Jagielo et al.: Demersal groundfish densities in trawlable and untrawlable habitats off Washington 



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Figure 2 



Geophysical map of the study area with associated overlays. Area outlined in bold is the submersible survey area. The grid squares 

 are 800 by 800 m sample units. (Top): Mosaic of side-scan imagery of the seafloor (Middle): Depth contours (1-m isobaths) obtained 

 from multibeam bathymetry. (Bottom): Locations of historical NMFS shelf trawl survey events. Hatched lines = chain drags, stars = 

 chain snags, unhatched lines = successful tows. 



skipped hauls, chain drags, and chain snags. Interviews 

 with knowledgeable fishermen were also conducted to 

 establish the locations of known trawling sites within 

 the area. The resulting geophysical map, with overlays, 

 provided a geographically accurate reference of the study 

 area that allowed a priori classification of the bottom into 

 trawlable and untrawlable habitat types (Fig. 2). The final 

 map consisted of the following layers: 1) a mosaic of side- 

 scan imagery of the bottom (Fig. 2, top); high-resolution 

 depth contours (1-m isobaths) obtained from multibeam 

 bathymetry (Fig. 2, middle); and 3) locations of historical 

 NMFS trawl survey events (Fig. 2, bottom). 



Experimental design 



Our survey design process made use of the detailed map 

 of the study area for 1) definition of the sampling unit, 2) 

 classification of all sampling units as trawlable or untraw- 

 lable habitats, and 3) specification of the in situ survey 

 area. A sample of units to be surveyed by submersible was 



selected from each habitat type by using computer-gener- 

 ated pseudo-random numbers. 



In defining the size of the sampling unit, we sought to 

 strike a balance between a spatial scale that was small 

 enough to have homogeneity but large enough to have 

 meaning as a trawlable or untrawlable space. We chose 

 square sample units of 800 by 800 m in size. This size 

 was smaller than the standard NMFS tow length of about 

 3,000 m and was well within the order of resolution of the 

 multibeam bathymetry and side-scan imagery used for 

 discerning rock outcrops. A grid consisting of the 800 by 

 800 m sampling units was prepared and overlaid onto the 

 map of the study area (Fig. 2). 



Classification of the sampling units into "trawlable" and 

 "untrawlable" habitats was facilitated by examination of 

 the geophysical map of habitat features, together with an 

 evaluation of historical NMFS trawl survey records. The 

 survey map layer helped us to interpret the appearance 

 of trawlable and untrawlable habitat on the bathymetric 

 and side-scan geophysical map layers. Trawlable bottom 



