Robertson and Chivers: Prey of Stenella attenuate 
335 
150 ° 140 ° 130 ° 120 ° 110 ° 100 ° 90 ° 80 ° 70 ° 
Figure 1 
Distribution of net-sets (n =103) from which 428 pantropical spotted dolphin stom- 
achs were collected between 1989 and 1991. For analysis of geographic variabil- 
ity in prey composition, the sample was divided into areas which correspond to 
recognized stock boundaries and different oceanographic regions: northeastern, 
southern, and western. 
rinsing them through a series of sieves with mesh 
sizes of 12.5 mm, 1.4 mm, and 500 g. Fish otoliths 
and other skeletal remains, cephalopod beaks, crus- 
taceans, gastropods, and parasitic nematodes were 
collected and enumerated. 
Left and right fish otoliths for each species were 
separated and counted. The highest count of either 
was used as the minimum number of fish present 
for that species. Fish species were identified to the 
lowest possible taxon by using voucher collections of 
otoliths at the Los Angeles County Museum of Natu- 
ral History (Lavenberg 1 ), the Southwest Fisheries 
Science Center (SWFSCK Pitman and Carretta 2 ), and 
otolith identification keys published by Fitch and 
Brownell (1968), Fitch (1969), and Butler (1979). 
Frigate mackerel ( Auxis thazard) were identified 
from vertebral characteristics rather than from 
otoliths (Clothier, 1950; Uchida, 1981). For cephalo- 
pods, upper and lower mandibles were separated and 
counted for each species; the highest count of either 
represented the minimum number present in the 
stomach. Species identifications to the lowest pos- 
sible taxon were made with the voucher collection of 
1 Lavenberg. R. 1993. Los Angeles County Mus. Natl. History, 
Ichthyology Dep., 900 Exposition Blvd., Los Angeles, CA 90007. 
2 Pitman, R., and J. Carretta. 1993. Southwest Fish. Sci. Cen- 
ter, Natl. Mar. Fish. Serv., NOAA, P.O. Box 271, La Jolla, CA 
92038. 
beaks at the Santa Barbara Museum of Natural His- 
tory 3 and identification keys published by Wolff 4 ) and 
Clarke (1986a). The relative importance of prey was 
determined by calculating the percent number and 
percent frequency of occurrence (Hyslop, 1980) for 
each individual species and family. 
Prey size 
For analysis of prey size, maximum length of otoliths 
(tip of the rostrum to the posterior margin) and lower 
rostral length (LRL) of beaks were measured with 
an ocular micrometer disc accurate to 0.1 p, only for 
those items that showed little sign of erosion. Re- 
gression equations and ratios of standard length to 
otolith length were used to convert measurements 
to prey lengths and weights (Butler, 1979; Clarke, 
1986a; Hecht; 1987; Wolff, 4 Pitman and Carretta 5 ). 
Length measurements of A. thazard were obtained 
by estimating total length from whole fish and skel- 
etons recovered from the stomachs. We used the prey 
3 Hochberg, E. 1992. Santa Barbara Museum of Natural His- 
tory, 2559 Puesta del Sol Road, Santa Barbara, CA 93105. 
4 Wolff, G. A. 1982. A study of feeding relationships in tuna 
and porpoise through the application of cephalopod beak 
analysis. Final Tech. Report for DAR-7924779, 231 p. 
5 Pitman, R., and J. Carretta. 1993. Southwest Fish. Sci. Cen- 
ter, Natl. Mar. Fish. Serv., NOAA, P.O. Box 271, La Jolla, CA 
92038. Unpubl. data. 
