Aquatic Animal Health Code
Fallowing in aquaculture
Gaps in aquaculture production at the same location are commonly recognised to be of value in resting or restoring the local environment. As part of this strategy, fallowing can break re-infection cycles by removing loci of a disease from a farm. Consequently, fallowing is often carried out as a regular disease management measure in aquaculture, especially prior to the introduction of new populations of aquatic animals into a previously used site. In order to promote improved health in aquaculture, the Aquatic Animal Health Service in a country may encourage the use of fallowing as a routine management strategy for many diseases. Account should be taken of the likely beneficial effects of fallowing in proportion to the economic costs involved. The Aquatic Animal Health Service should also consider such factors as the level of risk to the local and national aquaculture operations, previous knowledge of the severity of a disease(s), the infective period and distribution of the pathogenic agent(s), the socioeconomic conditions, and benefits pertaining to the general aquatic resources. When the infective period is not known, the farm may be fallowed for a period, the length of which should be based on a risk assessment.
However, where an official stamping-out policy is being carried out for a disease of concern, the Aquatic Animal Health Service should require that an infected aquaculture establishment, and all other aquaculture establishments in an officially established infected zone, be subjected to a required period of fallowing, if necessary synchronised.
In cases where fallowing may be a compulsory measure, for instance in the establishment or restoration of a disease free zone, countries should establish a legal framework for the implementation of fallowing procedures in aquaculture establishments. Legal provisions could include:
defining mechanisms based on risk assessment where individual disease-specific measures may be determined, including disinfection and the length of the fallowing period prior to the re-introduction of susceptible species;
Technical parameters for the implementation of a statutory fallowing plan
Fallowing of a farm should start immediately after:
removal of all species capable of acting as carriers of the disease of concern; and
if appropriate, removal of other species; and
removal of water in which infected stocks have been held, where feasible; and
equipment and other materials contaminated or otherwise capable of harbouring infection have either been removed or subjected to disinfection to standards approved by the Aquatic Animal Health Service.
The length of the statutory fallowing period should be based on scientific evidence of the likelihood of a pathogenic agent remaining infective outside its aquaculture host(s) in the local environment, at a level likely to cause an unacceptable risk of re-infection of the aquaculture establishment. Account should be taken of the extent of the disease outbreak, local availability of alternative hosts, the survival and infectivity characteristics of the pathogenic agent and the local climatological, geographical and hydrographical factors. In addition, the level of risk to the local aquaculture industry and wider aquatic resources may be included. A scientifically based risk assessment approach should be used to determine the length of the fallowing period.
Countries establishing fallowing procedures should develop a detailed set of instructions for disinfection of aquaculture establishments prior to fallowing. For this purpose, the instructions set out in Chapter 4.3. of the Aquatic Code and in Chapter 1.1.3. of the Aquatic Manual should be used as guidelines, taking into account current scientific knowledge on the efficacy of the treatments for the pathogenic agent of concern.
No aquaculture establishment that has been under compulsory fallowing should be restocked until the fallowing period has been completed and permission from the Competent Authority has been received. When restocking, care should be taken not to use stocks of aquatic animals that would compromise the objectives of the fallowing procedure.
To increase confidence in the effectiveness of the fallowing procedures, all farms subjected to compulsory fallowing should have a period of high level official surveillance after susceptible species have been restocked. The duration and intensity of the surveillance should be appropriate for the disease of concern and local conditions.
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