Estimating time to critical levels of Perkinsus marinus in eastern oysters, Crassostrea virginica

A Visual-Basic program was developed as part of an Excel spreadsheet to est imate the time to a critical level (t(Crit)) Of Perkinsus marinus, in easte rn oysters Crassostrea virginica. The estimate is accomplished by assuming that a weighted incidence (WI) of disease of 1.5 is critical, converting me asured WI values and the critical WI to parasite number, calculating a rate of change (r) of the parasite population using measured values of water te mperature (T) and salinity (S), and solving for t(Crit) by simulation. The model produces estimates of t(Crit) and r using a long-term data set of T, S, and WI from the Terrebonne estuary of Louisiana. The model does not pred ict future values of WI since it cannot predict future trends in T and S; h owever, regularly determining T and S, considering their interaction in a m odel, measuring WI at reasonable intervals, and iteratively estimating t(Cr it) should be useful to oyster management. Estimates of t(Crit) would suppo rt decisions concerning transplanting infected oysters to lower salinity ar eas, harvesting heavily infected populations early, and diverting freshwate r into high-salinity estuaries.