Williams and Ralston Distribution and co-occurrence of Sebastidae off California and Oregon 



847 



i 39 



100 200 300 400 



Depth (m) 



500 



Figure 5 



Contour map of species richness (number of species) from 

 the AFSC triennial bottom trawl survey for all samples 

 containing rockfish species. 



Discussion 



Our data were the result of pooling samples collected 

 during AFSC triennial continental shelf trawl surveys 

 conducted from 1977 to 1998. Because this interval rep- 

 resents a period of substantial fishery removals (Ralston, 

 1998), one might expect that rockfish assemblage struc- 

 ture changed over time as commercially important species 

 were serially removed (e.g. bocaccio and canary rockfish). 

 Although we did not perform year-specific analyses that 

 would allow us to address this possibility, results from 

 Weinberg (1994) showed that, over a comparable period 

 of time (1977-92), the trawlable assemblages of rockfishes 

 ofT Oregon and Washington were reasonably stable in 

 composition. In particular, his year-specific recurrent 

 group analyses revealed good agreement among surveys, 



from which he inferred the existence of three groups rep- 

 resenting a deep-water assemblage (shortspine thorny- 

 head. Pacific ocean perch, darkblotched, and redbanded 

 (S. habcocki) rockfish), a mid-shelf assemblage (canary, 

 yellowtail, and greenstriped rockfish). and a shelf-break 

 assemblage (sharpchin. rosethorn, and redstriped rock- 

 fish). Notably, there was substantial overlap between the 

 latter two groups, which is consistent with our findings. 



It is no surprise that rockfish distributions are re- 

 lated to bathymetric features, particularly when viewed 

 through bottom trawl survey samples, as we did in our 

 study. It is noteworthy that physical barriers, particularly 

 the Mendocino Escarpment and Monterey Canyon, seem 

 to affect the latitudinal distribution of certain rockfishes. 

 The former may act as a barrier to dispersal because 

 converging currents at Cape Mendocino create conditions 



