Miki Shirakihara

Finless porpoise abundance in Omura Bay, Japan : estimation from aerial sighting surveys

We used aerial surveys and line-transect methods to estimate the abundance of the finless porpoise (Neophocaena phocaenoides) in Omura Bay, Japan. We conducted 4 surveys once each season from August 1993 to May 1994. We surveyed 621.5 km and sighted 54 porpoise groups (87 animals). In the spring survey, we detected porpoises offshore as well as inshore, whereas porpoises were mostly inshore in all other seasons. The abundance of porpoises was estimated to be 187 animals (0.6 individuals/km 2 , CV = 20%).

A population size estimate of the finless porpoise,Neophocaena phocaenoides, from aerial sighting surveys in Ariake Sound and Tachibana Bay, Japan

We estimated the population size of the finless porpoise (Neophocaena phocaenoides) in Ariake Sound and Tachibana Bay of western Kyushu, southwestern Japan, from aerial sighting surveys using line transect methods. All 12 surveys were conducted from May 1993 to May 1994 (8 in Ariake Sound and 4 in Tachibana Bay). In addition to these, 14 surveys were also carried out to obtain information on porpoise occurrence in Tachibana Bay (5 surveys) and in neighboring Sumo Nada (5) and Yatsushiro Sound (4). In Ariake Sound, 225 porpoise groups (369 animals) were detected during all flights totalling 1,694.4 km. In Tachibana Bay, a total of 997.8 km was surveyed and 55 groups (290 animals) were sighted. However, no sightings were recorded in Sumo Nada (distance searched = 148.7 km) and Yatsushiro Sound (208.4 km). In Ariake Sound, few sightings were recorded from waters shallower than 5 m in depth. In Tachibana Bay all animals were detected from waters of less than 50 m depth. The population size was estimated as 1,983 animals in Ariake Sound (95% CI = 1,382-2,847), 1,110 in Tachibana Bay (95% CI = 642-1,920), and 3,093 in the 2 waters (1.3 individuals/km2, 95% CI = 2,278-4,201).

FOOD HABITS OF FINLESS PORPOISES NEOPHOCAENA PHOCAENOIDES IN WESTERN KYUSHU, JAPAN

Abstract We examined the stomach contents of 87 finless porpoises (Neophocaena phocaenoides) in the Ariake Sound–Tachibana Bay area and Omura Bay in western Kyushu, Japan, between 1987 and 1992. Fish (Gobiidae and Atherinidae) were the most numerous and most frequently occurring prey in Omura Bay, whereas both cephalopods (Octopodidae, Sepiidae, Sepiolidae/Sepiidae, and Loliginidae) and fishes (Clupeidae, Engraulidae, and Sciaenidae) were equally important in Ariake Sound–Tachibana Bay. Species compositions in the commercial catch differed between the 2 waters, suggesting that differences in prey availability may explain the geographical variation in diet. Finless porpoises in Ariake Sound–Tachibana Bay showed ontogenetic and seasonal variations in diet. The mean length at weaning was estimated to be 101 cm, corresponding to approximately 6 months of age. Calves fed on small-sized demersal fish and cephalopods. The predominant prey species for sexually mature individuals (including lactating females) consisted of konoshiro gizzard shad (Konosirus punctatus) and cephalopods. Seasonal availability of these organisms may be related to births in the fall–winter season in Ariake Sound–Tachibana Bay. Size-dependent prey selectivity was equivocal. Day–night difference in foraging time was not indicated by the index of stomach fullness.

A community split among dolphins: the effect of social relationships on the membership of new communities

Little is known about community splitting among dolphins because such events are rare in dolphin populations. A case of a community split was confirmed in a population of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in Japan, where a group of approximately 30 dolphins moved to a new habitat some 60 km from the original habitat. We examined the associations among the dolphins before the community split to determine whether the new community members were already socially different before the split, using 7-year identification data. Before the split, the males in the same community after the split more often associated with each other than they did with those in different community. In contrast, the association patterns among females and between sexes showed no relationships with their post-split community membership. These results indicate that the males of new community were socially different from the other males for a long time before the split, but the females might not have been different. Our findings suggest that at time of the community split, the factors determining the memberships of the subsequent communities are sex-linked. The long-term social relationships among males could be maintained in the subsequent communities.

