72 



Fishery Bulletin 103(1) 



El Nino related environmental changes on the energy 

 demands of blue rockfish (S. mystinus) under unfished 

 and fished conditions. Two relevant characteristics of 

 El Nino events in U.S. West Coast waters are elevated 

 temperatures and reductions in growth rates and re- 

 productive condition of Sebastes (Lenarz et al., 1995; 

 VenTresca et al., 1995; Woodbury, 1999). The bioener- 

 getics approach can incorporate these changes and can 

 therefore help to characterize the role of rockfish as 

 consumers in a dynamic environment. 



Methods 



Model structure 



I followed the basic structure of bioenergetics models 

 established for other fishes (e.g., Kitchell et al., 1977; 

 Hewett and Johnson, 1992), in which energy intake 

 (consumption) equals all energy outputs (respiration, 

 wastes, growth, and reproduction). The basic model 

 equation is 



C = (i? + A + S) + (F + U) + (AB + G) 



(1) 



where C = consumption, R = respiration, A = active 

 metabolism, S = specific dynamic action (digestive costs), 

 F = egestion, U = excretion, AB = somatic growth, and G 

 = gonad production. The respiration and active metabo- 

 lism portions of Equation 1 take the form 



R = RA x W RB x f(T) x ACT, 



(2) 



where RA and RB are constants that describe the allo- 

 metric respiration function, W is wet biomass, f(T) is a 

 temperature dependence function, and ACT is an activ- 

 ity multiplier (Kitchell et al., 1977). The function f(T) 

 (Kitchell et al., 1977) is a hump-shaped function that 

 requires estimates of optimal (RTO) and maximum 

 (RTM) temperatures for respiration, and a Q 10 (RQ). 



The terms S, U, and F all scale to total consumption 

 (Kitchell et al., 1977). One can thus think of them as a 

 general energy loss term 



Loss = (S + U) x (C -F) + F. 



(3) 



Model parameters 



Although parameters are derived from studies of many 

 rockfish species, I developed the present model to describe 

 energetic dynamics of S. mystinus, for which a consider- 

 able literature exists regarding diet and responses to 

 climate variability (e.g., Hallacher and Roberts, 1985; 

 Bodkin et al., 1987; Hobson and Chess, 1988; Lenarz et 

 al., 1995; VenTresca et al., 1995). 



Respiration parameter estimates came from studies 

 of other Sebastes species or related scorpaenid fishes 

 (Table 1). For RTM, I used published estimates for S. 

 thompsoni and S. schlegeli (Ouchi, 1977; Tsuchida and 

 Setoguma, 1997), and assumed that RTO would be 5°C 



cooler. The resulting RTO was similar to upper tem- 

 peratures at which juvenile S. diploproa experienced zero 

 growth while feeding (Boehlert, 1981). RQ was based on 

 low-temperature Q 10 values in several scorpaenid respi- 

 ration studies (Boehlert et al. 1991; Yang et al., 1992; 

 Kita et al, 1996; Vetter and Lynn, 1997). RA, the oxygen 

 consumption rate for a 1-g fish at RTO, was derived from 

 data for nongestating S. schlegeli (Boehlert et al., 1991). 

 RB, which describes the allometric scaling of respiration, 

 was also derived from data for nongestating S. schlegeli 

 spanning a range of roughly 0.7 to 1.9 kg body mass 

 (Boehlert et al., 1991). Respiration terms were converted 

 to energy units by an oxycalorific correction (13.56 J/mg 

 9 ), and then to biomass by assuming that rockfish en- 

 ergy density (ED) = 6,120 J/g wet mass (Perez, 1994). 



The ACT multiplier was assumed to equal 1. This 

 assumption is best justified in cases where routine res- 

 piration rates were used to determine parameters for 

 the model. Boehlert et al. (1991) stated that S. schlegeli 

 in their analysis were generally inactive, which implies 

 that rates derived from their data represent resting 



