682 



Fishery Bulletin 103(4) 



CD 

 CO 



w 



0.00 0.02 0.04 0.06 08 10 



F 



Figure 9 



Spawning biomass per recruit (SSB/R) against instan- 

 taneous fishing mortality (F) for silvergray rockfish 

 <Sebastes brevispinis). 



at the age of maximum fecundity ranged from 0.01 

 to 0.25 for rockfish. Fecundity at 50% maturity could 

 not be determined because we had no observations for 

 females less than 12 years of age. However, if we use 

 fecundity at age 12 (the youngest fish in our sample) 

 and fecundity at age 40 (the predicted age of maximum 

 fecundity), the ratio exceeds 0.40. This finding supports 

 the contention that age at 50% maturity for silvergray 

 rockfish is less than 12 years and adds credibility to 

 the observation that the age of 50% maturity is lower 

 than the age at 50% selectivity. 



Estimates of specific fecundity (fecundity/somatic 

 weight) were 356 and 482 ova/g for the 12-year-old and 

 40-year-old females, respectively. Given that the age at 

 50% maturity is probably less than 12 years; this range 

 in "relative investment" in reproduction appears aver- 

 age for rockfish (Haldorson and Love, 1991). As with 

 other rockfish, specific fecundity increases with size, 

 although it appears to reach an asymptote at age 40 

 for silvergray rockfish. 



Age at maturity and SSB/R 



An M of 0.06 places silvergray rockfish in the middle 

 to lower end of the mortality range for rockfishes. It 

 is higher than the estimates of 0.02-0.04 reported for 

 yelloweye rockfish (O'Connell and Fujioka 9 ; O'Connell et 



al. 10 ; Yamanaka and Lacko, 2001) but much less than 

 0.14 that has been used for yellowtail rockfish, or 0.28 

 used for black rockfish (S. melanops) (Dorn, 2002). 



The analysis of SSB/R indicates that an F- n , cor- 

 responds to F=0.072 or F=1.2M. This F to M ratio rep- 

 resents a more aggressive harvest strategy than the 

 range of 0.5-1.0 currently supported in the literature 

 (Patterson, 1992; Walters, 1998). This result is caused 

 by the special case of silvergray rockfish, anticipated 

 by Clark (1991), wherein recruitment at age is delayed 

 in comparison to maturity at age. If most females actu- 

 ally mature by age 11 or 12 years, but are still not 50% 

 vulnerable at age 14 (Fig. 9), then even at a relatively 

 high fishing mortality, most females can reproduce a 

 few times prior to capture. 



As stated above, recruitment to the fishery may be 

 driven more by the stage of maturation than by size or 

 age. Movement to areas and depths that are the source 

 for most fishery samples may be governed by behav- 

 ioral issues associated with maturation. If fish tend to 

 recruit as they become mature, somewhat independent 

 of size or age, then we may underestimate the age of 

 50% maturity. In this respect, it is interesting that the 

 fecundity data, compared to other rockfish data, also 

 indicate that the age of 50% maturity may be much 

 less than 12 years. 



Our suggestion to managers is that unless the non- 

 recruited population can be sampled to verify matu- 

 rity-at-age assumptions, then a more precautionary 

 approach is warranted than is implied by an F=1.2M 

 logic for harvest strategy. This silvergray rockfish ex- 

 ample emphasizes the sensitivity of an SSB/R harvest 

 logic to estimating age at maturity, which in turns em- 

 phasizes the often neglected issues of field classification 

 of maturity and the representativeness of samples. The 

 task of estimating age at maturity is perhaps too often 

 ignored at the expense of estimating other life history 

 parameters. 



Conclusion 



Owing to the small role that silvergray rockfish has 

 played in groundfish fisheries of the eastern North 

 Pacific Ocean, this species has received little research 

 attention. However, these less valuable stocks are begin- 

 ning to attract more attention owing to their potential to 

 disrupt precautionary management objectives within the 

 context of a multispecies fishery. With the shift to a more 

 precautionary paradigm, a lack of stock knowledge about 

 the status of any of the incidental species, such as silver- 

 gray rockfish, can be a basis for restricting the overall 

 fishery. Strategic allocation of resources by species or 

 stock can no longer be predicated on landed value. 



u O'Connell, V., C. Brylinsky, and D. Carlile. 1991. Demersal 

 shelf rockfish stock assessment and fishery evaluation report 

 for 2004. Alaska Dep. Fish and Game Regional Information 

 Report J03-39, 44 p. 304 Lake St. #103, Sitka, AK 99835- 

 7563. 



10 O'Connell, V., and J. Fujioka. 1991. Demersal shelf rock- 

 fish. In Status of living resources off Alaska as assessed 

 in 1991, p. 46-47. NOAA. Tech. Memo. NOAA-TM-NMFS- 

 F/NWC-211. 304 Lake St. #103, Sitka, AK 99835-7563. 



