Jaglelo et al.: Demersal groundfish densities in trawlable and untrawlable habitats off Wasfiington 



559 



not comprise a large portion of the area to be 

 assessed. Our data suggest, for instance, that 

 it would probably be unfeasible to estimate a 

 trawl survey bias correction factor for lingcod. 

 It appears that lingcod density can be estimat- 

 ed with relatively good precision in trawlable 

 areas by the trawl survey (CV=0.20, Table 10). 

 However, our submersible survey found high 

 variability across both habitat types (CV=1,17, 

 Table 9), which resulted in a relatively low-ef- 

 fect size-index threshold values for lingcod (e.g. 

 AJA dj=0.52, Table 10). The required sample 

 size rapidly exceeded n = 100 submersible dive 

 sites as the proportion of the management 

 area that was untrawlable increased above 5% 

 (P=80%, n=0.05; Fig. 8). In cases requiring such 

 large sample sizes, estimation of a trawl-survey 

 bias correction factor would probably not be an 

 acceptable alternative to direct, synoptic sur- 

 veys structured to obtain unbiased estimates 

 of abundance in untrawlable habitats. By contrast, the 

 trawl survey bias correction factor approach may be more 

 feasible for species where the ratio between the trawl 

 survey and submersible survey variation is smaller. Our 

 data suggest that flatfish may fall into this category. The 

 trawl survey precision (CV=0 .09, Table 10) in relation 

 to the submersible survey precision (CV=0.65, Table 9) 

 resulted in a relatively high-effect size-index threshold 

 value for flatfish at the proportion level of 5% for area that 

 was untrawlable (rf^=5.65. Table 10). The required sample 

 size was less than n = 25 submersible dive sites, even as 

 the ratio of A^/A (the proportion of the management area 

 that is untrawlable area) exceeded 30% (P=809(, a=0.05: 



Fig. 8). However, because our analysis aggregated flatfish 

 as a group, these results do not address the estimation of 

 a bias correction factor for individual species, which is a 

 requirement for any correction factor to be useful for stock 

 assessment purposes. 



As for any survey method, the visual transect survey 

 method has an array of advantages and disadvantages, 

 which have been well chronicled elsewhere (Uzmann et 

 al., 1977; Ralston et al., 1986; Butler et al., 1991; Adams et 

 al.,1995; Starr et al., 1996; CaiUiet et al., 1999). Some of the 

 disadvantages include 1) difficulties in fish identification, 

 particularly for small fish or fish with subtle coloration, 

 2) the potential for attraction or repulsion of fish from the 



