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Pathogen (disease and parasite) transfer among wild and farmed populations is a major issue, not only for the farming industry but also for consumers concerned about the sustainability of aquaculture products (ENS 2005; Phillip 2009). Diseases and parasites exist in virtually all food animal production systems (Collins and Wall 2004). In aquaculture, if farmed fish share an environment with wild fish that serve as pathogen hosts, the transfer of pathogens and parasites is a virtual certainty (Johnsen and Jensen 1991; Krkosek et al. 2007). The introduction, transmission (McVicar 1997), and amplification (DFO 2006) of pathogens are all potential risks in such a shared environment.
The formulation of a pathogens indicators proved to be especially difficult, and as such, a dedicated expert workshop was convened to address these particular challenges. It was determined that of importance is the potential effect of a rapid rate of disease onset within the wild community. The impact of diseases and pathogens coming from the farm is estimated using three variables: on-farm production loss, pathogenicity (the degree to which the pathogen causes diseases in the host), and biomass (the mass of the total number of living organisms in an ecosystem) of susceptible species in the ecosystem around the farm. Diseases or pathogens that are not known to cause mortality were not included in this calculation. On-farm production loss is used as a measure of the performance by the sector. It is assumed that an increase in on-farm losses due to pathogens will also result in a proportional increase in the wild pathogen load in susceptible species. The pathogens indicator uses the change in on-farm pathogen load to determine the magnitude of ecosystem impacts using the percentage of the total biomass of wild species in the ecosystem susceptible to the disease/pathogen.
PATH = ∑(Pathogen-Specific Wild Losses (mT))
Units: metric tonnes pathogen-specific wild losses
Sample Normalised Calculation: Bastard Halibut from the Republic of Korea, 2007
For each pathogen identified in a production system, the proportion of the biomass of susceptible fish in the ecosystem, pathogenicity, and life cycle are identified and used to predict the impact on wild populations (pathogen-specific wild losses).
The proportion of the biomass of susceptible fish in the ecosystem is the proportion of species in the ecosystem that is susceptible to the pathogen in question. It is assumed that all members of a taxonomic Family are susceptible to a pathogen when two or more Genera are known to be susceptible (Lafferty, pers. comm. 2009).(For more information visit the FAQ page)
Pathogenicity is the relative proportion of total production losses that is attributed to the pathogen in question.
If data on the total production loss from pathogens are not available, GAPI uses either:
If no proportional loss (relative pathogenicity) information is available, GAPI uses: