For the purposes of this review, fish in the Arctic can be divided into four general groups, the pelagic shallow and midwater species, anadromous species, and demersal fishes.  The bottom fish include nearshore, shelf, and deep water species.  Pelagic fish are the least diverse group representing 13% of the 242 recorded Arctic species (Bluhm et al. 2011; Mecklenberg et al. 2011).   There are 31 species considered to be anadromous.  The remaining species are marine demersal fish.

Throughout the Arctic, gadoids (cod) represent a critical link between the zooplankton community and higher trophic levels (e.g. seals, White whales).  Arctic cod (Boreogadus saida) and Polar cod (Arctogadus glacialis) are truly pan-arctic species occurring in all marine waters of the Arctic.  Cod are widely distributed throughout the Arctic, occupying nearshore, pelagic, and sea-ice habitats, residing both at depth and near the surface waters, depending upon age and season.  Both B. saida and A. glacialis can be found in small numbers or in large, densely packed schools.  In the scientific literature there is confusion on the common names “Arctic cod” and “Polar cod” at times referring to either B. saida or A. glacialis.  While these species are similar in appearance and life history, gene sequencing has shown that they are genetically distinct (Breines et al. 2008; Madsen et al. 2009). 

Both B. saida and A. glacialis have a diet dominated by pelagic or sympagic components (Sufke et al. 1998; Lonne and Gulleksen 1989; Bradstreet and Cross 1982).  Stomach contents of B. saida cod sampled in the pelagic ranges of the Beaufort Sea, northern Canadian waters, and the Barents Sea were dominated by calanoid copepods and gammarid amphipods.  Other prey items included hyperiid amphipods, mysids, and shrimps (Frost and Lowry 1984; Lonne and Gulliksen 1989).  A similar pelagic diet was observed for A. glacialis in the Northeast Water Polynya near Greenland (Sufke et al. 1998).  In the nearshore zone, the diet is dominated by copepods, gammarid amphipods, and young-of-the-year Arctic cod. 

Both B. saida and A. glacialis spawn under the ice in the winter months; however recent observations indicate that there are regional differences in the hatching season of B. saida (Figure 2-7).  Bouchard and Fortier (2011) noted that cod hatching started as early as January and extended to July in areas with significant freshwater input (Laptev/East Siberian Seas, Hudson Bay, and Beaufort Sea).  In contrast, hatching was restricted to April-July in regions with little freshwater input (Canadian Archipelago, North Baffin Bay, and Northeast Water).  The authors found that the different hatching periods resulted with different length and weight classes co-occurring throughout the Arctic.  Cod larvae generally occupy a depth of 10 – 30 m, settling to the bottom in September (Craig 1984; Graham and Hop 1995). 

As noted above, the distribution of B. saida and A. glacialis includes nearly all marine waters of the Arctic depending upon the age class and the season.  Arctic cod have been found to be a dominant component of both the pelagic and demersal communities at all depths; cod have been collected from the mixed water mass (<200 m), at the sharp halocline (200-300 m), and in the deeper Atlantic water mass (>300 to 1000 m; Majewski and Riest 2013; Norcross et al. 2012). One and two year old fish also occupy fissures and gaps in the ice pack, feeding on the sympagic fauna.  In the summer months, adult cod are dispersed in habitats ranging from coastal brackish waters to the demersal and pelagic zones of the shallow shelves, including the ice-water interface (Craig et al. 1982; Lonne and Gulliksen 1989; Gradinger and Bluhm 2004).  In autumn, as nearshore salinities increase, large shoals of cod are observed in shallow (<10 m) waters (Hop et al. 1997; Welch et al. 1993), presumably following shoreward fronts of plankton.

Acoustic surveys conducted as part of the BREA program as well as others conducted in the US Beaufort have found large shoals of Arctic cod in the water column and near the bottom along the entire shelf of the Beaufort Sea (Geoffrey et al. 2013; Parker-Stettner 2011).  There was a clear segregation between the young-of-the-year (YOY) and age 1+ fish in the summer months.  Age 1+ fish were found to aggregate in the deeper waters of the shelf  and slope at depths ranging from 200 to >1000 m while large shoals of YOY Arctic cod were found nearer to the surface (20 to 100 m) in nearshore waters extending into waters over the outer shelf and slope (Figure 2-8). The near bottom aggregations of Arctic cod at depths of 200 to 400 m appear to span the Canadian Beaufort shelf into the fall and winter months (Geoffrey et al. 2013; Benoit et al. 2008). Acoustic surveys during the winter months have found massive aggregations of adult cod at depths of 140 to 230 m under the pack ice in the Canadian Beaufort waters (Benoit et al. 2008) and at depths of 300 to 1,300 m near the North Pole (Geoffrey 2013). In winter months, adult cod appear to remain in schools at the deep inverse thermocline (160-230 m, -1 to 0°C) throughout the Polar night to avoid seal predation; whereas smaller cod (<25 g) periodically migrate into the isothermal cold intermediate layer (90-150 m) to feed on Calanoid copepods and then return to the deeper layer (Benoit et al. 2010).

