Figure 1. Study area and sample station locations within RACE bottom-trawl survey grid cells.
Yellowfin sole, northern rock sole, and Alaska plaice are three common flatfish that co-occur in the southern eastern Bering Sea (EBS) shelf at depths usually not exceeding 100 m. Their small mouths are adept at preying on infauna, especially polychaete worms. The average diet of Alaska plaice consists of almost 60% polychaetes by weight. For yellowfin sole, which has the most varied diet of the three species, polychaetes still comprise over a quarter of their diet by weight. An essential fish habitat (EFH) study in 2009 provided insights on the relationships between flatfish habitats, diets, and prey availability. Benthic grab samples and flatfish stomachs were collected at 31 RACE bottom-trawl survey stations across the shelf (Fig. 1). The two data sets were analyzed for correspondence between stomach contents and infauna assemblages across habitat types.
Polychaetes and clams were the most dominant groups, each comprising 35%-60% by weight of each infauna sample. They were also the only prey groups that frequently averaged over 50% of stomach content weight. Sediment grain size was the most important factor in determining the type of infauna assemblage in the habitat (Fig. 2). Grain size becomes smaller, i.e. sediment becomes muddier, the further from shore (or deeper the water). Clams dominated the infauna biomass on the sandy inner shelf (0-50 m depth) (Fig. 2). The “muddy sand” of the middle shelf (50-100 m) had the highest infauna biomass, which was dominated by deposit-feeding polychaetes.
Figure 2. Percentage composition of the four major infauna groups in the benthos (“Infauna”) and in the stomachs of each flatfish species (“Diet”) at each station. In the “Infauna” plot, the sediment type (in percentage of mud the sediment sample contains; sand % = 100 - mud %) of each station cell is also depicted. In the “Diet” plots, empty cells indicate no stomachs were collected.
Prey availability strongly influenced diet choices. Stomach contents of all three flatfish generally reflected the infauna assemblage of the habitat where they were collected (Fig. 2). Alaska plaice clearly adapted to prey availability – they ate mostly clams on the inner shelf, although their primary prey are polychaetes. All flatfish switched to eating more polychaetes on the middle shelf - even yellowfin sole, whose primary prey are amphipods and clams.
Polychaetes may not be obligatory prey for these flatfish, but they could very well be the choice prey - considering that polychaetes are overall the most dominant infauna group in the EBS and generally have a higher organic nutrient content than the other major groups: clams, amphipods, and brittlestars. Under this hypothesis, the biomass or abundance of polychaetes could indicate the quality of the habitat for flatfish in terms of prey availability. Alaska plaice, northern rock sole, and yellowfin sole obviously all eat polychaetes. Their actual proportion intake of polychaetes may be a result of interspecific competition.
Figure 3. The distributions of Alaska plaice catches and polychaete biomass at stations surveyed across the EBS shelf in this study, superimposed over the colormap of bottom water temperature in summer 2009. The temperature ranged from -1°C (blue) to 8°C (red). The 50, 100, and 200 m isobaths are delineated. Double dotted curve (black) approximates the inshore edge of the “cold pool” of <2°C bottom water. Highest concentration of Alaska plaice lies inshore of the cold pool, whereas highest polychaete biomass is mostly within the cold pool.
The middle shelf domain supports high infauna biomass because of high benthic productivity shaped mainly by hydrography. The nutrient environment and sediment type there particularly favor deposit-feeding polychaetes. This polychaete-rich habitat would presumably be highly suitable to flatfish, especially Alaska plaice. However, in 2009, bottom-trawl survey results showed that Alaska plaice were concentrated on the inner shelf (Fig. 3), like northern rock sole and yellowfin sole.
