Alatorre-Ramirez et al.: Trophic segregation of mixed schools of Thunnus albacares and Katsuwonus pelamis 
257 
Table 3 
Summary description of samples of yellowfin tuna (Thunnus albacares) and skipjack tuna (Katsuwonus 
pelamis) caught in 2005 in the eastern tropical Pacific Ocean and used for stomach contents analyses. 
The size classes for yellowfin tuna were small (S), 1-85 cm in fork length (FL), and large (L), >85 cm 
FL. The size classes for skipjack tuna were small (S) 1-50 cm FL, and large (L), 50 cm FL). Capture 
times were morning (0900-1059 h), afternoon (1100-1359 h), and evening (1500-1600 h). TS=total 
stomachs; SWC=stomachs with content; n/d=no data. 
Species Category 
Group 
Zone 1 
TS (SWC) 
Number of samples 
Zone 2 
TS (SWC) 
Zone 3 
TS (SWC) 
Thunnus albacares Males 
S 
64(64) 
45 (30) 
58(52) 
L 
6(6) 
15 (12) 
24(21) 
Females 
S 
48(48) 
50 (33) 
71 (63) 
L 
6(6) 
19 (16) 
33 (30) 
Month 
Jan. 
n/d 
89(51) 
n/d 
Feb. 
40(40) 
25(25) 
40 (37) 
Aug. 
45(45) 
n/d 
n/d 
Sep. 
24 (24) 
n/d 
20(19) 
Oct. 
n/d 
15 (15) 
126(110) 
Nov. 
15 (15) 
n/d 
n/d 
Capture time 
Morning 
22(22) 
24 (20) 
60(55) 
Afternoon 
55 (55) 
30 (28) 
49 (45) 
Evening 
47(47) 
75(43) 
77(66) 
Katsuwonus pelamis Males 
S 
1 (1) 
4(4) 
7(3) 
L 
43(29) 
34 (6) 
9(3) 
Females 
S 
12(10) 
6(5) 
10 (4) 
L 
48(35) 
35 (7) 
7(2) 
Month 
Jan. 
n/d 
30 (10) 
n/d 
Feb. 
n/d 
49 (12) 
19 (5) 
Aug. 
94 (65) 
n/d 
n/d 
Sep. 
n/d 
n/d 
3(3) 
Oct. 
10(10) 
n/d 
11 (4) 
Nov. 
n/d 
n/d 
n/d 
Capture time 
Morning 
25(20) 
30 (8) 
4(3) 
Afternoon 
54 (35) 
15 (6) 
6(3) 
Evening 
25(20) 
34 (10) 
23 (6) 
pelagic red crab, in August they primarily consumed 
jumbo squid and pelagic red crab, in September they 
primarily consumed pelagic red crab and Auxis spp., 
in November they primarily consumed the Panama 
lightfish [Vinciguerria lucetia]), and capture times 
(i?=0.310, P=0.01; in the morning individuals primarily 
consumed Cubiceps spp., in the afternoon they primar¬ 
ily consumed jumbo squid and pelagic red crab, and in 
the evening they primarily consumed Panama lightfish, 
jumbo squid, and pelagic red crab) (Fig. 3). 
Skipjack tuna prey items comprised 5 taxa (2 cepha- 
lopods, 2 crustaceans and 1 fish species). The %MN 
and %MW indices indicated that the most important 
prey items were a krill species, Nyctiphanes simplex 
(60.0%; 60.0%), pelagic red crab (20.0%; 20.0%), and 
tropical two-wing flyingfish {Exocoetus volitans) (12.0%; 
12.0%), respectively. According to the IRI, N. simplex 
(98.6%) and Argonauta spp. (1.3%) were the most im¬ 
portant components in the diet (Table 5). The diversity 
index was 1.1 and the was 0.003. The trophic level 
of the stomach contents was estimated at 3.5 (SD 0.4). 
The ANOSIM test indicated a similar diet composition 
for the sexes (i?=0.007), size class (iZ=0.094), months 
(i2=0.018), and capture times (i2=0.099). 
Diet composition in zone 2 
Yellowfin tuna prey items comprised 19 taxa (7 cepha- 
lopods, 1 crustaceans, and 11 fish species). Based on 
the %MN and %MW indices, the most important prey 
items were Argonauta spp. (33.1%; 20.6%), jumbo squid 
(27.9%; 27.7%), and Auxis spp. (9.5%; 19.9%), respec¬ 
tively. According to the IRI, jumbo squid (55.3%), Ar¬ 
gonauta spp. (30.4%), and Auxis spp. (11.2%) were the 
most important components in the diet (Table 6). The 
diversity index was 1.3 and the B; was 0.1. The tro¬ 
phic level of the stomach contents was estimated at 
4.7 (SD 0.2). The ANOSIM test indicated a similar 
