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

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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).

High Trawl Concentration		Mean
Table 1. Site 5, Eastern Aleutian Islands, Depth 101-200m. Five predominant demersal species based on standardized mean individuals/10,000m².
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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