Distribution of Risso's dolphin (Grampus griseus) in the east-central coastal waters of Taiwan based on whale-watching records

Distribution of Risso's dolphin (Grampus griseus) in the east-central coastal waters of Taiwan based on whale-watching records

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 whale-watching boats operated in 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 depths between ship tracks and dolphin sightings, we found that Risso’s dolphin commonly occurred on the continental shelf and slope, and mainly stayed in waters of 300~1500 m in depth, with a mean of 862.1±12.3 m (standard error). The frequency distribution of water depths 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 sightings per trip during the 17-year period may be related to variations in prey abundances, the confirmation of which requires future investigation.