330 



Fishery Bulletin 97(2), 1999 |»* 



I ill 



greater variation if repeated estimates were con- 

 ducted at greater time intervals. 



Over a one-week period and an area of over 100 

 km^, we have demonstrated precision (CV=15%) in 

 acoustic biomass estimates that is acceptable in a 

 stock assessment context for yellowtail rockfish. Vari- 

 ance among replicated transects is low enough that 

 survey effort can be distributed to maximize the num- 

 ber of different transects, either to increase the area of 

 the survey or the density of coverage. We have shown 

 that a survey of yellowtail rockfish can be conducted 

 throughout the diel cycle and thus reduce survey costs. 



This study also indicates that yellowtail rockfish 

 can aggregate within a well-defined bathymetric 

 range near topographic features. If these tendencies 

 are consistent over the whole range of this species, 

 they may provide the basis for stratification and pos- 

 sible further gains in efficiency. Our results indicate 

 that a simple systematic transect design with transects 

 oriented perpendicular to the long axis of the fish con- 

 centrations (the edge of the continental shelf) is a sat- 

 isfactory choice for the elementary sampling distance 

 unit (ESDU) (Simmonds et al., 1992; Simmonds and 

 Fryer, 1996). If the preferable depth range is narrow 

 over most of the coastline and the survey must thus 

 cover a long nan-ow corridor, then a systematic zig-zag 

 would be preferable (Simmonds et al., 1992). 



The affiliation of yellowtail rockfish with a minor 

 topographic feature within a depth range indicates 

 that the density distribution of yellowtail rockfish is 

 "nonstationary," in that the densities will not be ran- 

 domly distributed in a study area or stratum, even 

 after bathymetric stratification (Simmonds et al., 

 1992). This is an important characteristic of the spe- 

 cies and should be noted if more advanced survey 

 design and analysis procedures, such as geostatistical 

 spatial averaging and cokriging, are to be investi- 

 gated (Foote and Stefansson, 1993; Marcotte, 1991). 



Although the results of the experiment support the 

 potential for acoustic estimation of this species, the 

 generality of the conclusions will have to be tested over 

 a larger scale and more varied habitat. The study site 

 was chosen carefully to minimize the unknowns, in 

 particular the presence of other species. The hypoth- 

 eses will have to be re-examined over different depths 

 and topography. It is also possible that the annual cycle 

 of maturation, mating, and parturition for these live- 

 bearing fish may be associated with different behavior. 



Acknowledgments 



We thank R. Kronlund, J. Schweigert, and three 

 anonymous reviewers for constructive reviews of 

 earlier drafts. 



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