Seasonal ice cover creates a pool of cold bottom water on the eastern Bering Sea continental shelf each winter. The southern edge of this cold pool, which defines the ecotone between arctic and subarctic communities, has retreated approximately 230 km northward since the early 1980s. Bottom trawl surveys of fish and invertebrates in the southeastern Bering Sea (1982-2006) show a coincident reorganization in community composition by latitude. Survey catches show community-wide northward distribution shifts, and the area formerly covered by the cold pool has seen increases in total biomass, species richness, and average trophic level as subarctic fauna have colonized newly favorable habitats. Warming climate has immediate management implications, as 57% of variability in commercial snow crab (Chionoecetes opilio) catch is explained by winter sea ice extent. Several measures of community distribution and structure show linear relationships with bottom temperature, suggesting warming climate as the primary cause of changing biogeography. However, residual variability in distribution not explained by climate shows a strong temporal trend, suggesting that internal community dynamics also contribute to changing biogeography. Variability among taxa in their response to temperature was not explained by commercial status or life history traits, suggesting that species-specific responses to future warming will be difficult to predict.

/pdf/sea-ice-retreat-alters-the-biogeography-of-the-bering-sea-2vvfgqkzl3.pdf

Sea ice retreat alters the biogeography of the bering sea continental shelf

▪ Abstract The total world catch from marine and freshwater wild stocks has peaked and may be slightly declining. There appear to be few significant resources to be developed, and the majority of the world's fish stocks are intensively exploited. Many marine ecosystems have been profoundly changed by fishing and other human activities. Although most of the world's major fisheries continue to produce substantial sustainable yield, a number have been severely overfished, and many more stocks appear to be heading toward depletion. The world's fisheries continue to be heavily subsidized, which encourages overfishing and provides society with a small fraction of the potential economic benefits. In most of the world's fisheries there is a “race for fish” in which boats compete to catch the fish before a quota is achieved or the fish are caught by someone else. The race for fish leads to economic inefficiency, poor quality product, and pressure to extract every fish for short-term gain. A number of countries hav...

State of the World’s Fisheries

This initiative was supported by grants to AFS and the Virginia Institute of Marine Science by the Pew Charitable Trusts, NMFS Office of Protected Resources, National Fish and Wildlife Foundation, The Munson Foundation and the Homeland Foundation, J.A. Musick, Principal investigator.

/pdf/marine-estuarine-and-diadromous-fish-stocks-at-risk-of-d4xmux80zz.pdf

Marine, Estuarine, and Diadromous Fish Stocks at Risk of Extinction in North America (Exclusive of Pacific Salmonids)

The southeastern, middle shelf of the Bering Sea has exhibited extreme variability in sea ice extent, temperature, and the distribution and abundance of species at multiple trophic levels over the past four decades. From 1972–2000, there was high interannual variability of areal extent of sea ice during spring (March–April). In 2000, this shifted to a 5-year (2001–2005) period of low ice extent during spring, which transitioned to a 4-year (2007–2010) period of extensive sea ice. High (low) areal extent of sea ice in spring was associated with cold (warm) water column temperatures for the following 6–7 months. The ocean currents also differed between warm and cold years. During cold years, the monthly-mean currents over the shelf were largely westward, while in warm years the direction of currents was more variable, with northward flow during December–February and relatively weak flow during the remainder of the year. The types and abundance of zooplankton differed sharply between warm and cold years. This was especially true during the prolonged warm period (2001–2005) and cold period (2007–2010), and was less evident during the years of high interannual variability. During the warm period, there was a lack of large copepods and euphausiids over the shelf; however, their populations rebounded during cold period. Small crustacean zooplankton taxa did not appear to vary between and warm and cold years. For both walleye pollock and Pacific cod, year-class strength (recruitment) was low during the prolonged warm period, but improved during the following cold period. Year-class strength did not appear to vary as a function of warm and cold years during the period of high year-to-year variability. Also, recruitment of arrowtooth flounder (a predator of pollock and cod) did not appear influenced by the warm or cold years. Finally, the distribution and relative abundance of fin whales appeared to differ in warm and cold years, with fewer whales on the southeastern, middle shelf during warm years.

