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



765 



was maintained at the default value 

 of 0.9, higher stocking densities barely 

 increased the probability of the popula- 

 tion becoming self-sustaining (Fig. 5). 

 Absence of reproduction from undetected 

 diploids, and the lack of reproduction in 

 mosaic (reverted triploid) oysters until 

 age 3, well past harvest size, were the 

 principal determinants of lowered repro- 

 ductive risk. 



Interaction of stocking density and salinity 



At lower salinity sites, changes in stock- 

 ing density had less of an effect on the 

 likelihood of developing a self-sustain- 

 ing population than at higher salinities 

 when all other variables were set at 

 default values (Fig. 6). Lower salinity 

 decreases fecundity, which counteracts 

 the increased fertilization efficiency at 

 higher population densities. 



Interaction of stocking density and 

 certainty of harvest 



Stocking density could be increased with- 

 out increasing the risk of a self-sustaining 

 population by increasing certainty of har- 

 vest, and keeping all other variables set 

 at the default values (Fig. 7). Increased 

 certainty of harvest decreased the 

 number of oysters remaining in culture 

 that were able to revert and reproduce, 

 countering the increase in fertilization 

 efficiency from increased density. 



Discussion 



o 



.0,001 

 Detection 

 tfireshold 



-0.000 

 Detection 

 threstiold 



100 200 300 



Stocking density (oysters per square meter) 



Figure 5 



Relationship between stocking density and probability of a C. ariakensis popu- 

 lation becoming self-sustaining, keeping all other variables at default values. 

 The solid line represents the relationship when the detection threshold for 

 diploids is 0.001. The dashed line represents the relationship when the detec- 

 tion threshold for diploids is 0.000. 



The introduction of any non-native spe- 

 cies into a new environment poses a 

 number of potential ecological hazards. 

 In general, these are related to two root 

 causes: 1) the associated introduction of 

 epibionts, pathogens, or other infectious 

 agents, and 2) ecological disruption from 

 the persistence of the introduced species 

 (i.e. through reproduction, competition, 

 etc.). To a large degree, associated intro- 

 ductions (with the possible exception of 

 viruses) can be eliminated by adherence 

 to codes of practice for proper quarantine 

 and propagation, such as those set by 

 the International Council for the Exploration of the Seas 

 (ICES, 1994). The second risk of ecological disruption is 

 caused by reproduction and subsequent naturalization. To 

 address this hazard, sterile triploids have been proposed 

 as a means to introduce the Suminoe oyster for commercial 

 aquaculture. This model addresses those elements of risk 



a 13.5 



11 \ 



Stocking density (oysters per square meter) 



Figure 6 



Relationship between stocking density, salinity at deployment site, and the 

 probability of a C. ariakensis population becoming self-sustaining, keeping all 

 other variables constant at default values and stocking area set at 300 square 

 meters. 



associated with reproduction. Risk assessment modeling 

 will enable managers to anticipate which management 

 actions can have the greatest impact on decreasing the 

 likelihood of a self-sustaining population. According to our 

 results, risk reduction strategies include stocking Sumi- 

 noe oysters in relatively low salinity, growing oysters in 



