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Fishery Bulletin 101(4) 



confinements (i.e. floating cages, lantern nets, bags, 

 etc.) to maximize the certainty of achieving harvest, 

 harvesting at the earhest possible (and presumably, 

 economically feasible) opportunity, and maintaining 

 a low population density of stocked oysters. 



There are several key factors that affect the 

 model's overall predictive value. First, few of the 

 biological parameters that determine reproductive 

 potential of Suminoe oyster are well known. All key 

 parameters in the reproduction equations of the 

 model were based on eastern oyster data (Mann 

 and Evans, 1998) because of a lack of corresponding 

 information about Suminoe oyster. For example, the 

 parameter values in the equation for the relation- 

 ship of oyster density and fertilization efficiency are 

 not known for Suminoe oysters; therefore those for 

 eastern oysters were used. It is important to deter- 

 mine these parameter values for Suminoe oyster 

 so that the model may more accurately predict the 

 relationship between density and the probability 

 of the population becoming self-sustaining. Also, 

 parameter values relating salinity and fecundity 

 are unknown for Suminoe oyster; therefore eastern 

 oyster parameter values were used. For most oyster 

 species, gametogenesis is decreased or even nonex- 

 istent at lower salinities; however, the exact salinity 

 value at which gametogenesis is decreased or absent 

 may vary among species ( Amemiya, 1929; Calabrese 

 and Davis, 1970; Kennedy et al, 1996). The Sumi- 

 noe oyster seems to thrive in estuarine conditions 

 (Huang, 1962; Calvo et al, 2001) and there may be 

 biological parallels to the eastern oyster However, 

 details about lower reproductive potential in lower salinity 

 waters may be different for Suminoe oysters. In the wild, 

 some populations of Suminoe oyster spawn in early spring 

 in salinity as low as 10 ppt (Zhang and Lou 1956; Huang, 

 1962); therefore the model may slightly underestimate fe- 

 cundity of Suminoe oyster in low salinity environments. 



In this model, we also assumed that any oyster whose 

 gamete cells reverted from triploid to "reproductive" mosaic 

 or diploid recovered full fecundity. However, studies have 

 yet to quantify recovered reproductive function in reverted 

 triploids (Chandler et al., 1999). It is likely that reverted 

 oysters would exhibit lower fecundity than diploid oysters 

 because revertant oysters are mosaic; i.e. they comprise 

 both triploid gamete-producing cells that are unable to 

 produce viable gametes, and also diploid gamete-producing 

 cells that are able to produce viable gametes. Continuing 

 research with triploid Suminoe oyster should help us fill 

 this gap in knowledge; however, until then, we decided that 

 the model should err on the ecologically conservative side 

 with the assumption that all reverted triploids exhibit full 

 reproductive potential. 



Despite its limitations, the model clearly points out key 

 areas of concern, as well as highlights areas where more 

 information or improvements in technology could prove 

 critical. For example, advances in techniques for detect- 

 ing very low proportions of diploids could reduce risk of a 

 triploid Suminoe oyster population becoming self-sustain- 

 ing under high stocking rates. Currently, flow cytometry is 



0,6 Tl 



Figure 7 



Relationship between stocking density, certainty in obtaining the 

 desired harvest rate, and the probability of a C. ariakerisis popula- 

 tion becoming self-sustaining, keeping all other variables constant 

 at default values and stocking area set at 300 square meters. 



used for certifying triploid batches in subsampling larval 

 populations (Allen and Bushek, 1992). Up to hundreds of 

 thousands of larvae can be subsampled from a hatchery- 

 scale larval culture. Cells disaggregated from triploid (and 

 intermingled diploid) larvae then can be assayed. The 

 difficulty lies in detecting extremely low levels of diploid 

 cells within the mix. Improved detection by flow cytometry 

 through repeated sampling techniques could help decrease 

 the probability of stocking diploid individuals, thereby de- 

 creasing the subsequent chance for reproduction. Improved 

 detection would also allow watermen to stock more Sumi- 

 noe oysters in a smaller area. 



We developed our own demographic model instead of 

 using existing oyster models, such as the time-dependent, 

 energy flow eastern oyster model (Hofmann et al., 1992; 

 Dekshenieks et al., 1993; Hofmann et al., 1994; Powell et 

 al., 1994; Powell et al., 1995; Dekshenieks et al., 1996; Pow- 

 ell et al., 1996; Ford et al., 1999) for various reasons. First, 

 there were only two years of growth, mortality, and rever- 

 sion rate data available for Suminoe oysters in the Chesa- 

 peake Bay (Calvo et al., 2001 ) and very little information in 

 the literature about the Suminoe oyster in general. Hence, 

 we decided that a demographics-based population dynam- 

 ics model that tracked population size over time would be 

 the most defensible method for achieving the objective 

 of estimating the probability of a population becoming 

 self-sustaining. Although the time-dependent energy flow 

 model also tracks population size over time, all calcula- 



