254 
Fishery Bulletin 115(2) 
Map of zones (1, 2, and 3) in the eastern tropical Pacific Ocean fished by the Mexican tuna 
fleet. The zones are based on trophic relationships between pelagic top predators accord¬ 
ing to Galvan-Magana (1999), and on the subsurface currents that influence the zones: 
California Current (CC), North Equatorial Current (NEC), North Equatorial Counter Cur¬ 
rent (NECC), North and South subsurface Counter Current (NSSCC), according to Lavin 
et al. (1997). Squares, triangles, and diamonds indicate locations where tunas were caught 
in 2005 in sets associated (assoc.) with floating objects, in sets not associated with floating 
objects or dolphins), and in sets associated with dolphins, respectively. 
The diet data for both species were analyzed as mean 
proportions by number (%MN) and weight (%MW) for 
individual fish, then averaged for each prey type, and 
multiplied by 100 (Chipps and Garvey, 2007). Also, the 
index of relative importance index (IRI) was calculated 
with the formula described by Pinkas et al. (1971) and 
modified as a percentage (Cortes, 1997): 
IRI = {%N + %W) X %FO, (1) 
where %N and %W = the number and wet weight of 
each food item as percentages, 
respectively; and 
%FO = the percentage frequency of oc¬ 
currence (presence-absence) of 
each food item in all stomachs 
that contained food. 
The diet breadth (5,) of yellowfin and skipjack tu¬ 
nas was evaluated with the Levin's standardized index 
by sex, size class, month, and time interval of capture 
(Krebs, 1999). Index values ranged from 0 to 1; low 
values (<0.6) indicated specialist diets and higher val¬ 
ues (>0.6) indicated generalist diets (Labropoulou and 
Eleftheriou, 1997). 
where 5; 
n 
Levin’s index for predator I; 
the numerical proportion of the jth prey item 
in predator I’s diet; and 
the number of prey categories. 
The trophic level (TLj) of yellowfin and skipjack tu¬ 
nas based on stomach contents was calculated with the 
equation proposed by Christensen and Pauly (1992): 
rz,, = i + (E;„DCji)(rLj), (3) 
where TL; 
DCji 
TLj 
n 
the trophic level of predator species i; 
the diet composition in weight, in terms of 
the prey proportion j in the predator diet 
the trophic level of all prey species j; and 
the number of prey groups in the diet. 
The TLj for fish prey species (Table 1) were obtained 
from FishBase (Froese and Pauly‘S), and those for ceph- 
alopods and crustaceans were obtained from Cortes 
(1999). We calculated the mean and standard deviation 
(SD) to represent the variability of the individual TL; 
values. 
R = 
1 
1 
(n-1) 
--1 
^ Froese, R., and D. Pauly (eds.). 2006. FishBase. World 
Wide Web electronic publication. [Available from website, 
accessed November 2006.] 
