78 



Abstract— Increasing interest in the 

 use of stock enhancement as a man- 

 agement tool necessitates a better 

 understanding of the relative costs and 

 benefits of alternative release strate- 

 gies. We present a relatively simple 

 model coupling ecology and economic 

 costs to make inferences about optimal 

 release scenarios for summer flounder 

 (Paralichthys dentatus), a subject of 

 stock enhancement interest in North 

 Carolina. The model, parameterized 

 from mark-recapture experiments, 

 predicts optimal release scenarios from 

 both survival and economic standpoints 

 for varyious dates-of-release, sizes-at- 

 release, and numbers of fish released. 

 Although most stock enhancement 

 efforts involve the release of relatively 

 small fish, the model suggests that 

 optimal results (maximum survival 

 and minimum costs) will be obtained 

 when relatively large fish (75-80 mm 

 total length! are released early in the 

 nursery season (April). We investigated 

 the sensitivity of model predictions to 

 violations of the assumption of den- 

 sity-independent mortality by includ- 

 ing density-mortality relationships 

 based on weak and strong type-2 and 

 type-3 predator functional responses 

 (resulting in depensatory mortality 

 at elevated densities). Depending on 

 postrelease density, density-mortality 

 relationships included in the model con- 

 siderably affect predicted postrelease 

 survival and economic costs associated 

 with enhancement efforts, but do not 

 alter the release scenario (i.e. combina- 

 tion of release variables ) that produces 

 optimal results. Predicted (from model 

 output) declines in flounder over time 

 most closely match declines observed 

 in replicate field sites when mortality 

 in the model is density-independent 

 or governed by a weak type-3 func- 

 tional response. The model provides an 

 example of a relatively easy-to-develop 

 predictive tool with which to make 

 inferences about the ecological and 

 economic potential of stock enhance- 

 ment of summer flounder and provides 

 a template for model creation for addi- 

 tional species that are subjects of stock 

 enhancement interest, but for which 

 limited empirical data exist. 



Manuscript approved for publication 

 17 July 2003 by Scientific Editor. 



Manuscript received 20 October 2003 

 at NMFS Scientific Publications Office. 



Fish. Bull. 102:78-93 (2004). 



Coupling ecology and economy: 

 modeling optimal release scenarios for 

 summer flounder (Paralichthys dentatus) 

 stock enhancement 



G. Todd Kellison 



David B. Eggleston 



Department ol Marine, Earth, and Atmospheric Sciences, 



North Carolina State University 



Raleigh, North Carolina 27695-8208 



Present address (for G T. Kellison, contact author): National Park Service/ Biscayne National Park 



9700 SW 328 th St, Homestead, Florida 33033 

 E-mail address (for G T Kellison) todd_kellison 5 nps gov 



Commercially important marine fish 

 and invertebrate populations are 

 declining worldwide in response to 

 overexploitation and habitat degrada- 

 tion (Rosenberg et al„ 1993; FAO 1998). 

 This reduction in harvestable fishery 

 resources has stimulated increasing 

 interest in the use of hatchery-reared 

 (HR) animals to enhance wild stocks 

 (Munro and Bell, 1997; Travis et al., 

 1998; Cowx, 1999; Kent and Draw- 

 bridge, 1999). Unfortunately, many stock 

 enhancement programs proceed before 

 ecological concerns are adequately 

 addressed (Blankenship and Leber, 

 1996), and without the identification 

 of goals or the evaluation of the success 

 of enhancement efforts (Cowx, 1999). 

 If fishery managers can satisfactorily 

 determine that enhancement efforts 

 will have no ecologically significant 

 negative ramifications, then managers 

 should establish specific, quantifiable 

 goals and objectives of enhancement 

 efforts as part of a responsible approach 

 to stock enhancement (Blankenship 

 and Leber, 1996; Heppell and Crowder, 

 1998). Once such goals have been 

 established, managers should identify 

 stocking approaches that will lead to 

 the most cost-efficient realization of 

 enhancement goals — a process that 

 can be accomplished with the aid of 

 coupled ecological and economic models. 

 Although numerous (advanced) models 

 (conceptual and species-specific) exist 

 to predict the biological and ecological 

 impact of alternative enhancement 

 scenarios (e.g. Botsford and Hobbs, 

 1984; Salvanes et al„ 1992; Barbeau 

 and Caswell, 1999; Sutton et al., 2000), 



there are few models ( of which we are 

 aware) that have attempted to link the 

 biological and ecological results of stock- 

 ing efforts (e.g. addition of biomass to a 

 stocked population) with the economic 

 costs associated with various release 

 scenarios (e.g. Botsford and Hobbs, 1984; 

 Hobbs et al., 1990; Hernandez-Llamas, 

 1997; Kent and Drawbridge, 1999). Such 

 a link is critical to the responsible use 

 of funding to rebuild or manage fisher- 

 ies, and for the comparison of predicted 

 costs of enhancement versus alternative 

 management techniques. 



In North Carolina, there has been 

 recent interest in stock enhancement 

 with summer flounder (Paralichthys 

 dentatus) (Waters, 1996; Rickards, 

 1998; Waters and Mosher, 1999; Burke 

 et al., 2000; Copeland et al. ' ) because of 

 a combination of heavy commercial and 

 recreational exploitation, established 

 techniques for mass hatchery-rearing 

 (Burke et al., 1999), and considerable 

 knowledge of summer flounder life his- 

 tory (Powell and Schwartz, 1977; Burke 

 et al., 1991; Burke, 1995). Nevertheless, 

 there have been no large-scale release 

 experiments ( and subsequent collection 

 of data) by which to make empirical 

 inferences about stock enhancement 

 potential for this species. We present 

 a compartmental model, parameterized 

 from mark-recapture field experiments, 



Copeland, B. J., J. M. Miller, and E. B. 

 Waters. 1998. The potential for flounder 

 and red drum stock enhancement in North 

 Carolina. Summary of workshop, 30-31 

 March. 1998, 22 p. ' (Available from North 

 Carolina State Univ, Raleigh. NC 27695.] 



