A case study of the use of the structural model for the assessment of environmental impact on the coastal fisheries is described. The utility of this method is discussed in comparison with the other estimates.We have focused on the impact of sediment discharge due to reclamation work, and identified an impact indicator as differntial catch of small shrimps in the proposed area. Eight environmental factors including the increase of turbidity (C1) as input and the catch loss of the small shrimps (Fl) as output were selected, and interactions among them identified. The interactions were formulated by using remaining ratios as variables. Each remaining ratio is normalized over the range (0-1): the upper boundary for the present state and the lower boundary for the largest impacted state. By inputting predicted initial remaining ratio to Cl, we could quantify the possible impact on Fl.Looking at the effect sequence of the structural model used for the present estimate, it becomes clear that the reduction in the area of algal belt may have a major impact on the abundance of small shrimps. This suggests the significance of indirect effects, of which little notice was taken in the previous estimates.