Van Noord et al.: Oceanographic influences on the diet of myctophids in the eastern Pacific Ocean 
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140°W 130°W 120°W 110°W 100°W 90°W 80°W 
Distribution of the prey community sampled with an oblique bongo net in the eastern tropical 
Pacific Ocean. Each color represents a zooplankton prey species, other prey species, or non-prey 
species. Circle size reflects zooplankton displacement volume. The numbers 44, 110, and 241 
indicate zooplankton displacement volume (mL/lOOOm^). 
communities are structured and how energy is trans- 
ferred through the food web. 
Longitude had the greatest variable importance 
ranking among all predictors, and this is likely be- 
cause water masses and prey composition co-varied 
geographically along the NECC. The NECC is charac- 
terized by a shoaling thermocline and by increased pro- 
ductivity from west to east (Fiedler and Talley, 2006), 
and the classification tree allowed us to identify dis- 
tinct regions along the NECC where myctophid diet 
was different. The zooplankton samples collected from 
the stations in each of the 3 geographic regions had 
different percentages of prey groups that contributed 
to myctophid diet patterns. 
The offshore region was defined by a high abundance 
of pelagic ostracods in myctophid diets. This region was 
oceanographically distinct because of its mixed layer 
depth, low productivity, and high abundance of ostra- 
cods in contrast to the other regions. A deep MLD cor- 
responds to reduced mixing, lower nutrient availability 
in surface waters, and oligotrophic conditions (Fiedler 
and Talley, 2006). Pelagic ostracods are typically most 
abundant in such oligotrophic conditions because of 
their greater ability to exploit environments low in 
food availability (Le Borgne and Rodier, 1997; Angel 
et ah, 2007). 
The nearshore region was defined by a high abun- 
dance of euphausiids in myctophid diets. This region 
was oceanographically distinct because of its shallow 
MLD and low saline waters. It typically displays el- 
evated primary productivity and low oxygen levels 
(Fiedler and Talley, 2006). Extreme local rainfall and 
westward transport of water vapor across the Isthmus 
of Panama contribute to the low-salinity water mass in 
this location (Amador et ah, 2006). Euphausiid abun- 
dance is typically greatest in productive, nearshore 
waters (Brinton, 1979; Simard et ah, 1986), and that 
is the case here. Upwelling and biological production 
are greatest near the coast in the ETP, particularly the 
Gulf of Panama and the Costa Rica Dome than in other 
regions in the ETP (Lavin et ah, 2006). Additionally, an 
oxygen minimum zone exists in the ETP; low oxygen 
values extend south into the Gulf of Panama (Fiedler 
and Talley, 2006). Some euphausiids, such as Euphau- 
sia diomedeae and E. mutica that were consumed by 
the myctophids in this study, are tolerant of low oxygen 
(Brinton, 1979). 
Myctophids in the intermediate region had high 
numbers of copepods in their diets. This region, a tran- 
sition zone between the nearshore and offshore, showed 
moderate mixed layer depths, salinities, and surface 
chlorophyll values in comparison with the higher and 
lower values of the other regions, respectively. Cope- 
pods were abundant throughout the study area but 
