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Fishery Bulletin 101(3) 



Matthews and Richards, 1991; Stein et al.,1992; 

 O'Connell and Carlile, 1993; Gregory et al., 1997; 

 Krieger and Ito, 1999; Nasby, 2000; Yoklavich et al., 

 2000). Thus, there is considerable interest in evalu- 

 ating alternative survey tools. 48°30' 



One alternative to trawl surveys that has gained 

 increased attention in recent years is the method of 

 direct observation of the seafloor, typically conducted 

 with a remotely operated vehicle (ROV) or with an 

 occupied submersible (Auster et al., 1989; Krieger, 48°00' 

 1993; Caimi et al., 1993; Adams et al., 1995; Gregory 

 et al., 1997; Nasby, 2000). We evaluated the sampling 

 power of the benthic video-strip transect method, us- 

 ing videotapes of the sea floor collected in situ with 

 an occupied submersible. Our goal was to judge the 

 feasibility of using this approach to provide mean- 

 ingful comparisons of demersal groundfish densities 

 between trawlable and untrawlable habitats on spa- 

 tial scales large enough to be useful for west coast 

 fisheries management. 



We prepared a geophysical map of the bottom and 

 conducted a submersible survey at a study site located on 

 the continental shelf off Cape Flattery, Washington. Our 

 objective was to provide guidelines on sample sizes (num- 

 ber of submersible transects) that would be needed to 

 characterize differences in density between the two habitat 

 types, and specifically, sample sizes that would be needed 

 to estimate the trawl survey habitat bias in subsequent 

 studies designed to cover wider geographic areas. The 

 study was structured to answer the following questions: 1) 

 what species occupy trawlable and untrawlable habitats off 

 Washington; 2) what magnitude of density differences can 

 be expected between trawlable and untrawlable habitats; 

 3) what is the variability offish density within each habitat 

 type; and 4) what sample sizes are required to estimate 

 density differences between habitats, and the trawl survey 

 habitat bias, in a statistically reliable manner. Our focus 

 was on the benthic species and species groups that could 

 be assessed reliably with our submersible survey method; 

 primarily rockfish (Sebastes spp.), lingcod (Ophiodon elon- 

 gatus), and flatfish (Pleuronectiformes). 



Materials and methods 



Study site 



Selection of the study site was aided by examination of 

 historical NMFS trawl survey records and Washington 

 Department of Fish and Wildlife (WDFW) trawl fishery 

 logbook data. We chose a rectangular area west of the 

 Point of Arches, Washington, which extends from the Juan 

 de Fuca Canyon in the east (125°17'W) to Nitinat Canyon 

 in the west ( 125°37'W) and ranges from 48°13' in the south 

 to48°16' in the north (Fig. 1). We selected this area because 

 1) this portion of the Washington coast has been the site of 

 a productive groundfish fishery since the 1940s (Alverson 

 1951), 2) this location has been surveyed tri-annually since 

 1977 as part of the NMFS west coast shelf survey, 3) the 

 area has demersal groundfish species of interest, and 4 ) 



126°00' 



125°00' 



Stu dy jSit e ;l / 



Figure 1 



Location of the study area (marked "study site" on map) on the 

 continental shelf off Washington State. 



the area contains both trawlable and untrawlable habitats. 

 The seafloor of this area was sculpted and shaped by ice 

 movements during the late Pleistocene period (approxi- 

 mately 18-20 thousand years ago) and is characterized by 

 boulder fields resulting from glacial deposition that cover 

 substantial portions of the area (GoldfingerM. Planning 

 for the submersible survey required geodetically precise 

 knowledge of the seafloor characteristics of the study area. 

 This was facilitated by conducting geophysical surveys and 

 by preparing a detailed map, which was instrumental to 

 the submersible survey design. 



Geophysical surveys and map preparation 



Geophysical surveys of the study site were conducted by 

 collecting side-scan sonar and multibeam bathymetry 

 data simultaneously during a five-day effort on board the 

 USN Agate Passage (YP-697) in May 1998. Slant-range- 

 corrected side scan sonar data were collected by using a 

 Waverly widescan 100-kHz system, with a swath width of 

 800 m. Eighteen parallel track lines were conducted with 

 100% overlap. The resulting imagery was assembled into 

 a mosaic map of the bottom relief for a rectangular area 

 measuring approximately 5.6 by 24.8 km (13,888 hect- 

 ares). Bathymetric data, with resolution on the order of 

 ±0.4 m were collected with a Reson Model 8101 multibeam 

 bathymetry system. The multibeam bathymetry data were 

 processed to produce a detailed map of the bottom topogra- 

 phy with 1-m depth contour intervals. 



Map overlays were prepared that showed the locations 

 of trawl survey events and trawl fishery tows. Detailed 

 NMFS records were used to identify the location of various 

 events associated with historical surveys of the area. The 

 NMFS survey event types included good hauls, bad hauls, 

 short hauls (tows ended early because of rough bottom), 



' Goldfinger, C. 2001. Personal commun. Department of Geol- 

 ogy, Oregon State University, Corvalis, OR 97331. 



