6.6 NURSERY CULTURE

Bivalve nurseries serve as an interface between hatcheries and the growout phase, i.e. the culture of bivalves in suspension or off-bottom in the sea. They are cost efficient systems that eliminate the necessity of growing very small seed in fine-mesh nets such as Pearl nets, whose meshes readily clog with floating seaweed, sediment and the settlement of fouling organisms.

The purpose of nurseries is to rapidly grow small seed at low cost to a size suitable for transfer to growout trays, bags, or nets with mesh apertures of 7 to 12 mm. Larger mesh size growout trays are not as prone to rapid clogging and require less maintenance.
Nursery systems evolved in Europe and the USA in the 1970s and early 1980s as a natural adjunct to hatcheries. They can be regarded either as the final stage in hatchery production or the first stage in growout. The most efficient nurseries stock seed at high density in upwelling containers. Others may consist of floating or submerged tray units in productive waters with or without an element of forced as against passive flow, but these systems are more akin to growout and will not be considered here.

Nursery spat holding containers may be mounted on rafts or barges moored in productive estuaries or saltwater lagoons. Others are placed in troughs adjacent to or on upwelling rafts floating in natural or artificially constructed seawater ponds (Figure 103). Primary production can be enhanced in ponds and lagoons as already explained by the application of natural or artificial fertilizers to encourage blooms of algae, usually of naturally occurring species. In this respect, they are more amenable to management than sea-based nursery systems because the quantity and to some extent the quality of the available food supply can be manipulated and controlled.

Figure 103: A – a land-based nursery with food supplied by a pair of blooming ponds that are filled and fertilized at different times to promote a succession of blooms. Food is controlled by allowing a flow of water from the most productive pond – Pond 2 in the diagram – into the stock pond from which the troughs holding containers of spat are supplied. B – a floating barge or raft nursery that may be moored in a productive estuary or in a large coastal lagoon or system of ponds. Small floating nurseries may be powered by a low-head propeller (axial flow) pump and larger versions by a paddle wheel, both of which drain water from the discharge channel generating upwelling through the mesh bases of the spat holding containers.