An actuarial valuation shall be made within one year after the date on which any system established under this chapter becomes effective, and thereafter at intervals not to exceed three years. The valuation shall be conducted under the supervision of an enrolled actuary and shall cover the mortality, service, and compensation experience of the members and beneficiaries, and shall evaluate the assets and liabilities of the retirement fund. Upon the basis of the investigation, valuation, and recommendation of the actuary, the board shall, at least 60 days prior to the beginning of the succeeding fiscal year, recommend to the board of supervisors such changes in the rates of interest, in the rates of contributions of members, in county and district appropriations as are necessary, and appropriate mortality tables. In making recommendations to the board of supervisors, the board shall act with the care, skill, prudence and diligence under the circumstances then prevailing that a prudent person acting in a like capacity and familiar with such matters would use in the conduct of an enterprise of a like character and with like aims. No adjustment shall be included in the new rates for time prior to the effective date of the revision. The cost of actuarial valuations and investigations may, in the sound discretion of the board, be charged against the earnings of the retirement fund.