574 



Fishery Bulletin 91(3). 1993 



age groups for both areas. Otoliths were visually in- 

 spected without a microscope to look for gross morpho- 

 logical differences. 



Results and discussion 



Several unique features were observed in the landings 

 of widow rockfish from Cobb Seamount. Mean length- 

 at-age was similar for six-year-old widow rockfish in 

 both areas; however, size composition, age composi- 

 tion, and physical structure of the otoliths were very 

 different between the two areas. 



Mean length of six-year-old widow rockfish on the 

 Cobb Seamount was 35.3 cm FL (ra=72, SE=0.11) and 

 37.3 cm FL (re=140, SE=0.09) for males and females, 

 respectively. In the nearshore samples, the mean length 

 of six-year-old fish was 35.3cm FL (n=53, SE=0.22) 

 and 37.8 cm FL (ra=52, SE=0.26) for males and females, 

 respectively. This suggests that mean length-at-age, at 

 least for young fish, is similar between fish caught on 

 the Cobb Seamount and fish caught in the nearshore 

 area. Assuming that the fish on the seamount spent 

 the majority of their lives there, then it is reasonable 

 to assume that conditions affecting growth (tempera- 

 ture, food availability, competition, etc.) are probably 

 comparable to the conditions found in the nearshore 

 environment. 



In the northern Oregon-southern Washington area of 

 the Pacific coast, the mean length of widow rockfish in 

 the commercial landings is 38.4cm (Pearson and 

 High tower, 1991). In contrast, the mean length of widow 



rockfish from Cobb Seamount was 33.1cm. Age compo- 

 sition of the widow rockfish in the landings from the 

 Cobb Seamount is also very different from those in the 

 landings from the nearshore area (Fig. 1). Only 7.5% of 

 all fish collected from the Cobb Seamount were more 

 than six years old, while 85.7% of fish collected from 

 the nearshore samples were more than six years old 

 (Fig. 1). This difference in age composition cannot be 

 explained by differences in gear type because the same 

 gear and mesh size was used in both areas. There are 

 several potential explanations for these differences in- 

 cluding differences in fishing pressure, differences in 

 fishing behavior, differences in discard practices, and 

 actual differences in the population age composition. 



The argument for differences in fishing pressure re- 

 lies on the belief that widow rockfish on the Cobb Sea- 

 mount were heavily exploited prior to 1985 and there- 

 fore their population was reduced to very low levels. 

 This argument is supported by Sasaki (1986) who re- 

 ported that the seamount had, in fact, been heavily 

 fished. Widow rockfish were never specifically men- 

 tioned as having been caught by any fishery prior to 

 1991. While intense fishing pressure on the Cobb Sea- 

 mount may be responsible for the absence of older 

 widow rockfish, there are arguments against this ex- 

 planation. First, a sample of 50 harlequin rockfish (S. 

 variegatus) had a mean age of 15 years, much older 

 than would be expected if they had experienced simi- 

 lar fishing mortality to the widow rockfish. In addi- 

 tion, even if the Cobb Seamount had experienced ex- 

 tremely heavy fishing pressure, it seems unlikely that 

 virtually all the older fish would have been caught. 



50 



o 



c 



CD 

 3 



cr 



CD 



k_ 



LL 

 +-* 

 C 

 CD 



2 



cd 

 a. 



30 



Cobb Swunount 



n-724 



N««rahorB 



n-824 



F^ 



12 13 



Age (years) 



Figure 1 



Comparison of age composition from widow rockfish caught on Cobb Seamount and the 

 coastal area of northern Oregon. 



