Polar cod (Boreogadus saida) were the most abundant fish caught during this survey, both numerically and by weight. Polar cod are known to be a major component of the Beaufort Sea fish community and important prey for higher trophic levels such as seabirds (Hobson 1993) and marine mammals (Bradstreet and Cross 1982; Bradstreet et al. 1986; Welch et al. 1992). They are also the dominant consumer of zooplankton (Atkinson and Percy 1992) and are thus an important conduit for secondary production (Welch et al. 1992). One of the earliest documented records of polar cod in the Alaskan Beaufort Sea is from 1951 (unpublished data, University of British Columbia, N. J. Wilimovsky, H. A. Fehlmann). Previous studies in nearshore, often brackish waters, have also documented the distribution of polar cod (Craig et al. 1982; Craig 1984; Moulton and Tarbox 1987; Jarvela and Thorsteinson 1999), but this study is the first to quantify the abundance of polar cod in offshore marine waters of the Beaufort Sea.
The polar cod caught in this survey were primarily sub-adults, ages 1 and 2, although some age-3 and -4 fish were found. Large polar cod were distributed primarily in the deeper depths (100–500 m) while small cod were found primarily in the shallower depths (40–100 m). This difference in size by depth is likely not driven by net type, because the larger cod in the deeper depths were caught with the lined net, and the smaller cod in shallower depths were caught with the unlined net. Frost and Lowry (1983) documented a similar distribution pattern from their 1977 survey. They report that polar cod were larger in water deeper than 100 m, whereas cod in water less than 100 m were on average, smaller. Similarly, in the northeast Chukchi Sea, polar cod greater than age 3 found offshore were significantly larger than the same age fish found inshore (Gillispie et al. 1997).
We documented or confirmed extensions to the known ranges of four species of fishes: walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus), festive snailfish (Liparis marmoratus), and eyeshade sculpin (Nautichthys pribilovius). The Chukchi Sea survey in 1990 (Barber et al. 1997) reported Pacific cod at three stations located between 68°N and 69°N. Festive snailfish are a relatively rare species; only one specimen has been recorded in the northeast Bering Sea near St. Lawrence Island at 63°00′N, 169°20′W (Busby and Chernova 2001). Previous to this record, the species had only been documented in the Sea of Okhotsk. The northernmost record of the eyeshade sculpin previous to our survey was in the northern Chukchi Sea, west of Point Barrow (Barber et al. 1997). In addition to the range extensions of pollock and Pacific cod, Bering flounder (Hippoglossoides robustus) were caught in the present 2008 survey, but not in the 1977 survey (Frost and Lowry 1983). All three of these species are abundant in the Bering Sea and are commercially valuable.
We caught walleye pollock as far north as 71°59′N (154°25′W). The domestic groundfish fishery off Alaska is the largest US fishery by volume and walleye pollock make up the dominant portion of that catch (Hiatt et al. 2008). Pollock were recorded as far north as 71°23′N during a 2004 survey of the Chukchi Sea (Mecklenburg et al. 2007), and a specimen was collected at 69°26′N, during a 1990 survey of the Chukchi Sea on the Ocean Hope III (unpublished data; NMFS-AFSC-Resource Assessment and Conservation Engineering (RACE) Division, cruise 90–2). Two specimens were collected in the Beaufort Sea near the mouth of Elson Lagoon, east of Point Barrow at approximately 71°31′N, 156°32′W in 1951 and 1954 (unpublished data, University of British Columbia: N. J. Wilimovsky, J. E. Bohlke; D. Wohlschlag, and W. C. Freihofer). However, the specimens are missing and identification as walleye pollock is uncertain (K. Mecklenburg, pers. comm.). We found pollock in moderate densities throughout the survey area, so if the Elson Lagoon samples collected in 1954 were correctly identified as pollock, our results confirm the range extension and document that the species may be widespread in the Beaufort Sea.
