334 
Prey occurrence in pantropical 
spotted dolphins, Stenella attenuata, 
from the eastern tropical Pacific 
Abstract .—Identified prey of pan- 
tropical spotted dolphins, Stenella 
attenuata, include 56 species offish and 
36 species of cephalopods. Species iden- 
tifications were made from fish otoliths 
and cephalopod beaks recovered from 
428 stomachs collected throughout the 
eastern tropical Pacific between 1989 
and 1991. The most frequently found 
fish were lanternfish (family Mycto- 
phidae) at 40%, and the most frequently 
found cephalopods were flying squids 
(family Ommastrephidae) at 65%. The 
dominance of these primarily mesope- 
lagic prey species and a significantly 
higher stomach fullness index for stom- 
achs collected during the morning 
hours <x 2 =112.99, df=6, P<0.0001) sug- 
gest that pantropical spotted dolphins 
feed at night when many mesopelagic 
species migrate toward the surface. Sig- 
nificant differences in prey composition 
by season and geographic region indi- 
cate that pantropical spotted dolphins 
are flexible in their diet and may be 
opportunistic feeders. Comparison of 
the diets of pregnant and lactating fe- 
male dolphins revealed that lactating 
females increase both the proportion of 
squid in their diet and quantity of food 
consumed. 
Manuscript accepted 6 September 1996 
Fishery Bulletin 95:334-348 (1997). 
Kelly M. Robertson* 
Susan J. Olivers 
Southwest Fisheries Science Center 
National Marine Fisheries Service, NOAA 
RO. Box 271, La Jolla, California 92038 
*E-mail address: kroberts@its.ucsd.edu 
Previously published analyses of 
the prey of pantropical spotted dol- 
phins in the eastern tropical Pacific 
(ETP) have reported that many spe- 
cies are consumed and that the spe- 
cies composition and importance 
varies. Three previous studies re- 
ported that epipelagic species are 
the dominant prey and included 
species belonging to the families 
Ommastrephidae (flying squid), 
Onychoteuthidae (hooked squid), 
and Exocoetidae (flying fish) (Fitch 
and Brownell, 1968; Perrin et al., 
1973; Bernard and Hohn, 1989). 
However, mesopelagic species in the 
families Myctophidae (lantern fish) 
and Enoploteuthidae (enope squid) 
were also identified in high num- 
bers (Fitch and Brownell, 1968; 
Perrin et ah, 1973). A more recent 
study by Roberts (1994) examined 
only cephalopod prey and found 
that primarily mesopelagic squid 
species in the family Ommastrephi- 
dae were dominant. Another study 
reported on the prey of a spotted 
dolphin caught off Hawaii. The prey 
were predominantly mesopelagic 
species belonging to the families 
Myctophidae and Enoploteuthidae 
( Shomura and Hida, 1965). All these 
studies were based on either a small 
number of samples or samples that 
were collected from a restricted area 
of the Pacific Ocean and, therefore, 
may be limited in their represention 
of the prey of pantropical spotted 
dolphins. 
In this paper, we describe the 
prey of pantropical spotted dolphins 
collected throughout their range in 
the ETP. We calculate the percent 
number and percent frequency of 
occurrence for each prey species 
identified to quantify the relative 
importance of prey species. Variabil- 
ity in the diet due to geographic re- 
gion, oceanographic season, and, for 
females, due to reproductive condi- 
tion are examined. We also present 
the size distribution of prey con- 
sumed for species for which data 
were available in order to convert 
otolith and beak measurements to 
prey size. 
Methods 
Stomachs were collected from 428 
pantropical spotted dolphins in 103 
net sets by biological technicians 
placed aboard U.S. tuna purse-seine 
vessels fishing in the ETP between 
1989 and 1991 (Fig. 1; see Jefferson 
et al., 1994, for collection procedures). 
Only the contents in the esoph- 
ageal (fore) stomach were examined 
for our analyses because this stom- 
ach compartment contains the most 
recent meal and thus the most 
identifiable remains (Harrison et 
al., 1967). The forestomach of each 
specimen was weighed full and 
empty to the nearest 0.1 g with a 
Mettler PC4400 balance. The con- 
tents were sorted and recovered by 
