NOAA logo JAS 2000 Quarterly Rpt. sidebar

Resource Assessment &
Conservation Engineering Division

(Quarterly Report for July-Aug-Sept 2000) 


Eastern Bering Sea Crab-Groundfish Survey

The Bering Sea resource assessment group completed the 30th annual survey of the eastern Bering Sea shelf from 23 May to 20 July 2000.  The early surveys covered a much smaller area than our present efforts and were primarily aimed at assessing stocks of red king crab (Paralithodes camtschaticus).  As time passed, the surveyed area increased, and sampling methodology became more consistent.  Since 1982, the standard survey has covered over 463,000 km2, and the trawl gear and sampling pattern have remained the same.

After a record cold year in 1999, water temperatures returned to near the long-term mean in 2000 (2000 mean bottom temperature: 2.17oC, long-term mean: 2.42oC). Unlike last year, no ice was encountered during the survey.

The biomass estimate for walleye pollock (Theragra chalcogramma) was significantly higher in 2000 at over 5.1 million metric tons (t).  Pollock distribution was also more widespread in 2000, unlike 1999 where the cold water in the middle domain (waters 50 - 100 m) seemed to force the fish to the outer shelf.  The biomass estimate for Pacific cod (Gadus macrocephalus) continued a general decline shown since 1994 to an estimate of 528,000 t.  Of the major flatfish species, only Alaska plaice (Pleuronectes quadrituberculatus) showed a decline from 1999.  Northern rock sole (Lepidopsetta polyxstra) showed the greatest increase, up to 2.1 million t.

It has been known for some time that a portion of the yellowfin sole (Limanda aspera) stock was present in waters shallower and inshore of the standard survey area.  Yellowfin sole are spawning during the survey time and are therefore potentially missed and underestimated by the survey.  As part of a program to evaluate the magnitude of the problem, 20 inshore stations were sampled in 2000 during the progression of the survey.  These stations were along the Alaska Peninsula, in Togiak Bay and in Kuskokwim Bay.  The geographic area that these stations represent is presently being evaluated, so it is difficult to estimate the actual biomass for yellowfin sole in this area.  However, the raw catches were very high and amounted to nearly 40% of the survey catches.  It is expected that these new stations, or possibly more, will be sampled again next year to determine if they should be added to the standard survey in the future.

At the end of the standard survey another experiment in the continuing program of investigating trawl performance was performed.  The experiment was designed to determine how trawl speed and currents affect the footrope’s contact with the bottom. The results are presently being evaluated.

During the last 2 years the survey has started about 2 weeks earlier due to time constraints brought on by changes in fishery seasons near 1 August.  The ramifications of this early start are not known; however, one apparent effect is on the survey of red king crab.  Part of the crucial data collected is the stage of eggs present on the females.  In past years the females have already molted and have new eggs present.  In 1999 and 2000 many of the eggs were old eggs, not yet hatched.  To gauge the success of the hatch, we have had to return to inner Bristol Bay at the end of the standard survey and resurvey parts of that area after the red king crab have molted.  Whether this effect is due strictly to timing, or is a result of the severe cold anomaly of 1999 (since a large part of the maturation of the 2000 hatching eggs occurred in 1999) is unknown.  Further study of this process, plus discussion of resetting the survey timing is under way

By Gary Walters.


picture of attaching a video camera to a net

 An underwater video camera is attatched forward of the Aleutian wing trawl to observe walleye pollock behavior inside the net during the summer 2000 Bering Sea shelf echo integration-trawl survey aboard the NOAA research ship Miller Freeman

Bering Sea Survey

The Midwater Assessment and Conservation Engineering (MACE) program has monitored abundance of walleye pollock in U.S. waters of the eastern Bering Sea for more than two decades using echo integration-trawl survey (EIT) techniques.  Between 8 June and 3 August 2000, MACE scientists employed the NOAA ship Miller Freeman to survey pollock on the eastern Bering Sea shelf from Port Moller, Alaska, to the U.S./Russia Convention Line (Figure 1).  Principal objectives of the research cruise were 1) to collect echo integration and trawl data to estimate pollock abundance and distribution and 2) to collect pollock target strength information to validate the relationship between pollock length and target strength–a measure of acoustic reflectivity.  This midwater time series together with near-bottom abundance and distribution data derived from annual summer bottom trawl surveys, are used in stock assessment models and ultimately assist the National Marine Fisheries Service and North Pacific Fishery Management Council in managing the commercial fishery.