Analysis of pollock otoliths showed that most of the fish caught in the present 2008 survey were sub-adults (age-2). In 1990, an ichthyoplankton survey in the Chukchi Sea (Echeverria 1995) found juvenile walleye pollock northwest of Point Barrow. During the Russian-American Long-Term Census of the Arctic (RUSALCA) survey of the Chukchi Sea in 2004, Mecklenburg et al. (2007) recorded pollock ranging from 102 to 168 mm total length, indicating that these fish were likely sub-adults. So, although pollock are occurring in Arctic seas, fish of spawning age or size have not yet been documented, and the origins of the juvenile fish are not known. The fact that the pollock we caught in the Beaufort Sea were smaller at age than pollock in the Bering Sea may provide evidence that the fish were spawned in cold Arctic waters or were transported into such waters shortly after spawning. The size difference is manifested first at age-2; age-1 pollock from the Bering and Beaufort seas were similar in size. This lends support to the latter hypothesis that fish were spawned in north Pacific waters and transported into the Arctic sometime during their first year of life. Despite the potential northward shift in the distribution of some species, the fish communities of the Beaufort and Chukchi seas are still distinct from the Bering Sea. Polar cod are a dominant component of the Beaufort and Chukchi Sea fish communities, whereas pollock, Pacific cod, and flatfish dominate the Bering Sea. Although we document the presence of pollock and commercial-sized snow crab in the Beaufort Sea, their densities are far lower than in the Bering Sea and Chukchi seas.
Invertebrates dominated the demersal trawl catches both in terms of abundance and species diversity. The notched brittle star (Ophiura sarsi) dominated all of the trawls in the present 2008 survey. The 1977 survey reported that notched brittle stars were also the most abundant invertebrate captured and dominated the catch west of longitude 154°W (Frost and Lowry 1983). Several studies have documented the prevalence of notched brittle stars in the North Pacific and Arctic ecosystems. Dense carpets of notched brittle star were reported off the coast of Japan (Fujita and Ohta 1989) and were also reported as one of the most dominant epibenthic invertebrates in many parts of the Chukchi Sea (Grebmeier et al. 2006a, b). Both the notched brittle star and snow crab (Chionoecetes opilio) were the most abundant epibenthic invertebrates encountered in the Chuckchi sea during the RUSLCA cruises in 2004, 2007, and 2008 (Bluhm et al. 2009). The size and depth distribution of snow crab during the 2008 survey were unexpected based on previous studies in the Bering and Chukchi Seas. In 1990 and 1991, 48 stations were sampled in the northeast Chukchi Sea to examine the distribution and abundance of snow crab (Paul et al. 1997). Snow crabs were found at all stations, with the highest abundance and mean crab weight occurring in the stations directly west of Point Barrow (Paul et al. 1997). However, carapace width of the male crabs ranged from 20 to 74 mm (Paul et al. 1997), compared with the measured snow crab in the present Beaufort survey that ranged from 55 to 119 mm. In the 1977 Beaufort Sea survey, the maximum carapace width for a male snow crab was 75 mm (Frost and Lowry 1983).
Recently, snow crab has also been observed in the northeast Atlantic’s Barents Sea (Alvsvåg et al. 2008). Evidence of juveniles below 50 mm carapace width confirms that the population is established and reproductive with adult crabs ranging in size from 50 to 136 mm (Alvsvåg et al. 2008). The presence of female crabs with eggs during the 2008 Beaufort Sea survey is further evidence that this population is reproductive. Our survey found the highest CPUE and the largest crabs by carapace width and weight in water depths greater than 300 m and temperatures around 0.6°C. This result was also unexpected, as surveys in the Bering and Chukchi Seas indicate that snow crabs are found predominantly in waters less than 200 m in depth. However, the main population of crab found in the Barents Sea survey was located in depth ranges from 80 to 350 m and in waters less than 2°C (Alvsvåg et al. 2008). Also, the fact that Frost and Lowry (1983) only caught small snow crab (less than 80 mm) may be due to the fact that only one tow was made in water deeper than 200 m. The legal minimum carapace width for the commercial snow crab fishery in the Bering Sea is 78 mm; therefore, the 2008 survey is the first to document snow crab of commercial size in the North American Arctic.
