Robertson and Chivers: Prey of Stenella attenuate 
343 
Prey species 
Fish 
1 = Symbolophorus spp 
2 = M aurolaternatum 
3 = L. parvicauda 
4 = D. spiendidus 
5 = C pauciradiatus 
Cephalopods 
6=0. bartrami 
7 = 0. banksii 
8 = A. affinis 
9 = M. dentata 
10 = /-. dislocata 
□ West 
0 South 
■ Northeast 
520 
390 
260 
130 
0 
1 23456789 10 
Prey species 
Figure 3 
The percent number of the 10 most numerous prey species by season: winter and 
summer, and geographic area: northeast (/i=159), south (n= 72), and west (n=198). 
See text for full name of the species listed. 
dolphin species (Brown and Norris, 1956; Ross, 1979; 
Jones, 1981; Fiscus, 1982; Gaskin, 1982; Leather- 
wood et al., 1983; Evans, 1987; Young and Cockcroft, 
1995). Geographic and seasonal changes in prey com- 
position could be a result of migration of prey into or 
out of an area, prey spawning seasons, or simply dis- 
tributional boundaries of prey. It has been suggested 
that the movements of dolphin may correspond to 
the movement or availability of prey (Jones, 1981; 
Reilly, 1990; Young and Cockcroft, 1994). There is 
evidence that the distribution of pantropical spotted 
dolphin shifts westward along the 10°N latitude as 
the summer season progresses, and it has been hy- 
pothesized that this change in distribution is due to 
changes in prey distribution re- 
sulting from the equatorial cur- 
rents (Au and Perryman, 1985; 
Reilly, 1990). Unfortunately, in- 
formation on precise distribu- 
tions and seasonal movements 
of identified prey species in the 
ETP is limited; therefore this 
hypothesis cannot be addressed 
properly (Clarke, 1973; Oku- 
tani, 1974; Wisner, 1974; Clarke, 
1977; Roper et al., 1984; Clarke, 
1986, a and b). 
Reproductive condition 
Changes in diet composition 
between lactating and pregnant 
dolphins have been documented 
in a number of species (Perez 
and Mooney, 1986; Bernard and 
Hohn, 1989; Recchia and Read, 
1989; Cockcroft and Ross, 1990; 
Young and Cockcroft, 1994, 
1995). The physiological energy 
required to maintain lactation 
is quite high for mammals and 
may require a change in diet 
composition to include food with 
a higher caloric content 
(Clutton-Brock et al., 1982; 
Perez and Mooney, 1986; 
Recchia and Read, 1989; 
Iverson, 1993). In fact, Bernard 
and Hohn ( 1989) presented evi- 
dence for a shift in diet between 
pregnant and lactating 
pantropical spotted dolphins 
and suggested that it was due 
to the physiological demands of 
lactation. They found a higher 
proportion of flying fish (family Exocoetidae) in the 
diet of lactating females and a higher proportion of 
ommastrephid squid in the diet of pregnant females. 
We tested the same hypothesis for our sample, but 
our results were different. Although the proportion 
of fish (family Myctophidae) in the diet was higher 
than the proportion of squid (family Omma- 
strephidae) for both pregnant and lactating females, 
the proportion of squid was significantly higher in 
the diet of lactating females. Fish may provide most 
of the caloric intake for both lactating and pregnant 
females because both lanternfish and flying fish have 
a high caloric and lipid content in comparison with 
squid (Childress and Nygaard, 1973; Sidwell et al., 
