The most common harvest method of all is drain harvest where all the water is released from the pond at once and most or all of the shrimp exit the pond with the water. This technique requires that pond bottom be above the water level in the receiving ditch. In extensive ponds serviced by tidal water exchange, drain harvest can often only occur a few times a month, at appropriate tidal stage. Timing is critical in large ponds since complete draining may take 3 to 4 days. Once draining begins, shrimp will tend to move with the current and any cessation in flow may cause the shrimp to redispense in the pond and be stranded, with sometimes disastrous results. Close attention must also be paid to dissolved oxygen levels during draining. If these fall too low, new water must be pumped into the pond even if this means halting the drain. It may then be necessary to refill the pond and harvest latter to avoid excessive stress induced molts, or stranding problems (Fast, 1992).