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Fishery Bulletin 114(3) 
160°W 150°W 140°W ISO^W 120'’W IIO'W lOO'W 90°W 80°W 
Major currents and oceanographic features in the eastern tropical Pacific Ocean. Ar- 
rows indicate direction of the currents. Dashed ovals signify distinct oceanographic 
features. 
depth stratification throughout the water column, and 
certain, often larger, individuals remain deeper in the 
column (Collins et ah, 2008; Saunders et ah, 2015). 
Additionally, not all members of a surface-migrating 
population migrate each night, and surface-migration 
is likely spurred by feeding. Therefore, our samples 
are not representative of all size classes within this 
population; instead, we focused entirely on the surface- 
migrating myctophids that were found in surface wa- 
ters during the time of capture. Specimens were frozen 
whole at sea. 
Zooplankton were sampled with a bongo net (0.6- 
m mouth diameter, 333-pm mesh) towed obliquely to 
a depth of 200 m at an average ship speed of 1.75 kn. 
A flow meter was attached to the net to determine the 
amount of filtered seawater. We analyzed only net tows 
at the 32 stations where myctophids were collected 
(Fig. 2). The sampling depth of the bongo tows does not 
cover the entire depth range for myctophids because 
our study focused exclusively on the surface-migrating 
members of the population and we were interested in 
the prey that might be available to this subset of the 
fish community. Zooplankton also conduct diel vertical 
migrations from deep-water to near-surface waters and 
it is likely that some zooplankton had migrated from 
deeper than 200 m (Longhurst and Harrison, 1988). 
Depth-integrated zooplankton samples were used be- 
cause myctophids feed during migration (Watanabe et 
ah, 1999). Net tows commenced 30 min after the con- 
clusion of dipnet sampling and the zooplankton sam- 
ples were preserved at sea in 3.7% buffered formalin. 
Systematic oceanographic sampling was conducted 
during the surveys (for details, see Fiedler and Phil- 
brick^). Sea surface temperature (SST) and SSS values 
were recorded with a thermosalinograph at 2-min in- 
tervals along transects (Fig 3). Surface chlorophyll-a 
(SCHL) concentrations were measured at approximate- 
ly 55-km intervals along transects by using a fluorom- 
eter. Mixed layer depth (MLD), i.e. the depth at the top 
of the thermocline, was estimated as the depth (m) at 
which the temperature is 0.5°C less than the surface 
temperature. MLDs were derived from data obtained 
from expendable bathythermograph (XBT) and conduc- 
tivity, temperature, and depth (CTD) casts. XBT casts 
were made at approximately 55-km intervals along 
transects to a depth of 760 m. CTD casts were under- 
taken at sunrise and sunset each day to a depth of 
1000 m. Using these data, Barlow et al. (2009) created 
smoothed (using the Kriging method) maps of SST, SSS, 
MLD, and SCHL data (Fig. 3) and are presented here 
with permission. In the classification tree model we 
considered, only variables coinciding with the 32 dipnet 
stations at which myctophids were collected. 
In the laboratory, myctophids were thawed individu- 
ally, identified (by using keys devised by Wisner, 1974 
and Gago and Ricord, 2005), blotted, weighed to the 
nearest mg, and measured to the nearest mm (standard 
length, SL). Stomachs were dissected whole from each 
2 Fiedler, P. C., and V. A. Philbrick. 2002. Environmental 
change in the eastern tropical Pacific Ocean: observations 
in 1986-1990 and 1998-2000. NOAA, Natl. Mar. Fish. 
Serv., Southwest Fish. Sci. Cent. Admin. Rep. LJ-02-15, 16 
p. [Available at website.] 
