Year Class

A year-class, or cohort, is the group of fish spawned in the same year, the unit that recruitment and cohort analysis track through time. Year-class strength is set early, mostly during the larval and juvenile stages, and varies widely with environmental conditions, so a single strong year-class can dominate a fishery’s catch for a decade while weak ones leave gaps. Virtual population analysis reconstructs each year-class from its catch-at-age history, and length-frequency analysis identifies cohorts as modal peaks. Managing toward stable recruitment protects the sequence of year-classes that sustains the stock.