Dew et al: Model for assessing populations of Crassostrea anakensis in Cfiespeake Bay 



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oysters planted for aquaculture could become 

 a self-sustaining population of diploid Suminoe 

 oysters and introduce numerous unknown eco- 

 logical consequences. 



Another hazard associated with deployment 

 of triploid Suminoe oysters is the possibility 

 that nontriploids might be stocked inadver- 

 tently because of failure to detect them in a 

 mixed batch of triploid and diploid individu- 

 als. Although technology to produce "100%" 

 triploids is now available, as practiced on 

 Pacific oyster (Guo and Allen, 1994b; Guo et 

 al., 1996), the reliability of the approach for 

 producing "100%" triploids in Suminoe oyster 

 is yet undetermined. Diploids may enter the 

 population from several sources: chromosomal 

 nondisjunction in tetraploid males producing 

 haploid gametes, low level hermaphrodism 

 in diploid females yielding self-fertilized 

 embryos, and cross-contamination between 

 diploid and triploid cultures (cf Guo and Al- 

 len, 1997). Typically, flow cytometry has been 

 used to determine the presence or absence of 

 diploid cells (Allen, 1983). Flow cytometry has 

 the sensitivity to detect one diploid among a 

 thousand triploid oysters (Allen and Bushek, 

 1992); thus, the detection threshold is 0.001 with current 

 technology. Should the (nonzero) frequency of diploids be 

 greater than zero but less than one in a thousand, then 

 the batch would be certified 100% triploid. This failure to 

 detect diploid individuals in a mixed batch poses a hazard 

 for stocking other fertile diploid oysters in that batch into 

 culture systems. 



Before substantial commercial introduction of triploid 

 Suminoe oysters into the Chesapeake Bay, any environ- 

 mental hazards of reproduction associated with a range 

 of management scenarios should be assessed. Hazards 

 are defined as undesirable outcomes from an activity 

 (Hallerman and Kapuscinski, 1995). Stocking triploid 

 Suminoe oysters produces two hazards in this model: the 

 inadvertent stocking of diploids and the reproductively 

 effective reversion of triploids. These two hazards may 

 lead to the establishment of a self-sustained Suminoe 

 oyster population and the probability of this occurring 

 is defined as a risk. Risk assessment is the process of 1) 

 identifying hazards posed by management actions, such 

 as deployment of triploid Suminoe oysters, 2) quantifying 

 the associated risks of hazards being realized (Hallerman 

 and Kapuscinski, 1995), such as the population becoming 

 self-sustaining, and 3) evaluating the consequences of 

 the hazards. Quantitative models often are used to as- 

 sess risk (Lackey, 1994). Building upon data collected on 

 growth, mortality, and reproductively effective reversion 

 for Suminoe oysters, we have developed a quantitative 

 model to estimate the risk associated with large-scale 

 deployment of triploid Suminoe oysters under a range of 

 management scenarios. The model predicts the likelihood 

 of out-planted triploid Suminoe oysters giving rise to a 

 self-sustaining population at a given site in the Chesa- 

 peake Bay given user-specified stocking, reproductively 



Slocking, age-class zero | 



Surviving 



stocked 



age-class zero 



oysters 



Starting 



population 



size 



Next 

 year 



Grow to 



mean 

 shell 

 length 



Surviving juvenile 

 oysters 



^ Mortality ^ 



Reproduction 



Natural 

 mortality 



Reproductively 

 effective 



reversion and 

 detection 

 threshold 



Surviving adult oysters 



Final 

 population 



Figure 1 



Flow chart depiction of the annual time step in the model for estimating 

 likelihood of estabhshing self-sustaining reproduction in triploid Suminoe 

 oysters (C. ariakensis). 



effective reversion, reproduction, growth, and mortality 

 rates (both natural and harvest), as well as user-specified 

 management options. 



Methods 



Overview of model 



A quantitative population model of the Suminoe oyster was 

 developed to evaluate the consequences of hazards asso- 

 ciated with introducing triploid Suminoe oysters under 

 a range of environmental conditions and management 

 strategies. The model includes set demographic parameters 

 (length-fecundity, oyster density-fertilization efficiency, 

 and salinity-fecundity relationships) and user-specified 

 variables (reproduction, growth, and natural and harvest 

 mortality rates). It includes options for varying stock- 

 ing rates, harvest rates, and other management actions. 

 Because little is known about Suminoe oyster reproduc- 

 tion, we assumed that Suminoe oysters would behave like 

 the congeneric eastern oysters in Chesapeake Bay; hence, 

 an eastern oyster fecundity model (Mann and Evans, 1998) 

 was used to estimate fecundity of Suminoe oysters. The 

 model assumes that the Suminoe oyster population is 

 closed, i.e. that natural immigration and emigration do not 

 occur The model is age-structured, and a yearly time step is 

 used. The state variable tracked through time is population 

 size. Intrinsic population growth rate is exponential and 

 without density dependence. The final output of the model 

 is the predicted population size of Suminoe oyster assum- 

 ing specified demographic parameters and environmental 

 and management variables. The model was programmed in 

 Visual Basic (Microsoft Corp., Redmond, WA). 



