Increasing evidence has demonstrated that the life history traits of fishes have changed in many exploited populations, caused principally by intense fishing mortality and size-selectivity of the fishing gear. Broad and intensive trawl fishing over an extended period has the enormous potential to change the biological characters of exploited fish populations. An individual-based model was developed to explore the interactions between trawl fishing and evolutionary changes in length-at-age and age structure of an exploited fish population. A perennial fish population was simulated with a multiple age structure in the model to examine the effects of long-term trawl fishing on hairtail, Trichiurus lepturus, in the East China Sea. The results revealed that distribution of the body length-at-age and the age structure of the fish population were irreversibly changed under long-term trawl fishing. The simulated results confirm that the length-at-age is increasing shorter, the younger individuals dominate, the influence of trawl selectivity on the biological traits of the fish population is highly significant, and that these changes have potentially evolutionary consequences on the fish body length-at-age.