Fraser’s dolphin was described in 1956 based on a skeleton collected by E. Hose from a beach in Sarawak, Borneo in 1895. F. C. Fraser gave it the genus name Lagen-odelphis, due to what appeared to him as similarity of the skull to those of Lagenorhynchus spp. and Delphinus del phis. The external appearance of this species was not known until 1971 when specimens were found in widely separated areas: near Cocos island in the eastern tropical Pacific, South Africa, and southeastern Australia.
I. Characters and Taxonomic Relationships
Fraser’s dolphin is easily identified by its stocky body, short but distinct beak, and small, triangular or slightly falcate dorsal fin—the flippers and flukes are also small (Fig. 1). The color pattern is striking and varies with age and sex. For example, a distinct black stripe that extends from eye to anus is absent or faint in juveniles, wider and thicker in adult males, and variable in adult females. The same is true with the facial stripe or “bridle”: it is absent in calves, variable in females, and extensive in adult males, where it merges with the eve-to-anus stripe to form a “bandit mask.” Color pattern in the genital region may also be sexually dimorphic. The back is brownish gray, the lower side of the body is cream colored, and the belly is white or pink. Other features that appear to vaiy with age and sex are dorsal fin shape and the postanal hump. With some variability, the dorsal fin is slightly falcate in calves and females and more erect or canted in adult males. Similarly, the postnatal hump is either absent or slight in females and young of both sexes and well developed in adult males. From a distance, the eve-to-anus stripe makes Fraser’s dolphin look similar to the striped dolphin. Stenella coendeoalba. However, the distinctive body shape of Fraser’s dolphin rules out confusion with other species. The largest male recorded was 2.7 m long and the largest female 2.6 m with males over 10 years old significantly larger than females. Large males could weigh up to 210 kg. Based on a limited number examined, it is tentatively proposed that Fraser’s dolphins in the Atlantic are lar ger than those in the Pacific. Fraser’s dolphin belongs to the subfamily Delphininae. Based on cytochrome b mtDNA sequences, it is more closely related to Stenella, Tursiops, Delphinus, and Sou so than it is to Lagenorhijchus. Morphologically, the skull structure shows close similarity with that of the common dolphin, D. delphis, in terms of the presence of deep palatal grooves and with those of S. longirostris, S. coeruleoalba, and the clymene dolphin Stenella clymene in several other characteristics.
II. Distribution and Ecology
Fraser’s dolphin is a tropical species, distributed between 30°N and 30°S. Strandings outside this limit, such as in southeastern Australia, Brittany, and Uruguay, are considered unusual and are probably influenced by temporary oceanographic events. It is typically an oceanic species, except in places where deep water approaches the coast such as in the Philippines, Indonesia, and Lesser Antilles, where Fraser’s dolphins can be observed within 100 m from shore. In the eastern tropical Pacific, they were observed to occur at least 15 km offshore and, mostly, on high seas approximately 45-110 km from the coast where water depth was between 1500 and 2500 m. In the Sulu Sea, Philippines, highest sighting rates were in waters >500 m and up to 5000 m. although some animals were observed in shallower waters adjacent to the continental shelf. In the Gulf of Mexico, sightings have been around 1000 m depth, and the animals appear to be more common in the Gulf than anywhere else in the North Atlantic. Affinity to deep waters can be explained by the type of prey eaten by Fraser’s dolphins, which is composed of mesopelagic fish, crustaceans, and cephalopods. It is also suggested that Fraser’s dolphins feed selectively on larger prey that inhabit deeper waters. In the eastern tropical Pacific and the Sulu Sea, the most common fishes in the diet are the Myctophidae and Chauliodontidae, and the most common crustaceans are the Oplophoridae. Cephalopods were not reported eaten by Fraser’s dolphins in the eastern tropical Pacific, but this group comprised a significant amount of the diet in the Sulu Sea animals, i.e., about 30% by volume. Based on prey composition, it was hypothesized that Fraser’s dolphins in the eastern tropical Pacific feed at two depth horizons: the shallowest level of no less than 250 m and the deepest no less than 500 m. In the Sulu Sea. Fraser’s dolphins appear to feed from near surface to at least 600 m. Examination of myoglobin (Mb) concentrations in the skeletal muscles of Fraser’s dolphin support the distribution and feeding habits of this species. The value averages at 7.1 g Mb 100~g muscle and is comparable to those of the very best divers such as the Weddell seal (Lep-tonychotes weddellii). bottlenose whale (Hyperoodon ampulla-tus), and sperm whale (Physeter macrocephalus). However, in South Africa and in the Caribbean, Fraser’s dolphins were observed feeding near the surface.
Although no predation has been reported, Fraser’s dolphins may be preyed upon bv killer whales (Orcinus orca), false killer whales (Pseudorca crassidens), and large sharks. Cookie cutter sharks (Isistius brasiliensis) are thought to inflict circular wounds.
An external parasite, Xenobalanus sp., and internal parasites (Phyllobothrium delphini, Monorhygma grimaldi, Anisakis simplex, Tetrabothrius sp., Bolbosoma sp., Strobicephalus triangularis, Campula sp., and Stenurus ovatus) have been observed in Fraser’s dolphins.
Figure 1 Froser’s dolphins in the eastern tropical Pacific.
III. Behavior and Life History
Fraser’s dolphins often swim in tight fast-moving schools of 100 to 1000 individuals with the members of the school “porpoising in low-angle, splashy leaps” and have been reported to swim away from vessels in the eastern tropical Pacific. In the Philippines they were observed to ride the bow if the boat ran at less than 3 knots but were often displaced by melon-headed whales (Peponocephala electra).
In the eastern tropical Pacific and the Gulf of Mexico. Fraser’s dolphins are often found together with melon-headed whales. Although also sometimes seen with melon-headed whales (when in relativelv shallow waters), Fraser’s dolphins in the Sulu Sea are often seen with short-finned pilot whales. Glo-bicephala macrorhijnchus. Other species Fraser’s dolphins are seen with are the false killer whale. Risso’s dolphin (Grampus griseus), spinner dolphin, pantropical spotted dolphin (S. at-tenuata), bottlenose dolphin (Tursiops truncatus), and sperm whale. In the western tropical Indian Ocean, Fraser’s dolphins were not seen with any other species.
A sample of 108 dolphins from a school captured in Japan showed a mixed-age group and a ratio of 1:1 between males and females; the oldest was estimated to be 17.5 vears old. Males reach sexual maturity at about 7-10 years at 220-230 cm and the females at 5-8 years at 210-220 cm. Mating may be promiscuous. The annual ovulation rate is about 0.49, and the gestation period is about 12.5 months. The calving interval is approximately 2 years; in Japanese waters, calving appears to peak in spring and fall. Limited samples from South Africa suggest that calving occurs in summer. Length at birth is estimated to be about 100-110 cm.
IV. Interaction with Humans
Fraser’s dolphins are caught in drive nets in Japan and by harpoon in Lower Antilles, Indonesia, and (before they became protected) in the Philippines. Thev are also caught incidentally in purse seines in the eastern tropical Pacific and the Philippines, in traps nets in Japan, in gill nets in South Africa and Sri Lanka, in antishark nets in South Africa, and in drift nets in the Philippines.
