RESOURCE ECOLOGY & FISHERIES MANAGEMENT
(REFM) DIVISION (cont.)

2003 Stock Assessment Summary (cont.)

BSAI Rock Sole

The 2003 assessment incorporated new catch and survey information. This year’s EBS bottom trawl survey resulted in a biomass estimate of 1,135,000 t, a 12% increase relative to the 2002 estimate. An AI trawl survey was performed in 2002 and resulted in a biomass estimate of 57,700 t, which represents only 3% of the BSAI rock sole combined biomass estimate from the trawl surveys.

Survey catchability was estimated with a prior estimate on catchability from results of a herding experiment conducted by the RACE Division. Reliable estimates of B_40%, F_40%, and F_35% exist for this stock, which therefore qualifies rock sole 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 203,000 t, 0.176, and 0.210, respectively. Given that the projected 2004 spawning biomass of 424,600 t exceeds B_40%, the ABC and OFL recommendations for 2004 were calculated under sub-Tier “a” of Tier 3. The author recommended setting F_ABC at the F_40% (= 0.176) level, which is the maximum permissible level under Tier 3a. Projected harvesting at the F_40% level gives a 2004 ABC of 139,300 t. The OFL was determined from the Tier 3a formula, where an F_35% value of 0.213 gives a 2004 OFL of 166,300 t. Model projections indicate that this stock is neither overfished nor approaching an overfished condition. Stock projections for the rock sole stock continue to show declines, as do several other flatfish stocks, due to the low recruitment in the last decade.


BSAI Flathead Sole

The 2003 assessment incorporated new catch, age composition, length composition, and survey information. The 2003 EBS bottom trawl survey biomass estimate was 529,188 t, an 8% decrease relative to last year’s estimate. The 2002 AI trawl survey biomass estimate was 9,894 t.

The 2002 assessment was implemented using AD Model Builder as a modeling platform. The model was configured as a split-sex population. Age compositions were developed for each sex to account for differential growth across sexes. The relationship between temperature anomalies and survey biomass anomalies was investigated, and the survey catchability coefficient was modeled as a function of temperature anomalies. A Monte-Carlo Markov Chain algorithm was used to obtain estimates of uncertainty.

Reliable estimates of B_40%, F_40%, and F_35% exist for this stock and, therefore, is qualified 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 119,124 t, 0.3, and 0.373, respectively. Given that the projected 2004 spawning biomass of 204,866 t exceeds B_40%, the ABC and OFL recommendations for 2004 were calculated under sub-Tier “a” of Tier 3. The author recommended setting F_ABC at the F_40% (= 0.3) level, which is the maximum permissible level under Tier 3a. Projected harvesting at the F_40% level gives a 2004 ABC of 61,900 t. However, it is unlikely that the total ABC will be taken in 2004 because the total catch of flathead sole has been well below the ABC for several years. The OFL was determined from the Tier 3a formula, where an F_35% value of 0.373 gives a 2004 OFL of 75,234 t. Model projections indicate that this stock is neither overfished nor approaching an overfished condition.


BSAI Alaska plaice

The 2003 assessment for Alaska plaice incorporated new catch and survey information. In previous Alaska plaice assessments, length composition data were not used as input data. In addition, the number of read otoliths from the fishery samples was minimal, and fishery age composition data were obtained from applying age-length keys from survey samples to fishery length compositions. In this assessment, a transition matrix was constructed to covert modeled numbers-at-age to numbers-at-length, allowing use of length-frequency data as input data. This change resulted in 12 years of survey length composition data being added to the model, dropping all of the previously estimated fishery age compositions and using the fishery length compositions directly for most of these years, and adding several years of fishery length compositions. The estimated growth curve was recomputed from the available survey age samples to produce the matrix, and this growth curve was also used to estimate the weight at age vector. A single transition matrix was used for the model, and the stability of the estimated growth over time was evaluated. In addition, new data on fishery age compositions from 2000 and the survey age compositions from 2000-2002 were added to the model.

The 2003 biomass estimate from the EBS bottom trawl survey was 467,326 t for Alaska plaice, a 9% increase from the 2002 survey biomass estimate.

Reliable estimates of B_40%, F_40%, and F_35% exist for this stock complex and, therefore, it 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 115,100 t, 0.569, and 0.783, respectively. Given that the projected 2004 spawning biomass of Alaska plaice (261,140 t) exceeds B_40%, the Plan Team’s ABC and OFL recommendations for 2004 were calculated under sub-Tier “a” of Tier 3. For Alaska plaice, the author recommended F_ABC at the F_40% level (= 0.569), which is the maximum allowable under Tier 3a. Projected harvesting at the F_40% level gives a 2004 ABC of 203,056 t for Alaska plaice. It is unlikely that the total ABC will be taken in 2004 because the total catch of Alaska plaice has been well below the ABC for several years. Stock projections of Alaska plaice show that the population will gradually decline due to low recruitment.

The OFL for Alaska plaice was determined from the Tier 3a formula, where, for F_35% value (= 0.783) gives a 2004 OFL of 257,929 t. Model projections indicate that Alaska plaice is neither overfished nor approaching an overfished condition.


BSAI Other Flatfish Complex

The other flatfish assessment incorporated biomass estimates from the 2003 EBS trawl survey and the 2002 AI trawl survey. The 2003 EBS survey biomass estimates for other flatfish species was 90,300 t. The 2002 AI survey biomass for other flatfish was 8,801 t. Relative to 2002, other flatfish decreased slightly in the EBS.

The recommended ABC and OFL for other flatfish species was based on a Tier 5 criteria. The recommended 2004 ABC and OFL for the other flatfish complex were 13,549 t and 18,065 t respectively.


BSAI Pacific Ocean Perch

Pacific Ocean perch (POP) are managed as a single BSAI population. In 2003, the POP assessment incorporated new information on fishery harvest levels, 2002 survey age compositions, and 2002 fishery length compositions. The 2002 biomass estimate for POP from the AI survey was 468,588 t, an 8% decrease from the 2000 survey estimate. Reliable estimates of B_40%, F_40%, and F_35% exist for this stock; therefore, the stock qualified 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 130,358 t, 0.048, and 0.057, respectively. Projected spawning biomass for 2004 is 122,515 t, placing POP in the EBS in sub-Tier “b” of Tier 3. The maximum F_ABC value allowed under Tier 3b is 0.045. Projected harvesting at a fishing mortality rate of 0.045 gives a 2004 catch of 13,297 t, which is the recommended ABC. The OFL fishing mortality rate is 0.054 under sub-Tier 3b. Projected harvesting at a fishing mortality rate of 0.054 gives a 2004 catch of 15,761 t, which is the recommended OFL. Model projections indicate that this stock is neither overfished nor approaching an overfished condition.


BSAI Northern Rockfish

An age structured stock assessment was introduced for northern rockfish assessment in 2003. The change in assessment methodology was made possible by the reading of archived otoliths from AI trawl surveys. The change in assessment methodology results in management recommendations based on Tier 3 criteria. The estimated 2004 spawning stock biomass was 43,730 t well above the spawning biomass at B_40% (26,892 t) placing the stock under sub-Tier 3a. The 2004 ABC for northern rockfish was 6,881 t, and the 2004 OFL for northern rockfish was 8,136 t.

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AFSC Quarterly
Research Reports
Oct-Nov-Dec 2003