Polar Biol DOI 10.1007/s00300-012-1241-0 ORIGINAL PAPER Pelagic fish and zooplankton species assemblages in relation to water mass characteristics in the northern Bering and southeast Chukchi seas Lisa Eisner • Nicola Hillgruber • Ellen Martinson • Jacek Maselko Received: 18 June 2012 / Revised: 22 August 2012 / Accepted: 23 August 2012 Ó Springer-Verlag 2012 Abstract This research explores the distributions and were dominated by bivalve larvae and copepods (Centro- community composition of pelagic species in the sub-Arctic pages abdominalis, Oithona similis, Pseudocalanus sp.). and Arctic waters of the northern Bering and central and Pelagic community composition was related to environ- southern Chukchi seas during September 2007 by linking mental factors, with highest correlations between bottom pelagic zooplankton and fish assemblages to water masses. salinity and large zooplankton taxa, and latitude and fish Juvenile saffron cod (Eleginus gracilis), polar cod (Bore- species. These data were collected in a year with strong ogadus saida), and shorthorn sculpin (Myoxocephalus northward retreat of summer sea ice and therefore provide a scorpius) were most abundant in warm, low salinity Alaska baseline for assessing the effects of future climate warming Coastal Water (ACW) of the central Chukchi Sea, charac- on pelagic ecosystems in sub-Arctic and Arctic regions. terized by low chlorophyll, low nutrients, and small zoo- plankton taxa. Adult Pacific herring (Clupea pallasii) were Keywords Arctic Á Bering Sea Á Chukchi Sea Á more abundant in the less stratified Bering Strait waters and Community composition Á Water mass characteristics Á in the colder, saltier Bering Shelf Water of the northern Zooplankton distribution Á Polar cod Á Pelagic fish Bering and southern Chukchi seas, characterized by high chlorophyll, high nutrients, and larger zooplankton taxa. Juvenile pink (Oncorhynchus gorbuscha) and chum Introduction (O. keta) salmon were most abundant in the less stratified ACW in the central Chukchi Sea and Bering Strait. Abun- In recent years, sub-Arctic and Arctic marine ecosystems dances of large zooplankton were dominated by copepods have been experiencing the effects of substantial climate (Eucalanus bungii, Calanus glacialis/marshallae, Metridia change. Under increasing greenhouse gas scenarios, the pacifica) followed by euphausiids (juvenile Thysanoessa Arctic is predicted to be ice-free in summer by 2050 and raschii and unidentified taxa), whereas small zooplankton sea surface temperatures (SSTs) to increase by as much as 10 °C from 2000 to 2100 (Arzel et al. 2006; Lin et al. 2006; Stroeve et al. 2008; Wang and Overland 2009). Potential & L. Eisner ( ) Á E. Martinson Á J. Maselko increases in resource extraction and ship travel make it Auke Bay Laboratories, Alaska Fisheries Science Center, National Marine Fisheries Service (NOAA), Ted Stevens critical to collect baseline data on fisheries resources and Marine Research Institute, 17109 Pt. Lena Loop Road, their relationships between ocean and ecosystem compo- Juneau, AK 99801, USA nents in these high-latitude regions (Grebmeier et al. 2010). e-mail: [email protected] A better understanding of the water mass characteristics N. Hillgruber responsible for the distribution and abundance of pelagic School of Fisheries and Ocean Sciences, University of Alaska fish is an essential first step in interpreting the effects of Fairbanks, 17101 Pt. Lena Loop Road, Juneau, AK 99801, USA climate change on the pelagic fish communities in this region. N. Hillgruber Institute of Fisheries Ecology, Thu¨nen Institut, While the distribution and abundance of demersal fish Wulfsdorfer Weg 204, 22926 Ahrensburg, Germany assemblages in the northern (N.) Bering and southeastern 123 Polar Biol Chukchi seas have been explored in several disjointed exists between AW and BSW, which promotes the forma- studies (Lowry and Frost 1981; Barber et al. 1997; Cui tion of a combined water mass, the Bering Shelf Anadyr et al. 2009; Norcross et al. 2010), a similar exploration of Water (BSAW) characterized by high primary and sec- the pelagic fish fauna to date is missing. For example, little ondary production (McRoy et al. 1972; Alton 1974; Stoker is known about the ecology and habitat preferences of polar 1978, 1981; Grebmeier 1987; Grebmeier et al. 1988; cod (Boreogadus saida) in the N. Bering and southern (S.) Springer 1988; Walsh et al. 1989). While high primary and central (C.) Chukchi seas (Quast 1974; Lowry and production is fueled by a high and continuous supply of Frost 1981; Barber et al. 1997; Cui et al. 2009; Norcross nitrogen from AW (Grebmeier et al. 1988), high secondary et al. 2010). Another gadoid species that has attracted even production is largely due to the transport of oceanic zoo- less scientific attention is the saffron cod (Eleginus graci- plankton northward into the Chukchi Sea (Springer et al. lis). While in recent years, this species appears to have 1989; Weingartner 1997). undergone a remarkable range extension into the northern Zooplankton distributions are