264 
Fishery Bulletin 115(2) 
Table 8 
Results of the similarity of percentages analysis of taxa contributing to dissimilarity be¬ 
tween yellowfin tuna (Thunnus albacares) and skipjack tuna {Katsuwonus pelamis) caught 
in 2005 in zones 1, 2, and 3 of the eastern tropical Pacific Ocean, including average num¬ 
ber of individuals (Av. abund), dissimilarity with standard deviation ratios (Diss/SD), and 
percentage of contribution to the overall Bray-Curtis similarity between assemblages of 
the 2 tuna species (Contrib%). 
K. pelamis T albacares 
Av. abund Av. abund Diss/SD Contrib% 
Zone 1 
Average dissimilarity=95.10 
Prey species 
Nyctiphanes simplex 
614.7 
0.00 
1.14 
53.66 
Pleuroncodes planipes 
3.31 
24.56 
0.71 
21.87 
Dosidicus gigas 
0.11 
9.26 
0.48 
9.04 
Vinciguerria lucetia 
0.00 
11.08 
0.29 
5.60 
Zone 2 
Average dissimilarity=99.27 
Prey species 
Argonauta spp. 
0.00 
6.47 
1.01 
28.21 
Dosidicus gigas 
0.04 
6.65 
0.77 
24.97 
Exocoetus volitans 
1.70 
0.02 
0.91 
17.45 
Vinciguerria lucetia 
0.00 
1.74 
0.30 
5.65 
Zone 3 
Average dissimilarity=97.4 
Prey species 
Vinciguerria lucetia 
2.08 
8.26 
0.71 
24.50 
Exocoetus volitans 
2.08 
0.00 
0.84 
22.12 
Pleuroncodes planipes 
0.00 
9.46 
0.53 
18.22 
Argonauta spp. 
0.00 
1.46 
0.59 
12.58 
Roman-Reyes (2000), whereas mesopelagic species (e.g., 
Panama lightfish) were consumed in zone 3. 
The presence of mesopelagic prey species in the diet 
of yellowfin and skipjack tunas may reflect the vertical 
migration of prey species. Jumbo squid and Panama 
lightfish make vertical migrations to the epipelagic 
area at night to feed (Olson and Boggs, 1986; Galvan- 
Magaha, 1988). 
Comparison of diet between sexes and size classes 
For both tuna species in all zones in our study, there 
were no significant differences in feeding between the 
sexes (ANOSIM values close to 0). Nakamura (1965), 
for skipjack tuna, and Alverson (1963), for yellowfin 
tuna, reported high diet similarity for both sexes, sug¬ 
gesting that males and females frequented the same 
areas. Between size classes, however, our data show 
dietary differences: in zone 1 for yellowfin tuna and in 
zone 3 for skipjack tuna. Olson and Boggs (1986) re¬ 
ported that the yellowfin tuna trophic spectrum (diet) 
in the ETPO depends on predator size—a result that 
we found in our data as well. Trophic level values were 
3.9 for smaller fish and 4.5 for larger fish. 
Changes in diet related to size can be attributed to 
differences in the energy requirements at distinct on¬ 
togenetic stages in the development of a fish (Olson 
and Galvan-Magana, 2002; Graham et ah, 2007). For 
example, stage-specific dietary differences have been 
reported for skipjack tuna in other areas of the Pacific 
Ocean (Nakamura, 1965; Ankenbrandt, 1985). Our re¬ 
port of juvenile skipjack tuna feeding mainly on fishes 
(e.g., zone 3) is in agreement with that of Roman- 
Reyes (2000), although the prey species differed. The 
main prey species of skipjack tuna in our study was 
the tropical two-wing flying fish, whereas Roman-Reyes 
(2000) found the main prey species to be the mesope¬ 
lagic fish V; lucetia. For yellowfin tuna, our data show 
that the juveniles fed on small abundant prey, such 
as pelagic red crab in zone 1, whereas adult yellow¬ 
fin tuna fed on cephalopods or, as reported by Roman- 
Reyes (2000), on fishes. 
Diet variation of yellowfin and skipjack tunas is 
most likely to be related to the spatiotemporal distri¬ 
bution of the predators, and to the prey availability 
in different areas of the ETPO rather than to strict 
ontogenetic changes in predator size. Alverson (1963) 
mentioned that yellowfin tuna juveniles were usually 
