Dew et al: Model for assessing populations of Crassostrea ar/okensis in Ctiespeake Bay 



761 



L, , = mean shell length (mm) at time t for age- 

 class / greater than one; and 



Af, , = population size (number of diploid oysters) 

 at time t for age-class i greater than one. 



We recognize that growth forms in oysters are fluid, and 

 that fecundity may be more closely related to weight than 

 to length. However, available growth data for C. ariakensis 

 described length-at-age, and therefore we modified Mann 

 and Evans' (1998) fecundity equation to use length as the 

 independent variable. 



Equation 1 is used when determining the number of 

 eggs produced by diploid oysters that survived from the 

 previous year. Equation 1 must be modified to determine 

 the number of eggs produced by triploid oysters undergo- 

 ing reproductively effective reversion. The reproductively 

 effective reversion rate comprises two reproduction-related 

 processes, nondetection of diploids (T^ ;) and reproductively 

 effective reversion (R^ ,) of triploids. First, diploid individu- 

 als may enter the population through a failure to detect 

 them at frequencies lower than 0.001 in mixed batches 

 with triploids. We make the ecologically conservative as- 

 sumption that diploid individuals will enter the first age 

 class at the detection threshold of 0.001. After age-class 

 2, reproductively effective reversion is the percentage of 



the population that reverts from triploidy to mosaicism 

 (i.e. contains both triploid and diploid gamete cells) and 

 therefore has the potential to reproduce. We conservatively 

 assume that reverted triploids will have full reproductive 

 capabilities, even though this has not yet been observed 

 (Allen, unpubl. data). Default values for reproductively 

 effective reversion were obtained from Calvo et al. (2001). 

 The Mann and Evans ( 1998) fecundity model was modified 

 to include reproductively effective reversion in the follow- 

 ing way: 



Frevert,, = 39.06x[0.000423x /,;,'""]-"' xA',, x(/?,, -hT;,), (2) 



where Frevert,^ = total fecundity (number of eggs pro- 

 duced) for reverted triploid oysters at 

 time t for age-class i greater than one; 

 iJ, ^ = reproductively effective reversion rate 

 at time t for age-class i greater than 

 one; and 

 Ti , = diploid delection threshold at time t for 

 age-class i greater than one. 



The variable Fs is the effect of salinity on fecundity 

 obtained by using the mean salinity value for the area in 

 Chesapeake Bay where a particular population of Suminoe 