Ecosystems comparisons and historical observations
The comparisons of the Arctic ecosystems (Chukchi and Beaufort seas) to the North Pacific ecosystem (Bering Sea) show differences in both species presence/absence and overall abundance. In general, the mean CPUE for species caught in the Beaufort Sea and in the Chukchi Sea fell outside the confidence intervals of CPUE for the same species caught in the Bering Sea (Table 5). Polar cod was the most prevalent fish species in both the present 2008 Beaufort Sea survey and the 1990 Chukchi Sea survey (Table 5). In the Bering Sea, walleye pollock was the most abundant fish species, at 61.2 kg/ha compared with 0.13 kg/ha in the Beaufort Sea and 0.02 kg/ha in the Chukchi Sea (Table 5). In addition, the flatfish species that were dominant in the Bering Sea (arrowtooth flounder (Atheresthes stomias), Bering flounder (Hippoglossoides robustus), flathead sole (Hippoglossoides elassodon), Greenland halibut (Reinhardtius hippoglossoides), rock sole (Lepidopsetta polyxystra and Lepidopsetta bilineata), and yellowfin sole (Limanda aspera)) were absent or found in low densities in the Chukchi and Beaufort seas (Table 5). Saffron cod was more abundant in the Chukchi Sea than the Bering Sea, but was absent from the present 2008 Beaufort Sea survey (Table 5).
Of the 34 taxa captured and identified from the present 2008 Beaufort Sea survey, 17 of those had also been documented in the 1977 survey (Table 6). Although CPUE was not calculated during the 1977 survey, the number of fish caught was recorded at each station. Polar cod was the most numerous fish species found in both surveys. Bering flounder and walleye pollock were fairly abundant relative to other species in the 2008 survey but were not observed during the 1977 survey. Also, Pacific cod (Gadus macrocephalus), festive snailfish (Liparis marmoratus), eyeshade sculpin (Nautichthys pribilovius), and bigeye sculpin (Triglops nybelini) were caught during the 2008 survey (albeit in relatively small numbers), but were absent from the 1977 survey.
Eelpouts were common during both surveys, but different species were dominant; marbled eelpouts (Lycodes raridens) were the most abundant eelpout in the 2008 survey, whereas Canadian eelpouts (Lycodes polaris) and fish doctors (Gymnelus viridis) were most abundant in the 1977 survey. Snailfish were fairly common during both surveys. Variegated snailfish (Liparis gibbus) and gelatinous seasnail (Liparis fabricii) were the most abundant snailfish species in the 2008 survey, but snailfish were not identified to species in the 1977 survey. Sculpins were caught during both surveys, but they ranked higher in abundance during the 1977 survey. In addition, different species were caught: warty (Myoxocephalus verrucosus) and ribbed sculpin (Triglops pingeli) were most common during the 2008 survey while spatulate (Icelus spatula) and twohorn sculpin (Icelus bicornis) were the dominant species during the 1977 survey. The twohorn sculpin was the third most prevalent species in the 1977 survey and did not occur in the 2008 survey. These differences in the fish species composition between 1977 and 2008 are suggestive of changes in the marine fish community of the Beaufort Sea since the late 1970s. Nonetheless, without more extensive surveys, it is difficult to conclude that changes in species communities have occurred.
Assessment of the impacts of climate change, northerly expansion of marine species, future offshore oil and gas exploitation, and potential fisheries development will require monitoring of the distribution and abundance of marine offshore fishes. Net mensuration and gear standardization are recommended for future monitoring studies and would provide quantitative estimates to compare with the present 2008 survey, and with future fishery surveys. Standardized, comparable surveys can serve as an index of change without bias due to changes in gear type and survey methods.