碩士 / 國立臺灣大學 / 動物學研究所 / 91 / In order to research the distribution and social organization of Risso''s dolphins (Grampus griseus) in the central-eastern waters off Taiwan, a photo-identification survey was conducted in the waters near Shir-ti, Hualien, during the periods from April to September 2001 and 2002. Observations were not conducted during winter due to the rough sea state. Data regarding group size and composition was used to analyze time-scale and space-scale distribution. Photos obtained in 2001 were used to identify individuals by their dorsal fins. Half-weight index and the SOCPROG program were used to analyze association patterns of resighted animals. A total of 204 groups of Risso''s dolphins were observed. Group sizes in summer were significantly larger than in spring and fall. This suggested a higher summer abundance of Risso''s dolphins in the study area. This seasonal increase might correspond to an increase in summer prey, Sthenoteuthis oualaniensis. Sightings of Risso''s dolphins were significantly higher in 500 - 1500 m water depth. A shoreward movement from spring to fall was also observed. This seasonal switch in habitat might correspond to a high winter abundance of its predominant winter-prey, Enoploteuthis chunii, in the nearshore water of the study area. The group compositions of Risso''s dolphins were categorized into 4 groups based on the skin color patterns and presence of calves: grayish-adult, whitish-adult, mixed, and calving groups. Except for grayish-adult groups, the sizes of the other three groups were significantly correlated with the daylight hours of 10:00 to 18:00, with group size increasing from noon to dusk. Sightings of large schools containing 120 to 200 animals were significantly greater in the afternoon. This phenomenon could be related to the nocturnal feeding habits of Risso''s dolphins. The large schools might be feeding aggregations. Sightings of calves were highest in summer and fall. Courting behaviors occurred more often in spring and summer and were often observed in the mixed groups. This finding suggests that the function of mixed groups might be related to courtship. Summer might be an important mating season because the occurrence of mixed groups peaked in this season. At least 670 individuals were identified and of these, 94 animals were resighted in 2001. Some of the resighted individuals inhabited the study area for at least 3-4 months. Whitish adults showed significantly non-random and stronger association among whitish individuals, while grayish adults showed random association. Previous studies and this data suggest that whitish adults are likely males and grayish adults are likely females and this subject merits further investigation.
| Identifer | oai:union.ndltd.org:TW/091NTU00312014 |
| Date | January 2003 |
| Creators | Ching-Yi Lin, 林靜宜 |
| Contributors | Lien-Siang Chou, 周蓮香 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | en_US |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 83 |
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