Preliminary cruise results show that pollock were present on all but the second transect of the survey (Figure 1).  East of the Pribilof Islands, the highest pollock concentrations were observed along transects 7-8, north of Unimak Island.  Pollock abundance was lower between Unimak and the Pribilofs.  West of the Pribilof Islands, pollock increased and peaked in abundance on transects 20 and 21 southwest of St. Matthew Island.  In the far west, pollock were heavily concentrated in a few spots along the U.S./Russia border (transects 26-28, Figure 2).   In general, water temperatures were warmer in 2000 than in 1999 and, on most transects, pollock were distributed somewhat farther north in 2000 than in 1999.  Overall pollock biomass was similar to that observed in 1999.

One of several additional research projects conducted during the cruise involved mounting an underwater video camera on the midwater trawl to observe pollock behavior in front of the the codend as part of ongoing research on fish behavior in relation to trawl gear (see photo above).

By Taina Honkalehto.

Gulf of Alaska Survey

diagram of echo integration-trawl methods
Figures 4a, 4b.  Echo integration-trawl (EIT) methods require combining acoustic data, which are represented in the echogram of adult walleye pollock (top), with associated trawl samples (bottom) to estimateabundance and distribution patterns for the targeted species. Illustrative data are from Barnabus gully during the August 2000 EIT survey.  The trawl catch of fish being sorted by scientists in the photograph are primarily adult walleye pollock.

picture of trawl haul samples aboard ship

A feasibility study was conducted off the east side of Kodiak Island between 8-20 August 2000 as a collaborative effort between scientists of the Center’s RACE and REFM Divisions. The purpose of the study was to evaluate the suitability of this location for conducting a multiyear summertime field experiment to evaluate the effect of commercial fishing activity on the availability of walleye pollock to Steller sea lions (Eumetopias jubatus).

Principal objectives for the feasibility study were:

  1. to use standard EIT survey methodologies to describe the abundance and distribution patterns of walleye pollock within Barnabas and Chiniak gullies

  2. to determine whether acoustic back-scattering from nontargetted species would prohibit meeting the first objective

  3. to determine the spatio-temporal variability in pollock abundance and distribution patterns within and between the two gullies over the duration of the study.

The researchers will use this information to assess the proposed experimental design for more comprehensive field work in subsequent years. The more comprehensive work will require that the gullies serve as treatment and control sites where commercial fishing would be allowed in one location and prohibited in another.

The EIT survey operations included the collection of 38 and 120 kHz acoustic data, as well as net catch data from large midwater and bottom trawls (Figures 4a, 4b above).  These data were collected along a series of uniformly spaced (i.e., 3 nmi) parallel transects during the 2-week survey.   Two complete survey passes were conducted in both Barnabas and Chiniak gullies.

Preliminary survey results indicated that it was possible to use EIT survey methods to assess the summer distribution of pollock within the study area (Figure 5).  Substantial back-scattering was attributed to “adult” pollock  as well as “small fishes” which included age-0 pollock, age-1 pollock, and capelin (Mallotus villosus).  It was difficult, however, to determine the relative contribution of capelin and age-0 pollock to the “small fishes” back-scattering.  Additional analyses of the data are in progress to quantify the spatio-temporal distribution patterns and abundance of pollock and capelin.  Interpretation of these results will provide information to determine whether these two submarine gullies off of the east side of Kodiak Island are appropriate sites for conducting a more comprehensive field experiment next summer.

By Chris Wilson.

Halibut Excluder for Cod Trawlers

diagram of full halibut excluder
Figure 6.  Full halibut excluder, including the skate deflector (solid panel), which was made of 20 x 20 cm (8 x 8 inch) square meshes. (the top and near-side panels of the trawl are not shown).

diagram of ring grate detail
Figure 7.  Detail of ring grate (rings 22 cm (8.5 inch) diameter, one panel in aluminum hoop and three panels behind it) and slot sections (slots 35 x 7 cm [14 x 2.75 inch]). 

