Resource Ecology & Fisheries Management  (cont.)

(2001 Stock Asessment Summary: part 8)
GOA Flatfish

The flatfish group is subdivided into deep-water flatfish, rex sole, shallow-water flatfish, and flathead sole.  The 2002 exploitable biomass for each category is based on a delay difference model that includes estimates of growth, natural mortality, and recruitment, as well as biomass estimates from the 1996 and 2001 bottom trawl surveys.  ABC and OFL were calculated by species, with individual species identified as Tier 4, 5, or 6 depending upon the available data.  The 2001 GOA bottom trawl survey biomass was used as the current biomass for calculation of ABC.

The overfishing levels for the flatfish groups are determined by the fishing mortality rates determined from the tier structure and the exploitable biomass estimates of the individual species.  The catch levels associated with those fishing mortality rates are:

 
GOA Arrowtooth Flounder

The 2002 estimated female spawning biomass of arrowtooth flounder was 1,098,130 t.  This estimate was based on abundance estimates derived from an age-structured model developed with AD Model Builder software.  Similar to the previous assessment, the model accommodated a higher proportion of females in the larger size intervals of both survey and fishery data by giving males a higher mortality rate than females.  Exploitable biomass in 2002 is estimated to be greater than B_40%, and ABC was determined to be 146,264 t based on Tier 3a calculations (F_40% = 0.134).  Using Tier 3a criteria, the overfishing level based on F_35% = 0.159 is estimated at 171,057 t.

 
GOA Atka Mackerel

Prior to 1997, exploitable biomass and ABC for Atka mackerel were based on triennial bottom trawl survey estimates.  However, schooling behavior, patchy distribution, and habitat preference make this species difficult to sample with standard trawl survey gear.  Atka mackerel are also poor targets for hydroacoustic surveys because they lack swim bladders.  Re-evaluation of historical survey data indicated abundance estimates prior to 1997 were also compromised by high variability.  Thus, existing GOA bottom trawl survey data has limited utility for either absolute abundance estimates or indices for Atka mackerel.

The Plan Team continues to support a bycatch only fishery as a conservative harvest policy for Atka mackerel because:  1) there is no reliable biomass estimate; 2) localized depletion may occur; and 3) this species has previously exhibited a particular vulnerability to fishing pressure in the GOA.  The Team recommends an ABC of 600 t in 2002 to satisfy bycatch needs in other fisheries.

 
GOA Thornyheads

In 2001, the thornyhead assessment was updated with several new types of data including: 1999 - 2000 harvest levels by gear, biomass estimates from the 2001 GOA bottom trawl survey, and relative population numbers from the 2001 sablefish longline survey.  Alternate models examined assumptions regarding natural mortality and length at age.  The recommended ABC and spawning stock biomass levels were 2,494 t and 23,235 t, respectively.

By Anne Hollowed.

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AFSC Quarterly
Research Reports
Oct-Dec 2001