S.F.P.D. uses terahertz (THz) electromagnetic wave technology to detect plastic particle residues in fish cargo, conducts plastic material detection before the fish is selected, separates the fish cargo containing plastic particles, and processes the fish to remove plastic particles. Then use S.F.P.D. again for inspection and make pre-shipment clearance. S.F.P.D. can be used in fish processing plants, integrated with existing conveyor systems, and rapidly automated fish and cargo classification and grading shunts for subsequent processing, and modular machine designs can independently apply terahertz (THz) electromagnetic wave detectors to traditional fish wholesale markets to avoid people miseating plastics.