Ferm et al.: Foraging habits and dietary overlap of Limanda aspera and Lepidopsetta polyxystra in the Bering Sea 9 
both predator species. Conversely, hyperbenthic copepods 
(low PC1 scores) had the shallowest depth of occurrence 
in the diets of both predator species. Annelids (the most 
common prey items for both species) were consumed by 
both species at stations with fine sand (low PC2 score). 
The PCA plot (Fig. 5) also reflects some separation in for- 
aging niche (including prey species and foraging habitat 
characteristics) between species: yellowfin sole consumed 
bivalves at relatively shallow, muddy stations, in compar- 
ison with the stations where this species consumed other 
prey. In contrast, northern rock sole consumed bivalves 
at deeper, sandier stations. Cnidaria (anemone tentacles) 
were not a common prey item of either species, but their 
high PC2 score indicates their consumption by yellowfin 
sole in areas with large grain sediments. 
Discussion 
Many flatfish species rely upon juvenile nursery habitats 
in shallow, coastal areas whose spatial extent is markedly 
smaller than that of adult distribution (Gibson, 2005). 
As such, flatfishes are considered particularly prone to 
density-dependent growth and recruitment limitations 
associated with interspecific competition for habitats and 
prey (“space concentration hypothesis;” Beverton, 1995). 
Furthermore, these coastal nurseries are commonly inhab- 
ited by multiple flatfish species, indicating the potential for 
interspecific competition and density dependence (Vinagre 
et al., 2005; Mariani et al., 2011). Consequently, numerous 
studies have examined the patterns of niche overlap and 
partitioning with regard to habitat associations and forag- 
ing of juvenile flatfishes (e.g., Carter et al., 1991; Vinagre 
et al., 2005; Tomiyama and Omori, 2008; Mariani et al., 
2011). In this study, we examined the foraging habits and 
dietary overlap of juvenile yellowfin sole and northern rock 
sole in previously uncharacterized nearshore bays in the 
SEBS and examined functional differences in the foraging 
habits of these species. These observations indicate that 
direct competition for prey resources between juveniles of 
these co-occurring species is reduced by a combination of 
differential habitat associations and prey selection. How- 
ever, this differentiation indicates that climate change in 
the SEBS may differentially affect the forage base and 
productivity of these species. 
The diets of juvenile yellowfin sole and northern rock 
sole captured along the Alaska Peninsula of the SEBS in 
our study were generally similar to observations in other 
parts of the ranges of these species. Both species are gen- 
eralist benthic invertebrate predators, with polychaetes 
and amphipods generally dominating their diets (Corco- 
bado Onnate, 1991; Holladay and Norcross, 1995; Lang 
et al., 1995; Hurst et al., 2007; Yeung and Yang, 2017). The 
incorporation of secondary diet components varied across 
studies. For example, bivalves were rare in the diets of 
both species in our sampling area, common in the diets 
of yellowfin sole (but not in those of northern rock sole) in 
broader sampling of the Bering Sea, (Lang et al., 1995), 
and common in diets of northern rock sole (but not in those 
of yellowfin sole) in coastal waters of the GOA (Holladay 
and Norcross, 1995; Hurst et al., 2007). 
The largest difference between diets of yellowfin sole 
and northern rock sole was the higher occurrence of mysids 
and euphausiids in the diets of northern rock sole. These 
taxa composed >40% (by weight) of the diets of northern 
rock sole but were only minor components (by weight) of 
the diets of yellowfin sole (mysids: 7%; euphausiids: 1%). 
Mysids and euphausiids appear to be consistently rare in 
the diets of juvenile yellowfin sole, with no indication of 
a significant contribution reported from previous studies. 
The incorporation of these taxa in the diets of northern 
rock sole appears more variable across studies, potentially 
varying over space and time. Mysids were rare in 2 pre- 
vious studies in the SEBS (Lang et al., 1995; Yeung and 
Yang, 2017). Mysids were important in the diets of juve- 
nile northern rock sole in one GOA study (Holladay and 
Norcross, 1995) but not in another (Hurst et al., 2007). 
Interestingly, in both studies in the GOA, mysids were the 
dominant prey of juveniles of other co-occurring flatfishes, 
indicating that their rarity in the diets of northern rock 
sole and yellowfin sole is a reflection of prey selection as 
opposed to availability. 
The consumption of euphausiids by northern rock sole 
observed in our study appears to be greater than that 
observed in previous studies of similarly sized northern 
rock sole. Because euphausiids are highly aggregated, 
pelagic prey that undertake diel vertical migrations (Scha- 
betsberger et al., 2000), it is possible that the conditions 
under which northern rock sole consume them are more 
limited than those for consumption of other types of prey. 
Dietary niche partitioning 
In waters of Alaska, yellowfin sole and northern rock 
sole have overlapping distributions at both the juvenile 
and adult stages and have been the most abundant fish 
species in our sampling area (Hurst, 2016). The degree of 
dietary overlap is an important indicator of the potential 
for interspecific competition between species in their juve- 
nile nurseries. Lang et al. (1995) found high dietary over- 
lap and potential competition between juvenile yellowfin 
sole and northern rock sole (fish <200 mm SL in depths 
<50 m: SI=0.70). Across our study area, the dietary over- 
lap of these species was low (SI=0.39). The differential 
consumption of mysids and euphausiids contributed to the 
low levels of dietary overlap in our study and is likely facil- 
itated by morphological differences between the 2 species 
(Barnes et al., 2021). Although flatfishes share broadly 
similar body morphologies, variations between species 
are associated with specific foraging habits (Livingston, 
1987, Russo et al., 2008). Of the 2 species we studied, the 
northern rock sole has the more extendable jaw and a nar- 
rower field of vision, both characteristics that favor the 
consumption of more mobile prey (Lang et al., 1995). 
The dietary niche partitioning between the spe- 
cies, indicated by the low overall SI values across the 
sampling area, was also in part due to fine-scale hab- 
itat segregation (Mariani et al., 2011). Although both 
