| Summary: | 博士 === 國立臺灣大學 === 生態學與演化生物學研究所 === 107 === Risso’s dolphin occurs in tropical and temperate waters worldwide, and is one of the most common cetacean species in Taiwanese waters. This study attempted to reveal its distribution in water-depth ranges and variations in group size and composition (with or without mother-calf pairs). Based on sighting records from experienced whale-watching guides on board the whale-watching boats operated on eastern Taiwan, 1630 sightings of Risso’s dolphin were recorded from 3939 touring trips near Hualien County from 1998 to 2014. . Comparing the frequency distribution of water depth between ship tracks and dolphin sightings, we found that Risso’s dolphin commonly occurred on the continental shelf and slope, and mainly stayed at waters of 300~1500 m in water depth, with a mean of 862.1 ± 12.3 m (standard error). The frequency distribution of water depth showed a significant difference between sites (deeper in the Shirti area than in the Hualien area), but no significant difference was detected among various group sizes. Nevertheless, the spatial distribution significantly varied with group composition, but not with group size. Groups with mother-calf pairs stayed in significantly shallower water, which was proposed to be related to predator avoidance. In addition, the large fluctuation in annual sighting ratios of Risso’s dolphin that ranged 0.1~ 0.7 during the 17-year period may be related to variation in prey abundances, which require future investigation.
We also took whale-watching sighting data achieved during the years of 1998-2014, at two areas in the central eastern coast of Taiwan. We used ecological niche modelling (Maxent) to analyze Risso’s dolphin habitat preference with topographic and climatic factors. Our results demonstrate that Risso’s dolphins frequently occurred in deep water habitats along the edge of continental shelf in the study area. The important variables associated with their distribution include water depth, the distance from river mouth and sea surface temperature.
This study attempted to understand the movement paths, including the swimming speed, movement direction, and linearity were compared among groups of different sizes and groups with and without mother–calf pairs. Based on the focal group follows conducted on the boat surveys during 2010-2012. Mean speed between two 5-min units (leg-speed) of Risso’s dolphin groups was 4.1 ± 0.1 km/hr (S.E. n=420, range: 0.1-13 km/hr). No difference in speed in group size classes and composition. Most groups heading moved to south and with high linearity.
This study shows that the movement range of Risso’s dolphins is wide in the eastern sea area of Taiwan, and the range of activities overlaps with the geographic locations of the sighting spots, indicating the importance of this habitat for Risso’s dolphins.
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