545 



Abstract — Demersal groundfish den- 

 sities were estimated by conducting a 

 visual strip-transect survey via manned 

 submersible on the continental shelf off 

 Cape Flattery, Washington. The purpose 

 of this study was to evaluate the statis- 

 tical sampling power of the submersible 

 survey as a tool to discriminate density 

 differences between trawlable and 

 untrawlable habitats. 



A geophysical map of the study area 

 was prepared with side-scan sonar 

 imagery, multibeam bathymetry data, 

 and known locations of historical 

 NMFS trawl survey events. Submers- 

 ible transects were completed at ran- 

 domly selected dive sites located in each 

 habitat type. Significant differences in 

 density between habitats were observed 

 for lingcod (Ophiodon elongatus), yel- 

 loweye rockfish (Sebastes ruberrimus), 

 and tiger rockfish (S. nigrocinctus) 

 individually, and for "all rockfish" and 

 "all flatfish" in the aggregate. Flatfish 

 were more than ten times as abundant 

 in the trawlable habitat samples than 

 in the untrawlable samples, whereas 

 rockfish as a group were over three 

 times as abundant in the untrawlable 

 habitat samples. 



Guidelines for sample sizes and 

 implications for the estimation of the 

 continental shelf trawl-survey habitat- 

 bias are considered. We demonstrate an 

 approach that can be used to establish 

 sample size guidelines for future work 

 by illustrating the interplay between 

 statistical sampling power and 1) 

 habitat specific-density differences, 2) 

 variance of density differences, and 3) 

 the proportion of untrawlable area in 

 a habitat. 



Manuscript approved for publication 

 12 February 2003 by Scientific Editor. 



Manuscript received 4 April 2003 at 

 NMFS Scientific Publications Office. 



Fish Bull. 101:545-565 (2003). 



Demersal groundfish densities in trawlable and 

 untrawlable habitats off Washington: implications 

 for the estimation of habitat bias in trawl surveys 



Thomas Jagielo 



Annette Hoffmann 



Jack Tagart 



Washington Department of Fish and Wildlife 



600 Capitol Way N. 



Olympia, Washington 98501-1091 



E-mail address (for T Jagielo) lagiethim'dfw wa gov 



Mark Zimmermann 



National Marine Fisheries Service 

 7600 Sandpoint Way NE 

 Seattle, Washington 98115-0070 



Despite their utility, trawl surveys C£ui- 

 not obtain quantitative samples from 

 rough, rocky habitats, and thus have 

 a limited ability to sample all habitats 

 representatively (Uzmann et al., 1977; 

 Kulbicki and Wantiez, 1990; Krieger, 

 1993; Gregory et al., 1997). Since 1977, 

 triennial bottom trawl surveys have 

 been used to estimate the abundance 

 of commercially and recreationally 

 exploited groundfish species in the 

 continental shelf waters off Washing- 

 ton, Oregon, and California (Shaw et 

 al., 2000). The data generated from 

 these NMFS surveys are often a key 

 component of groundfish stock assess- 

 ments which are used to set levels of 

 acceptable biological catch (ABC) for 

 selected species (PFMC, 2001). Clearly, 

 proper interpretation of these survey 

 data with respect to fish habitat prefer- 

 ences is an important part of developing 

 unbiased stock assessments for fisher- 

 ies management. 



In trawl survey methodology, popula- 

 tion biomass is related to CPUE by the 

 following equation (Dark and Wilkins, 

 1994): 



S, =-L CPUE.x- 

 a. \ q 



where i 

 B. 



area-depth stratum; 

 estimated biomass in the ith 

 area-depth stratum; 



A, = total area in the ith stratum; 

 a, = total area swept during a 

 standard trawl haul in stra- 

 tum i; 

 CPUE, = mean catch per unit of effort 

 in the ith stratum; and 

 q = the catchability coefficient of 

 the sampling trawl. 



For this model to be an unbiased esti- 

 mator of abundance, it is necessary to 

 assume that the area sampled by the 

 trawl is representative of the entire 

 area-depth stratum of interest (i.e. 

 a, is representative of A,). Validating 

 this assumption becomes particularly 

 important where untrawlable habitat 

 comprises a significant proportion of 

 the total area assessed, and where 

 species composition and density vary 

 between habitats. We shall refer to 

 error in trawl survey estimates of abun- 

 dance due to differences in groundfish 

 density between habitat types as the 

 trawl-survey habitat-bias. 



The trawl-survey habitat-bias may 

 be substantial on the west coast conti- 

 nental shelf because of the considerable 

 spatial extent of untrawlable habitat 

 in some management regions (Shaw et 

 al., 2000). It is also widely recognized 

 that demersal groundfish species com- 

 position and density can vary consider- 

 ably by bottom type (Richards, 1986; 



