Fish Population Dynamic and Stock Assessment

2.2.5 Virtual population analysis

The virtual population analysis (VPA) and cohort analysis are the methods used in fish population dynamics.  This analysis is basically an analysis of catches of commercial fisheries obtained from fisheries statistics along with detailed information on the contribution of each cohort to the catch data.

In VPA analysis, once the history of population is known the future catches could be predicted.

The version of VPA suitable for pocket calculator is the cohort analysis. This analysis is identical to VPA and the calculation is also made simple.  In VPA, the catch is taken continuously and in cohort analysis, the assumption is made that all fish are caught on one single day.  The day is chosen to be I^st July i.e. when one half of year has elapsed and natural mortality (M) is assumed to be constant throughout the life span of a cohort. The two methods namely Pope’s cohort analysis and Jones length based cohort analysis are used for prediction of catches.