A method of calculating indices of annual year class strength from abundance data only is introduced. It involves a mathematical technique based on difference equations. The method is applied to data for six commercially harvested species of Green Bay, Lake Michigan. Estimated year class strengths are then compared with relative abundance indices of the parental spawning populations. Only for smelt and perch was there a significant correlation between the two. The same species were also the only ones for which the exponential relation of Ricker between spawners and year class strengths could be established.