Polychaete assemblage as surrogate for prey availability in assessing southeastern Bering Sea flatfish habitat

Abstract

The flatfish yellowfin sole (Limanda aspera), northern rock sole (Lepidopsetta polyxystra), and Alaska plaice (Pleuronectes quadrituberculatus) in the southeastern Bering Sea prey mainly on infauna. Spatial correspondence between their stomach contents and infauna assemblages across habitat types was examined to identify indices of prey availability for flatfish habitat characterization and quality assessment. Benthic samples and flatfish stomachs were collected in 2009 near the Alaska Peninsula in the southeastern Bering Sea. Polychaetes and bivalves were the most dominant infauna groups, each comprising 35–60% by weight in each infauna sample. These two were also the only prey groups that frequently averaged > 50% of stomach content by weight. Bivalves dominated the infauna biomass on the relatively sandy inner shelf (0–50 m depth). The muddier middle shelf (50–100 m) had the highest infauna biomass, which was dominated by polychaetes. Diet compositions of the flatfish varied spatially in correspondence with the infauna assemblage. Polychaetes were prevalent in all flatfish diets on the middle shelf, even yellowfin sole whose typical primary prey are amphipods and bivalves. Polychaete-rich habitats are potentially prime for flatfish as polychaetes are readily utilized where available and generally have high nutritional value. Flatfish did not select for specific polychaete taxa, so an index of habitat quality could be based on the biomass of aggregate polychaetes or on dominant polychaete families of the region. Under normal environmental conditions, the three flatfish have slightly-offset spatial distributions, enabling each to utilize different infauna assemblages across the shelf. However, during cold phases in the Bering Sea ecosystem, as when this study was conducted, a cold pool of < 2 °C bottom water from the spring ice melt extends over the middle shelf in summer. This physiological barrier displaces all three flatfish to the inner shelf, intensifying competition for prey resources.

Introduction

The southeastern Bering Sea (SEBS) continental shelf accounts for approximately half of the total US fishery production (National Research Council, 1996). The flat, shallow shelf ranges from 500 to over 800 km wide from shore to the shelf break at about the 170-m isobath, with an average cross-shelf gradient of 0.2 m km^− 1 (Kinder and Schumacher, 1981). Surficial sediment becomes finer with distance from shore and water depth (Sharma, 1979). Most of the surficial sediment can be characterized as predominantly sandy, except for nearshore areas of mixed sand and gravel, and mixed sand and mud further offshore (Smith and McConnaughey, 1999). Oceanographic fronts during the summer are defining features. The Inner Front located near the 50-m isobath and the broad Middle Front about the 100-m isobath divide the shelf into the inner, middle, and outer domains of different hydrographic conditions and productivity (Coyle et al., 2007, Kinder and Schumacher, 1981, Stabeno and Hunt, 2002). Another dominant hydrographic feature on the shelf is the “cold pool” — a tongue of cold bottom water < 2 °C formed by the spring melting of sea ice that normally extends southward over the middle shelf. The extent and intensity of the cold pool are driven by the Bering Sea climate regime (Overland and Stabeno, 2004).

Flatfish are a major fishery resource in the SEBS. Yellowfin sole (Limanda aspera), northern rock sole (Lepidopsetta polyxystra), and Alaska plaice (Pleuronectes quadrituberculatus) are relatively small-sized species among the seven ecologically and commercially important flatfish in the SEBS (Lee et al., 2010). These benthivores have overlapping distributions in depths of ≤ 110 m. Yellowfin sole has the highest biomass among flatfish in the SEBS, and supports the largest flatfish fishery in the world (Wildebuer et al., 2005).

Understanding the ecological linkages that define suitable or essential fish habitat is crucial for ecosystem-based fishery management (Link et al., 2002, Shucksmith et al., 2006). Habitat definitions are theoretically as complex and dynamic as ecological linkages, but in fact most are simply based on observed fish distributions and environmental variables (DeLong and Collie, 2004, Norse, 2005). Environmental variables that are relatively easy to measure and spatially contiguous (e.g. surficial sediment type, water temperature, salinity, and water depth) have been most useful in explaining fish distributions.

On the gently-graded and mostly featureless SEBS shelf, sediment grain size distribution or substrate type is the main physical characteristic of flatfish habitat. It is often the variable with the highest correlations with flatfish distribution in soft-sediment habitats (Amezcua et al., 2003, McConnaughey and Smith, 2000, Yeung and McConnaughey, 2008). Flatfish generally prefer fine sediments for efficient burial, as demonstrated in laboratory experiments (Moles and Norcross, 1995). Granulometry also influences the availability of suitable prey (Able et al., 2005, Stoner and Ottmar, 2003). Finer sediments usually have higher organic content (Sharma, 1979) that favors higher total benthic biomass (Feder et al., 2007, Grebmeier et al., 1989), but hydrography and sea ice climatology are the principal drivers of pelagic primary production — the main supplier of organic matter to the benthos (Grebmeier et al., 1988, Overland and Stabeno, 2004, Stabeno and Hunt, 2002).

Yellowfin sole, northern rock sole, and Alaska plaice have small mouths suited for feeding on benthic infauna (Link et al., 2002, Yeung et al., 2010), particularly polychaetes, which comprise the bulk of the SEBS infauna (Haflinger, 1981, Stoker, 1981). The proportion of polychaetes in the diet by weight averaged 26% for yellowfin sole, and as high as 60% for northern rock sole and Alaska plaice (Yeung et al., 2010). Adult plaice (Pleuronectes platessa) abundance in the English Channel was related to the density of polychaete tubes (Shucksmith et al., 2006). The condition of adult plaice in the Irish Sea was related to the production of suitable-sized infauna (Hiddink et al., 2011). Polychaete density was found to be a significant habitat suitability variable for juvenile sole (Solea senegalensis) in the Tagus estuary, Portugal (Vinagre et al., 2008a). In the SEBS, the relationships of substrate to prey availability and prey demand have not been quantitatively defined. Prey availability is certainly an ecological basis for habitat selection, but is difficult and expensive to measure, particularly of the diverse infauna prey of many flatfish. In this study, we examined prey availability to the three flatfish in different SEBS habitats as defined mainly by sediment properties. We examined differences in the infauna assemblages among habitats and the spatial correspondence between flatfish diets and infauna assemblages. The ultimate goal is to identify potential quantitative indices of prey availability for flatfish habitat characterization and quality assessment.