generally associated with Gulf of Alaska (Johnson et al. 2009), knowledge about the water masses (e.g., Hopcroft et al. 2010). However, the biology and ecology and even taxonomy of this species is distribution of pelagic early life stages of fishes may be limited or outdated (Wolotira 1985; Johnson et al. 2009). more closely tied to bathymetry (e.g., Duffy-Anderson Even fewer studies specifically targeted the pelagic larval et al. 2006), topography and current patterns (Doyle et al. and juvenile stages of these ecologically important Arctic 2002), and to water mass characteristics (e.g., Norcross taxa (Quast 1974; Welch et al. 1992, 1993; Norcross et al. et al. 2010, Siddon et al. 2011). The strong contrasting 2010, Parker-Stetter et al. 2011). A better understanding of physical and biological characteristics of water masses in these small pelagic fishes, however, is important because of the northeastern Bering Sea and Chukchi Sea are expected their trophic link to many piscivorous predators, such as to define distinctly differing habitat characteristics for seabirds (Springer et al. 1984; Piatt et al. 1989; Gaston pelagic zooplankton and fish taxa in these regions. There- et al. 2003) and marine mammals (Seaman et al. 1982). fore, this study is directed at identifying the main water Juvenile stages of Pacific salmon (Oncorhynchus spp.) and masses and characterizing patterns in pelagic zooplankton whitefish (Coregonus spp.) are also parts of these pelagic and fish distributions in response to environmental vari- fish assemblages in Arctic and sub-Arctic waters and are ables associated with these water masses. Specifically, we important subsistence resources of local communities examined nutrient concentrations, biomass, size fraction- (Jarvela and Thorsteinson 1999). ation and production of phytoplankton, light attenuation, In the N. Bering and the C. Chukchi and S. Chukchi seas, large and small zooplankton abundance, and pelagic fish several water masses can be discerned, which are likely to abundance. The main objectives of this study are to (1) impact the distribution of pelagic zooplankton and fish, characterize the physical and biological properties of the namely Alaska Coastal Water (ACW), Bering Shelf Water major water masses in the study area; (2) identify single (BSW), and Anadyr Water (AW). These water masses have species and community composition of zooplankton and a north–south orientation (Coachman et al. 1975), with fish in relation to water masses and geographic location; ACW on the east, BSW in the middle, and AW on the west. and (3) examine whether the distributional patterns of The ACW originates along the coast over the inner shelf in zooplankton and pelagic fish communities are related to the eastern Bering Sea; it develops annually from the input environmental parameters. The data used in this study are of river water and melting ice from western Alaskan rivers the result of a northern extension of an ongoing Bering and its temperature increases rapidly through spring and Aleutian Salmon International Survey (BASIS) program summer from about 0 to 10 °C (Springer et al. 1984). The initiated by the North Pacific Anadromous Fisheries BSW originates on the middle Bering Shelf, south of St. Commission to study the effect of climate change and Lawrence Island. The AW originates from the Gulf of variability on Bering Sea pelagic ecosystems (Farley Anadyr at depth along the continental slope of the Bering 2009). Sea (Springer et al. 1989). The ACW is less saline (\*31.8–32.2), warmer (2–13 °C), and has lower con- centrations of nutrients and chlorophyll a than BSW and Methods AW (Coachman and Shigaev 1992; Weingartner 1997). In contrast, BSW and AW are cooler (0–10 °C), more saline Sample collection, laboratory analysis, and processing (BSW:*31.8–33; AW:*32.3–33.3), and have substan- tially higher chlorophyll a and nutrient concentrations A fisheries oceanography survey was conducted aboard the (Sambrotto et al. 1984; Walsh et al. 1989; Coachman and NOAA ship R/V Oscar Dyson in the S. and C. Chukchi Sea Shigaev 1992; Weingartner 1997). While strong frontal and N. Bering Sea, September 4–17 2007 (Fig. 1). Station gradients separate ACW and BSW, only a gradual interface spacing was 38–55 km from latitude 70°N–64°N and 123 Polar Biol Fig. 1 Stations sampled by the R/V Oscar Dyson, 4–17 September 2007. Dashed line is the international date line. Bathymetry contours are every 50 m. Latitudinal regions are circled longitude 164°W–172°W, on the US side of the date line. (Gordon et al. 1994). Chlorophyll a samples were stored at Bottom depths ranged from 25 to 60 m. The surface mixed -70 °C and analyzed with a Turner Designs (TD-700) layer depth ranged from 10 to 30 m. laboratory fluorometer (Parsons et al. 1984). In situ fluo- Oceanographic data were obtained from conductivity– rometric data were calibrated with discrete chlorophyll temperature–depth (CTD) vertical profiles from the surface a samples to estimate chlorophyll a concentrations to 5–10 m above the bottom using a SBE (Seabird Elec- (r2 = 0.78).