140 



Fishery Bulletin 101(1) 



— I — I — I — I — I — I — I — r — I — I — I — I — 1 — I — I — I 



3.0 4.0 5.0 60 70 



loQc (catch witti 505-(.im net) 



Figure 9 



Paired comparison of log^-transformed larval shortbelly 

 rockfish catches taken in bongo nets with different mesh 

 sizes (all samples collected during February 1991). Also 

 shown is the line of equality. 



505) resulted mx= -0.1458, n = 23J = -1.24, and P = 0.23, 

 indicating no difference in the catch of the larger mesh net 

 in relation to the smaller net. We conclude that the 505-pm 

 net was an effective sampler of rockfish larvae. 



Examination of the standardized catches from the MOC- 

 NESS tows conducted in 1992 revealed that shortbelly 

 rockfish larvae were not caught in depths below the 120- 

 160 m interval (Fig. 10). The mean catch rate in that 

 depth range was 16.8 larvae/1000 ni'^ which amounted to 

 only l.S'/f of the combined average catch at all depths, i.e. 

 98.7% of all larvae were captured at depths < 120 m. Be- 

 cause the bongo net was deployed to a maximum depth of 

 140 m in 1991, and because the mixed layer depth was de- 

 pressed during MOCNESS sampling in 1992 due to strong 

 El Nino conditions (Lynn et al., 1995), we concluded that 

 the 1991 bongo net survey sampled the entire depth range 

 where shortbelly rockfish larvae occurred. 



For this assessment, a total of 2203 shortbelly rockfish 

 larvae were subsampled from the 505-pm bongo net catch- 

 es and were aged by using optical microscopy (see Ralston 

 et al., 1996). Results from that work indicated that the ages 

 were quite precise (849^ agreement among three readers to 

 within ±1 d) and there were, moreover, no increment in- 

 terpretation differences between optical observations and 

 those made with a scanning electron microscope (SEM). 



The horizontal distribution of very young (0-2 d) short- 

 belly rockfish larv-ac was strongly associated with the 

 continental shelf break (Fig. 11) and, latitudinally, with 

 the Pioneer Canyon area. Due to the coincidence of this 

 distribution with the locus of adult sampling sites (Fig. 

 1), we concluded that the trawl survey sampled the adults 

 that produced the lai-vae captured in the iclithyoplankton 

 survey. This finding supports the coupling of our popula- 

 tion weight-specific fecundity statistic (102.17 larvae/g of 

 female) with larval production to estimate the total bio- 



Sample depth (m) 



Figure 10 



Mean catch rate of shortbelly rockfish larvae in seven 

 depth strata taken by a MOCNESS sampler during March 

 1992. Means are bracketed bv ±1.0 standard error 



mass of shortbelly rockfish in the Pioneer Canyon region. 

 Although not shown, older age classes of larvae tended to 

 be more dispersed and to occur increasingly to the north- 

 west — a pattern consistent with hydrographic conditions 

 at the time of the survey (Sakuma et al., 1995). 



When Sette-Ahlstrom weights were used to expand age- 

 specific standardized bongo net catches to the entire study 

 region, the composite age-frequency distribution of short- 

 belly rockfish larvae appeared to support a log^-linear 

 model with constant exponential mortality (Z |/d| ), i.e. 



iV^ = AT^e^^r iog^.(7Vj,) = log,,(A^o) - ZT. 



where Nj = the total abundance of larvae of age Ttd); 



Z = the instantaneous larval mortality rate (/d); 

 and 

 7V„ = the lai^val renewal rate (i.e. daily production 

 of larvae). 



The model was fitted over the first 25 days of life, which 

 largely represents the preflexion stage. In addition, the N-j. 

 were first coded by scaling all obsei-vations to 1x10" and 

 0.5 was added to all larval ages as a continuity correction 

 (see Ralston et al., 1996). Results show (Fig. 12, Table 2) 

 a satisfactory fit, although there is some suggestion of 

 an aberrant, serially correlated pattern in the residuals 

 from age 0-9 d. The ,v-mtercept term (log.liV,,! = -0.3775), 

 when back-transformed iwith bias-correction 1 yields N,, = 

 7.562x10'" age-0 lai-vae. 



Given estimates of daily larval production (7.562x10'" 

 larvae) and population weight-specific fecundity (4>= 

 102.17 lai-vae/g of female), we calculated from the ratio 

 of these two quantities that 740 t of female shortbelly 

 rockfish spawned each day during the bongo net cruise 

 (DSJ-9102), which is equivalent to a daily biomass of 1366 

 t/d (sexes combined). 



