Final answer:
The average size of the fish population would decrease due to a form of artificial selection favoring smaller fish, altering the genetic profile of the population.
Step-by-step explanation:
When fishermen historically kept larger adult fish and left smaller adult fish, over time, the average size of the fish population would decrease. This outcome is due to a form of artificial selection, where the reproductive potential of larger fish is reduced because they are removed from the ecosystem before they can breed. Consequently, smaller fish with earlier maturity rates and smaller adult sizes become more prevalent, as these traits are not selected against by fishing practices. This evolutionary pressure can lead to a change in the genetic profile of the fish population, where genes associated with smaller size and earlier maturity become more common, thereby reducing the average size of fish in future generations.