Stanley et al.: Diel vertical migration by Sebastes flavidus 



321 



■5I°00' 



BRITISH \ 



COLUMBIA [\ 



-N- 



50 100 



-48''00'-o 

 O 



en 



CM 



STUDY 

 AREA 



VANCOUVER 



ftNAQA . 

 Tj.S^A. 



KILOMETRES 



Figure 1 



Location of study area off the southwest coast of Vancouver Island, British Co- 

 lumbia, Canada. 



mates. In this report, we summarize our observa- 

 tions from this study and discuss the implications of 

 the results to hydroacoustic survey design for yel- 

 lowtail rockfish. 



Methods 



Study area 



We first surveyed a 250-km^ area along the shelf edge 

 off Nootka Island to locate concentrations of rock- 

 fish (Fig. 1). This area produces 1000-3000 t of yel- 

 lowtail rockfish per year (Rutherford, 1996). An ini- 

 tial grid included nine parallel transects 9-13 km in 

 length and 2.8 km apart. They were oriented south- 

 west to northeast, perpendicular to the isobaths. The 

 southwestern end of the track lines extended well 

 beyond the shelf break, whereas the shallower north- 

 eastern end extended onto the shelf to the 130-m 

 isobath. This A series was covered during one 24-h 

 period. The northern half of the A series showed the 

 greatest concentrations of rockfish. From this north- 

 ern area, we selected a B series of transects to con- 

 duct the study (Fig. 2). No further use was made of 

 A-series measurements because the respective 

 transects were not comparable with the B-series 

 transects. The distance between transects was halved 

 to 1.4 km, and the length of each was reduced to span 

 depths from the 140-m isobath to just beyond the 

 shelf break (180 m). The A series had shown that 



essentially all fish concentrations were between these 

 limits. The B series was repeated in the same se- 

 quence (B9-B2) through four 24-h cycles to produce 

 a balanced design of four diurnal and four nocturnal 

 observations per transect. In addition to the acous- 

 tic estimates for the eight replicates of B2-B9, we 

 selected B5 and B6 for continuous soundings, result- 

 ing in ten additional observations for these two 

 transects. 



The bottom topography of transects B2-B9, with 

 the exception of B8, was characterized by one or two 

 underwater cliffs They appeared to be 10-15 m high 

 and followed a north-south axis through the study 

 area, landward of the shelf break (Figs. 2-4). The 

 transects were oriented perpendicular to the axes of 

 the cliffs. At the southernmost transect ( B9 ), the cliffs 

 were approximately 1.5 km inshore of the shelf break. 

 This distance increased to about 7.0 km by transect 

 B2. The cliffs were found at a progressively shallower 

 depth. This topographic feature is well known to fish- 

 ermen as the "clay bumps" on the Nootka fishing 

 grounds. Fishermen report that catches of yellow- 

 tail rockfish can be made year-round by towing bot- 

 tom trawl nets in an east to west (shallow to deep) 

 direction over the cliffs. Transect B5 corresponds to 

 a commonly used commercial fishery tow (Figs. 2-4). 



We excluded the results of transect Bl from our 

 analyses. It passed over an extended underwater 

 ridge such that the modal depth was less than 130 m 

 rather than the targeted depth of 150 m. These 

 depths are shallower than the November habitat 



