Ferm et al.: Foraging habits and dietary overlap of Limanda aspera and Lepidopsetta polyxystra in the Bering Sea 5 
Foraging patterns 
We discerned the functional foraging patterns of yellowfin 
sole and northern rock sole with a principal components 
analysis (PCA) of the environmental and biophysical fea- 
tures of the predation conditions for each prey taxa. This 
approach differs from the nonmetric multidimensional 
scaling analysis frequently applied in diet studies. 
Whereas the nonmetric multidimensional scaling 
approach is an ordination of samples based on the degree 
of co-occurrence of prey types in the diets of individual 
predators (or pooled by sampling location), our approach 
focuses on ordination of the characteristics that describe 
the predation event. For example, in this approach, prey 
taxa would be clustered in the ordination space if they 
were consumed at similar depths, at similar times of day, 
by similarly sized predators, even if they rarely co-occurred 
in the diets of individual predators. 
For this analysis, prey taxa were grouped into broad 
taxonomic and habitat categories (e.g., epibenthic cope- 
pods) and retained in the analysis if they occurred 
>3 times in the diets of either predator species. The matrix 
for each prey taxa was created by using the mean of each 
variable from all observations of that prey taxa. We lim- 
ited the number of response variables in the prey taxa 
matrix to reduce occurrences of highly correlated vari- 
ables. However, because of their traditional importance to 
habitat definitions, bottom water temperature and depth 
were retained despite their strong negative correlation 
(coefficient of determination [r7]>0.7), and all sediment 
fractions were retained despite an r’>0.7 for some combi- 
nations. Variables included in the analysis were consumer 
SL, K (Fulton’s condition factor), and stomach fullness; 
log-transformed catch per unit of effort for the species 
at the station; and the corresponding 
environmental variables of time of day, 
depth, bottom water temperature, sedi- 
ment proportions of silt or clay, fine sand, 
medium sand, coarse sand, granules, 
and bivalve shells. Fractions of sediment 
composition were logit transformed prior 
to analyses, and all variables were stan- 
dardized prior to analyses. A single PCA 
of the combined prey of northern rock 
sole and yellowfin sole was conducted. 
ine) 
n 
n 
® 
= 
= 
= 
fa 
(6) 
oO 
iS 
je) 
2 
dp) 
Results 
Collections 
Within the PM-HB system, juvenile yel- 0800 
lowfin sole were more abundant than 
northern rock sole, but northern rock 
5] Species 
yellowfin sole were caught (Fig. 1). At least one of the spe- 
cies was captured at 47 of the 75 sampling stations, with 
both species being captured at 24 stations. Yellowfin sole 
(35-99 mm SL) and northern rock sole (44-90 mm SL) 
analyzed for this study were age 1 and age 2 (based on 
length—frequency distributions). A cohort of smaller age-0 
northern rock sole (<35 mm SL) (Hurst, 2016) captured 
during sampling is not described here. 
In total, diets of 443 juvenile flatfish were examined 
(260 northern rock sole and 183 yellowfin sole). Overall, 
mean stomach fullness was significantly higher in north- 
ern rock sole (1.77% [standard error of the mean (SE) 
0.14]) than in yellowfin sole (0.22% [SE 0.02]) (analysis 
of variance: P<0.001). Although this pattern was consis- 
tent across sampling regions, it was most pronounced in 
the BC region, with mean stomach fullness of northern 
rock sole at 2.18% (SE 0.19). Although samples were col- 
lected only during daylight hours (0800-2100), there was 
a clear pattern of feeding periodicity for northern rock sole 
with maximum stomach fullness observed in fish caught 
during 1600-1800 (Fig. 3). There was no temporal pattern 
in stomach fullness observed for yellowfin sole. 
Diet breadth 
We identified 91 distinct prey types in the diets of yellow- 
fin sole and 85 distinct prey types in the diets of north- 
ern rock sole. Some of these prey types were common prey 
items that were identified to life stage within a species; 
whereas, other types were identifiable only to order or 
family. Estimates of cumulative prey richness of the diet 
of yellowfin sole and northern rock sole were 151.1 (SE 
30.0) and 129.8 (SE 22.6), respectively, and estimates 
were not significantly different between predator species 
1000 1200 1400 1600 1800 2000 
Time of day 
Figure 3 
sole were more abundant in the coastal 
waters outside the mouth of the PM-HB 
system. Both species were found in all 
sampling areas except for the innermost 
stations of the HB region where only 
Temporal pattern in stomach fullness of yellowfin sole (Limanda aspera) 
(YFS) and northern rock sole (Lepidopsetta polyxystra) (NRS) captured in 
August 2012 in coastal areas of the southeastern Bering Sea. Error bars indi- 
cate standard errors of the mean. 