Stomach contents of an Indo‐Pacific bottlenose dolphin stranded in Amakusa, western Kyushu, Japan

The food habits of toothed whales are usually determined by examining the stomach contents of harvested, accidentally killed or stranded individuals. For species taken directly by fisheries, a comparatively large number of specimens have been used for analysis. However, for small populations of whales that are not target species of fisheries, large numbers of specimens cannot be expected to be collected. For small populations of coastal bottlenose dolphins, the stomachs of stranded or by-caught dolphins are collected over a long period of time. A local population of Indo-Pacific bottlenose dolphins Tursiops aduncus inhabit the coastal waters around Amakusa-Shimoshima Island (130°07′E, 32°33′N), which is located at the mouth of Ariake Sound, Kyushu, Japan. The population is resident year-round, and the mean population size was estimated to be 218 individuals in 1995–1997. Despite their use of coastal habitats, little information is available on the food habits of these dolphins. The redfin velvetfish Hypodytes rubripinnis and a cephalopod beak were found in the stomach of a juvenile dolphin taken accidentally (Shimizu M, pers. comm., 2006). The authors had an opportunity to examine the stomach contents of an Indo-Pacific bottlenose dolphin stranded on the west coast of Oyano Island (130°25′E, 32°34′N), at the mouth of Ariake Sound, on 3 June 2002. The dolphin examined was a male with a body length of 258 cm. To obtain the maximum information on prey species from dolphin specimens, the authors observed all bones and otoliths of fish using a method that was developed to identify animal remains from archaeological sites. The results of the stomach content examination are herein reported. After both the lower part of the esophagus and the upper part of the small intestine were tied with string, the entire stomach was removed. The foreand main-stomach contents were washed, strained in a 0.5-mm sieve, and sorted into fishes and cephalopods. Undigested or partially digested fishes retrieved from the stomach were identified based on external morphology and the standard length was measured, if possible. Whole fishes and the remnants of other fishes were boiled to collect bones and otoliths. Fish bones and otoliths were used for prey identification with the aid of the reference collection. This collection contains over 150 species and has been used for identifying faunal remains found during archaeological excavations. Squid lower beaks were identified by referring to the cephalopod beak collection of the National Science Museum, Tokyo. Prey were identified to the lowest taxonomic level possible. To estimate the number of prey, the number of bones identified for each bone element was divided by the number of such bone elements present in a skeleton, and then the highest number was used as the number of that prey. Symmetric elements of fish bones were identified as left or right, and the total number of fishes was estimated from the largest number of bones from either side. Axial elements were divided by one; vertebrae were divided by the number of vertebrae in a reference skeleton. Cephalopods were quantified using the number of lower beaks. *Corresponding author: Tel: 81-052-789-4179. Fax: 81-052-789-4179. Email: shanqijian@yahoo.co.jp Received 28 November 2006. Accepted 31 July 2007. FISHERIES SCIENCE 2008; 74: 1195–1197

Alliance Formation of Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) off Amakusa, Western Kyushu, Japan

Abstract. Indo-Pacific bottlenose dolphins (Tursiops aduncus) off Amakusa-Shimoshima, approximately 200 individuals, form relatively large groups frequently exceeding 100 individuals and show high site fidelity to the area around Tsuji Island, northern coast of Amakusa-Shimoshima. This suggests that individual dolphins may have long interaction times with many other individuals. Consequently, competition between males is likely to be high and formation of alliances may be expected. However, this has not yet been confirmed. With photo-identification data collected between 2010 and 2014, we examined individual associations. Pairs of males formed significantly non-random associations for multiple years, and were seen surrounding females, many of whom were considered to be receptive at that time. Our results suggest that male Indo-Pacific bottlenose dolphins form alliances in this population, where dolphins form large groups and show high site-fidelity.

Patterns of association among female Indo-Pacific bottlenose dolphins ( Tursiops aduncus ) in a population forming large groups

Indo-Pacific bottlenose dolphins (Tursiops aduncus) around Amakusa-Shimoshima, Japan form large groups of >100 individuals, a remarkable feature of this population, which is much larger than previously studied bottlenose dolphins. Using photo-identification data collected on 33 days in 2012, associations among 70 females were investigated using two scales of association measure (photograph- and group-based association) to know whether they associate based on their reproductive status, as is typically observed in other populations with smaller group size. Significant differences in associations between females of the same reproductive status category and those in different categories were detected, and a female that lost her calf within a year decreased associations with females with calves of the same age. These suggests that even in a population with large group size, the reproductive status of females is one of the factors influencing their associations.