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Fishery Bulletin 101(4) 



sustaining. For example, when minimum shell length-at- 

 harvest was lowered from 76.7 mm to 66.7 mm, oysters 

 could be grown in higher salinity areas without increasing 

 the probability of the population becoming self-sustaining 

 (Fig. 2). By harvesting the oysters sooner, there is decreased 

 likelihood of reproductively effective reversion. 



Certainty in harvest rate 



As the certainty of obtaining the desired harvest rate 

 increased, the probability of the population becoming 

 self-sustaining decreased (Fig. 3). For example, if oysters 

 were grown on the bottom, inability to reliably recapture 

 all individuals is such that the certainty of obtaining the 



o 



.0,001 

 Detection 

 threshold 



.0.000 

 Detection 

 threshold 



Certainty in harvest rate 



Figure 3 



Relationship between the certainty in obtaining a specified harvest rate 

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

 when area is 300 m^ and all other variables are set 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 detection threshold for diploids is 0.000. 



desired harvest rate would be lower than if cages were 

 used. Lower certainty of harvest would increase the like- 

 lihood of reproductively effective reversion among older 

 individuals remaining on site, thereby increasing prob- 

 abilities of both reproduction and the development of a 

 self-sustaining population. In contrast, if oysters were 

 kept in confinements such as wire cages, the certainty 

 in obtaining the desired harvest rate would be high. As a 

 result, this would decrease the likelihood of reproductively 

 competent individuals remaining on site and giving rise to 

 a self-sustaining population. 



We also examined the relationship between certainty 

 in obtaining the desired harvest rate and probability of 

 a population becoming self-sustaining when the diploid 

 detection threshold was zero, meaning 100% 

 triploids were stocked (Fig. 3). This procedure 

 distinguished between the effects of the tech- 

 nical problem of detection from the biological 

 problem of reversion. It modeled the ideal sce- 

 nario where flow cytometry detected any and all 

 diploids in a batch but still allowed reproduc- 

 tively effective reversion to occur in age classes 

 greater than two. When the detection threshold 

 was set at zero and as certainty in obtaining the 

 desired harvest rate increased, the probabil- 

 ity of a population of triploid oysters becoming 

 self-sustaining was decreased in relation to a 

 detection threshold of 0.001. For example, when 

 the detection threshold is 0.001 and certainty 

 of harvest is set at 0.75, the probability of the 

 population becoming self-sustaining is 0.082. In 

 contrast, when the detection threshold is 0.000 

 and certainty of harvest is set at 0.75, the prob- 

 ability of the population becoming self-sustain- 

 ing is 0.006. 



0.8 



06 



0.4 



0.2 



70 75 80 85 90 



Minimum shell-length-at-harvest (mm) 



95 



100 



Figure 4 



Relationship between minimum shell length-at-harvest (mm) and prob- 

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

 all other variables at default values. 



Minimum shell-length-at-harvest 



As the minimum shell-length-at-harvest was 

 increased, the probability of the population 

 becoming self-sustaining increased (Fig. 4). 

 Increased probability of self-sustainability is 

 due to the probability of reproductively effective 

 reversion increasing the longer oysters are in the 

 water, and the number of offspring produced by 

 undetected diploids becoming higher. 



Stocking density 



With a 0.001 threshold for detecting diploids in 

 triploid batches, and certainty of harvest of 0.9, 

 the probability of the population becoming self- 

 sustaining increased with increased stocking 

 density (Fig. 5). Increased probability of self- 

 sustainability is due to an increase in gamete 

 fertilization efficiency as density increases 

 (Mann and Evans, 1998). In contrast, when 

 the diploid detection threshold was decreased 

 to 0.000, meaning that all oyster spat stocked 

 were indeed triploid, and certainty of harvest 



