342 
Fishery Bulletin 95(2), 1 997 
Dophin length class (cm) 
Figure 2 
Mean estimated length of squid (A) and fish (B) with 2 SE 
shown for each 5 cm dolphin length class. Significant dif- 
ferences were found in the size of prey consumed with in- 
creasing dolphin length for both squid (r 2 =0. 791, P<0.0001 ) 
and fish (r 2 =0.648, PcO.OOl). 
no fresh prey was recovered from stomachs collected 
after 1200 h. More recently, the collection of data on 
dive patterns has provided additional evidence of 
nighttime feeding by spotted dolphins. The dive pat- 
terns show marked diurnal changes and both deeper 
and longer dives at night. In particular, the dawn- 
dusk diving patterns suggest that dolphins were fol- 
lowing the ascent and descent of the deep scattering 
layer (Scott et al. 7 ). 
The capture of prey by dolphins in the deep scat- 
tering layer may be facilitated by abundance of prey, 
schooling size of prey, and bioluminescence of prey 
(Clarke, 1973; Crawford, 1981; Clarke, 1986b). The 
top three prey families in our study (Myctophidae, 
Enoploteuthidae, Ommastrephidae) are all abundant 
in the ETP and have bioluminescent organs (Clarke, 
1973, 1978; Wisner, 1974; Okutani, 1974; Clarke, 
1977; Crawford, 1981; Roper et al., 1984; Harman 
and Young, 1985; Clarke, 1986b). Myctophids, which 
accounted for 49.7% of all prey recovered in our 
sample, are the most abundant deep scattering layer 
species, representing 25% of the biomass of all me- 
sopelagic fishes (Karnella, 1987). Myctophids are 
small in size, school in large numbers, and have bi- 
oluminescent organs, all characteristics that might 
facilitate detection by dolphins (Crawford, 1981). 
Prey size 
There does appear to be some selectivity of prey by 
size because larger dolphins tend to eat larger prey 
(Fig. 2). Our results support the supposition that 
because most dolphins have been observed to con- 
sume their prey whole, the size of prey ingested is 
limited by the size that can be swallowed (Fiscus and 
Kajimura, 1981; Fiscus and Jones, 1990; Wolff 4 ). The 
size of prey in our study indicated that prey consumed 
by juveniles through adults ranged from 2.4 to 320.9 
mm (Okutani, 1974, Butler, 1979; Uchida, 1981; 
Roper et al., 1984; Clarke, 1986a; Smith and 
Heemstra, 1986; Karnella, 1987; Murata and Hayase, 
1993; Welch and Morris, 1993). Most of our size 
ranges corresponded with those reported by Wolff, 4 
who examined the squid prey of spotted dolphins from 
the Perrin et al. (1973) study, except for A. affinis, 
which had a narrower size range in our study, and 
O. banksii, which encompassed a broader size range. 
Wolff 4 also showed an increase in the size of squid 
consumed by spotted dolphins; there was a 30-mm 
mantle length increase for squid found in dolphins 
from 140 to 205 cm in length. He also found that a 
broader size range of squid was consumed for larger 
dolphins, which was also the case in our study. 
Geographic and seasonal variability 
The composition of prey species consumed by 
pantropical spotted dolphins changed both tempo- 
rally and spatially (Fig. 3), suggesting that they are 
opportunistic feeders, as is the case with many other 
7 Scott, M. D., S. J. Chivers, H. Rhinehart, M. Garcia, R. Lindsey, 
R. L. Olson, W. Armstrong, and D. A. Bratten. 1997. Move- 
ments and diving behavior of pelagic spotted dolphins. Inter- 
Am. Tropical Tuna Comm., c/o Scripps Institution of Oceanog- 
raphy, Univ. Calif., San Diego, CA 92037. 
