71 



Abstract — Fish bioenergetics models 

 estimate relationships between energy 

 budgets and environmental and physi- 

 ological variables. This study presents 

 a generic rockfish (Sebastes) bioen- 

 ergetics model and estimates energy 

 consumption by northern California 

 blue rockfish (S. mystinus) under 

 average (baseline I and El Nino con- 

 ditions. Compared to males, female 

 S. mystinus required more energy 

 because they were larger and had 

 greater reproductive costs. When El 

 Nino conditions I warmer tempera- 

 tures; lower growth, condition, and 

 fecundity) were experienced every 3-7 

 years, energy consumption decreased 

 on an individual and a per-recruit 

 basis in relation to baseline conditions, 

 but the decrease was minor (<4% at 

 the individual scale, <7% at the per- 

 recruit scale) compared to decreases 

 in female egg production (12-19% at 

 the individual scale. 15-23% at the 

 per-recruit scale). When mortality in 

 per-recruit models was increased by 

 adding fishing, energy consumption 

 in El Nino models grew more similar 

 to that seen in the baseline model. 

 However, egg production decreased 

 significantly — an effect exacerbated 

 by the frequency of El Nino events. 

 Sensitivity analyses showed that 

 energy consumption estimates were 

 most sensitive to respiration param- 

 eters, energy density, and female 

 fecundity, and that estimated con- 

 sumption increased as parameter 

 uncertainty increased. This model 

 provides a means of understand- 

 ing rockfish trophic ecology in the 

 context of community structure and 

 environmental change by synthe- 

 sizing metabolic, demographic, and 

 environmental information. Future 

 research should focus on acquiring 

 such information so that models like 

 the bioenergetics model can be used to 

 estimate the effect of climate change, 

 community shifts, and different har- 

 vesting strategies on rockfish energy 

 demands. 



Effects of El Nino events on energy demand 

 and egg production of rockfish 

 (Scorpaenidae: Sebastes): 

 a bioenergetics approach 



Chris J. Harvey 



Northwest Fisheries Science Center 

 National Marine Fisheries Service 

 2725 Montlake Blvd. E 

 Seattle, Washington 98112 

 E-mail address Chris. Harveyignoaa gov 



Manuscript submitted 20 October 2003 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 2 August 2004 by the Scientific Editor. 



Fish. Bull. 103:71-83 (2005). 



Over 90 species of rockfish (Sebastes 

 spp.) are found in kelp beds, rocky 

 reefs, pelagic habitats, and continental 

 shelf and slope zones of the temperate 

 and subarctic North Pacific; these spe- 

 cies feed on a range of organisms, from 

 zooplankton to fish (Love et al., 20021. 

 Although they are a key component of 

 groundfish fisheries on the U.S. Pacific 

 Coast, many rockfish have declined 

 considerably in recent decades, owing 

 to overfishing and climate-induced 

 downturns in production (Parker et 

 al., 2000). Conservation efforts, rang- 

 ing from coast-wide fishery closures to 

 establishment of marine reserves, have 

 been enacted in order to rehabilitate 

 rockfish stocks. The efficacy of such 

 actions depends in part on the dynam- 

 ics of the communities in which rock- 

 fish exist. Key among these dynamics 

 are trophic interactions, as influenced 

 by abiotic factors and rockfish popula- 

 tion structure. 



Although rockfish are widely dis- 

 tributed and important to the ecolo- 

 gy, fisheries, and conservation efforts 

 of the Pacific Coast, little is known 

 about their trophic dynamics. For ex- 

 ample, of the 65 rockfish species that 

 live along the North American West 

 Coast, quantitative diet data are 

 available for only 15 species (Murie. 

 1995). Better information on the food 

 habits and energetics of both juvenile 

 and adult rockfish would facilitate 

 a greater understanding of the role 

 they play in their communities, and 

 how their role is affected by external 

 forces. This is particularly true given 

 observations that environmental vari- 

 ation can have strong effects on rock- 

 fish growth and condition (Lenarz et 

 al., 1995; Woodbury, 1999). 



Fish bioenergetics models relate 

 the energy consumption, growth, and 

 energy allocation patterns of fishes 

 to environmental and physiological 

 variables such as temperature, food 

 quality, body size, and reproductive 

 status (Kitchell et al., 1977). These 

 models, founded in thermodynamic 

 laws of mass and energy balance, 

 can successfully predict patterns of 

 energy demands by fish (Madenjian 

 et al., 2000). At the scale of the indi- 

 vidual fish, bioenergetics models can 

 estimate effects of a fish on its com- 

 munity (in terms of the amount of 

 prey it consumes) and effects of the 

 environment on the fish, such as how 

 changes in temperature or food avail- 

 ability influence energy consumption 

 and growth (Rice et al., 1983). When 

 coupled to population models, bio- 

 energetics models can predict prey- 

 predator supply-demand relationships 

 (Negus, 1995) and determine how 

 different fishery management poli- 

 cies will affect prey resources in the 

 community from which the targeted 

 fish is extracted (Kitchell et al., 1997; 

 Essington et al., 2002; Schindler et 

 al., 2002). Thus, these models may 

 facilitate a more community- or eco- 

 system-level approach to rockfish 

 management. 



In this study, I develop a generic 

 Sebastes bioenergetics model. My first 

 objective is to detail the parameters 

 and the sensitivity analysis of the 

 model, thereby offering a synthesis 

 of what is known about Sebastes en- 

 ergetic physiology and identifying pa- 

 rameters for which greater informa- 

 tion is desirable. The second goal is 

 to present a simple application of the 

 model: an estimation of the effects of 



