130 



Fishery Bulletin 101(1) 



ity of Pioneer Canyon (CalCOFI line 63). Later research 

 by Laidig et al. (1991) resulted in the development of a 

 detailed growth model for young-of-the-year shortbelly 

 rockfish, from extrusion through the late pelagic juvenile 

 stage (-180 d), and verified the feasibility of a daily ag- 

 ing protocol by validating a one-to-one correspondence 

 between counts of daily increments and elapsed time in 

 days. More recent work by Ralston et al. (1996) showed 

 that larval shortbelly rockfish can be accurately aged by 

 using optical microscopy. 



For the year 2000 the Pacific Fishery Management 

 Council (PFMC) revised the shortbelly rockfish acceptable 

 biological catch (ABC) downwards from 23,500 to 13,900 

 fyr (PFMC-). The new ABC is based on the low end of the 

 estimated MSY range presented in Pearson et al. (1991); 

 it was reduced due to a probable natural decline in stand- 

 ing stock during the 1990s arising from poor ocean condi- 

 tions (MacCall, 1996). The original range, however, was 

 derived by using quite variable data from unpublished hy- 

 droacoustic surveys and Pearson et al. (1991) considered it 

 a strictly preliminary estimate. Given that the biomass of 

 shortbelly rockfish along the central California coast was 

 once thought to be very large (Gunderson and Sample, 

 1980; MacGregor, 1986), and that the species is still proba- 

 bly the single largest rockfish contribution available to the 

 west coast groundfish fishery, data are needed to estimate 

 the size of the stock more precisely than results available 

 from these previous investigations. 



The goal of this study was to develop an analytical ap- 

 proach to estimate the total biomass of shortbelly rockfish 

 in the region of Pioneer and Ascension Canyons and to 

 gather field data to evaluate the method. Successful appli- 

 cation of the method to shortbelly rockfish would provide 

 to the PFMC information useful for management. On a 

 more fundamental level, it would also assist in developing 

 fishery-independent survey techniques capable of assess- 

 ing other, more highly exploited, species of rockfish. 



The assessment approach 



The basic premise of egg and larval surveys is that it is 

 easier to estimate the absolute abundance of ichthyo- 

 plankton than it is to estimate that of adults (Saville, 

 1964; Gunderson, 1993). This is especially true when the 

 spatial distribution of adults exhibits some type of size- or 

 age-specific pattern. Shortbelly rockfish is one such spe- 

 cies (Lenarz, 1980) and obtaining a representative sample 

 of the adult population is challenging (Lenarz and Adams, 

 1980). Conversely, early life history stages (i.e. eggs and 

 preflexion larvae) can be sampled effectively with stan- 

 dard plankton nets (Smith and Richardson, 1977). Due to 

 the direct coupling between egg [iroduction and spawning 

 biomass, mediated through population weight-specific 



■^ PFMC (Pacific Fishery Management Council). 1999. Status 

 of the Pacific coast proundfish fi.shcry through 1999 and recom- 

 mended acceptable biological catches for 2000, 44 p. + .54 tables 

 and 5 figs. Pacific Fishery Management Council. 21.30 SW 

 Fifth Ave.. Portland, OR 97201. 



fecundity(^[eggs/gl), egg and larval surveys have proven 

 successful for estimating spawning biomass in many 

 applications (e.g. Houde, 1977; Parker, 1980; Richardson, 

 1981; Lasker, 1985; Armstrong et al., 1988; Hunter et al, 

 1993). 



Members of the scorpionfish genus Sebastes are distinc- 

 tive because they are primitive viviparous livebearers 

 (Wourms, 1991), resulting in parturition of advanced 

 yolksac larvae (Bowers, 1992). This reproductive strategy 

 lends itself to a larval production stock assessment be- 

 cause the age of all spawning products can be accurately 

 determined from otolith microstructure (Laidig et al., 

 1991; Ralston et al., 1996). In contrast, in egg surveys, 

 egg age is back-calculated to the time of spawning by 1 ) 

 defining a series of developmental stages, 2) estimating 

 the relationship between stage-specific developmental 

 rates and temperature, 3) assigning a thermal history to 

 each egg, and 4) determining the time required to account 

 for embryo development from spawning to the obsei-ved 

 stage (see for example Lo, 1985; Moser and Ahlstrom, 

 1985). Because a distinctive extrusion check forms on 

 the otoliths of Sebastes larvae at the time of parturition 

 (Ralston et al., 1996), a rockfish lai-val production estimate 

 does not require information on temperature-dependent 

 developmental rates and the ambient thermal history of 

 egg samples. 



In the approach presented in the present study, a spa- 

 tially extensive but temporally restricted ichthyoplankton 

 survey was conducted. The age composition of larvae in 

 each plankton tow was determined by subsampling the 

 catch, aging the subsample, and expanding the subsample 

 age composition back to that of the tow total. The total 

 age-specific abundance of larvae in the study region was 

 calculated by weighting the larval catch-at-age in each 

 plankton sample by the polygonal area around it (see 

 Sette and Ahlstrom, 1948). Characterization of the declin- 

 ing trend in total larval abundance at age with an expo- 

 nential mortality model allowed estimation of the produc- 

 tion rate of day-0 larvae and the larval mortality rate at 

 the time the ichthyoplankton cruise was conducted. 



Information on adult reproduction was obtained from 

 contemporaneous data collected during two separate 

 cruises conducted during the spawning season. In particu- 

 lar, the following functional relationships were estimated 

 from the data collected: 1 ) weight as a function of total 

 length, 2) sex-specific weight at age (i.e. male and female 

 von Bertalanffy growth equations), 3) fecundity as a func- 

 tion of total weight, 4) maturity as a function of age, and 

 5) the population sex ratio. From these relationships, a 

 life table was constructed, based on an estimate of natu- 

 ral mortality (/yr), that yielded estimates of population 

 weight-specific fecundity (O). As defined here, population 

 weight-specific fecundity includes the biomass contribu- 

 tions to total population size from males and immature 

 females. 



Together, these estimates (daily larval production and 

 population weight-specific fecundity) can be used to cal- 

 culate the "daily" total biomass of fish in the population 

 required to produce the obsei-ved abundance of lai^vae. 

 The long-term mean seasonal distribution of shortbelly 



