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Fishery Bulletin 103(1) 



dition (the ratio of actual to expected weight, based on 

 length-weight relationships), and fecundity were based 

 on empirical measures of S. mystinus during El Nino 

 years (Lenarz et al., 1995; VenTresca et al., 1995). As 

 in the baseline model, 1 expressed energy consumption 

 by both sexes at individual and per-recruit scales. 



Finally, I ran simulations at the per-recruit scale in 

 which the total mortality rate (Z) was increased by add- 

 ing a fishing-induced mortality rate (F) in increments of 

 0.05 to M; fishing mortality was imposed on fish greater 

 than 20 cm, the size at which S. mystinus enters fisher- 

 ies in California waters (Laidig et al., 2003). The range 

 of Z examined was 0.2 (natural mortality only) to 1.0 

 (a heavily overfished condition). These simulations were 

 run under baseline conditions and El Nino conditions 

 to determine if there was any interaction between El 

 Nino effects and Z. 



Sensitivity analysis 



To measure sensitivity of the Sebastes bioenergetics 

 model to different parameters, I used a Monte Carlo 

 error analysis method (Bartell et al., 1986). In this 

 method, parameters are drawn randomly from normal 

 distributions with means equal to parameter estimates 

 (Table 1) and with a coefficient of variation (CV) of either 

 2%, 10%, or 20%. Cases where randomly drawn RTO 

 was greater than RTM were discarded. Female and 

 male models were run 1000 times for each of the three 

 CVs. Individual simulations ran to age 30 at 0.25-year 

 increments; seasonal temperatures were those used in 

 the baseline model. Parameter influence on 30-year 

 cumulative consumption estimates was judged accord- 

 ing to the parameters' relative partial sums of squares 

 (RPSS), which quantify the influence of a parameter 

 after all other parameters have been accounted for. 

 RPSS for all parameters were calculated with SYSTAT 

 (version 10.2, SYSTAT Software Inc., Richmond, CA). 

 Additionally, means and standard deviations of con- 

 sumption estimates from RPSS analyses were calculated 

 to capture the range of energy consumption possible over 

 the lifetime of female and male S. mystinus. 



Results 



Northern California S. mystinus baseline energy demands 



Baseline energetic demands of northern California S. 

 mystinus were a function of size, sex, and the scale of 

 calculation (i.e., individual versus per recruit). As size 

 increased, more energy was allocated to respiration, 

 elimination of wastes, and reproduction, and steadily 

 less energy was allocated to growth (Fig. 1). At the 

 individual scale, females consumed more than males at 

 all ages. The sexes diverged markedly as fish matured 

 (beginning at age 3 for females, age 4 for males), and 

 continued to diverge as fish approached asymptotic 

 sizes (Fig. 2A). The disparity was related to sex-based 

 differences in growth rate, maximum size, GSI, and 



the increased respiration of gestating females. Cumu- 

 lative consumption through age 30 was 285.0 MJ for 

 individual females, and 174.6 MJ for individual males. 

 Assuming a prey energy density of 1500 J/g (given S. 

 mystinus diets [Hobson and Chess, 1988] and prey- 

 density measurements of the same or related prey spe- 

 cies [Paine and Vadas, 1969; Thayer et al., 1973; Foy 

 and Norcross, 1999]), this energy density equates to 

 a long-term average energy consumption rate of 2.7% 

 body mass per day for females and 2.8% body mass per 

 day for males. 



Females also had greater requirements than males 

 at the per-recruit scale, although mortality gradually 

 lessened the contribution of older age classes (Fig. 2B), 

 nullifying some of the disparity between the sexes at 

 the individual scale. Cumulative female and male per- 

 recruit energy consumption was 20.7 MJ and 14.8 MJ, 

 respectively. Per-recruit energy consumption, the prod- 

 uct of age-specific consumption rate and relative fish 

 abundance, peaked at ages 4-6, indicating that those 

 age groups have the greatest potential to affect their 

 prey species. 



Effects of El Nino on S. mystinus energetics 



El Nino events changed S. mystinus energy consumption 

 compared to that in the baseline model, but the direction 

 and magnitude of change were dependent on sex, age, 

 scale of calculation (individual vs. per recruit), and the 

 number and frequency of El Nino events experienced by 

 a given cohort. To demonstrate this change, I modeled 

 growth of two cohorts that experienced El Nino regimes 

 of moderate or high intensity. The first cohort ("cohort 

 A") experienced five El Nino events by age 30, whereas 

 the second cohort ("cohort B") experienced eight El Nino 

 events (Figs. 3 and 4). 



At the scale of individual fish, cohorts A and B experi- 

 enced lower energy consumption in El Nino events, par- 

 ticularly among females. During El Nino years, which 

 first occurred at age 3 for cohort A and at age 1 for co- 

 hort B, consumption by females was always lower than 

 the baseline value (Fig. 3A). In immature females, the 

 disparity was 7-10% lower than the baseline value and 

 was 12-13% lower for mature females. These reductions 

 in consumption were a function of lower growth rates, 

 poor condition factor, and reduced fecundity during El 

 Nino years. In contrast, consumption by males during 

 El Nino years was 4-9% lower than the baseline value 

 among immature individuals, but was roughly equal to 

 the baseline value for mature individuals (Fig. 3B), in 

 part because males did not experience drastic changes 

 in reproductive condition during El Nino years. Both 

 sexes experienced years when energy consumption was 

 greater than the baseline value, particularly two years 

 after an El Nino event when the somatic condition fac- 

 tor returned to normal and greater-than-average growth 

 for that age occurred. By age 30, sizes of fish in both 

 El Nino models were close to the asymptotic maxima 

 and were therefore similar to baseline sizes (Table 3). 

 Cumulative 30-year energy consumption values were 



