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Groundfish Assessment Program

Annual Bottom Trawl Survey of the Eastern Bering Sea Continental Shelf

David Somerton with red king crab
RACE Division scientist David Somerton displays some of the red king crab specimens encountered aboard the charter vessel American Eagle.

The eastern Bering Sea (EBS) Crab and Groundfish Trawl Survey conducted by the National Marine Fisheries Service (NMFS) commenced on 3 June 2007, continuing an annual series of assessment surveys, which began in 1971. The chartered fishing vessels F/V Arcturus and F/V Aldebaran will be used to sample 376 stations (Fig. 1) covering about 0.5 million km2 of the Bering Sea shelf between the depths of 20 m and 200 m and from Bristol Bay to St. Lawrence Island. Data from the survey provides the North Pacific Fishery Management Council with up-to-date information on distribution, abundance, and population biology of key groundfish, crab, and other invertebrate species within the EBS continental shelf ecosystem.

  figure 1, see caption
Figure 1.  Established sampling grid used for the EBS crab and groundfish bottom trawl survey.

After 3 days of preparations, the vessels departed Dutch Harbor and headed for Bristol Bay to conduct a gear test to compare red king crab catches between the standard 83-112 Eastern trawl, used for the AFSCs EBS assessment survey, and the Nephrops trawl, used in crab surveys in the Canadian Gulf of St. Lawrence for the Bering Sea Research Foundations (BSRF) pilot crab assessment survey. The purpose was to compare estimates of Bristol Bay red king crab abundance and biomass between two surveys taking into account dissimilar gear, sampling design, and methodologies.

The BSRFs charter vessel F/V American Eagle alternately made 20 side-by-side paired tows with the Arcturus or Aldebaran on 8-9 June using a nonrandom complete block experimental design. Red king crab were weighed, sexed, enumerated, and measured.

Other special studies in addition to the BSRF gear comparison are being done routinely during survey operations, and
they include:

  • using high-quality acoustic data from survey charter vessels for improving the walleye pollock assessment during years without Midwater Assessment and Conservation Engineering (MACE) Division acoustic surveys
  • assessing the effect of light intensity and penetration on the behavior and distribution of pollock
  • reducing the variance of trawl biomass estimates of pollock using acoustic data from survey charter vessels
  • assessing summer zooplankton biomass
  • determining weight-length relationships for commercial crab species
  • studying reproductive potential of Bristol Bay red king crab and Chionoecetes crabs
  • collecting specimens for developing a key to decapod crustaceans
  • continuing seabird and fishery interactions studies
  • continuing trophic interactions and feeding ecology studies
  • collecting vertebrae for studying age and growth of Alaska skate
  • life history of Bering Sea octopi
  • validating Pacific cod visual maturities
  • studying sand lance (Ammodytes) taxonomy
  • studying coral taxonomy
  • collecting specimens for stable isotope analysis to study trophic ecology
  • continuing study of bitter crab syndrome of Chionoecetes
  • collecting DNA for prey item identification library
  • studying biology and pathogenesis of Ichthyophonus in walleye pollock
  • researching maturity and fecundity of the yellow Irish lord
  • collecting live skate egg cases for rearing experiments
  • improving trawl gear monitoring.

Similar to 2006, bottom water temperatures were colder than average, and the cold pool (< 2C) extended well into Bristol Bay. Lower than average water temperatures are implicated in an observed delay in the molting and spawning of female red king crab in Bristol Bay again this year, necessitating the resampling of 32 red king crab stations by the Aldebaran in Bristol Bay near the end of the survey, as was done previously in 1999, 2000, and 2006.

By Bob Lauth

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