To cause a general investigation to be made by a competent actuary, at least once every 3 years, of the retirement, disability, separation, mortality, interest, and employee earnings rates; to recommend, as a result of each such investigation, the tables to be adopted for all required actuarial calculations; and to cause an annual determination to be made by a competent actuary of the liabilities and reserves of the system and an annual determination of the amount and distribution of the required employer contributions.