Alatorre-Ramirez et al.: Trophic segregation of mixed schools of Thunnus albacares and Katsuwonus pelamis 
265 
found in coastal areas where crustaceans are abun¬ 
dant, while adults were found in oceanic areas where 
cephalopods and fishes predominate. Thus, the high 
diet similarity between tuna of different sizes and the 
temporal (monthly, yearly) variation reported previ¬ 
ously (Alverson, 1963; Nakamura, 1965; Roman-Reyes, 
2000) indicate that the consumption of distinct prey 
by organisms of different sizes may be more closely re¬ 
lated to spatiotemporal segregation (oceanic in contrast 
with coastal segregation) of predators and prey than to 
size-related developmental shifts, such as a shift be¬ 
cause of the added energy demands for reproduction. 
Diet breadth 
Values of indicate that yellowfin and skipjack tu¬ 
nas are specialist predators; however, because the prey 
species were different in the 3 zones and form large 
aggregations (e.g., pelagic red crab and jumbo squid), 
these tunas can better be described as opportunistic 
predators, assuming that prey items are consumed in 
proportion to their abundance and availability in each 
zone. This conclusion is justified because tunas tend to 
feed often and on the most abundant prey (Olson and 
Boggs, 1986; Galvan-Magana, 1988). For example, in 
zones 1 and 3, the yellowfin tuna diet was dominated 
by pelagic red crab, which is abundant in upwelling 
areas of subtropical zones, generating large quanti¬ 
ties of food that span several trophic levels (Alverson, 
1963; Blackburn, 1969; Galvan-Magana, 1988; Cabrera- 
Chavez-Costa et al., 2010; Tripp-Valdez et al., 2010). 
In zone 2, yellowfin tuna consumed large quantities of 
jumbo squid, which is one of the main prey species in 
the ETPO. The jumbo squid lives in the mesopelagic 
zone but can also be found over the continental slope, 
mainly in upwelling areas rich in nutrients (Ehrhardt 
et al., 1986; Markaida-Aburto, 2001). 
Trophic levels 
Our calculations indicated that the prey of skipjack 
and yellowfin tunas have trophic level values between 
3.5 and 4.1, and between 3.9 and 4.6, respectively. 
These estimates are consistent with trophic position 
estimates for the diet of yellowfin and skipjack tu¬ 
nas in the ETPO based on stomach contents and stable 
isotopes of nitrogen (Olson and Watters, 2003; Popp et 
ah, 2007; Olson et al., 2010; Hunsicker et al., 2012). 
Interspecific comparison of trophic level shows that 
skipjack tuna feed lower in the food web than yellow¬ 
fin tuna—a characteristic that is likely related to body 
size. Cortes (1999) mentioned that trophic levels of top 
predators increased with size, with larger predators 
having higher trophic levels than those of their smaller 
counterparts (Magnuson and Heitz, 1971). Also, several 
authors noted that trophic level may increase intraspe- 
cifically as fish grow (Cousins, 1980, Cohen et al., 1993) 
because they have access to different habitats (Graham 
et al., 2007). In addition, as predator size increases, 
prey capture efficiency also increases (Torres-Rojas et 
al., 2012). Our yellowfin tuna specimens were larger 
than our skipjack tuna specimens, and were thus able 
to capture a wider range of prey. 
Comparisons of diet 
The low diet similarity and interspecific differences 
in diet diversity and S; for co-occurring yellowfin and 
skipjack tunas indicate that the association between 
these 2 species is not because they feed on the same 
prey in the ETPO. Although both tuna species are con¬ 
sidered epipelagic, and they were caught together in 
the same sets, their primary prey were different in all 
3 of the geographic zones examined. One explanation 
for this was offered by Giller (1984), who mentioned 
that subtle differences in size or morphological struc¬ 
tures can lead to differences in the prey consumed and 
reduce competition and facilitate coexistence. In the 
case of yellowfin and skipjack tunas, diet differences 
may be related to differences 1) in the anatomy of 
the gill raker apparatus of the species (Ankenbrandt, 
1985), and 2) in body size (Graham et al., 2007). For ex¬ 
ample, Magnuson and Heitz (1971) attributed the pres¬ 
ence of euphausids (e.g., N. simplex) in the stomachs of 
skipjack tunas in the ETPO and their absence in the 
stomachs of yellowfin tuna (that consumed fish) to the 
small size of euphausids and to the smaller gaps be¬ 
tween gill rakers in skipjack tuna compared with those 
in yellowfin tuna. 
Body size and morphological differences between 
these 2 tuna species, as described above, are possible 
reasons for the observed differences in the prey con¬ 
sumed that likely reduce competition and facilitate 
coexistence. Therefore, an alternative hypothesis to 
explain the occurrence of mixed schools is that these 
2 tuna species accompany each other to protect them¬ 
selves from predators, as has been suggested for dol¬ 
phins and tunas (Scott and Cattanach, 1998); however, 
more studies on this issue should be carried out in or¬ 
der to clarify this unique behavior. 
Acknowledgments 
The authors thank the following organizations for 
academic and financial support: Consejo Nacional de 
Ciencia y Tecnologfa, Inter-American Tropical Tuna 
Commission, Instituto Politecnico Nacional (IPN), In- 
stituto de Ecologia, Pesquerias y Oceanografla del 
Golfo de Mexico, Universidad Autonoma de Campeche, 
Programa Integral de Fortalecimiento Institucional, 
Estimulos al Desempeno de los Investigadores, and 
IPN Comision de Operacion y Fomento de Actividades 
Academicas. 
Literature cited 
Allen, G. R., and D. R. Robertson. 
1994. Fishes of the tropical eastern Pacific, 332 p. Univ. 
Hawaii Press, Honolulu, HI. 
