252 
National Marine 
Spencer F. Baird 
Fisheries Service 
Fishery Bulletin 
First U.S. Commissioner 9 
NOAA 
fb" established in 1881 •<?. 
of Fisheries and founder aLtX 
of Fishery Bulletin 
Trophic segregation of mixed schools of 
yellowfin tuna iThunnm albacares} and 
skipfack tuna iKatsuwonus pelatnis} caught 
in the eastern tropical Pacific Ocean 
Email address for contact author: galvan.felipe@gmail.com 
' Institute Politecnico Nadonal 
Centro Interdisciplinario de Ciencias Marinas 
Avenida Instituto Politecnico Nacional s/n 
Colonia Playa Palo de Santa Rita 
23096 La Paz, Baja California Sur, Mexico 
^ Instituto de Ecologia Pesquerias y Oceanografia del Golfo de Mexico 
Universidad Autonoma de Campeche (EPOMEX-UAC) 
Campus 6 
Avenida Heroe de Nacozari 480 
24029 San Francisco de Campeche, Campeche, Mexico 
^ Inter-American Tropical Tuna Commission 
8901 La Jolla Shores Drive 
La Jolla, California 92037-1509 
Abstract —One of thie major chal¬ 
lenges in trophic ecology is to under¬ 
stand how organisms interact with 
each other and to apply this knowl¬ 
edge to the management of popula¬ 
tions, communities and ecosystems. 
Our goal was to examine fine-scale 
variability in the feeding habits and 
trophic position of yellowfin tuna 
(Thunnus albacares) and skipjack 
tuna (Katsuwonus pelamis) caught 
together in mixed schools by purse 
seine to examine the null hypothesis 
that the association between these 2 
tuna species is not related to trophic 
interactions. In total, 439 yellowfin 
tuna and 216 skipjack tuna were 
collected in 3 different areas in the 
eastern tropical Pacific Ocean dur¬ 
ing 2005. The stomachs of both tuna 
species contained prey at different 
stages of digestion, which indicated 
intermittent feeding throughout the 
course of the day. Yellowfin tuna 
consumed mainly epipelagic crus¬ 
taceans and mesopelagic squids, 
whereas epipelagic euphausiids and 
epipelagic flyingfish were the most 
abundant prey species in the stom¬ 
ach contents of skipjack tuna. Our 
results suggested that both tuna 
species employed an opportunistic 
predation strategy, but significant 
dietary differences showed that they 
occupy different trophic levels, and 
that there is no food competition be¬ 
tween yellowfin and skipjack tunas 
in the eastern tropical Pacific Ocean. 
Manuscript submitted 10 April 2016. 
Manuscript accepted 2 March 2017. 
Fish. Bull. 115:252-268 (2017). 
Online publication date: 28 March 2017. 
doi: 10.7755/FB.115.2.11. 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Vanessa G. Alatorre-Ramirez' 
Felipe Galvan-Magaiia (contact author)^ 
Yassir E. Torres-Rojas^ 
Robert J. Olson^ 
The eastern tropical Pacific Ocean 
(ETPO) is one of the most produc¬ 
tive oceanic provinces on the planet 
(Picaut, 1985; Fiedler et ah, 1991; 
Pennington et ah, 2006). The tuna 
fishery in the ETPO is one of the 
most important in the world; the 
2012 annual catches of yellowfin 
tuna (Thunnus albacares) and skip¬ 
jack tuna (Katsuwonus pelamis) ex¬ 
ceeded 209,000 and 271,000 metric 
tons, respectively (lATTC^). 
Yellowfin and skipjack tunas are 
schooling species that are frequently 
found together in large aggregations 
(Scott et ah, 2012); in the ETPO they 
are often co-occurring and captured 
by purse-seine near the sea surface. 
' lATTC (Inter-American Tropical Tuna 
Commission). 2014. Tunas and bill- 
fishes in the eastern Pacific Ocean in 
2013. Inter-Am. Trop. Tuna Comm., 
Fish. Status Rep. 12, 177 p. [Available 
from website.) 
Despite intensive research on these 2 
tuna species, the association between 
them remains unexplained. The best 
documented and understood associa¬ 
tion between large marine organisms 
in the ETPO is that between yellow¬ 
fin tuna and dolphins (Stuntz^; Scott 
and Cattanach, 1998). The tuna- 
dolphin association appears to be a 
strategy to reduce the risk of preda¬ 
tion for one or both species (Scott et 
ah, 2012). 
Associations among marine organ¬ 
isms can result in increased feeding 
success of one or both of the associ¬ 
ated species (Nikolsky, 1963). Yel¬ 
lowfin and skipjack tunas require 
large amounts of energy to sustain 
themselves (Olson and Boggs, 1986; 
^ Stuntz, W. E. 1981. The tuna-dolphin 
bond: a discussion of current hypoth¬ 
eses. NOAA Southwest Fish. Cent., 
Admin. Rep. LJ-81-19, 9 p. 
