392 



Abstract— Fisheries often target indi- 

 viduals based on size. Size-selective 

 fishing can create selection differen- 

 tials on life-history traits and, when 

 those traits have a genetic basis, may 

 cause evolution. The evolution of life- 

 history traits affects potential yield 

 and sustainability of fishing, and it is 

 therefore an issue for fishery manage- 

 ment. Yet fishery managers usually 

 disregard the possibility of evolution, 

 because little guidance is available to 

 predict evolutionary consequences of 

 management strategies. We attempt, 

 to provide some generic guidance. We 

 develop an individual-based model of 

 a population with overlapping genera- 

 tions and continuous reproduction. 

 We simulate model populations under 

 size-selective fishing to generate and 

 quantify selection differentials on 

 growth. The analysis comprises a 

 variety of common life-history and 

 fishery characteristics: variability 

 in growth, correlation between von 

 Bertalanffy growth parameters (K 

 andL,.), maturity rate, natural mor- 

 tality rate (M), M/K ratio, duration 

 of spawning season, fishing mortality 

 rate (F), maximum size limit, slope of 

 selectivity curve, age at 50% selectiv- 

 ity, and duration of fishing season. 

 We found that each characteristic 

 affected the magnitude of selection 

 differentials. The most vulnerable 

 stocks were those with a short spawn- 

 ing or fishing season. Under almost 

 all life-history and fishery character- 

 istics examined, selection differentials 

 created by realistic fishing mortality 

 rates are considerable. 



Effects of fishing on growth traits: 

 a simulation analysis 



Erik H. Williams 



Kyle W. Shertzer 



Center for Coastal Fisheries and Habitat Research 

 101 Pivers Island Road 

 Beaufort, North Carolina 28516 

 E-mail address EnkWilliams@noaa.gov 



Manscript submitted 16 April 2004 

 to the Scientific Editor's Office. 



Manuscript approved for publication 



20 December 2004 by the Scientific Editor. 



Fish. Bull. 103:392-403 (20051. 



Fishing is typically size selective. 

 It almost always targets the larger 

 individuals of a population and can 

 thus shift the spawning stock towards 

 smaller, slower-growing individuals. If 

 somatic growth has some genetic basis, 

 size-selective fishing may cause evolu- 

 tion toward a smaller size-at-age. 



Changes in somatic growth are 

 well documented in field data, and 

 several studies implicate fishing 

 (Ricker, 1981; Harris and McGovern, 

 1997; Haugen and Vollestad, 2001; 

 Sinclair et al., 2002). However, with 

 typical field data, it is difficult to rule 

 out other explanations. Changes in 

 growth could result from fluctuations 

 in population density or the environ- 

 ment. Furthermore, they may not be 

 evolutionary, but instead expressions 

 of phenotypic variability. Because of 

 such possibilities, the idea that fish- 

 ing can cause evolution has often 

 been accepted because of compelling 

 theoretical arguments, rather than 

 on empirical support. However, the 

 laboratory experiments of Conover 

 and Munch (2002) demonstrated that 

 size selection can cause evolution of 

 growth traits. More and more, fish- 

 ing-induced evolution is considered 

 not just possible, but prevalent (Law, 

 2000; Stockwell et al., 20031. 



The evolution of growth traits, de- 

 spite wide acknowledgement of the 

 potential for evolution of these traits, 

 is usually a low priority in fishery 

 management. However, it raises at 

 least four management concerns. 

 First, any reduction in growth rate 

 or maximum size can decrease rec- 

 reational and economic value (Miller 

 and Kapuscinski, 1994). Second, size 

 selection could reduce genetic vari- 

 ability (Falconer and Mackay, 1996). 



unpredictably altering correlated 

 traits and population fitness. Third, 

 evolution may not easily be reversed, 

 even with after-the-fact management. 

 Fourth, the evolution of growth and 

 other life-history traits can modify 

 population dynamics (Bronikowski et 

 al., 2002; Shertzer and Ellner, 2002) 

 and therefore potential yield (Edley 

 and Law, 1988; Heino 1998). Evolu- 

 tion in fishes can be rapid (Reznick et 

 al., 1997; Hendry et al., 2000; Quinn 

 et al., 2001), so that evolutionary, 

 population, and fishery dynamics oc- 

 cur on similar time-scales (Sinervo 

 et al., 2000; Shertzer et al., 2002; 

 Yoshida et al., 2003). These dynam- 

 ics imply that evolution matters for 

 fishery management on the time-scale 

 of years or decades. 



For fishing to cause evolution, two 

 conditions must be met. There must 

 be a selection differential on a pheno- 

 typic trait and a genetic basis must 

 exist for the trait's expression (i.e., 

 the trait must be heritable). Selec- 

 tion differential is defined as the dif- 

 ference in the mean phenotypic trait 

 value of parents before and after se- 

 lection (e.g., size-selective fishing). 

 Stokes and Law (2000) argued that, 

 under exploitation levels in many of 

 today's fisheries, "selection differ- 

 entials on body size should be sub- 

 stantial and measurable." Even so, 

 attempts to estimate selection differ- 

 entials of actual fish stocks have been 

 rare (but see Law and Rowell, 1993; 

 Miller and Kapuscinski, 1994). This 

 lack of estimates is surprising, given 

 that the data needed are often avail- 

 able, as noted by Law (2001). 



The second necessary condition, her- 

 itability, is defined as the proportion 

 of phenotypic variability in offspring 



