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MESA Archives: Spatial Variation in Demersal Communities in Areas of High and Low Bottom Trawl Effort

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  thumbnail of spatial variation map1 (14137 bytes) Map 1


thumbnail of spatial variation map2 (19152 bytes) Map 2

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Spatial variation of demersal communities was examined by comparing locations of bottom trawl fishing effort, research catch data, and bathymetry.  Community structure of predominant species of same depth was described for regions of high and low trawl effort.  Using GIS layers to describe spatial patterns of demersal community structure may provide managers a tool to help document habitat areas of particular concern on small geographic scales.


Methods:

  • Twenty-one regions of high bottom trawl effort (> 364 days/25km2 area) were plotted based on observer data from 1990-1998 for the Gulf of Alaska and Aleutian Islands
    (Map 1).
  • Locations of NMFS groundfish assessment surveys (GAS) from 1990-1997 were overlaid with bathymetric strata and the bottom trawl effort data.  An example in the Eastern Aleutians region (Sequam Pass) is displayed in Map 2.
  • Regions of high trawl concentration (HTC) that contained GAS data were compared with neighboring areas of low trawl concentration (LTC) by four depth ranges (1-100, 101-200, 201-300, and 301-500m).
  • Catch composition of research trawls (GAS) were combined by depth stratum for each HTC/LTC area.  The top five species by mean individuals/10,000m2 are summarized below (Table 1).
Table 1.  Site 5, Eastern Aleutian Islands, Depth 101-200m.
Five predominant demersal species based on standardized mean individuals/10,000m2.
High Trawl Concentration Mean
Common Name Latin Name Contribution ind/10,000m2
Pacific halibut
northern rock sole
bigmouth sculpin
whiteblotched skate
ribbed sculpin
Hippoglossus stenolepis
Lepidopsetta peracuata
Hemitripterus bolini
Bathyraja maculata
Triglops pingeli
44.5%
11.9%
10.3%
10.3%
  6.3%
1021.3
546.3
157.4
235.0
286.6
 


Low Trawl Concentration


Mean
Common Name Latin Name Contribution ind/10,000m2
Pacific ocean perch
Pacific halibut
arrowtooth flounder
northern rock sole
northern rockfish
Sebastes alutus
Hippoglossus stenolepis
Atherestes stomias
Lepidopsetta peracuata
Sebastes polyspinis
56.8%
  9.0%
  6.2%
  5.6%
  3.8%
2096.3
276.4
381.4
511.8
350.2
 


Results:

  • Eight sites of high and low bottom trawl effort were summarized by depth with available catch composition data for the Gulf of Alaska and Aleutian Islands.
  • Predominant species groupings of the top five fish/invertebrates species were quantified for each of the four depth categories where research trawls occured.  Common dominant species included:  Pacific Ocean perch (Sebastes alutus), Northern rockfish (S. polyspinis), Arrowtooth flounder (Atheresthes stomias), and two species of sculpin for eight sites in the analysis.
  • Species associations can be plotted with potential indicator species of bottom or sediment types to determine a robust description of habitat.

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