The study tries to suggest one kind of method for measuring expost commutative fisheries damages caused by a large scale coastal reclamation. The situation of fisheries damages which the paper is considering is different from those of general cases:the latter is a priori investigation but the former is a posteriori one. Therefore we need a different approach for exact measurement of such kind of fisheries damages. The key contribution of the paper is to try to estimate a decreasing production function using the results of present investigation and several statistical data about our coastal fisheries productivity and environmental deterioration. Using the function, the paper tries to derive the expected catch amount and damage amount.