Physical methods of disease prevention and control are based on the physiological tolerance of disease agents to adverse conditions such as increased or low temperature, absence of moisture, presence of deleterious irradiation; and the removal of pathogen sources or presence of physical barriers to prevent contact between the disease agent and the host.
Potential pathogens can be removed by ultraviolet radiation and through the process of microfiltration. Chemical pollutants can be eliminated by carbon filtration, biofiltration and water dilution. Exposure of tank or pond to heated water and sun-drying can also eliminate some microbial flora. Infected fish must be removed quickly and destroyed.
Such health classification scheme has been proposed by Ghittino and deKinkelin (1975):
• Fish free of specific pathogenic organisms (SPF) – refer to fish free of all species-specific pathogens. The water supply must be completely sterile and exchanges of fish is possible only between SPF classified establish-ments.
• Fish free of coded pathogenic organisms (CPF) – include fish free of all diseases appearing in a list drawn up by an international agreement. For Southeast Asian countries, such a list has yet to be drafted. Water supply would have to be pretreated. CPF classified farms can receive SPF or CPF fish but cannot dispatch fish to SPF farms.
• Fish free of specified diseases (SDF) – relate to fish reared in water supplies in which pathogens could exist, multiply or be disseminated by wild fish. Disease could occur but readily controlled by therapy. Certification for free-dom from certain diseases can be issued but guarantees only for the dis-eases listed in the document. Such a farm can receive fish from SPF or SDF farms as well as enterprises of similar sanitary level.
• Uncontrolled fish consist of fish not checked for the presence of disease or pathogens. Fish exchange is possible only with farms of similar category but can receive fish from the three foregoing ones.
This sanitary classification of fish farms can be used as basis for issuance of permits for fish import, export, exchange or restocking.
The International Council for the Exploration of the Seas (ICES) have recom-mended policy measures dealing with the introduction of aquatic species and guidelines for implementation, including methods to minimize the possibility of disease transfers. Such recommendation is the Revised Code of Practice to Reduce the Risks of Adverse Effects Arising from the Introduction and Transfers of Marine Species (Sinderman and Lightner, 1988). The ICES Code of Practice is as follows:
• A recommended procedure for all species prior to reaching a decision re-garding new introductions;
• Recommended action if the decision is taken to proceed with the introduc-tion;
• A suggestion that regulatory agencies use the strongest possible measures to prevent unauthorized introductions;
• A recommended procedure for introduced or transferred species which are part of current commercial practice; and
• A note recognizing that countries will have different attitudes toward the selection of the place of inspection and control of the consignment.