Alatorre-Ramirez et al.: Trophic segregation of mixed schools of Thunnus albacares and Katsuwonus pelamis 
259 
Table 4 
Number of prey items present in the diet of yellowfin tuna (YT; Thunnus albacares) and skipjack tuna (ST; Katsuwonus 
pelamis) caught in 2005 in fishng zones 1-3 in the eastern tropical Pacific Ocean, by digestive state (DS), geographic zone, 
capture time (morning [M], afternoon [A], and evening [E]) and category of prey species (fish [F], cephalopod [C], and crus¬ 
tacean [Cr]). DS level 1= food includes recently consumed items; DS level 2=food items with little to no skin remaining; 3 
DS level=presence of fish skeletons; and DS level 4=presence of hard structures like fish otoliths, crustacean remains, and 
cephalopod beaks. 
Zone 1 Zone 2 Zone 3 
Capture Predator Prey 
time 
species 
species 
DS 1 
DS 2 
DS3 
DS 4 
DS 1 
DS 2 
DS 3 
DS 4 
DS 1 
DS 2 
DS3 
DS 4 
M 
YT 
F 
0 
9 
159 
1 
0 
2 
2 
0 
0 
6 
103 
16 
C 
0 
0 
0 
3 
0 
0 
1 
79 
0 
29 
21 
235 
Cr 
2 
2 
0 
0 
0 
0 
0 
0 
35 
637 
563 
0 
ST 
F 
0 
0 
17 
1 
0 
0 
0 
0 
0 
0 
0 
0 
C 
0 
0 
0 
1 
0 
0 
0 
0 
0 
0 
0 
0 
Cr. 
0 
4744 
1196 
0 
0 
0 
0 
0 
0 
0 
0 
0 
A 
YT 
F 
0 
86 
423 
0 
0 
1 
9 
30 
0 
0 
36 
0 
C 
56 
175 
102 
384 
0 
0 
17 
475 
0 
1 
0 
38 
Cr 
218 
1498 
329 
0 
0 
0 
0 
1 
0 
0 
0 
0 
ST 
F 
0 
0 
7 
0 
0 
0 
26 
3 
0 
0 
0 
0 
C 
0 
0 
0 
5 
0 
0 
0 
1 
0 
0 
0 
0 
Cr 
2227 
32,251 
2159 
0 
0 
0 
0 
1 
0 
0 
0 
0 
E 
YT 
F 
0 
4 
948 
1 
0 
6 
204 
14 
0 
7 
1229 
2 
C 
0 
216 
0 
641 
0 
111 
29 
626 
0 
0 
7 
160 
Cr 
66 
545 
362 
1 
0 
0 
1 
0 
0 
26 
268 
0 
ST 
F 
0 
0 
0 
0 
0 
1 
0 
25 
0 
2 
44 
4 
C 
0 
0 
0 
3 
0 
0 
0 
0 
0 
2 
0 
0 
Cr 
0 
3628 
120 
0 
0 
0 
0 
0 
0 
0 
0 
0 
flyingfish (2.49%) were the most important components 
in the diet. The diversity index was 1.1 and the was 
0.1. The trophic level of the stomach contents was es¬ 
timated at 4.1 (SD 0.2). The ANOSIM test indicated a 
similar diet composition among sexes (i2=0.009), size 
classes (i?=0.020), months of capture (i2=0.007), and 
capture times (/?=0.120). 
Diet composition in zone 3 
Yellowfin tuna prey items comprised 23 taxa (8 ceph- 
alopods, 2 crustaceans and 13 fishes). The %MN and 
%MW indices indicated that the most important prey 
items were pelagic red crab (23.8%; 25.6%), Argonau- 
ta spp. (20.5%; 13.3%), and Panama lightfish (16.2%; 
17.1%) respectively. According to the IRI, pelagic red 
crab (46.2%), Panama lightfish (24.7%), and Auxis spp. 
(17.4%) were the most important components of the 
diet (Table 7). The diversity index was 1.3 and the 5; 
was 0.1. The trophic level of the stomach contents was 
estimated at 4.5 (SD 0.4). The ANOSIM test indicated 
a similar diet composition among sexes (jR=0.014), size 
classes (R=0.060), months of capture (J?=0.120), and 
capture times (R=0.067). 
Skipjack tuna prey items comprised 3 taxa (1 cepha¬ 
lopod and 2 fish species). The %MN and %MW indi¬ 
ces indicated that the most important prey items were 
tropical two-wing flyingfish (66.7%; 66.8%) and Panama 
lightfish (29.2%; 25.0%) respectively. According to the 
IRI, tropical two-wing flyingfish (83.8%) and Panama 
lightfish (15.8%) were the most important components 
of the diet (Table 7). The diversity index was 0.8 and 
the Rj was 0.6. The trophic level of the stomach con¬ 
tents was estimated at 4.1 (SD 0.3). The ANOSIM test 
indicated a similar diet composition for sexes (i?=0.098) 
and months of capture (72=0.180), and differences be¬ 
tween size classes (72=0.212, P=0.04; small individuals 
primarily consumed tropical two-wing flyingfish, and 
large individuals primarily consumed Panama lightfish 
and tropical two-wing flyingfish) (Fig. 5). The ANOSIM 
test for capture times could not be performed because 
of the presence of empty stomachs. 
Comparisons of diets 
The ANOSIM showed differences in diet composition 
between yellowfin tuna and skipjack tuna in zone 1 
(72=0.40, P=0.01), zone 2 (72=0.60, P=0.01), and zone 3 
(P=0.25, P=0.01). A SIMPER analysis indicated chang¬ 
es in the contribution of each prey species between 
