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MESA Archives: Spatial Patterns of Commercial Bottom Trawls in the Gulf of Alaska and Aleutian Islands

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Between 1990-1998 there were over 157,000 estimated bottom trawls in the Gulf of Alaska and Aleutian Islands.   Many of these fishing locations are repeated due to high catch productivity.   Categorizing trawl location is an important consideration in understanding the spatial extent of fishing induced disturbance.


Abstract:

  • GIS was utilized to examine the spatial patterns of bottom trawl effort to determine areas of high fishing activity since many fishing vessels target in the same area repeatedly.
  • Trawl locations were provided by the National Marine Fisheries Service (NMFS) groundfish observer program database.
  • Arc/Info software was utilized to either generate or import coverages for the trawl data, geography, and bathymetry.
  • Density of bottom trawl effort (number) and duration (time) was calculated by generating a polygon fishnet coverage of area (25 km2), and was intersected with the trawl (point) data. Display are in trawl duration.
  • The resulting data were classified to show areas of low (.007-73.817), medium (73.817-364.099) and high bottom trawl duration (364.099-1292.661) in days/25km2 area. [.007 days = 10 minutes ]
  • Effort data was intersected with depth strata to locate bathymetric areas with the highest number of bottom trawls.

Methods:

  • Observed bottom trawl locations were created into a point coverage using Arc/Info.  A polygon coverage utilizing the fishnet option was generated to overlay a 5km2 grid over the entire state of Alaska.
  • The point data was intersected with the polygon data.
  • Arc/Info calculated a frequency table based on the sum of trawl time for all points in each 5kmpoly-ID.
  • The data was displayed as a theme in ArcView.  The theme properties displayed separate classes of trawl duration by choosing the theme with graduated symbols.  The polygons containing < 10 minutes were deleted from the display to reduce noise and omit possible outliers.
  • The trawl data was intersected separately with bathymetry polygons (NMFS strata) to determine the locations based on depth where the highest estimated number of bottom trawls occurred.

Results:

thumbnail of spatial patterns map2 (9771 bytes)
Figure 1
thumbnail of spatial patterns map1 (10001 bytes)
Figure 2
thumbnail of spatial patterns map3 (9516 bytes)
Figure 3


(click on image to enlarge)

  • The point-polygon intersection effectively displays bottom trawl fishing effort to encompass repetitive fishing locations. Any field (columns) in your table can be summarized and displayed (Figure 1).
  • Enlarging a view allowed closer examination of spatial patterns on a smaller regional scale.  The Aleutian Island enlargement indicates 11 regions of high (> 364 days/25km2) bottom trawl duration. (Figure 2).
  • Areas that had bottom trawl duration > 365 days/25km2 can be displayed separately to locate specific depth contours within the regions of intensive trawling. (Figure 3).
  • The area with the highest estimated number of bottom trawls was on the continental shelf at depths of 101-200m for the Gulf of Alaska and Aleutian Islands.  The highest number of days trawled/km2 occurred in the Chirikof region at depths of 301-500m.

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