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Wild population data Collecting data to aid in the analysis of disease in wild populations is much more challenging than reviewing records on a farm .
Depending on the fishery , the level of knowledge about existing stocks will vary considerably . A commercially valuable and regulated species may be carefully managed by authorities or the local industry body with robust estimates of existing biomass . This data may have been captured by regular and proven quantification methods such as transect counts used in the abalone commercial catch industry .
For highly pelagic species estimating fish populations is much more difficult . The level of mortality in the pilchard die‐off of 1995 will never be accurately known .
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Calculating the denominator on farms and in the wild
Working out the number of fish in the population at risk can be challenging ! Farmers will often know the approximate number of fish in a culture unit . Where this number is quite uncertain , an approximation will need to be made and treated with caution . We need to assess the accuracy of calculating the biomass and therefore number of animals in a unit and consider how often counts are undertaken and potential sources of bias .
In wild populations , best estimates from industry / government sources , transect counts , or researchers , can be used as outlined above .
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