Williams and Shertzer: Effects of fishing on growth traits: a simulation analysis 



401 



compressed. We modeled fishing seasons at the begin- 

 ning of the year and spawning seasons at the end of 

 the year, and in a single-year simulation, the annual 

 timing of the fishing and spawning seasons will affect 

 selection differentials. For example, if the one-month 

 fishing season had been modeled at the end of the year, 

 the selection differential would be smaller because of 

 the 11 months of spawning prior to fishing mortality. 

 Over multiple years, however, the annual timing of the 

 fishing and spawning seasons is less important than 

 their duration and overlap. 



Our model simulated selection differentials at the 

 onset of a fishery. As a fishery progresses, selection 

 differentials should decrease as life-history parameters 

 shift in the direction of selection. A multiyear simula- 

 tion of evolution would require knowledge or assump- 

 tions about heritability and trait distributions, both of 

 which are likely to be dynamic. Even so, a short-term 

 simulation, where selection differentials and heritabil- 

 ity are assumed to be static, may be an informative 

 approximation. 



We simulated evolution of the base-model population, 

 assuming a static heritability of 0.2 and selection differ- 

 entials of 2.5% for L v and 1.2% for K (values from Tables 

 2 and 3 with 20% CV's in both parameters). Two simu- 

 lations were conducted with different values for fishing 

 mortality. With F = AM, five years of evolution led to a 

 9.0% decrease in the capacity of spawning biomass. With 

 F = M, five years led to a 2.3% decrease. 



With real fishery data it is often impossible to docu- 

 ment conclusively that fishing causes a genetic change 

 in growth. Any such change may be hard to measure, 

 fall within the range of statistical variability due to 

 sampling, or be masked by strong year classes. Selec- 

 tion for reduced growth may be compensated by den- 

 sity-dependent effects (for example, lower abundance 

 leaving more resources for survivors to allocate towards 

 growth). Even when a change can be demonstrated, 

 fishing is just one potential explanation. Alternative 

 explanations include environmentally driven evolution 

 and reaction norms (i.e., phenotypic expressions of a 

 genotype-environment interaction). 



Nonetheless, size-selective fishing is widespread and 

 often accompanies changes in somatic growth rates 

 (Ricker, 1981; Harris and McGovern, 1997; Haugen and 

 Vollestad, 2001; Sinclair et al., 2002). Until recently, 

 the question was whether fishing can cause changes in 

 growth that are evolutionary, and the answer was "yes 

 . . . probably." The laboratory experiments of Conover 

 and Munch (2002) removed any doubt. However, those 

 experiments represented an extreme fishery in terms 

 of its potential to inflict a selection differential: high 

 F compressed in time (90% of population removed in 

 one day), knife-edge selectivity, non-overlapping gen- 

 erations, and a population where all individuals are 

 susceptible. 



The goal of our study was to shed light on selection 

 differentials created by fishing under realistic ranges of 

 life-history and fishery characteristics. Understanding 

 how life-history characteristics affect selection differen- 



tials is important for identifying which stocks are most 

 susceptible to evolution of growth traits. For example, 

 susceptibility increases with compression of the spawn- 

 ing season. Fish species with compressed spawning 

 seasons, such as many anadromous species, may be at 

 higher risk of evolution from size-selective fisheries. 



Understanding how fishery patterns affect selec- 

 tion differentials has direct management implications 

 because it is the fishery parameters that can be con- 

 trolled. For example, our results indicate that size-selec- 

 tive fisheries compressed in time are apt to cause high 

 selection differentials. Managers should avoid "derby" 

 style harvests, such as the annual Pacific herring sac- 

 roe fisheries, which are completed in only a few days. 

 Other management strategies could reduce selection 

 differentials, such as slot limits, reduction in the slope 

 of selectivity curves, and partial selectivity after the 

 age at maturity. However, because no size-selective 

 fishing pattern can preclude some directional selection 

 on growth, management by area closures may be the 

 best option for avoiding fishery-induced evolution of 

 growth traits. 



As fishing technology improves, so does the ability 

 to fully and rapidly exploit fish populations, and thus 

 increase the potential for evolutionary responses. Still, 

 when overfishing depletes a stock, low abundance is 

 usually the paramount concern. With appropriate man- 

 agement, stock abundance may recover, but pre-fishing 

 growth capacity may recover more slowly or not at all 

 if genetic variation is lost. Given plausible heritabili- 

 ties of growth traits, this analysis shows that under a 

 wide variety of life-history and fishery characteristics, 

 selection differentials are large enough to allow for 

 rapid evolution. 



Acknowledgments 



We thank R. Munoz, M. Prager, and D. Vaughan for 

 comments on the manuscript. This work was supported 

 by the National Marine Fisheries Service through its 

 Southeast Fisheries Science Center. 



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