Another rough rule of thumb is that the volume of spat should double every 7 to 10 days. Growth is the best indicator of a good nursery and, given suitable water temperatures, the volume of newly stocked spat should increase steadily. This transfer is usually not attempted until the spat are at least large enough to be retained on 3-4 mm mesh and will be dependent on the type of nursery equipment used for estuarine growout. But this allows for differences in growth rates within each batch of spat and requires that spat are progressively removed from the system when they reach a size at which they can be transferred to the leases. Each 10-screen upweller unit described is capable of handling batches of 750,000 to 1 million spat at a time. As the spat grow the number of spat on each screen is reduced. When the spat are large enough to be retained on 1.25 mm mesh, they are moved to 1 mm screen.

The spat remain on the 350 micron screen until they are large enough to be retained on 670 micron mesh, at which time they are moved to 500 micron screen.

It is common to stock about 250,000 spat (120 ml) on each 350 micron screen. This is done in case the shell of spat is abraded during grading and transport to the field nursery. Spat from the hatchery are usually large enough to be retained on at least a 500 micron mesh sieve, however, for safety these spat are placed on a] 350 micron screen in the nursery. Stocking densities for the screens vary with the size of the spat. Click here for commercial upweller systems. Flows however should not be strong enough to lift the spat off the screen into the water column as they will clog the outlet and cause the screen to overflow. It is impossible to predict in advance what will be required, however, as a rule of thumb we allow 20 L/minute for each 45 cm upweller screen. Flow rates of water through the screens are an important consideration and vary in accordance with the numbers of spat held, the size of the spat and the amount of suitable food in the water. Upweller tanks can be constructed from fiberglass, plywood, barrels or moulded plastic and are designed to hold single or multiple screens depending on the water flow that can be delivered to this type of system. The example (Figure 3) is 2.7 m long x 1.25 m wide x 0.5 m deep, with a central trough or channel 12 cm wide. Plastic mesh secured with a cable tie or similar is placed over the outlet as a back-up screen to prevent spat being accidentally flushed from the screen. A small (1.5 cm) hole in the wall of the screen, opposite the outlet, is used to hold a rod or peg that braces the screen (Figure 2). This sleeve serves to bayonet mount the screen to the inner wall of the tank connecting it to the sump. An outlet (sleeve) is glued or welded through the upper wall of the screen. Fine mesh is glued to the base of the cylinder and then secured with an additional ring of plastic that is heat–shrunk around the rim of the screen.

The example (Figure 2), is the most common in NSW and is made from sheet plastic that is welded in to a 45 cm diameter cylinder. Upweller screens can be made from plastic, wood or fibreglass. There are variations on this design including floating upweller systems or FLUPSYS. The example pictured (Figure 3), is a twin tank system with a central overflow sump. Seawater is pumped into each tank, which then “wells up” through the screen before passing to waste through a screened outlet into the sump. The name upweller is derived from a system in which spat are held on a screen in a tank or trough. Upweller nurseries of the type illustrated in Figures 2 & 3, are one way that these spat are on-grown to a size at which they are suitable for estuarine growout techniques.

Size of oyster spat