260 
Fishery Bulletin 115(2) 
Table 5 
Summary of food categories in stomachs of yellowfin tuna (Thunnus albacares) and skipjack tuna (Katsuwonus pelamis) 
caught in 2005 in zone 1 of the eastern tropical Pacific Ocean, expressed as percentages of the mean proportion by number 
(%MN), mean proportion by weight (%MW), frequency of occurrence (%FO), and relative importance index (%IRI). x=not 
present in the diet; SWC=stomachs with content; TN=total number of prey examined; TW=total weight (in grams) of prey 
examined. 
Thunnus albacares 
(SWC=124) 
Katsuwonus pelamis 
(SWC=75) 
Prey item 
TN 
%MN 
TW 
%MW 
%FO 
%IRI 
TN 
%MN 
TW 
%MW 
%FO 
%IRI 
Cephalopoda 
Teuthoidea 
6 
0.93 
26.0 
1.10 
1.62 
0.01 
X 
X 
X 
X 
X 
X 
Loliginidae 
Lolliguncula (Loliolopsis) 
diomedeae 
213 
3.54 
702.8 
4.08 
4.87 
2.61 
X 
X 
X 
X 
X 
X 
Ommastrephidae 
Dosidicus gigas 
1139 
20.66 
1122.9 
16.59 
54.47 
28.26 
8 
6.66 
0.1 
6.66 
6.66 
0.01 
Sthenoteuthis oualaniensis 
2 
0.15 
0.1 
0.01 
1.62 
0.01 
X 
X 
X 
X 
X 
X 
Thysanoteuthidae 
Thysanoteuthis rhombus 
3 
0.14 
0.1 
0.01 
2.43 
0.01 
X 
X 
X 
X 
X 
X 
Mastigoteuthidae 
Mastigoteuthis dentata 
4 
0.20 
0.1 
0.01 
2.43 
0.01 
X 
X 
X 
X 
X 
X 
Octopoda 
Bolitaenidae 
Japetella diaphana 
15 
0.65 
0.5 
0.01 
8.13 
0.02 
X 
X 
X 
X 
X 
X 
Argonautidae 
Argonauta spp. 
186 
3.67 
18.93 
0.09 
24.39 
1.00 
1 
1.33 
0.1 
1.33 
1.33 
0.01 
Octopodidae 
Octopus rubescens 
9 
0.51 
0.1 
0.01 
5.69 
0.01 
X 
X 
X 
X 
X 
X 
Crustacea 
Euphausiidae 
Nyctiphanes simplex 
X 
X 
X 
X 
X 
X 
46107 
60.00 
1750.4 
60.00 
60.00 
98.60 
Galatheidae 
Pleuroncodes planipes 
3021 
48.89 
1692.3 
54.20 
59.35 
61.12 
248 
20.00 
133.3 
20.00 
20.00 
1.29 
Penaeidae 
Penaeid shrimps 
2 
0.01 
1.1 
0.01 
0.81 
0.01 
X 
X 
X 
X 
X 
X 
Teleostei 
Clupeidae 
Harengula thrissina 
2 
0,36 
32.2 
0.58 
0.81 
0.01 
X 
X 
X 
X 
X 
X 
Phosichthyidae 
Vinciguerria lucetia 
1363 
3.70 
410.7 
3.90 
13.82 
5.11 
X 
X 
X 
X 
X 
X 
Exocoetidae 
Exocoetus spp. 
1 
0.05 
0.2 
0.01 
0.81 
0.01 
X 
X 
X 
X 
X 
X 
Exocoetus uolitans 
X 
X 
X 
X 
X 
X 
25 
12.00 
8.5 
12.00 
12.00 
0.05 
Carangidae 
Seriola lalandi 
4 
0.68 
4.4 
1.57 
0.81 
0.01 
X 
X 
X 
X 
X 
X 
Coryphaenidae 
Coryphaena spp. 
1 
0.22 
5.5 
0.90 
0.81 
0.01 
X 
X 
X 
X 
X 
X 
Chaetodontidae 
Chaetodontids 
1 
0.90 
1.5 
0.90 
0.81 
0.01 
X 
X 
X 
X 
X 
X 
Scombridae 
Auxis spp. 
4 
1.14 
30.3 
1.20 
3.25 
0.03 
X 
X 
X 
X 
X 
X 
Nomeidae 
Cubiceps pauciradiatus 
253 
13.36 
338.4 
13.94 
0.81 
1.81 
X 
X 
X 
X 
X 
X 
tuna species. The prey species that contributed most to 
dissimilarity were N. simplex (53.7%) and pelagic red 
crab (21.9%), in zone 1\ Argonauta spp. (28.2%), jumbo 
squid (25.0%), and tropical two-wing flyingfish (17.5%) 
in zone 2; Panama lightfish (24.5%), tropical two-wing 
flyingfish (22.1%), and pelagic red crab (18.2%) in zone 
3 (Table 8). 
Discussion 
Digestive state of prey species 
The variation in the degree of digestion of yellowfin 
tuna prey is related to the type of prey (fishes, cephalo- 
pods, and crustaceans) and the time of day when feed¬ 
ing occurs (Magnuson, 1969; Brill, 1987; Galvan-Maga- 
ha, 1988). Olson and Boggs (1986) found that squid soft 
tissue passed through the stomach of yellowfin tuna 
in 5-10 h, whereas the beak was the only cephalopod 
structure that persisted in the stomach because it is 
composed of digestion-resistant chitin. In contrast, 
the soft tissue of fishes passed through the stomach 
in about 6-18 h. Tunas are captured primarily dur¬ 
ing the day in the ETPO (Ortega-Garcfa et al., 1992). 
The low occurrence of prey found in digestive state 1 
in the morning, but high in the afternoon and evening, 
indicated peak feeding activity in the afternoon and 
evening for both tuna species. This finding coincides 
with observations reported for different areas of the 
ETPO (Ortega-Garcfa et al., 1992; Roman-Reyes, 2005). 
Because of the occurrence of prey in digestion state 2 
and 3 (little fresh cephalopod tissue) in the afternoon 
and evening, we deduce that yellowfin and skipjack tu¬ 
nas actively feed on cephalopods at night and during 
early morning as they migrate to the surface (Olson 
and Boggs, 1986). 
Also the presence of mesopelagic prey species in 
the diet of yellowfin and skipjack tunas may reflect 
the vertical migration habits of prey. Jumbo squid and 
