Results and Discussion
According to reported information, the average concentrationof Cd is about 0.05 lg Cd/L in unpolluted water,while in coastal waters it tends to increase to over 0.1 lgCd/L and even over 10 lg Cd/L in some areas due toanthropogenic input, local geological conditions, andhuman activities, according to reported data (Soegiantoet al. 1999a). Likewise, Zn concentrations in coast waterhad also been detected as 4–800 lg Zn/L (Bryan 1976).Thus, the impact of heavy metals Cd and Zn should be seriously considered when coastal seawater is directly usedfor shrimp culture and the human health is concerned.
Estimation of LT50 values of L. vannamei was based onthe results of mortality and the determined LC50 values.Regression curves were drawn from LC50 values and exposure times as shown in Table 1 and Fig. 1. LT50 values were calculated based on the regression equations as follows: Y = 23.80323 ? 71.54851 9 (e((X - 0.02938)/0.07509)) (R2 = 0.999) for the Cd set; and Y = 3.68762 ? 93.35289 (e(-(X - 0.13033)/0.30121)) (R2 = 0.989) for the Zn set, whereY is the exposure time (h) and X is the LC50 metal concentration(mg/L) for either Cd or Zn. Standard checks of bothregression equations were between -10% and 10% as shownin Table 1. The results of the estimation of LT50 values aftercalculations based on LC50 data sets are shown in Table 2.According to our results, LT50 values of L. vannamei for 1.0,
1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 mg/L concentrations were estimated as shown in Table 2.