262 
Fishery Bulletin 115(2) 
Table 6 
Summary of food categories in stomachs of yellowfin tuna {Thunnus albacares) and skipjack tuna (Katsuwonus pelamis) 
caught in 2005 in zone 2 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 Katsuwonus pelamis 
(SWC=91) (SWC=22) 
Prey item 
TN 
%MN 
TW 
%MW 
%FO 
%IRI 
TN 
%MN 
TW 
%MW 
%FO 
%IRI 
Cephalopoda 
Teuthoidea 
1 
8.98 
0.1 
2.89 
12.63 
0.01 
X 
X 
X 
X 
X 
X 
Lepidoteuthidae 
Pholidoteuthis boschmai 
3 
0.41 
0.1 
0.35 
3.15 
0.01 
X 
X 
X 
X 
X 
X 
Ommastrephidae 
Dosidicus gigas 
612 
27.95 
2055.4 
27.66 
51.59 
55.32 
1 
1.85 
0.1 
0.01 
3.70 
0.05 
Thysanoteuthidae 
Thysanoteuthis rhombus 
11 
0.685 
1.1 
1.77 
9.47 
0.01 
X 
X 
X 
X 
X 
X 
Mastigoteuthidae 
Mastigoteuthis dentata 
20 
0.87 
0.1 
0.97 
8.42 
0.12 
X 
X 
X 
X 
X 
X 
Octopoda 
Bolitaenidae 
Japetella diaphana 
22 
0.86 
0.1 
0.98 
8.42 
0.14 
X 
X 
X 
X 
X 
X 
Argonautidae 
Argonauta spp. 
595 
33.11 
8.6 
20.64 
66.31 
30.42 
X 
X 
X 
X 
X 
X 
Crustacea 
Squillidae 
Squillid mantis shrimps 
1 
0.14 
1 
0.55 
1.05 
0.01 
1 
1.85 
0.1 
0.01 
3.70 
0.05 
Teleostei 
Clupeidae 
Harengula thrissina 
1 
0.26 
11 
0.10 
1.05 
0.01 
X 
X 
X 
X 
X 
X 
Phosichthyidae 
Vinciguerria lucetia 
160 
6.39 
55.9 
8.47 
8.42 
1.17 
1 
3.70 
0.2 
3.70 
3.70 
0.06 
Hemiramphidae 
Oxyporhamphus micropterus 
21 
4.19 
75.3 
2.59 
10.52 
0.40 
4 
4.44 
59.5 
5.31 
11.11 
2.49 
Exocoetidae 
Exocoetids 
2 
0.05 
30.6 
1.12 
2.10 
0.02 
1 
1.85 
2.5 
3.70 
3.70 
0.08 
Exocoetus spp. 
3 
0.79 
34.8 
0.27 
1.05 
0.01 
4 
4.93 
21.0 
3.80 
7.40 
0.89 
Hirundichthys spp 
X 
X 
X 
X 
X 
X 
1 
1.85 
13.0 
3.70 
3.70 
0.19 
Exocoetus monocirrhus 
1 
0.02 
0.1 
0.02 
1.05 
0.01 
X 
X 
X 
X 
X 
X 
Exocoetus volitans 
2 
0.14 
18.8 
2.20 
2.10 
0.01 
39 
79.50 
207.2 
79.77 
66.66 
96.16 
Carangidae 
Jacks 
1 
1.46 
14.35 
2.86 
1.05 
0.01 
X 
X 
X 
X 
X 
X 
Scombridae 
Scombrids 
1 
4.20 
13.0 
6.67 
1.05 
0.01 
X 
X 
X 
X 
X 
X 
Auxis spp. 
34 
9.50 
1725.3 
19.89 
21.05 
11.20 
X 
X 
X 
X 
X 
X 
Diet composition by zone 
The diets of both tuna species are consistent with pre¬ 
vious reports for the ETPO; however, we found fewer 
prey species in the fish stomachs than those of pre¬ 
vious studies (Galvan-Magana, 1988; Roman-Reyes, 
2000). For yellowfin tuna, we recorded a total of 29 
prey species, whereas Galvan-Magana (1988) reported 
a total of 53 prey species. For skipjack tuna, we re¬ 
corded the occurrence of only 9 prey species, whereas 
Roman-Reyes (2000) reported a total of 55 prey species 
in skipjack tuna stomachs. This variability in prey di¬ 
versity may be associated with the number of stomachs 
analyzed; Galvan-Magana (1988) analyzed 1299 yellow¬ 
fin tuna stomachs and Roman-Reyes (2000) analyzed 
611 stomachs of skipjack tuna. Despite the difference 
in the total number of prey species recorded, the most 
important prey have remained unchanged for both 
tuna species during the last 30 years in the ETPO. 
These include pelagic red crab, jumbo squid, and tropi¬ 
cal two-wing fiyingfish (Galvan-Magana, 1988; Roman- 
Reyes, 2000; present study). 
Crustaceans (primarily the pelagic red crab) were 
an important diet component by number, weight, and 
frequency of occurrence for both tuna species in zone 
1, but only for yellowfin tuna in zone 3, which is in 
agreement with data reported by Alverson (1963). The 
pelagic red crab is a crustacean that has a pelagic juve¬ 
nile phase, is present in high abundance at that phase, 
and is distributed vertically throughout the water col¬ 
umn on the west coast of Baja California Sur, Mexico 
(zone 1). This region is influenced by the California 
Current during spring and summer. The abundance of 
the pelagic red crab in zone 3 may also be related to 
the intensity of the California Current (Lavin et al., 
1997). Because the pelagic red crab is a passive swim¬ 
mer, it can be transported to this zone by the Cali¬ 
fornia Current and therefore it becomes easy prey for 
several predators, including Panama hake (Merluccius 
angustimanus) (Balart and Castro-Aguirre, 1995), silky 
shark {Carcharhinus falciformis) (Cabrera-Chavez-Cos- 
ta et al., 2010), and dolphinfish {Coryphaena hippurus) 
(Tripp-Valdez et al., 2010), in addition to yellowfin and 
skipjack tunas in both zones 1 and 3. 
