Resource Ecology & Fisheries Management  (cont.)

(2001 Stock Asessment Summary: part 3)
Bering Sea Walleye Pollock

This year’s pollock chapter features new data from the 2001 bottom trawl survey and from the 2000 fishery.  The 2001 bottom trawl survey estimated a biomass of 4,140,000 t, a decrease of 19.5% relative to the 2000 estimate and following a 44% increase from 1999 to 2000.  The most recent summer echo-integration trawl (EIT) survey occurred in 2000.  The estimated biomass from the 2000 EIT survey was 3,050,000 t and the age-composition estimates for this survey were updated.  Minor changes to the assessment model were made in 2001.  These included adding an environmental effect (bottom temperature) to survey catchability and developing alternative specifications for selectivity forms for the fisheries and surveys.  In previous years, the Council’s scientific and statistical committee (SSC) determined that reliable estimates of B_MSY and the probability density function for F_MSY exist for this stock, and that eastern Bering Sea (EBS) walleye pollock therefore qualified for management under Tier 1.  The maximum permissible value of fishing mortality (F_ABC) under Tier 1a is based on the harmonic mean value of F_MSY (which requires estimates of the uncertainty on this quantity).  This translates into a 2002 catch of 2,108,000 t.  The authors noted that a fixed catch of 1,400,000 t would maintain the stock above B_40% level of spawning in the near term.  No change was made to the AI  region harvest recommendations.  The 2000 bottom trawl survey of the AI region resulted in a biomass estimate of 105,554 t, an increase of 13% relative to the 1997 estimate.  Aleutian Island and Bogoslof Island pollock qualify for management under Tier 5.  For the AI region, this gives an ABC of 23,750 t at a biomass of 105,554 t.  For the Bogoslof region, the 2001 EIT survey of the Bogoslof region resulted in a biomass estimate of 232,000 t.  The adjusted value (considering that this region is treated as an index site for the Bering Sea basin or Donut Hole stock) for ABC is 4,310 t.  (Additional research on pollock and the environment is presented elsewhere in this edition.)

 
BSAI Pacific Cod

The 2001 assessment updated last year’s assessment, incorporating new catch and survey information.  This year’s EBS bottom trawl survey resulted in a biomass estimate of 830,479 t, a 57% increase from last year’s estimate and the lowest observed value for the survey.  The AI region was surveyed in 2000, and the biomass increased 63% from 1997.  Estimates of abundance are higher for the 2001 assessment compared to the 2000 assessment.  For example, projected 2002 spawning biomass for the BSAI stock is 425,000 t, up about 15% from last year’s F_ABC projection for 2001.  The SSC has determined that reliable estimates of B_40%, F_40%, and F_35% exist for this stock, and that this stock therefore qualifies for management under tier 3 of the BSAI Groundfish fishery management plan (FMP).  The updated point estimates of B_40%, F_40%, and F_35% from the present BSAI cod assessment are 431,000 t, 0.30, and 0.36, respectively.

The Bayesian meta-analysis, which has formed the basis for a risk-averse ABC recommendation in each of the 1996-99 assessments, was not performed for the present assessment.  The ratio between the recommended F_ABC and F_40%  given in the 1999 assessment (0.87) was assumed to be an appropriate factor by which to multiply the 2001 maximum permissible F_ABC to obtain a recommended 2002 F_ABC.  A 2002 catch of 223,000 t would represent an increase of 19% relative to the 2001 ABC.  Spawning biomass projected for 2002 is 425,000 t.

The recommended overfishing level (OFL) was determined from the Tier 3 formula times 0.87, where fishing at a rate of 0.31 gives a 2002 catch of 294,000 t.  Model projections indicate that this stock is neither overfished nor approaching an overfished condition.

 
BSAI Yellowfin Sole

The 2001 assessment incorporated new catch and survey information.  This year’s EBS bottom trawl survey resulted in a biomass estimate of 1,855,200 t, an increase of 17% from last year’s survey, but still a 20% decline from 1998.  The sharp decrease in 1999 was attributed in part to cold water which might have decreased availability.  However, both the 1999-2001 trawl survey lower estimates may be due to the survey being performed earlier, when a significant portion of the stock is still at the spawning grounds in shallow water.  Extra tows were done outside the normal trawling area (in shallow waters), and concentrations of yellowfin sole were encountered.  The catchability of yellowfin sole is related to water temperature.  Therefore, an alternative assessment model was introduced that assumed that q was a linear function of bottom temperature.  This model was used for future stock projections.

Reliable estimates of B_40%, F_40%, and F_35% exist for this stock and therefore qualifies for management under Tier 3 of the BSAI Groundfish FMP.  The updated point estimates of B_40%, F_40%, and F_35% from the present assessment are 392,200 t, 0.11, and 0.13, respectively.  Given that the projected 2001 spawning biomass of 453,700 t exceeds B_40%, the ABC and OFL recommendations for 2001 were calculated under sub-tier “a” of Tier 3.  The author recommended setting F_ABC at the F_40% (=0.11) level, which is the maximum permissible level under Tier 3a.  Projected harvesting at the F_40% level gives a 2001 ABC of 114,900 t.  The OFL was determined from the Tier 3a formula, where an F_35% value of 0.13 gives a 2001 OFL of 136,400 t.  Model projections indicate that this stock is neither overfished nor approaching an overfished condition.  Stock projections show the yellowfin sole stock will continue to decline despite low exploitation rates.  The decline is due to the low recruitment in the last decade.

Cont. >>>

AFSC Quarterly
Research Reports
Oct-Dec 2001