Van Noord et al.: Oceanographic influences on the diet of myctophids in the eastern Pacific Ocean 
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Table 1 
Geographic, oceanographic, and biologic predictor variables used in the analysis to determine the diet of 3 
myctophids in the eastern tropical Pacific Ocean. Fish were collected from August through November 2006. 
Predictor variable 
Type of variable 
Mean (Min. -Max) 
Longitude 
Spatial 
140.7°W to 80.4°W 
Latitude 
Spatial 
11.0°S to 10.8°N 
Myctophid species 
Biological 
Myctophum nitidulum, Symbolophorus 
reversus, Gonichthys tenuiculus 
Myctophid standard length 
Biological 
50.4 mm (25-80) 
Ostracod zooplankton (ZP) composition 
Biological 
10.9% (1.2-37.0) 
Copepod ZP composition 
Biological 
72.2% (45.0-87.3) 
Euphausiid ZP composition 
Biological 
4.4% (1.8-20.7) 
Zooplankton volume 
Biological 
97.5 mL (l,000)/m3 (44-241) 
Mixed layer depth (MLD) 
Oceanographic 
38.1 m (6-93) 
Sea surface salinity (SSS) 
Oceanographic 
33.25 psu (31.21-35.58) 
Sea surface temperature (SST) 
Oceanographic 
21.2'’C (21.9-29.2) 
Surface chlorophyll (SCHL) 
Oceanographic 
0.184 mg/m3 (0.002-0.382) 
We categorized the myctophid diet into 17 prey 
groups in the CART analysis (14 are shown in Table 
3). These groups ranged in taxonomic level from fam- 
ily (e.g., Euchaetidae) to phylum (e.g., Mollusca) be- 
cause the taxonomic resolution of prey identifications 
varied and because some rare prey were combined 
into broader taxa along with unidentifiable specimens. 
For the CART analysis, all mollusks were grouped to- 
gether, as were all euphausiids, and all cyclopoid co- 
pepods. Rare calanoid copepods, defined as types con- 
tributing less than 1% MNi to all 3 myctophid species 
were grouped as “other copepods.” Rare amphipods, 
defined as types contributing less than 0.3% in MNi 
were grouped as “other amphipods.” The “other” co- 
pepod and amphipod groups contained a majority of 
unidentifiable specimens. Decapods (0.5% combined 
mean MNi), fish eggs (0.3%), and the one cephalopod 
(<0.1%) were not included in the analysis because of 
their scarcity. 
Results 
Oceanographic variables 
The MLD, SSS, SST, and SCHL concentrations each 
showed distinct geographic patterns within the study 
area. MLD deepened from east to west along the NECC, 
between the equator and 10°N (Fig. 3). At its shallow- 
est, MLD (mean 38.1 ±20.5 m standard deviation [SD], 
averaged over the 32 dipnet stations) was 6 m deep 
nearshore (8°N, 91°W) and reached 93 m at the station 
farthest offshore (6°N, 140°W). SSS (mean 33.3 ±1.20 
practical salinity units [psu]) was lowest (31.21 psu) 
near the coast of Central America, and became more 
saline farther offshore and south of the equator (max. 
of 35.5 psu). SST (mean 21.2 ±1.90° C) showed little 
variation along the NECC, where the majority of sta- 
tions occurred. Surface chlorophyll-a values (mean 0.20 
±0.08 mg/m^) were greatest along the coast of southern 
Mexico and Ecuador and decreased offshore (Fig. 3). 
Myctophid size-composition 
The individuals collected at the surface nightly by dip 
net differed morphologically (Fig. 4). Symbolophorus re- 
versus (71=199; 172 with identifiable prey remains) was 
the largest of the 3 species in length (mean=55.9 ±8.8 
mm SD) and weight (2.66 ±1.27 g SD). Myctophum niti- 
dulum (?7=299; 275 with identifiable prey remains) was 
intermediate in length (47.9 ±6.6 mm) and weight (1.82 
±0.80 g). Gonichthys tenuiculus (7i=82; 26 with identifi- 
able prey remains), was the smallest species in length 
(37.8 ±4.3 mm) and weight (0.54 ±0.21 g [Fig. 4]). 
Diet composition 
Prey composition data for each of the 3 species are 
summarized by the 3 diet indices, MNi, MWi, and Oj in 
Table 2. We focused our data analysis on the numeric 
diet index for consistency with the numeric data on 
prey availability. We included the weight and occur- 
rence indices in Table 2, however, so that our data are 
comparable with other published data. 
Zooplankton community 
Seventeen unique taxonomic groups of zooplankton 
were identified and enumerated (?i=178,090 individu- 
als) in the net tows. Copepods were by far the most 
abundant group, representing as much as 87.3% of 
the community sampled and never less than 45.0% at 
any station (Table 3, Fig. 5). Ostracods were the sec- 
ond most abundant group overall (8.65 ±10.1%). Eu- 
phausiids and amphipods each contributed <5% of the 
sampled community. 