An exception to the Arctic cod dominated pelagic food web is in the Barents Sea and White Sea.  The Barents Sea is located between the Arctic and boreal oceanic systems and is influenced by the variations in the Atlantic current and the Polar front.  In the southern portions of the Barents Sea, where the Atlantic water is more predominant, capelin (Mallotus villosus) is the primary link between pelagic crustaceans (e.g. copepods and amphipods) and higher trophic levels (Blanchard et al. 2002; Hamre 1994; Mehl and Yaragina 1992; Titov et al. 2006).  The importance of capelin to the Barents Sea ecosystem was demonstrated when capelin stocks decreased markedly in the 1980s, resulting in decreases in stocks of the commercially important Atlantic cod (Gadus morhua) and ringed seals.  Subsequent studies have demonstrated linkages between the location of the polar front, the population of capelin and subsequent changes in the population of Atlantic cod (Titov et al. 2006).  Capelin are also an important forage fish in the northern boreal waters of Greenland, the Sea of Okhotsk and the Bering Strait.  Unlike Arctic cod, Atlantic cod are more demersal in nature, generally feeding near the bottom.  The diet of Atlantic cod is remarkably varied, with Mehl and Yaragina (1992) reporting with over 180 prey species.  While capable of feeding on a variety of invertebrate and vertebrate prey, capelins appear to be the primary energy source.

Figure 2-8. Arctic cod (Boreogadus saida) vertical distribution during the ice-free season along the Mackenzie Slope 2012 (from Geoffrey et al. 2013).

Pacific herring (Clupea harengus pallasi) are another nearshore forage fish that represents an important link between zooplankton and higher level consumers, particularly anadromous fishes of the Bering-Chukchi-Laptev and Lofoten-Barents Sea systems (Mehl and Yaragina 1992; Hamre 1994).  Herring feed primarily in the water column on copepods, euphausiids, and mysids.  Herring spawn en masse and their eggs and larvae are a vital food source for nearshore migrating anadromous fish in the Arctic such as salmonids, cisco, and char. In the Arctic they are generally found in the nearshore waters and avoid the colder, Arctic water (Craig 1984).  While numerically less important than the Arctic cod, capelin and herring are important forage species for upper trophic level consumers.

Anadromous fish include those species with a life history that includes both freshwater and marine habitats.  In the Arctic, utilization of marine waters is typically limited to the brackish waters found in nearshore corridor immediately following breakup.  Anadromous fishes in the Arctic include Arctic char (Salvelinus alpinus), least and Arctic cisco (Coregonus sardinella and C. autumnalis), broad and humpback whitefish (C. pidschian and C. nasus), Inconnu (Stenodus leucichthys), and several species of salmonids (Craig 1984; Mecklenburg et al. 2011).  These species spawn in fresh water and typically do not enter coastal waters until months, or often years, after hatch.  Thus, the most sensitive life stages for anadromous fish are spent in the Arctic rivers and lakes.  Use of marine waters is limited to feeding and migration.  Coregonids are the most common anadromous fish in the Laptev-Kara-East Siberian waters, with Arctic cisco more commonly found in the marine waters than Least cisco (Sherman and Hempel 2008; Craig 1984).  Arctic char range widely from their stream of origin and might be found in more open water during high flow years (Jarvela and Thorsteinson 1999; Johnson 1980).  When occupying the nearshore brackish water, anadromous fish feed nearly exclusively on epibenthic fauna (e.g. polychaetes, mysids, and amphipods; Dunton et al. 2012).  In turn, anadromous fish become an important food source for seals as well as subsistence fishers.

A number of salmonid species are found in river systems throughout the Arctic; however, their use of marine waters is limited.  Atlantic salmon are the most common form found in the Lapotov-Barents Sea and Hudson Bay river systems (Sherman and Hempel 2008).  Pink and chum salmon (O. gorbuscha) are the most abundant species of Pacific salmon documented in the Arctic, with populations in Russian (Yana and Lena Rivers), Canadian (Mackenzie River), Alaskan, and Norwegian waters (Sherman and Hempel 2008; Mecklenburg et al. 2002).  With the exception of some juvenile use of the brackish nearshore waters, adults are the most common life stage found in marine waters.

