58 



Fishery Bulletin 94(1). 1996 



Figure 2 



A sampling diagram depicting the 20 one-minute transects where 

 juvenile blue rockfish, Sebastes mystinus, were counted. Counts 

 began offshore and ended inshore. 



to a certain point and then levels off to an asymp- 

 tote. We feel that the asymptotic level-off is real, but 

 because of the large variability in the daily counts 

 (see Fig. 3), it may be a sampling artifact. Both 

 Lockwood (1980) and Meyers and Cadigan (1993) 

 found that density-dependent mortality occurred in 

 juvenile marine fishes. Lockwood ( 1980) proposed a 

 two-stage density-dependent mortality mechanism 

 for juvenile plaice where normally piscivorous preda- 

 tors cause a baseline level of natural mortality and, 

 in years when juvenile numbers are high, other 



predators switch to preying on juveniles. We feel that 

 a similar process may occur with juvenile blue rock- 

 fish and that the natural mortality asymptote is the 

 point at which these secondary predators are saturated. 

 For blue rockfish, density-dependent mortality in 

 the benthic juvenile stage would reduce the inter- 

 annual variation between subsequent adult year 

 classes. An example of how density-dependent mor- 

 tality would reduce interannual variation can be 

 made by using the 1986 (a weak year class, see Fig. 

 4) and 1987 (a strong year class) data from Dark 



