Estimates the internal rate of return on costs of investment in a ‘standard’ giant clam farm involved in the ocean phase of giant clam mariculture as a function of the period of ocean growout of Tridacna gigas using Australian data obtained from trials at Orpheus Island Marine Research Station. The standard ocean 'farm is assumed each year to place 100,000 seed clams of approximately one year of age. The optimal length of time to hold them is estimated to be 10 years when they are sold for their meat at $5 per kg at the farm gate. This yields an estimated internal rate of return of 19.5% and maximises the net present value or capitalised value of the farm. The method used to estimate the optimal rotation or harvest cycle for giant clams is similar to that used in the forestry economics literature for determining forest rotation cycles.