Robertson and Chivers: Prey of Stenella attenuate 
341 
Tabie 3 
Mean, standard deviation, and range of fish length (total length) and weight of fish species consumed by spotted dolphin, calcu- 
lated with regression equations (see Footnote 2 in the main text) and ratios of otolith length to fish length (Butler, 1979; Hecht, 
1987). ‘Number’ represents the total number of otoliths that were measurable, without being broken or worn. For A. thazard, ‘8’ 
represents the number of fish measured. 
Estimated prey length Estimated weight (g) 
Prey species 
No. 
Mean ±SD 
Range 
Mean ±SD 
Range 
Total 
Fish 
Symbolophorus spp. 
273 
66.6 ±7.0 
40.9-82.4 
4.1 ±1.3 
0.8-7. 9 
1,105 
Myctophum nitidulum 
14 
56.4 ±5.2 
46.7-63.1 
Ceratoscopelus warmingii 
114 
49.3 ±4.8 
33.8-61.5 
Exocoetus volitans 
68 
158.7 ±22.2 
90.9-187.4 
Exocoetus monocirrhus 
19 
164.7 ±16.8 
136.9-193.7 
50.9 ±8.3 
37.2-65.2 
967 
Oxyporhamphus micropterus 
40 
143.4 ±15.1 
122.2-180.1 
Cubiceps pauciradiatus 
233 
109.1 ±9.8 
74.6-129.8 
Cubiceps baxteri 
3 
140.9 ±4.9 
136.1-145.8 
Auxis thazard 
8 
211.5 ±22.9 
190.0-260.0 
Stomach fullness index (SFI) 
The SFI ranged from 0.4% to 86.4% (average: 29.4%) 
after all measures were scaled to the maximum SFI. 
A statistically significant difference was found in the 
SFI between quarters of the day (% 2 = 112.99, df=6, 
P<0.0001). A SFI >60% was calculated for 42.0% of 
the animals collected between 0600 and 0900 h, 
whereas only 1.0% of the sample collected between 
1501 and 1800 h had a SFI >60% (Fig. 4). 
Reproductive condition 
The mean number of squid differed significantly be- 
tween pregnant and lactating females (Student’s t- 
test=-2.65, P=0.010); however, no significant differ- 
ence was found in the mean number of fish consumed 
(Student’s (-test=0.25, P-0.803). When stratified by 
time-of-day, the mean SFI was significantly higher 
for lactating dolphins during all time periods 
(X 2 =46.98, df=6, P<0.0001; Table 4). 
Discussion 
Mesopelagic prey were found to dominate the diet of 
pantropical spotted dolphins; myctophid fish, and 
enoploteuthid and ommastrephid squid accounted for 
69% of all prey consumed (Table 1). These mesope- 
lagic species are associated with the deep scattering 
layer and most undergo diel vertical migration, mov- 
ing into the upper 200 m at dusk to feed and retreat- 
ing to depth at dawn to avoid predation (Gibbs and 
Roper, 1971; Clarke, 1973, 1978; Wisner, 1974; Roper 
Table 4 
The average stomach fullness (%) for lactating and preg- 
nant spotted dolphins throughout the day. The day is di- 
vided into three hour increments from 0600 h to 1800 h. ‘No.’ 
is equal to the number of stomachs in each time category. 
Average stomach fullness 
Lactating Pregnant 
Time (h) 
% 
No. 
% 
No. 
0600-0900 
52.7 
15 
38.6 
6 
0901-1200 
37.3 
18 
23.1 
10 
1201-1500 
27.2 
15 
15.2 
7 
1501-1800 
26.1 
9 
10.9 
14 
Overall average 
44.3 
57 
19.5 
37 
and Young, 1975; Roper et al., 1984; Smith and 
Heemstra, 1986). The SFI, which we found to be high- 
est in the morning hours (i.e. 0600-0900 h, Fig. 4), 
suggests that pantropical spotted dolphins feed dur- 
ing the night when these prey are nearest to the sur- 
face. In fact, Shomura and Hida (1965) hypothesized 
that the spotted dolphin caught off Hawaii fed just 
before dawn, prior to descent of the deep scattering 
layer, because fresh mesopelagic prey were found in 
its stomach (enoploteuthid squid and myctophid 
fishes). Evidence of nighttime feeding by pantropical 
spotted dolphins has also been presented by Scott 
(1991), who reported that the highest proportion of 
undigested prey was recovered from spotted dolphin 
stomachs collected between 0700 and 0930 h and that 
