MESOPELAGIC FISHES EATEN BY FRASER'S DOLPHIN, 



LAGENODELPHIS HO SEP 



Bruce H. Robison 2 and James E. Craddock 3 

 ABSTRACT 



Examination of the stomach contents of three specimens of the rare pantropical dolphin, Lagenodelphis 

 hosei, showed them to have been feeding on a mixed diet of mesopelagic fishes, shrimps, and squids, with 

 fishes by far the most important component. Ecologically and morphologically the prey fishes comprised 

 three types: A group of elongate, solitary, vertically mobile species; deep-bodied, aggregative, nonmigratory 

 fishes; and thick-bodied, dark colored nonmigrators. Based on the known vertical distribution patterns of the 

 prey species, the three dolphins had been feeding at depths near 250 and 500 m. The large sizes and species 

 composition of the prey fauna indicate that the dolphins were feeding selectively, ignoring the smaller, more 

 abundant vertically migratory species that dominate the upper mesopelagic midwater fish fauna of the east- 

 ern tropical Pacific. The estimated nutritional value of the ingested prey is similar to the values reported for 

 related cetaceans maintained in captivity. 



Until recently Fraser's dolphin, Lagenodelphis hosei, 

 was known only as a skeleton, collected before 1895 

 from a beach in Sarawak, Borneo, and deposited in 

 the British Museum of Natural History. From these 

 bones, F. C. Fraser described the species in 1956, but 

 it was not until 1971 that living specimens were ob- 

 served and recognized as L. hosei (Perrin et al. 

 1973a). Subsequent reports have appeared which 

 suggest a pantropical distribution (Tobayama et al. 

 1973; Caldwell et al. 1976; Miyazaki and Wada 

 1978). Most of the accumulating information on this 

 rare and little-known dolphin is concerned with its 

 distribution and anatomical distinctions, although 

 Tobayama and his colleagues briefly described the 

 stomach contents of a specimen found at Kamo- 

 gawa, Japan. 



Fitch and Brownell (1968, 1971) have demon- 

 strated the usefulness of fish otoliths found in ceta- 

 cean stomachs as reliable indicators of prey identity 

 and, in some cases, of feeding depths. The shape of 

 these characteristic structures is often species- 

 specific, and they are relatively resistant to digestion. 

 Other bones, such as dentaries, urohyals, and oper- 

 culars, can also be helpful as indicators if digestion 

 has not progressed too far (Miyazaki et al. 1973). 

 There is a nearly direct relationship between otolith 



'Contribution No. 5148 of the Woods Hole Oceanographic In- 

 stitution. 



2 Oceanic Biology Group, Marine Science Institute, University of 

 California, Santa Barbara, CA 93106. 



'Woods Hole Oceanographic Institution, Woods Hole, MA 

 02543. 



Manuscript accepted August 1982. 

 FISHERY BULLETIN: VOL. 81, NO. 2, 1983. 



dimensions and standard length for adult fishes 

 (Fitch and Brownell 1968); thus the sizes of the in- 

 gested fish can be quantified by comparing otolith 

 measurements with otolith and standard length data 

 from fish collected by trawling. These size data can in 

 turn be used to estimate the nutritional value of the 

 ingested fish by referring to data on their chemical 

 composition (e.g., Childress and Nygaard 1973). 



In addition to the information they provide about a 

 predator's feeding habits (Perrin et al. 1973b), 

 stomach content analyses are also valuable for es- 

 timating predation pressure on the prey fauna. With 

 regard to predation by nekton on micronekton, such 

 data may be of particular value because this major 

 trophic link is one of the most poorly understood as- 

 pects of oceanic community dynamics. 



MATERIAL AND METHODS 



We examined the stomach contents of three female 

 specimens of Lagenodelphis hosei that were captured 

 by purse seine in the eastern tropical Pacific (lat. 5°N, 

 long. 122°22'W) in May 1972 and acquired by Har- 

 vard University's Museum of Comparative Zoology. 

 The first specimen (MCZ 52979) was about 230 cm 

 long and carried a well-developed fetus. The sizes of 

 the second and third individuals (MCZ 54379, MCZ 

 56572) were about 215 and 210 cm. The stomachs 

 were removed intact from the specimens, which had 

 been frozen since capture. The stomachs were 

 thawed and opened, and their contents were gently 

 washed through a graded series of screens to 



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