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Fishery Bulletin 101(4) 



oysters is located. The value of Fs ranges from zero (mean- 

 ing zero fecundity) to one (meaning no effect of salinity on 

 fecundity). For the eastern oyster, when salinity is below 

 8.0 ppt, Fs is equal to zero, thereby making fecundity zero 

 (Mann and Evans, 1998). When salinity is between 8.0 ppt 

 and 13.5 ppt, there is a positive relationship between salin- 

 ity and fecundity as described below: 



Fs 



(5-8) 

 5.5 



(3) 



where Fs = the effect of salinity on fecundity; and 



S = salinity (ppt) between 8 ppt and 13.5 ppt. 



When salinity is greater than 13.5 ppt, Fs is equal to one 

 denoting no effect of salinity on fecundity. When salinity is 

 greater than 35 ppt, Fs is equal to zero, making fecundity 

 equal to zero (Mann and Evans, 1998). Low or no fertility 

 at high salinity is apparently the case for C. ariakensis as 

 well (Langdon and Robinson, 1996). 



The variable for disease prevalence, F^, has a value be- 

 tween 1.0 (no mortality from disease) and 0.0 (all oyster 

 spat die from disease). Recent field studies have suggested 

 that the Suminoe oyster is resistant to diseases on the east 

 coast of North America (Calvo et al., 2001); therefore, the 

 default value of Fd was set at one. Nevertheless, we re- 

 tained this variable in the model to account for future data 

 sets or other diseases so that the user can select F^ based 

 on the prevalence of disease in the area to be modeled. 



Oyster density is determined from the area over which 

 the population occurs. Density affects gamete fertilization 

 efficiency such that more dense oyster deployments (farms, 

 reefs, etc.) exhibit an increased fertilization rate. Levitan 

 (1991) reported the influence of body size and population 

 density on fertilization success and reproductive output, 

 and his equation was rewritten by Mann and Evans ( 1998) 

 as 



F/ , = 0.0049 X D° 



(4) 



where Ff, , = fertilization efficiency at time t for age-class 

 i greater than one ranging from zero (mean- 

 ing zero fertilization) to one (meaning all 

 gametes become fertilized); and 

 Z), , = oyster density (number of oysters per square 

 meter) at time t for age-class ; greater than 

 one. 



For diploid oysters, oyster density is equal to the number of 

 oysters in the population divided by the area (m''^). However, 

 the density value for Equation 4 will differ with triploid 

 populations because not all oysters may be able to repro- 

 duce; thus we modified the density equation to reflect the 

 density of only undetected diploids and reverted triploids: 



O. =■ 



A', X ( ff + ■/; , ) 



(5) 



The variable for sex ratio, Fq^, of females to males in the 

 population per age class is a value from 0.0 to 1.0 (Mann 

 and Evans, 1998). Fq^ modifies fecundity so that popula- 

 tion size in Equations 1 and 2 comprised females only. The 

 ratios of female-to-male Suminoe oysters at ages 1, 2, 3, 

 and 4 are 0.28, 0.66, 1.00, and 1.00, respectively (Yingya 

 etal., 1992). 



Hence, total number of offspring produced per each age 

 class at each time step modified with the previous variables 

 (Mann and Evans, 1998) is as follows: 



Ftotal, = V ( Frevert^j xFsx Fq, xFdx Ff,, ), 



(6) 



where Ftotal, = modified total number of offspring pro- 

 duced at time t summed across all age 

 classes. 



The number of recruits obtained from the model that 

 survive to the next time step, thereby becoming age class 

 one, depends on the total number of offspring produced 

 from reverted oysters in older age classes, daily mortality 

 rate (ranging from 0.07 to 0.1) until settlement (21 days 

 after fertilization) (Mann and Evans, 1998), the probability 

 of successful completion of metamorphosis (0.25) (Mann 

 and Evans, 1998), and total mortality for settled oysters 

 less than one year old (Thorson, 1966). Hence, the number 

 of individuals that will survive to enter age-class one at the 

 next time step is given by the equation below: 



N,^, = K, +i Ftotal, X [Pmet x (1 - Lmortf x (1 - M„ ))). 



(7) 



where A = area (square meters) 



where K, = the number of oyster spat stocked at time t; 

 Pmet = probability of successful completion of meta- 

 morphosis; 

 Lmort - daily larval mortality rate until settlement at 

 21 days; and 

 Mq = total mortality rate for settled oysters less 

 then one year old. 



The mortality variables in the model for adult oysters 

 are natural mortality and harvest mortality. Natural 

 mortality determines the proportion of oysters in each age 

 class of the population from nonharvest causes each year. 

 Default natural mortality rates were taken from Calvo et 

 al. (2001) (Table 2). Stochastic values of these variables 

 are chosen from a log normal distribution of the variance 

 around the mean mortality rate for each age class of the 

 population. Harvest-mediated mortality in the population 

 was imposed by randomly selecting individuals for harvest 

 in the age classes whose mean shell length is greater than 

 the set minimum shell length-at-harvest. The harvest rate 

 was a percentage of the population removed from the total 

 population each year 



Certainty of harvest is defined as how certain we are 

 that harvest occurs at a desired harvest rate. This vari- 

 able in the model captures the effects of different harvest 

 strategies. For example, if oysters are contained in wire 

 cages, the certainty of obtaining a given harvest rate could 

 be 100%. However, for oysters planted on the bottom, the 



