558 



Fishery Bulletin 101(3) 



nature of the respective study designs. The Oregon and 

 California studies both targeted particular substrate types 

 to characterize fish assemblages and fish habitat associa- 

 tions. Our study in Washington was structured to conduct 

 random sampling within each of the two broad habitat clas- 



Effect size Index (d) 



Figure 5 



Sample size (n=number of submersible dives in each habitat type) as 

 a function of the effect size index (rf) for comparisons of fish density 

 between trawlable and untrawlable habitats (a=0.05). 



20 

 Sample size (n) 



Figure 6 



Statistical Power as a function of sample size (n=number of submersible 

 dives in each habitat type) for the lowest [d=l.l) and highest id=2.0) 

 values of the effect size index observed in the present study (a=0.05). 



600 

 5.00 

 400 

 300 

 200 

 1 00 



000 



Flatfish 



— — Rockfish 



\ 



V 



0.05 10 0.15 0.20 0.25 30 0.35 

 Proportion untrawlable 



0.40 0.45 050 



Figure 7 



Trawl-survey habitat-bias 



proportion of untrawlable habitat (A„/A) in a management area for two 



categories of fish. 



fleet size-index (ci,,) as a function of thi' 



sifications and thus did not focus purposively on particular 

 local features (e.g. individual rock outcrops) which could 

 serve as areas of more concentrated fish density. Another 

 factor could be the nature of the fishing history of the 

 study areas; the Washington site has long been subjected 

 to heavy fishing pressure, whereas the other 

 sites, particularly portions of the Soquel canyon 

 site, may have received relatively less fishing 

 pressure (Yoklavich'^). It is also possible that 

 zoogeographic differences, interannual vari- 

 ability, and the relatively small spatial scales 

 of the sampled areas could also explain the 

 differences in densities observed between the 

 studies. 



The level of concordance among the habitat- 

 specific studies reviewed in the present study 

 suggests that the potential exists for differ- 

 ences in fish density between trawlable and 

 untrawlable habitats. These differences can 

 be of great importance in the interpretation 

 of trawl survey results for groundfish stock 

 assessments. The presently available data are 

 insufficient, however, to accurately quantify the 

 magnitude of the trawl-survey habitat bias for 

 west coast groundfish stock assessment and 

 management. First, the absolute magnitude of 

 such a bias will depend largely on the amount 

 of untrawlable habitat present, which is not 

 well estimated at this time. Modern benthic 

 mapping technology and geographic informa- 

 tion systems are capable of yielding detailed 

 habitat maps over large spatial scales for 

 habitat area quantification, but such maps 

 are not yet available for most of the west coast 

 (Nasby, 2000). Second, although many of the 

 habitat-specific studies conducted to date tend 

 to support the notion of significant fish density 

 differences between trawlable and untrawlable 

 habitats on small scales, studies with larger 

 geographic scope are needed in order to be 

 relevant to the assessment and management 

 of west coast benthic fishery stocks. In particu- 

 lar, studies structured a priori with stratified 

 random sampling designs can afford improved 

 statistical inference by providing representa- 

 tive observations and unbiased parameter 

 estimates across a spectrum of habitat types. 



Estimation of the trawl-survey habitat-bias 

 may not be the preferred solution to address 

 habitat-specific density differences for all 

 groundfish species. The approach is likely to 

 work best for situations where 1 ) variability in 

 the density estimates obtained from the survey 

 used to sample both habitats (in our case, vi- 

 sual transects collected by submersible) is rela- 

 tively small compared to the variability in the 

 trawl survey, and 2) untrawlable habitat does 



-' Yoklavich, M. 2001. Personal commun. 

 Santa Cruz, California 95060. 



NMFS, 



