3.4.6 Extensive outdoor culture

Intensive culture systems, described above, are closely controlled and highly productive, providing food for larvae, small juveniles and hatchery-held broodstock. An alternative, particularly suited to the provision of food for larger juveniles, is extensive outdoor tank culture, which makes use of natural light (Figure 31). This involves the fertilization of a large volume of seawater with the basic nutrients necessary for production, namely nitrogen, phosphorus and silica in one form or another. Here, the objective is not necessarily to induce a monospecific bloom, but a mixed flagellate and diatom population at a density greater than would normally occur in the sea.

It is possible to induce monospecific blooms by prior fine (<2 ?m particle retention) filtration of the impounded seawater and the introduction of an inoculum of the required species, as long as it is hardy and vigorous. The use of seawater, or suitably saline brackish water, drawn from wells will also serve this purpose. However, it is difficult to maintain such blooms for long periods because they rapidly become contaminated with other microorganisms.
Multispecific blooms are more easily managed and rely on the natural phytoplankton content of the seawater utilized as the inoculum.

While the species composition will vary from one blooming to another, according to season and environmental conditions, algae produced in this manner is nutritionally valuable in growing juveniles and also for maintaining broodstock.

Figure 31: Examples of large-scale, outdoor algal production. A – circular, covered, semi transparent, fibreglass tanks at a hatchery in British Columbia, Canada; B – 450 000 l concrete tanks used to bloom natural phytoplankton in support of spat culture at the Fisheries Laboratory, Conwy, UK; C – large concrete tanks with sloping bases used for monospecific algal production at Turpiolito, Venezuela: D – 2,500 l fibreglass “fish boxes” at a hatchery in Nova Scotia, Canada.
At the Fisheries Laboratory, Conwy, large, outdoor concrete tanks ranging in volume from 60 m3 to 450 m3 have been used for extensive algal production in support of the nursery culture of bivalve seed. These tanks are filled with seawater of 28 to 32 PSU salinity from the adjacent estuary at approximately 2-week intervals. In this form of culture, fertilizers are added 3 days before the tank is needed to produce algae as food for juvenile bivalves. The chemicals added are:
Urea NH2CONH2 (46% N) 1.50 g per m3 Triple superphosphate P2O5 (20% P) 1.56 g per m3 Sodium metasilicate Na2SiO3.5H20 (13% Si) 10.60 g per m3
Concentrations of NH2N are 50 ?g atoms per l; PO4-P, 10 ?g atoms per l and of SiO3-Si, 50 ?g atoms per l. More crudely, the application of poultry or other animal manure at 500 kg per hectare for tanks and ponds of about 1 m depth can be an effective and less costly source of nutrients.
The rate of development of a bloom is related to the initial species composition and density of algae in the seawater, day length, the amount of incidental illumination falling on the surface of the water, nutrient levels and temperature. The surface area/volume relationship of the tank or pond is important. Shallow tanks and ponds of about 1 m depth are more effective than deeper water, permitting better light penetration. Aeration of the tanks or ponds is beneficial to production.
The duration of the bloom depends on a number of factors related to the species of algae that develop in the prevailing conditions and the rate at which the algae are grazed by the bivalves. Usually, a bloom of useful density for feeding purposes can be maintained for 7-10 days after which the tank is drained, cleaned and refilled with fresh seawater.
The species composition of blooms can be manipulated to some extent by altering the types of fertilizers added. For example, by omitting Si, flagellate species may dominate because the natural Si content of the water upon which diatoms depend will rapidly become depleted. In smaller tanks it is possible to inoculate the fertilized water with a species grown in intensive culture systems. Whether or not this species will become dominant in the bloom depends on environmental conditions and the presence or absence of competing species. In general, the use of artificial fertilization of impounded seawater is a valuable technique in bivalve culture, particularly in nursery systems for juveniles. It is often possible to improve phytoplankton production by a factor of 5 or more compared with open sea conditions. The cost in fertilizers is small per 1 000 l of seawater compared with the considerable benefits in the increased commercial value of the faster growing juveniles.