Demersal fishes are those that are found in close association with the bottom.  Sculpins (Cottidae) and eelpouts (Zoarcidae) are the most speciose fish taxa in the Arctic, comprising over 50% of the species in polar waters (Mecklenberg et al. 2011).  Both taxa are well adapted to living in the dominant substrates found in the Arctic (sand, silt, and mud), as well as rocky bottoms, with most species spending the majority of their life cycle in close association with the bottoms.  Adults often deposit eggs directly on benthic substrates or on bottom oriented vegetation.  Larval and early juvenile forms may remain on the bottom near the adults or move into the water column or into vegetation before descending to the bottom as young fish.  Common and pan-Arctic sculpin species include the Arctic Staghorn sculpin (Gymnocanthus tricuspis), the large Short-horned sculpin (Myoxocephalus scorpius: 90 cm length), and Spatulate sculpin (Icelus spatula; Mecklenberg et al. 2011).  Two genera account for most Arctic eelpouts, Gymnelus and Lycodes.  Sculpin and eelpout diets vary, but many sculpin and eelpouts feed on benthic infauna and epifauna, including polychaetes, benthic amphipods, small mollusks, and epibenthic crustaceans, with larger species feeding on fish, including cods, flounders, and smelts (Dunton et al. 2012).  Anti-freeze proteins have been found in several species of Myoxocephalus sculpins, showing their adaptation to Arctic waters (Fletcher et al. 1982).

Flatfishes or flounders live on the bottom, usually in shallow marine waters, and burrow into the surface sediment to rest and wait for prey. They eat worms, mollusks, echinoderms, crustaceans, other benthic invertebrates, and fishes. Arctic flounder (Pleuronectes glacialis) are a pan-Arctic species that prefer coastal and nearshore waters (Mecklenberg et al. 2011).

The deep-sea fishes are perhaps the poorest known group in the Arctic.  Recently there have been several targeted efforts to sample the deep basins in the Arctic (Reist and Majewski 2013; Dolgov et al. 2009; Jorgensen et al. 2005). In the deep Canadian Beaufort, Arctic cod were the dominant species in the water column and associated with the demersal community (Reist and Majewski 2013). Other species found in midwater trawls included species found in other deep ocean basins, including Myctophids (lanternfish) and Gonostomids (bristlemouths).  The myctophids, Benthosoma glaciale and Protomyctophium arcticum, spawn above the Polar front (Dolgov et al. 2009) and were found in deep water trawls (Riest and Majewski 2013).  Other species have been recently collected from the Kara Sea, includingMyctophum punctulum (Dolgov et al. 2009; Weinrroither et al. 2010).  The wide-spread gonostomid, Cyclothone microdon, has been observed in trawls in Baffin Bay (Riest and Majewski 2013; Mecklenburg et al. 2011).  Based on observations of myctophids and bristlemouths throughout the world, their diet is dominated by pelagic crustacean, migrating to the surface from depth to feed.  It is not known if the diel vertical migrations occur in the Arctic.

Deepwater demersal fish taxa are generally similar to those found in other oceanic basins and include Zoarcids (eelpouts) and Macrourids (grenadiers).   The globally distributed macrourids, Coryphaenoides rupestris and Macrourus berglax have been found in the Baffin Bay (Jorgensen et al. 2005).  The Glacial eelpout (Lycodes glacialis) is one of the most dominant demersal fishes in the Arctic basins.  This large eelpout (~70 cm) moves along the bottom stirring up the bottom sediments to feed on small benthic crustaceans and mollusks; L. glacialis also feeds on other fishes and cephalopods (Mecklenberg and Mecklenburg 2011).  This is a truly deep water species, spending its entire life cycle at depths >1,000m.  The Greenland halibut (Reinhardtius hippoglossoides) is a right-eyed flounder typically associated with deep waters of the Arctic (200 – 1600 m), as well as deep waters of the Atlantic and Pacific Oceans.  The Greenland halibut is epibenthic, and feeds on epibenthic crustaceans, demersal fish, and other invertebrates.  Deepwater rays found in the southern Arctic waters in Alaska and Baffin Bay include Rajella spp. and Amblyraja radiata (Dolgov et al. 2009; Mecklenburg et al. 2002).