Ralston et al : An approach to estimating rockfish biomass from larval production 



131 



rockfish larvae and spawning activity 

 was then determined by a retrospective 

 analysis of all CalCOFI ichthyoplankton 

 samples collected in the vicinity of Pioneer 

 Canyon. Based upon the timing of our lar- 

 val survey with respect to the long-term 

 mean distribution of spawning activity, 

 total "annual" biomass was calculated by 

 expansion. Finally, the robustness of the 

 total biomass estimate was evaluated 

 through a simulation study and sensitiv- 

 ity analyses. 



Methods 



Trawl sampling of adults 



38- 



3 



37- 



lOO-fathom contour 



124 



Specimens of adult shortbelly rockfish 

 were collected during February-March 

 1991 by the FV New Janet Ann and the RV 

 Novodrutsk (Table 1). Shortbelly rockfish 

 aggregations in and around Ascension and 

 Pioneer Canyons (Fig. 1) were targeted 

 from acoustic sui-veys. A total of 28 trawls 

 (12 bottom and 16 midwater) were con- 

 ducted over bottom depths ranging from 

 115 to 384 m, at an average net depth 

 of 126 m (range: 18-210 m). The bottom 

 trawl codend mesh size was 3.8 cm and the 

 midwater trawl mesh was 5.1 cm. Dura- 

 tion of the trawls ranged from 3 to 73 min 

 (x=26 min). All landings were either fully 

 weighed or subsampled, depending on the 

 size of the catch. Landings were highly 

 variable, ranging from 23 to 36,300 kg. 



Two subsamples were taken from each trawl landing. 

 The first was used to determine the overall age, size, sex, 

 and maturity composition of the catch; it was obtained by 

 randomly selecting and examining 100-300 individuals 

 from each trawl, depending on the availability of both 

 time and fish. For these specimens, total lengths (TL) 

 were measured to the nearest mm. gonads were examined 

 to determine sex and to assign gi-oss maturity stages (see 

 below), and otoliths were removed for later age determina- 

 tion by the break-and-burn method (Pearson et al., 1991). 

 A second smaller subsample of 25-50 specimens was also 

 taken to estimate length-weight and fecundity relation- 

 ships. For each of these fish.TL was measured, weight was 

 determined to the nearest mg, the otoliths were extracted, 

 and late vitellogenic ovaries were dissected from females 

 and fixed in Gilsons fluid. 



We rated females in terms of maturity based on gross 

 gonadal condition. A summary of the scale we used is the 

 following: 1.0 = immature; 2.0-2.9 = vitellogenic oocytes 

 (yolk deposition with associated oocyte and ovary enlarge- 

 ment); 3.0-3.9 = fertilized eggs (embryos to hatched lar- 

 vae); 4.0 = spent; and 5.0 = reorganization and recovery. 



Fecundity of female shortbelly rockfish was estimated 

 gravimetrically from vitellogenic ovaries after 2-4 months 



Pioneer Canyon 



+ + 



Ascension Canyon 



-I- 



+ bongo station 



* adult trawl station 



* CalCOFI station 63.55 



123 



122 



Longitude 



Figure 1 



Map of the central California study region showing bongo net and adult 

 trawl sampling locations. The annual spawning season was estimated by 

 the long-term seasonal distribution of shortbelly rockfish larvae at CalCOFI 

 station 63.55. 



fixation with periodic stirring. Entire fixed ovaries from 

 each female were blotted dry and weighed to the nearest 

 1.0 mg. Duplicate subsamples of both ovaries were weighed 



