Cohort Analysis

Cohort analysis, also called virtual population analysis (VPA), reconstructs the historical size of a year-class by working backward through its catch-at-age record. Starting from the oldest age and a terminal fishing-mortality assumption, it solves the catch equation iteratively to estimate the number of fish alive at each earlier age and the fishing mortality they faced. Pope’s cohort analysis gives an algebraic approximation that avoids the iterative solution. The method needs aged catch data and an estimate of natural mortality M, and its early-year estimates are reliable while the most recent years stay sensitive to the terminal-F guess.