During cruises in June and August, RACE scientists developed and tested a halibut excluder for trawl fisheries which target Pacific cod. The resulting design was then tested in September during a fishery conducted under an exempted fishery permit sponsored by two trawl industry associations, the Groundfish Forum and the At Sea Processors Association (ASPA).  The most effective configuration of the device released 80% of the Pacific halibut encountered, while losing only 15% of the Pacific cod catch.

The initial cruise, conducted on the fishing vessel Hickory Wind, during 18-28 June, used an excluder based on designs previously tested for halibut exclusion in sole fisheries. The device had a grate in the section ahead of the codend to guide halibut to an escape hole, while allowing the sole to pass through to the codend. Instead of square grate openings used in previous designs, the grates were constructed with rigid circular holes, “ring grates,” to better allow cod passage while excluding halibut.  An auxiliary net attached to the trawl with a second codend was used to recapture the escaping fish to allow tow-by-tow estimates of escape rates.

Ring grates with 22-cm (8.5-inch) diameter openings effectively excluded large halibut while allowing nearly all of the cod to pass through to the codend.  However, most of the halibut encountered were less than 70 cm in length.  Reducing the ring diameter to 19 cm (7.5 inches) excluded some of the smaller halibut but greatly increased the loss of cod.  Skates were encountered frequently during the cruise, and underwater video showed that they often blocked large portions of the excluder grate.

During the second cruise, conducted on the fishing vessel Hazel Lorraine during 12-16 August, additional modifications were installed to exclude small halibut and to prevent skates from blocking the grates.  Two vertical panels with horizontal slots 9 cm (3.5 inch) high were installed behind the ring (22-cm, 8.5-inch diameter) grates to form a narrowing chute.  Animals passing through the slots in the vertical panels escaped.  The purpose of these vertical panels with narrow slots was to allow smaller halibut to escape, while retaining all but the smallest cod.  To keep skates from blocking the grates, a downward sloping mesh panel with large openings (20 x 20 cm (8 x 8 inch) square mesh) was installed ahead of the grate section.  Nearly all fish except for large skates were able to pass through this mesh.

The skate excluder panel was effective at preventing the grate from becoming blocked by large skates.  The combination of slots and rings allowed 75% - 80% of the halibut to escape across all of the sizes present.  However, medium- and small-sized cod (up to 70 cm length) were also escaping at rates from 30 to 70%. Underwater video showed that while only the smallest cod could pass through the slots in an upright position, when crowding induced strong escape efforts, the cod would turn on their sides, flattening their gill plates and thus narrowing the widths of their heads to pass through. Measurements of the compressed widths of cod and halibut heads and their lengths were taken to better optimize the slot openings.

The excluder system tested during the Groundfish Forum/ASPA exempted fishery, conducted 17 - 23 September the vessel Legacy, was similar to that used on the previous cruise except that the slots were narrowed to 7 cm (2.75 inch) in width (Figures 6 & 7 above).  Initial tows excluded 77% of the halibut, but also allowed 46% of the cod to escape, with escape rates consistent across cod sizes.  Restricting access to the escape hole below the ring grate in two steps reduced the cod loss to 31% and then to 15%, while the halibut escape rate remained consistent at 80%.  A final test with the ring section removed retained all but 2% of the cod, but only excluded 25% of the halibut.

This series of cruises developed and demonstrated an effective halibut excluder for trawl fisheries that harvest cod.  The resulting system was actually three excluders in sequence, a mesh panel to deflect skates, a grate with circular holes to direct large halibut to an escape opening, and vertical panels with long, narrow slots to exclude small halibut.  Depending on the mix nd sizes of halibut, cod, and skates encountered, one or more of these components might not be needed.  Further development of the durability and ease of handling of this system will improve its usefulness to all cod trawlers.  Field work to accomplish this should occur within the next year.

By Craig Rose.