A model on ‘The dynamics of mean-size statistics in a changing fishery’ proposed by Henderson (FAO) was tested using the Kainji Lake experimental gill-net data collected between 1969 and 1977. The results revealed distinct trends in mean weight and population abundance (catch/effort) for members of the family Citharinidae. Simultaneous plots of mean weight against log10 of population abundance allowed the ‘diagnosis’ of the causes of change. Changes were attributed to the effects of fishing mortality, intensive recruitment into the fishery and change in selectivity coupled with the effect of growth. The pattern of change depicted for Citharinus citharus Geoffroy St. Hilaire revealed a three yearly cycle of intensive recruitment. The regression of mean weight on mean length for C. citharus gave a linear relationship thus facilitating the extrapolation of the mean weight values to the mean lengths of fish caught in the different meshes of gill-nets. The conversions permitted the selection of suitable mesh nets for capturing fish at appropriate levels of mean weights in order to safe-guard the survival of sexually mature adults for an intensive spawning cycle.