Comparison of warm and cold years on the southeastern Bering Sea shelf and some implications for the ecosystem

Microsatellites have proved to be useful for the detection of weak population structure in marine fishes and other species characterized by large populations and high gene flow. None the less, uncertainty remains about the net effects of the particular mutational properties of these markers, and the wide range of locus polymorphism they exhibit, on estimates of differentiation. We examined the effect of varying microsatellite polymorphism on the magnitude of observed differentiation in a population survey of walleye pollock, Theragra chalcogramma . Genetic differentiation at 14 microsatellite loci among six putative populations from across the North Pacific Ocean and Bering Sea was weak but significant on large geographical scales and conformed to an isolation-by-distance pattern. A negative relationship was found between locus variability and the magnitude of estimated population subdivision. Estimates of F ST declined with locus polymorphism, resulting in diminished power to discriminate among samples, and we attribute this loss to the effects of size homoplasy. This empirical result suggests that mutation rates of some microsatellite loci are sufficiently high to limit resolution of weak genetic structure typical of many marine fishes.

/pdf/inverse-relationship-between-f-and-microsatellite-5ctyr558xf.pdf

Inverse relationship between F and microsatellite polymorphism in the marine fish, walleye pollock (Theragra chalcogramma): implications for resolving weak population structure.

Aspects of the feeding migration of walleye pollock (Theragra chalcogramma) in the eastern Bering Sea (EBS) were investigated by examining the relationship between
temperatures and densities of fish encountered during acoustic and bottom trawl surveys conducted in spring and summer between 1982 and 2001. Bottom temperature was used as an indicator of spring and summer warming of the EBS. Clusters of survey stations were identified where the density of walleye pollock generally increased or decreased with increasing water temperature. Inferences about the direction and magnitude of the spring and summer feeding
migration were made for five length categories of walleye pollock. Generally, feeding migrations appeared to be northward and shoreward, and the magnitude of this migration appeared to increase with walleye pollock size
up to 50 cm. Pollock larger then 50 cm showed limited migratory behavior. Pollock may benefit from northward
feeding migrations because of the changes in temperature, zooplankton production, and light conditions. Ongoing climate changes may affect pollock distribution and create new challenges for pollock management in the EBS.

Variation in the distribution of walleye pollock (Theragra chalcogramma) with temperature and implications for seasonal migration

Changes in the input data The 2001 NMFS bottom-trawl survey estimates of population numbers-at-age were available for analysis in this assessment. In terms of biomass, the bottom-trawl survey estimate for 2001 is 4.14 million tons, down by 19.5% from the 2000 estimate of 5.14 million tons. This level of decline was expected given the estimated age-structure of the population relative to availability patterns of pollock to the bottom-trawl survey. For the echo-integration trawl (EIT) surveys we used an updated estimate of abundance-at-age from the 2000 summer survey and also used revised sample-size values for the time series of EIT abundance-at-age data that reflects the number of trawl-hauls.

Eastern Bering Sea Walleye Pollock Stock Assessment

The Resource Assessment and Conservation Engineering Division of the Alaska Fisheries Science Center conducts annual bottom trawl surveys to monitor the condition of the demersal fish and crab stocks of the eastern Bering Sea continental shelf. The standard study area, surveyed each year since 1979, encompasses a major portion of the eastern Bering Sea shelf between the 20-m and the 200-m isobaths and from the Alaska Peninsula north to approximately the latitude of St. Matthew Island (60E50NN). In 2004, two chartered trawlers, the 40-m F/V Arcturus and the 40-m F/V Aldebaran, again surveyed this area. Demersal populations were sampled by trawling for 30 minutes at stations centered in a 20 × 20 nautical mile grid covering the survey area. At each station, species composition of the catch was determined. Length distributions and age structure samples were collected from ecologically and commercially important species. 395 stations were sampled. Fish biomass was dominated by walleye Pollock and Pacific cod (4.4 metric tons). A total of 229 invertebrate species were identified in the survey. Survey results presented in this report include relative fishing powers of the survey vessels, abundance estimates for fish and invertebrates, geographic distributions of important fish species and size composition of principal fish species. Surface and bottom temperatures recorded at each sampling station are also presented. Appendices provide station data, species listings, and detailed results of analyses of abundance and biological data of the sampled populations.

2004 bottom trawl survey of the eastern Bering Sea continental shelf

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Variation in the distribution of walleye pollock (Theragra chalcogramma) with temperature and implications for seasonal migration

Eastern Bering Sea Walleye Pollock Stock Assessment

2004 bottom trawl survey of the eastern Bering Sea continental shelf