Zusammenfassung: The purpose of this study is to construct a model that can predict the impact on fisheries caused by coastal development activities. To accomplish this, many species of fish must be incorporated in the model, because each type of fish has a different value as a fishery resource and a different behavior for the same impact. In addition the effect of fish swimming must be considered because even when the total resource amount of fish does not change, fish may still disappear from the object area. These factors make the model structure complicated. Meanwhile, a new concept called “object oriented programming” (OOP) is attracting the attention of modellers. In this paper, we have constructed a model called the “shallow-sea ecological model” (SSEM), by using Smalltalk, which is representative of the OOP language. This showed that the fishing ground environment are expressed very flexibly and plainly by Smalltalk.