Krieger and Sigler: Catchability coefficient for rockfish 



287 



in other studies. Uzman et al. ( 1977) compared sub- 

 mersible counts with trawl catches offish and inver- 

 tebrates and concluded that continuous visual as- 

 sessment of the benthos provides the best approxi- 

 mation of species abundance and diversity. Adams 

 et al. (1995) compared remotely operated vehicle 

 counts with trawl catches of groundfish and con- 

 cluded that for strongly bottom-associated ground- 

 fish, ROV abundance estimates were significantly 

 higher and were less variable than bottom-trawl 

 abundance estimates. 



Acknowledgments 



We thank the crew of the submersible Delta, for safe 

 and successful dives, and the crew and scientists 

 aboard the NOAA ship John N. Cobb for their de- 

 tailed sampling that allowed comparisons of trawl 

 catches with submersible observations. We also 

 thank John Karinen for participating in the submers- 

 ible diving, and Jon Heifetz, Scott Johnson, Steve 

 Ignell, Richard Carlson, and Sara Kraft for their thor- 

 ough reviews of this manuscript. The Delta was char- 

 tered by NOAA's National Undersea Reserch Center. 



Application to stock assessment 



The catchability coefficient estimated in this study 

 with a 400 Eastern trawl may not be directly appli- 

 cable to Alaska trawl surveys that use a Nor 'eastern 

 trawl. The main differences between the Nor 'eastern 

 trawl and 400 Eastern trawl are net height and 

 footrope design: the Nor'eastern net height is 6-10 

 m and roller gear (36-cm rubber bobbins separated 

 by 10-cm rubber disks) is attached to the footrope, 

 whereas the 400 Eastern has a maximum 1.8-m net 

 height with no roller gear on the footrope. The 15- 

 17 m wingtip spread of the Nor'eastern is similar to 

 the 15.8 m average spread of the 400 Eastern trawl, 

 whereas the 47-57 m door spread of the Nor'eastern 

 is wider than the average 45-m door spread of the 

 400 Eastern. The Nor'eastern trawl is assumed to 

 be more efficient for capturing rockfish because of 

 its higher vertical opening. Roller gear attached to 

 the Nor'eastern trawl may either increase or decrease 

 catch efficiency of rockfish. A repeat of this study 

 using the Nor'eastern trawl is recommended for de- 

 termining a catchability coefficient for Alaska trawl 

 survey data. 



Accurate indexing of Pacific ocean perch seems 

 possible from bottom-trawl surveys because of their 

 substrate preference and susceptibility to bottom- 

 trawl gear. Pacific ocean perch concentrate on 

 trawlable substrates (Krieger, 1993) and were effec- 

 tively sampled in this study. Assuming that fish oc- 

 cupy the water column above the opening of the trawl 

 and inhabit untrawlable areas, Balsiger et al. ( 1985) 

 reported that CPUE data from bottom-trawl surveys 

 probably underestimate Pacific ocean perch biomass. 

 Submersible studies, however, indicate Pacific ocean 

 perch biomass may be overestimated from bottom- 

 trawl surveys for two reasons: 1) the catchability 

 coefficient of the survey trawl may exceed 1.0 for the 

 distance between the wingtips; and 2 ) only trawlable 

 substrates where Pacific ocean perch are concentrated 

 are sampled during surveys, but CPUE from trawlable 

 substrates is applied to untrawlable substrates. 



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