Harvey: Effects of El Nino events on consumption and egg production of Sebastes spp. 



73 



metabolism. I chose to keep ACT at 1, however, because 

 I could find no data describing a reasonable activity 

 multiplier. Thus. Sebastes model outputs may underes- 

 timate energy consumption under conditions in which 

 individuals are especially active. 



I obtained growth (AB in Eq. 1) terms using von 

 Bertalanffy length-at-age curves and data for length- 

 to-mass conversions for S. mystinus as summarized 

 by Love et al. (2002). Because female S. mystinus are 

 larger at age than males, growth was modeled with 

 sex-specific von Bertalanffy curves with the difference 

 equation method of Gulland (1983). Digestion and waste 

 terms S, F, and U were derived from previous teleost 

 models (Hewett and Johnson, 1992). 



I estimated gonad production (G) with gonadosomatic 

 indexes (GSI) and size-fecundity relationships (females 

 only), assuming that female and male S. mystinus ma- 

 ture gradually over the range of lengths observed by 

 Wyllie-Echeverria (1987), and reproduce once annually. 

 For males, I assumed that gonads have the same ED as 

 somatic tissue; for females, I assumed that gonadal en- 

 ergy density (GED) = 8,627 J/g, which was the average 

 of gonadal energy density at the onset of embryogenesis 

 for S. flavidus and S. jordani (MacFarlane and Norton, 

 1999). Estimated maximum female GSI was based on 

 a fecundity-length relationship: 



fecundity = GA x TL GB , 



(4) 



where GA and GB were taken from a generic rockfish 

 length-fecundity relationship (Love et al., 2002) and TL 

 is total length in cm. Fecundity was converted to bio- 

 mass units by assuming that each egg weighed 0.0003 g, 

 which I derived from Love et al. (1990) by dividing the 

 mean maximum female gonad weight by the estimated 

 fecundity of modal mature females for several species. 

 For mature males, I assumed a constant maximum GSI 

 based on data for other species (Love et al., 1990). Post- 

 spawning GSI was assumed to be 10% of the maximum 

 for each sex, as with other rockfish (Love et al., 1990). 

 The G terms were the difference between the maximum 

 and minimum GSIs for each sex, expressed as mass 

 (and, in females, adjusted by multiplying by GED/ED). 

 Rockfish are viviparous, and developing larvae may 

 receive energy from both yolk and maternal sources 

 (Love et al., 2002). During gestation in a laboratory, 

 female S. schlegeli consumed 35% to 117% more oxygen 

 than nongestating fish of similar size (Boehlert et al., 

 1991). To account for the possibility that blue rockfish 

 may also be matrotrophically viviparous, I increased 

 female respiration by 50% during the gestation period 

 (assumed to be 45 days per year based on gestation 

 times of other species [Boehlert et al., 1991]). 



Model application: effects of El Nino 

 on blue rockfish energy consumption 



To examine the effects of El Nino on S. mystinus energy 

 consumption, I created two model conditions: a baseline 

 model and an El Nino model that estimated S. mystinus 



energy demands, in megajoules (MJ), required for neces- 

 sary growth, reproduction, and related metabolic costs. 

 I used MJ rather than prey biomass as the currency 

 because quantitative, seasonal diet data for S. mystinus 

 in northern California were available for average years 

 (Hobson and Chess 1988) but not for El Nino years. During 

 the 1982-83 El Nino, Lea et al. (1999) found that central 

 Californian S. mystinus consumed large numbers of the 

 pelagic crab Pleuroneodes planipes, which is typically 

 found south of Point Conception during average years. 

 During the same time period, S. fnystinus ate few tuni- 

 cates or scyphozoans (Lea et al., 1999), which were the 

 predominate prey of S. mystinus in average years (Hobson 

 and Chess, 1988). These findings suggest a major shift in 

 S. mystinus prey composition during El Nino events. 



The baseline model simulates energy consumption of 

 northern California S. mystinus from age to age 30, 

 based on quarterly growth estimates from sex-specific 

 von Bertalanffy curves (Love et al., 2002) and seasonal 

 temperature data from Hobson and Chess (1988). Mature 

 females released larvae in the fourth quarter of each 

 year, and mature males released gametes in the third 

 quarter (Wyllie-Echeverria, 1987). Energy consumption 

 for both sexes from ages to 30 was expressed at two 

 scales: for the 30-year life span of an individual; and on 

 a per-recruit basis (under the assumption that there was 

 no fishing mortality and that the natural mortality rate 

 [M] was 0.2, applied in quarterly time steps). 



The El Nino model was similar to the baseline model, 

 except an El Nino occurred every three to seven years. 

 During these years there were changes in temperature, 

 growth, condition, and fecundity (Table 2). Temperature 

 increases in El Nino years were similar to temperature 

 anomalies in northern California waters during major 

 El Nino events from 1957 to 1993 (Lenarz et al., 1995). 

 Changes in growth (in terms of length increment), con- 



