6.6.1 Land-based nurseries

Land-based pond nurseries are generally located on low-lying land close to the sea. The ponds are flooded at high tide via a sluice, through a culvert with flap valves that opens to the sea, or by low-head pumping. They can be drained by gravity at low tide (see Figure 103).

A land-based nursery system usually comprises a number of shallow, large surface area ponds or tanks interconnected by channels or pipes with sluices or valves. Most of the ponds are used to bloom naturally occurring microalgae species present in the water at the time of filling. Blooming can be controlled and enhanced by the application of agricultural-grade nitrogen and phosphorus fertilizers and a soluble form of silica (section 3.4.6) although reliance on the natural fertility of the water is the more common approach. These algal ponds are used in rotation to supply “bloomed” water to a pond adjacent to the upwelling seed containment unit. Excess water from the pond drains back to the sea and in many cases there is a regular or continuous partial replacement of the water direct from the sea to control the food density and to flush out waste and metabolites. Water is pumped from the supply pond to the upwelling unit, which operates according to the same basic principle as upwellers in the hatchery. Alternatively, if the upwelling unit is a floating structure, water flow is generated by propeller pumps or paddle wheels.

Examples of land-based nurseries are shown in Figures 104 and 106.

 

 

Figure 104: Examples of land-based nurseries. A and B – concrete spat holding tanks containing upwelling cylinders of spat (Tinamenor S.A., Pesues, Spain). Water is pumped from ponds into the tanks and is discharged into a drainage trough at the base of the tanks. C and D – a upwelling nursery system supplied from a nutrient enriched 450 m3 concrete tank at the Fisheries Laboratory, Conwy, Wales, UK. Water is delivered to the spat holding unit (D) by a high capacity submers

ible pump. E and F – the progenitor of most European bivalve nurseries developed by Seasalter Shellfish at Reculver, Kent, England.

The biomass of spat stocked in a land-based nursery is dependent upon the productivity of the ponds or tanks and this can be influenced by such factors as temperature and salinity as well as nutrient levels. Shallow pond systems of large surface area and volume act as heat sinks and will gain temperature from solar irradiation. They will often be at a significantly higher temperature than the adjacent seawater, which is beneficial for the growth of warm water species but requires careful management since blooms may be sudden and short-lived (Figure 105). There is always the risk that excessive blooming of algae will result in oxygen depletion of the pond water. Algae, which normally output oxygen as a by-product of photosynthesis, switch to a net uptake of oxygen for respiration during the hours of darkness when unable to photosynthesise. During intense blooming, sufficient oxygen is withdrawn from the water by the algae that the level of oxygen saturation can drop to as little as 20% over the course of a few hours, usually reaching a low point in the early hours of the morning. This can give rise to unexpected mass mortalities of the small bivalves. It is a wise precaution to have oxygen monitoring equipment connected with an alarm installed in the system. Careful management is exercised to control blooming by water exchange between ponds – assuming that there is more than one – and by diluting blooms with water drawn directly from the sea. If the sea is at a lower temperature than the ponds then it will have a higher oxygen content. Aeration equipment is often used to help maintain oxygen levels in pond systems.

Figure 105: Data from a land-based nursery pond system in Nova Scotia, Canada, operated from early May to the end of October: A – the temperature advantage of the ponds over ambient sea temperature; B – mean weekly temperature of the pond system; C – mean weekly suspended particulate matter (as thousands of particles per ml) in the size range 2.5 to 5.0 ?m diameter (green histograms) and 5.0 to 10.0 ?m (brown histograms). Particulates were determined using a Coulter Counter. Samples were examined by microscopy to ascertain that the particles were mainly of algal origin.

Salinity in the ponds can be lowered by heavy rainfall and by unexpected sources such as freshwater seepage through the ground or by springs or streams that may be seasonal in nature. As in site selection for hatcheries, careful research needs to be undertaken before committing to the development of a nursery at an unknown location.

Determining the biomass of spat that can be held in a pond system is largely a matter of trial and error. A general rule is that 1 hectare surface area of shallow pond will support the production of between 1 and 3 tonnes biomass of seed, depending on levels of algal productivity, over the course of a growing season. This represents the maximum sustainable biomass that can be maintained with careful management. The areas covered by many European nurseries can be measured in the tens of hectares. Spat are managed in much the same way as in hatcheries. They are regularly graded and redistributed so that any spat container will hold spat of a particular grade. Grading is usually accomplished with mechanical graders (Figure 106). Management also

Figure 106: Examples of raft or barge-type nurseries: A to C – raft floating in a man-made pond connected to a large network of blooming ponds with interconnecting channels (Tinamenor S.A., Pesues, Spain); B – detail of the raft showing the cylindrical spat holding cylinders and the lifting device; C – the same raft with detail of the paddle wheel which drives water from the raft’s discharge channel into the pond on the other side of the dam. Clam spat are being hand

graded on the work platform. D – a mechanical seed grader (right) as part of an oyster hatchery/ nursery operation in Atlantic Canada. E – a barge operating on the same upwelling principle but in an estuary in Prince Edward Island, Canada. F – loading the base of a spat container with small oysters from an insulated “cooler” in which they were transported from the hatchery. In this example the stainless steel base is detachable from the fibreglass body of the container.

involves controlling the blooming of algae and this requires regular observation on some parameter or parameters connected with algal production i.e. determinations of suspended particulate material, either as numbers per unit volume (Figure 105C) or as weight per unit volume, chlorophyll determination, or by microscopy. A reference to methodologies can be found in the suggested reading list at the end of Part 6 (Strickland and Parsons, 1968).

While it is generally possible to raise primary production in ponds to levels significantly higher than those prevailing in the sea it cannot always be guaranteed that the types of algae growing are of the size, digestibility, and nutritional value appropriate to the seed in culture. On occasion it may be necessary to alter the mix of fertilizers being used and “spike” a pond with a sufficient quantity of cultured algae to promote a bloom of the required composition (see section 3.4.6).