129 



Abstract — An assessment of the total 

 biomass of shortbelly rockfish t.Sebastes 

 jordani) off the central California coast 

 is presented that is based on a spatially 

 extensive but temporally restricted ich- 

 thyoplankton survey conducted during 

 the 1991 spawning season. Contempor- 

 aneous samples of adults were obtained 

 by trawl sampling in the study region. 

 Daily larval production (7.56 x 10"^ lar- 

 vae/d) and the larval mortality rate 

 (Z=0.11/d) during the cruise were es- 

 timated from a larval "catch curve." 

 wherein the logarithm of total age-spe- 

 cific larval abundance was regressed 

 against larval age. For this analysis, lar- 

 val age compositions at each of the 

 150 sample sites were determined by 

 examination of otolith microstructure 

 from subsampled larvae (n=2203), 

 which were weighted by the polygonal 

 Sette-Ahlstrom area surrounding each 

 station. Female population weight-spe- 

 cific fecundity was estimated through 

 a life table analysis that incorporated 

 sex-specific differences in adult g^rowth 

 rate, female maturity, fecundity, and 

 natural mortality (M). The resulting 

 statistic (102.17 larvae/g) was insensi- 

 tive to errors in estimating M and to 

 the pattern of recruitment. Together, 

 the two analyses indicated that a total 

 biomass equal to 1366 metric tons (t)/d 

 of age-l+ shortbelly rockfish (sexes 

 combined) was needed to account for 

 the observed level of spawning output 

 during the cruise. Given the long-term 

 seasonal distribution of spawning ac- 

 tivity in the study area, as elucidated 

 from a retrospective examination of 

 California Cooperative Oceanic Fisher- 

 ies Investigation (CalCOFI) ichthyo- 

 plankton samples from 1952 to 1984, 

 the "daily" total biomass was expanded 

 to an annual total of 67,392 t. An 

 attempt to account for all sources of 

 error in the derivation of this estimate 

 was made by application of the delta- 

 method, which yielded a coefficient of 

 variation of 19%. The relatively high 

 precision of this larval production 

 method, and the rapidity with which 

 an absolute biomass estimate can be 

 obtained, establishes that, for some 

 species of rockfish {Sehastes spp.), it is 

 an attractive alternative to traditional 

 age-structured stock assessments. 



An approach to estimating rockfish 

 biomass based on larval production, 

 with application to Sebastes jordani* 



Stephen Ralston 



James R. Bence 



Maxwell B. Eldridge 



William H. Lenarz 



Southwest Fisheries Science Center 



National Marine FIshenes Service 



1 10 Shaffer Road 



Santa Cruz, California 95060 



E mail address (for S Ralston, contact auttior); Steve RalstoniS)noaa gov 



Manuscript accepted 20 September 2002. 

 Fish. Bull. 101:129-146 (2003). 



Shortbelly rockfish (Sebastes jordani) 

 is an underutilized species that is dis- 

 tributed from Vancouver Island to 

 northern Baja California (Eschmeyer, 

 198.3), although it is especially abun- 

 dant along the central California coast. 

 Based on a swept-area bottom trawl 

 survey of demersal rockfish, Gunder- 

 son and Sample ( 1980) estimated there 

 were 24,000 metric tons (t) of shortbelly 

 rockfish in the Monterey International 

 North Pacific Fishery Commission 

 (INPFC) area (35°30'N-40°30'N). This 

 biomass estimate was far greater than 

 that of any other species of rockfish in 

 any area and, significantly, it did not 

 include the midwater portion of the 

 stock. Hydroacoustic estimates of short- 

 belly rockfish biomass in the shelf-slope 

 area between Ascension Canyon and 

 the Farallon Islands (37°00'-38°00'N), 

 a distance of only 110 km, have ranged 

 from 153,000 to 295,000 t (Nunnely'). 



Although at present there is no di- 

 rected fishery for this species (Low, 

 1991), much is known of its biology. Ear- 

 ly work by Phillips (1964) provided ba- 

 sic information about the length-weight 

 relationship, growth as estimated from 

 scale annuli, spawning seasonality (i.e. 

 parturition), fecundity, maturity, and 

 the food habits of shortbelly rockfish. 

 Lenarz (1980) later studied shortbelly 

 rockfish growth using ages from whole 

 otoliths and provided preliminary cal- 

 culations of the effect of fishing on the 

 stock. He also demonstrated marked 

 spatial variation in age and length 

 composition along both latitudinal and 

 depth gradients. Growth was re-esti- 



mated by Pearson et al. (1991) using 

 ages determined from broken and 

 burnt otoliths. From the hydroacoustic 

 biomass estimates cited above and an 

 estimated range for the natural mortal- 

 ity rate (0.20-0.35 yr), they concluded 

 that the maximum sustainable yield 

 (MSY) of the shortbelly rockfish stock 

 in the Ascension Canyon-Farallon Is- 

 lands area was 13,400-23,500 t. 



Shortbelly rockfish is one of the few 

 Sebastes spp. that can be readily identi- 

 fied at all life history stages. Descrip- 

 tions of the early life stages of short- 

 belly rockfish, from preflexion larvae 

 through the pelagic juvenile stage, were 

 provided by Moser et al. ( 19771. Extend- 

 ing that work, MacGregor (1986) pro- 

 vided a summary of the spatiotemporal 

 distributions of shortbelly rockfish 

 larvae taken in California Coopera- 

 tive Oceanic Fisheries Investigation 

 (CalCOFI) cruises conducted in five 

 different years. His results showed that 

 99.1"^^ of all shortbelly rockfish lar\'ae 

 (-4-10 mm) were captured within 

 90 km of shore and that 65. 4"^* were 

 sampled during the month of Febru- 

 ary. Moreover, a strong peak in larval 

 abundance (i.e. 34.7Cf of the coastwide 

 total) was concentrated in the vicin- 



* Contribution 111 of the Santa Cruz Labor- 

 atory, Southwest Fisheries Science Center, 

 National Marine Fisheries Service, Santa 

 Cruz, CA 95060. 



' Nunnely, E. 1989. Personal commun. 

 Alaska Fisheries Science Center, 7600 Sand 

 Pomt Way N.E., Bin C 15700. Seattle, WA 

 98115-0070. 



