Ralston and Howard: Year-class strength and cohort variability In Sebastes mystinus and 5. flavidus 



71 



year-class strength are feasible under a variety of 

 scenarios, if abundance is indexed following juvenile 

 metamorphosis. 



The reproductive biology and early life history of 

 Sebastes is distinctive (Boehlert and Yamada, 1991). 

 All rockfishes are livebearers, displaying a primitive 

 form of viviparity (Boehlert et al., 1987; Wourms, 

 1991). Along the west coast of North America, most 

 commercial species of rockfish copulate around Sep- 

 tember, but fertilization may not occur until weeks, 

 or even months, later (Wyllie Echeverria, 1987). 

 Hatching occurs in the ovary after 25-35 days of 

 embryonic development, and parturition occurs ap- 

 proximately five days later at about the time of yolk- 

 sac absorption (Eldridge et al., 1991; Yamada and 

 Kusakari, 1991). In the central and northern Cali- 

 fornia region, parturition of most commercial spe- 

 cies occurs in late fall, winter, and early spring but 

 is most concentrated from January to March (Wyllie 

 Echeverria, 1987; Moser and Boehlert, 1991). 



Rockfish larvae typically are found in the upper 

 mixed layer (Ahlstrom, 1961; Ralston et. al. 1 ), where 

 they grow slowly until flexion is complete at an age 

 of about 25 days (Laidig et al., 1991). Late larvae 

 (10-20 mm SL) are distributed well offshore (Moser 

 and Boehlert, 1991); at this stage growth rate in- 

 creases. Late larvae then metamorphose into a pe- 

 lagic juvenile stage characterized by attainment of 

 mature meristics and pelagic coloration (Moser et al., 

 1977; Matarese et al., 1989; Moser and Boehlert, 

 1991). Growth of pelagic juveniles, which feed pri- 

 marily on copepods and on both larval and juvenile 

 stages of euphausiids (Reilly et al., 1992), can be quite 

 rapid (0.3-0.6 mmd" 1 ) (Woodbury and Ralston, 

 1991). As pelagic juveniles approach sizes that are 

 competent to settle (30-90 mm SL, depending on the 

 species), they move deeper in the water column 

 (Lenarz et al., 1991) and closer to shore (Larson et 

 al., 1994). These changes in spatial distribution oc- 

 cur at a time of maximum offshore Ekman transport 

 of the ocean's surface layer and onshore recircula- 

 tion of subsurface waters (Mooers et al., 1978; Largier 

 et al., 1993). Peak settlement to demersal nearshore 

 habitats occurs during the upwelling season from 

 May to July (Carr, 1983; Love et al., 1991), after the 

 fish have spent from 3 to 6 months as plankton and 

 micronekton. Settlement usually occurs in relatively 

 shallow water and, after a period of several months 

 to a year, many species begin to move into adult habi- 

 tats located in deeper water (Love et al., 1991). 



In this study, we attempt to determine at what 

 point in the life history year-class strength becomes 

 fixed in two species of rockfish, i.e. blue rockfish, 

 Sebastes mystinus, and yellowtail rockfish, Sebastes 

 flavidus. Of particular interest is the influence of 

 settlement on interannual variations in reproductive 

 success. We compare abundance estimates of young- 

 of-the-year juvenile rockfish that were gathered be- 

 fore and after settlement to determine if this life his- 

 tory transition alters relative year-class strength. 

 The comparison is based on two separate fishery-in- 

 dependent surveys conducted over a 10-year period 

 (1983-92). In addition, we examine the relationship 

 between sea-surface temperature (SST) during the 

 larval period and estimates of year-class strength. 



For the arguments developed here, we assume that 

 during the study period variation in year-class 

 strength of blue and yellowtail rockfish was due to 

 interannual differences in reproductive success and 

 not to fluctuations in spawning biomass. Given the 

 generally weak relationship between recruitment 

 and spawning stock (Cushing, 1973), this is not an 

 unreasonable assumption, particularly since Sebastes 

 are slow-growing species with low rates of natural 

 mortality (Leaman and Beamish, 1984). Nonetheless, 

 yellowtail rockfish have been the focus of a substan- 

 tial commercial fishery for many years, even though 

 the fishery operates primarily to the north of our 

 study region (Tagart 2 ). 



Materials and methods 



Midwater trawl surveys 



Annual trawl surveys designed to estimate the dis- 

 tribution and abundance of pelagic juvenile rock- 

 fishes along the central California coast have been 

 conducted aboard the RV David Starr Jordan since 

 1983 (Wyllie Echeverria et al., 1990). Cruises have 

 been conducted during May and June when the pe- 

 lagic juvenile-stage fish are most susceptible to cap- 

 ture by midwater trawling. These surveys use a modi- 

 fied 26 x 26 m Cobb midwater trawl, with a codend 

 liner of 1.27-cm stretched mesh. Beginning in 1986, 

 three spatially replicated "sweeps" of a series of stan- 

 dard stations were conducted in a study area bounded 

 by Point Reyes and Cypress Point (Fig. 1); from 1983 

 to 1985 only one sweep was completed per year. As 

 part of the survey design, the area was subdivided 



1 Ralston, S., J. R. Bence, M. B. Eldridge, and W. H. Lenarz. 

 1993. Estimating the spawning biomass of shortbelly rockfish 

 (Sebastes jordoni) in the region of Pioneer and Ascension Can- 

 yons using a larval production method. Natl. Mar. Fish. Serv., 

 NOAA, 3150 Paradise Dr., Tiburon, CA 94920. Unpubl. manuscr. 



2 Tagart, J. V. 1993. Status of the yellowtail rockfish resource 

 in 1993, Appendix E. In Appendices to the Status of the Pa- 

 cific Coast groundfish fishery through 1993 and recommended 

 acceptable biological catches for 1994. Pacific Fishery Man- 

 agement Council, 2000 SW First Ave., Suite 420, Portland. OR. 



