long as possible, sometimes up to 7 h. Dolphin tracks with durations of less than 30 min were not included in this analysis. Throughout the survey, the researchers recorded dolphin behavior every 5 min (Fig. 4). Details such as current swimming direction, diving time and angle, inter-individual distance, and swimming speed were documented. This information was complemented by video footage and photographs to serve as a basis for assessing behavioral states (see Table 1). To ensure that the dolphins were recorded as undisturbed as possible, we maintained a distance of over 200 m from the observed group using binoculars. We approached them closely during the final collection of the underwater sounds and close-up shots.

Table 1. An ethogram for Risso’s dolphin
Behavioral State Definition
Feeding The appearance of predatory states involving encircling and chasing prey (such as fish) or the observation of individuals with food in their mouths. Risso’s dolphins exhibit behavior characterized by spin dives.
Social More than half of the individuals within the group swim in inconsistent, intersecting patterns. They engage in various social behaviors such as rubbing, touching, colliding, chasing, biting, genital and other forms of contact, or mating. In social states, water splashes caused by rolling are visible on the surface, often accompanied by various display behaviors.
Travel More than half of the individuals within the group swim in unison, moving in a fixed direction.
Mill More than half of the individuals within the group frequently change their swimming direction or move back and forth within a range of approximately 1 km, without clear orientation, but with the overall direction of the individuals within the group being roughly the same.
Rest More than half of the individuals within the group gather closely on the surface, slowly moving together towards a specific direction or floating quietly on the surface, maintaining stable positions in terms of head direction and swimming speed. Moreover, there may be a few individuals within the group engaging in surface activities, creating splashes.

Note: Approaches by whale-watching boats to cetacean groups or instances where cetaceans from the main group actively swim towards the boat's course are not included in the analysis.

Can diving behavior indicate Risso’s dolphins are preparing to feed?


Fig. 5. Left-Risso’s dolphin’s foraging spin dive, Right-Risso’s non-spin dive (Visser et al. 2021)


Fig. 6. Risso’s dolphin's spin dive, photo captured inshore of Hualien

During our survey, we observed several instances of Risso’s dolphins engaging in a behavior called "spin diving" (see Fig. 5), which was described in research conducted in the Azores. Scholars have fitted Risso’s dolphins with suction-cup data tags and observed that when Risso’s dolphins engage in spin diving, they quickly descend into deeper waters within a short period. Furthermore, after the dive, the suction cup data loggers recorded dolphins emitting both click and buzz sounds, indicating prey searching. In particular, a buzzing sound is emitted by odontocetes when they attempt to feed, thus proving that after spin diving, Risso’s dolphins attempt to feed underwater. We recorded Risso’s dolphins engaging in spin diving inshore of Hualien (Fig. 6), indicating that Risso’s dolphins are likely to feed in this area.

Risso’s dolphin habitat use analysis

This analysis used data from 43 Risso’s dolphin groups, totaling approximately 98 h of the survey track, and employed a 3 × 3 km² grid to analyze the utilization areas of various behavioral states. Looking at data from the past three years, we observed that Risso’s dolphins most frequently travel inshore of Hualien (Fig. 7), primarily ranging from the south of the Liwu River (立霧溪) to approximately 15 km offshore from Baqi (芭崎), with a few areas of high proportions observed to the north. As for the second most common behavioral state: social, it was divided into three core areas, from north to south: Dazhoushui River (大濁水溪), Chongde (崇德), and offshore of Baqi, within approximately 5 km offshore. The locations where Risso’s dolphins rest lie mainly within approximately 7 km of the mouth of the Liwu River. Milling mainly occurs within approximately 7 km offshore from Qixingtan (七星潭) and Yanliao. The feeding behavioral state is observed within approximately 10 km offshore from the mouth of the Hualien River (花蓮溪口) to Yanliao (Fig. 8). Additionally, we observed that 44.1% of Risso’s dolphin group sightings involved mother-calf pairs, indicating that the surveyed area is an important region for Risso’s dolphin nurturing.


Fig. 7. Proportions of five behavioral states of Risso’s dolphins inshore of Hualien (proportions calculated based on observation time)
Fig. 8. The distribution of five behavioral states of Risso’s dolphins (percentages counted by: duration of each state in each grid/total observation time in each grid; grid size is 3 × 3 km²).
 
As the research vessel tracked dolphins, it followed them parallel to their sides. By analyzing the speed of the vessel during tracking, we could roughly estimate the average swimming speed of the dolphins (Fig. 9). After analyzing the swimming speeds of the tracked groups, we observed that Risso’s dolphins had an overall swimming speed ranging from 0.3 to 4 knots, with faster speeds observed during milling, ranging from 2 to 4 knots. Among the five behavioral states, resting is the slowest on average, ranging from 1.2 to 3 knots; feeding and traveling exhibit a wider speed range, between 0.5 and 4 knots. We aimed to understand whether frequent whale-watching activities in the waters near Hualien affected Risso’s dolphins. By isolating the tracks of approaching whale-watching boats, we observed that there was not much variation in swimming speed. However, in a few experiences with approaching whale-watching boats, we observed a few changes in the group's behavior, such as inconsistent swimming directions or the group becoming more dispersed after the whale-watching boat departed.

Fig. 9. Swimming speeds of Risso’s dolphins in five behavioral states. The green box depicts the swimming speed when whale-watching vessels approach.

Through habitat use analysis, we gained preliminary insights into the key life cycle activities of Risso’s dolphins inshore in Hualien. Additionally, we observed that a few Risso’s dolphin groups were influenced by whale-watching boats. In the future, we will consolidate this analytical data to provide practical operational management recommendations for conservation strategies. Based on the findings of this analysis, we call upon readers to support and promote businesses that advocate dolphin-friendly whale-watching practices, enabling cetaceans inshore of Hualien to continue to thrive. Conducting habitat-use surveys in the Ocean Oases Project requires prolonged patient observation. Today's achievements are the result of the dedication of all participating researchers, collaborating boat operators, and captains, for whom we express our sincerest gratitude. We extend our thanks to the corporate and individual sponsors and government agencies that have donated to support the execution of the Ocean Oasis Project.

Reference

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