This study attempts to suggest a new approach of the estimation of range and degree of fisheries damages caused by a large scale of reclamation undertaken in coastal area using the central limit theorem(CLT) in statistics. The key result of the study is the introduction of the new concept of critical variation of environmental factor($d_{c}$). The study defines $d_{c}$ as a standard deviation of the sample mean($ar{X}$) of environmental factor(X), in other words, $frac{sigma}{ sqrt{n}}$. The inner bound of $d_{c}$ could be the area of fisheries damages caused by public coastal undertaking. The study also defines the decreasing rate of fisheries production$delta_{varepsilon}$, in other words, degree of fisheries damages, as the rate of change in the distribution of sample mean(($ar{X}$), caused by the continuous and constant variation of environmental factor. Therefore $delta_{varepsilon}$ can be easily calculated by the use of table of the standardized normal distribution.