Van Noord et al.: Oceanographic influences on the diet of myctophids in the eastern Pacific Ocean 
275 
scribed in myctophids in the Gulf of Mexico (Hopkins 
and Gartner, 1992; Hopkins and Sutton, 1998), Cen- 
tral Pacific (Clarke, 1980), western tropical Pacific (Van 
Noord 2013), Kuroshio Current (Watanabe et al., 2002), 
California Current (Suntsov and Brodeur, 2008), South- 
ern Ocean (Cherel et al., 2010; Shreeve et al., 2009), 
and North Atlantic (Pusch et al., 2004). Conversely, 
generalist behavior has been described by Kinzer and 
Schulz (1985), who found that 7 myctophid species in 
the equatorial Atlantic fed opportunistically on similar 
calanoid copepods. Pakhomov et al. (1996) also found 
that 4 myctophid species in the Southern Ocean fed op- 
portunistically on the same mesozooplankton, whereas 
Tyler and Pearcy (1975) found that myctophids in the 
California Current fed on a diverse and overlapping 
diet. These previous studies, although valuable, were 
primarily descriptive in nature, did not provide mea- 
surements of prey availability, were restricted to geo- 
graphically small areas, and therefore do not provide a 
broad picture of myctophid feeding and the variables 
that govern their diet. Understanding the factors that 
influence the diet of myctophid fishes will provide in- 
sight into food-web dynamics and the structure of re- 
lated communities. If myctophids are opportunistic 
feeders, for example, bottom-up forcing or environmen- 
tal changes in the system would have a dynamic effect 
on their feeding patterns, which, in turn, would rever- 
berate throughout the food web (Fiedler et al., 2013). 
Little is known about the ecology or biology of myc- 
tophids from the eastern tropical Pacific (ETP) but in 
surveys of larval fish in the ETP, high densities, as well 
as a diversity, of myctophid species have been encoun- 
tered (Ahlstrom, 1972, 1971). Studies investigating the 
ecological role of myctophids in this region have lumped 
species together as a forage base for top predators (e.g., 
Pitman et al.^; Maas et al. 2014) and myctophids have 
been documented as prey for cetaceans (Perrin et al., 
1973; Scott et al., 2012), tunas, swordfish, and other 
large pelagic fish (Moteki et al., 2001), squids (Shchet- 
innikov, 1992), and seabirds (Spear et al., 2007) in the 
ETP. Given their importance as prey, their feeding be- 
havior can have ramifications on how energy is trans- 
ferred from lower to higher trophic levels. 
The eastern tropical Pacific Ocean encompasses ar- 
eas of upwelling and oligotrophy (Fiedler and Talley, 
2006), and this oceanographic variability produces di- 
verse zooplankton prey assemblages (Fernandez-Alamo 
and Farber-Lorda, 2006). There are also diverse and 
abundant communities of myctophids (Ahlstrom, 1972, 
1971). With the abundance of myctophids and their 
zooplankton prey in this region, the ETP presents an 
opportunity to assess feeding strategies for these fish. 
Using samples collected across a productivity gra- 
dient along the North Equatorial Countercurrent 
^ Pitman, R. L., L. T. Bailance, and P. C. Fiedler. 2002. Tem- 
poral patterns in distribution and habitat associations of prey 
fishes and squids. NOAA, Natl. Mar. Fish. Serv., Southwest 
Fish. Sci. Cent. Admin. Rep. LJ-02-19, 52 p. [Available at 
website.] 
(NECC), we quantified the diets of 3 common surface- 
migrating myctophids, and assessed the availability 
of prey for these species from zooplankton net hauls. 
Then, using a bagged (i.e., bootstrap aggregating, where 
classifications of randomly generated training sets are 
combined to improve overall model performance) clas- 
sification tree (Kuhnert et al., 2012), we investigated 
the influence of spatial, oceanographic, and biologic 
(prey and predator) variables on myctophid diets. We 
postulate that, if dietary resources were partitioned, 
the diets of each species would be unique, whereas if 
feeding was opportunistic, myctophid diets would be 
related to broad-scale patterns of prey availability and 
oceanography. 
Materials and methods 
Study area and data coliection 
The study area is located in the ETP between the sub- 
tropical gyres of the North and South Pacific (Fiedler 
and Talley, 2006). The ETP contains 3 major surface 
currents: the North Equatorial Current (NEC), North 
Equatorial Countercurrent (NECC), and the South 
Equatorial Current (SEC [Fig. 1]). The NECC is a 
warm, eastward flowing current in which upwelling 
causes shoaling of the thermocline along an east-to- 
west gradient near 5°N (Fiedler and Talley, 2006). Two 
eastern boundary currents at the northern (California 
Current) and southern (Peru Current) extent of the 
ETP bring cold, nutrient-rich water into the system 
(Fiedler and Talley, 2006). Nearshore waters associated 
with the Gulf of Panama have characteristically low 
sea-surface-salinity (SSS) values owing to high local 
rainfall (Amador et al., 2006). 
Myctophids (n=580) were collected aboard National 
Oceanic and Atmospheric Administration (NOAA) re- 
search vessels RV MacArthur II and RV David Starr 
Jordan during surveys conducted in the ETP by NO- 
AA’s Southwest Fisheries Science Center (SWFSC) 
from August to November 2006. Myctophids were sys- 
tematically collected every night in dip nets at prede- 
termined stations (Fig. 2) located on line transects (Fig. 
3). A randomized subsample of myctophids captured at 
32 stations was available for this study. Dipnet sam- 
pling began one hour after local sunset and lasted for 1 
hour. Long-handled (~6-m) dip nets with 1-m wide bas- 
kets and 0.5-cm mesh size were used to catch mycto- 
phids under deck lights that illuminated approximately 
10 m2 of the water surface (Goad, 1998). Researchers 
at the SWFSC have collected specimens using this 
standardized method for decades (Fiedler et al., 2013; 
Pitman et al.^). The dipnet method is unique in com- 
parison with that of traditional net tows because fish 
are collected from the ocean environment individually 
and not retained in a net, thus excluding the possibility 
of postcapture feeding. Net avoidance associated with 
the bow-wave of large towed equipment is also negated 
with handheld dip nets. Myctophids exhibit size-related 
