Alatorre-Ramirez et al.: Trophic segregation of mixed schools of Thunnus albacares and Katsuwonus pelamis 
263 
Table 7 
Summary of food categories in stomachs of yellowfin tuna (Thunnus albacares) and skipjack tuna (Katsuwonus pelamis) 
caught in 2005 in Zone 3 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. 
Prey item 
Thunnus albacares 
(SWC=166) 
TN %MN TW %MW %FO %IR1 
Katsuwonus pelamis 
(SWC=12) 
TN %MN TW %MW %FO %1R1 
Cephalopoda 
Onychoteuthidae 
Onychoteuthis banksii 
27 
0.90 
0.1 
0.72 
3.10 
0.07 
X 
X 
X 
X 
X 
X 
Ommastrephidae 
Dosidicus gigas 
170 
15.06 
20.9 
12.15 
28.57 
4.01 
2 
4.16 
1.0 
8.32 
8.33 
0.40 
Sthenoteuthis oualaniensis 
8 
1.37 
0.5 
1.70 
3.10 
0.01 
X 
X 
X 
X 
X 
X 
Thysanoteuthidae 
Thysanoteuthis rhombus 
24 
3.19 
56.0 
3.80 
11.18 
0.81 
X 
X 
X 
X 
X 
X 
Mastigoteuthidae 
Mastigoteuthis dentata 
9 
0.72 
0.1 
1.26 
5.59 
0.03 
X 
X 
X 
X 
X 
X 
Octopoda 
Bolitaenidae 
Japetella diaphana 
16 
3.02 
0.1 
3.26 
8.69 
0.09 
X 
X 
X 
X 
X 
X 
Argonautidae 
Argonauta spp. 
235 
20.52 
3.6 
13.31 
39.13 
6.59 
X 
X 
X 
X 
X 
X 
Octopodidae 
Octopus rubescens 
6 
0.19 
0.1 
0.33 
2.48 
0.01 
X 
X 
X 
X 
X 
X 
Crustacea 
Galatheidae 
Pleuroncodes planipes 
1523 
23.85 
654.3 
25.64 
26.70 
46.16 
X 
X 
X 
X 
X 
X 
Squillidae 
Squillid mantis shrimps 
1 
0.01 
1.0 
0.02 
0.62 
0.01 
X 
X 
X 
X 
X 
X 
Teleostei 
Phosichthyidae 
Vinciguerria lucetia 
1330 
16.18 
435.5 
17.14 
18.01 
24.71 
25 
29.16 
3.1 
25.00 
33.33 
15.78 
Exocoetidae 
Hirundichthys spp. 
1 
0.62 
0.1 
0.62 
0.62 
0.01 
X 
X 
X 
X 
X 
X 
Exocoetus monocirrhus 
3 
0.14 
12.8 
1.10 
1.24 
0.02 
X 
X 
X 
X 
X 
X 
Exocoetus volitans 
X 
X 
X 
X 
X 
X 
25 
66.66 
3.0 
66.67 
66.66 
83.81 
Carangidae 
Jacks 
1 
0.10 
1.5 
0.62 
0.62 
0.01 
X 
X 
X 
X 
X 
X 
Coryphaenidae 
Coryphaena hippurus 
2 
0.06 
0.5 
0.05 
0.62 
0.01 
X 
X 
X 
X 
X 
X 
Bramidae 
Brama spp. 
2 
0.64 
2.8 
0.82 
1.24 
0.01 
X 
X 
X 
X 
X 
X 
Gempylidae 
Gempylus spp. 
1 
0.02 
0.2 
0.40 
0.62 
0.01 
X 
X 
X 
X 
X 
X 
Scombridae 
Scombrids 
1 
0.10 
0.1 
0.60 
0.62 
0.01 
X 
X 
X 
X 
X 
X 
Auxis spp. 
40 
10.66 
1179.4 
13.37 
14.28 
17.38 
X 
X 
X 
X 
X 
X 
Nomeidae 
Cubiceps pauciradiatus 
12 
2.48 
1.4 
2.79 
2.48 
0.03 
X 
X 
X 
X 
X 
X 
Balistidae 
Batistes polylepis 
1 
0.02 
5.1 
0.17 
0.62 
0.01 
X 
X 
X 
X 
X 
X 
Ostraciidae 
Lactoria diaphana 
1 
0.05 
0.5 
0.61 
0.62 
0.01 
X 
X 
X 
X 
X 
X 
In zone 2, yellowfin tuna preyed primarily on ceph- 
alopods, followed by fishes and crustaceans. This is 
in agreement with previous reports for this species 
in the ETPO (Alverson, 1963; Galvan-Magana, 1988, 
1999; Roman-Reyes, 2000, 2005; Olson et al., 2014). 
The main cephalopod prey items in the trophic spec¬ 
trum of yellowfin tuna were jumbo squid of the fam¬ 
ily Ommastrephidae and Argonauta spp. of the fam¬ 
ily Argonautidae. Members of the Ommastrephidae 
family undertake nightly vertical migrations toward 
the surface to feed (Markaida-Aburto, 2001), whereas 
members of the genus Argonauta are mostly epipelag- 
ic species that feed primarily during the day (Nesis, 
1977). These cephalopods were also present in skip¬ 
jack tuna stomachs, but were not important to their 
overall diet. In contrast, Nakamura (1965) reported 
that cephalopods were an important dietary compo¬ 
nent for skipjack tuna in the southern Pacific Ocean. 
Our data show, however, that skipjack tuna preyed 
primarily on fishes and yellowfin tuna preyed primar¬ 
ily on cephalopods in zone 2, highlighting the dietary 
difference between these 2 predatory species within 
this geographic region. 
In all 3 zones (1, 2, and 3), the most important prey 
species of yellowfin and skipjack tunas were those 
that form aggregations, such as the pelagic red crab, 
jumbo squid, tropical two-wing fiyingfish, and Panama 
lightfish; this finding confirms those of Galvan-Magana 
(1988) and Alverson (1963). The prey consumed by yel¬ 
lowfin tuna were mainly epipelagic species (pelagic red 
crab in zones 1 and 3, and Argonauta spp. in zone 2), 
whereas mesopelagic species (e.g., jumbo squid in zones 
1 and 2, and Panama lightfish in zone 3) were con¬ 
sumed in smaller amounts. Watanabe (1958) reported 
that yellowfin tuna fed mainly in the epipelagic zone, 
which is a direct result of its distribution in the water 
column (Eslava et al., 2003). The main prey items found 
in skipjack tuna stomachs were epipelagic species (e.g., 
N. simplex, Auxis spp., smallwing fiyingfish, and tropi¬ 
cal two-wing fiyingfish), confirming the information in 
