Course of Action I: While treating the water to identify the virus may seem important, it may not be the most immediate action needed in response to mass mortality. Immediate treatment might not be practical without understanding the scope and implications first.
Course of Action II: This course of action is strong because temporarily stopping the catching of shrimps can help protect the remaining shrimp population and prevent potential transmission of the virus to other areas.
Course of Action III: This is also a valid course of action. Asking fishermen to be vigilant for similar phenomena in nature can help in early detection of such issues in the future, contributing to better management and response.
So, the correct option is (D): Only II and III follow.