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NOAA Technical Memorandum NMFS-AFSC-253

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Calculation of the geographic sizes used to create population indices for the Alaska Fisheries Science Center longline survey

Abstract

Geographic area sizes (km2) are used for computing abundance indices for sablefish and other groundfish species caught in the Alaska Fisheries Science Center’s annual longline survey.  These areas were devised before geographic information systems (GIS) and accurate bathymetric maps were available. In addition, there was poor documentation for how the currently used area measurements were determined. With new technology, area sizes can be defined using GIS to ground truth the area sizes currently being used for computing abundance indices.  Additionally, because the survey was originally intended for sablefish, and their habitat is primarily in depths from 200 to1,000 m, area sizes were not determined for areas shallower than 200 m.  A substantial amount of survey effort is placed in the 151-200 m depth range and several species of interest are commonly caught in these depths.  Using ArcView GIS 9.3 and GEBCO_08 bathymetry source, geographic area sizes were recalculated and compared with existing values.  In addition, area sizes for shallow stratum, consisting of depths between 151 and 200 m, were created, and documentation is provided for how the currently used area measurements were determined. Recalculated area sizes and geographic area sizes utilized in the AFSC longline and RACE trawl surveys were generally similar across the WGOA and CGOA, with larger differences occurring in the EGOA and in the gullies where boundary lines were hard to decipher.  Where differences occurred, recalculated values were generally smaller.

 


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