Five devices for the protection of cultured juvenile giant clams (Tridacna gigas) in the ocean-nursery phase were investigated. These were known as trays, boxes, lines, covers and exclosures. The emphasis was a gradual reduction in the material cost and maintenance, and an increase in the size. The merits of each design were assessed. Clams at 0.83, 2.0, 2.6 and 3 years were dissected and the wet tissue mass, shell mass and total mass recorded. From these data the potential production from the ocean-nursery phase was calculated. A survival rate of 40% from the lines and covers for the period covering clams aged 6-8 months to 3 years was obtained. This was used for the calculation of potential production figures. A general figure of 29 t/ha of wet tissue was calculated.