Crassostrea virginica spat
Bivalve aquaculture production is more dependent on high quality microalgae than any other aquaculture sector. Because bivalves filter-feed almost exclusively on microalgae, the quantity and quality of available microalgae is critical at every life stage, and especially every stage of hatchery production: Broodstock, larvae, hatchery set, remote set, and nursery. Bivalve hatcheries today employ a combination of algae naturally found in local waters with algae produced in their own facilities to meet this demand.
The microalgae bottleneck
One of the greatest bottlenecks in bivalve production is an inadequate supply of quality microalgae when it is needed most. Algae production can be compromised by weather and season (where natural sunlight is used), equipment failures, or human error, and it must be timed to match the demands of the hatchery. Short day lengths, low light levels and suboptimal temperatures dramatically reduce winter algae production. To overcome this shortage, hatcheries culture algae, usually indoors, and production of microalgae is responsible for a major fraction of the infrastructure, labor, and other operating costs of a bivalve hatchery.
Algal culture requires specialized equipment and skilled labor, which entail costs with no return during the seasons when algae are not needed. Any shortfall in algae production can result in reduction or even failure of bivalve production.
Even when conditions for algae production are good, hatchery cultures can experience unexplained crashes or contamination by unwanted algae or zooplankton, limiting the availability of nutritionally critical microalgal species. Algae that is produced but cannot be used (because timing of production was misjudged, or an anticipated hatch was not successful) is an often overlooked, although significant, contributor to the total cost of algae production. An additional concern is that algae from both local waters and hatchery cultures can be sources of shellfish pathogens.
As an option commercially-available microalgae concentrates provide an economical supplement or replacement for ambient phytoplankton or hatchery-cultured microalgae.
Refrigerated or frozen algae concentrates or “pastes” have been in commercial production for more than 15 years. These products, which are most often viscous liquids, have proven to be effective feeds for all life stages of bivalves. The concentrated microalgae are suspended in media that preserve cellular integrity and nutritional value, although the cells are usually non-viable. Non-viable concentrates have the advantage that they pose no risk of introducing exotic algal strains to a locality. Concentrates produced at inland facilities free of pathogen vectors greatly reduce the danger of introducing diseases along with the feed.
The best refrigerated products typically have a shelf-life of several months, and the frozen products several years. This means that a reliable supply of algae can be kept on hand, available for use in any season or if an unexpected need arises. Algae costs become predictable, and often prove to be less than the cost of on-site production when total production costs are accounted for. What’s more, algal concentrates make it possible for small facilities with limited space and labor to undertake more ambitious projects that require occasional heavy feeding, or feeding with several different algae species.
Microalgae concentrates can be formulated with well-defined biomass densities, so the algae can be continuously and accurately dosed into bivalve cultures with a metering pump, matching feed delivery to the demands of the cultures to maximize feeding efficiency. This provides a significant advantage compared to the great difficulty of regulating feed dosing when feeding with hatchery-grown phytoplankton cultures, with their variable growth rates and concentrations.
Larviculture depends on availability of healthy broodstock. The limited supply of microalgae that often prevails in winter often results in poor broodstock condition. The hatchery production season must be delayed until broodstock animals can consume sufficient microalgae to support development and maturation of gonads, and ultimately gametes. Algae concentrates made with nutritionally optimized blends of microalgae are used to enhance the fecundity of broodstock and accelerate gonad maturation, allowing the production season to be started weeks before wild bivalve populations would be capable of supplying reproductively mature animals.
Partial or complete replacement
Microalgae concentrates can be used as a total replacement for live algae in bivalve larviculture. It has been shown that Crassostrea virginica larval growth rates using a 4-species microalgae concentrate are comparable to that achieved by feeding with live microalgae cultures, at a cost per million larvae of US $6.88 assuming a 14-day growout [Rikard & Walton 2012]. Because such products can be as much as several thousand-fold more concentrated than cultured microalgae, they are appropriate for implementing new innovative high-density larviculture methods that can dramatically reduce water use.
Minimizing water use is especially important where local water conditions are unfavorable (e.g. affected by extreme temperatures, acidification, toxic algae or bacterial blooms) and the cost of treating sufficiently large volumes of seawater is economically prohibitive, or even impossible.
Feeding spat at remote sites
Microalgae concentrates are also convenient for feeding spat at remote-set locations distant from a hatchery. Even for hatcheries that culture their own algae for on-site use, use of commercially-available concentrates eliminates the need for equipment and labor to concentrate the algae before transport to a remote site.
Specific Pathogen-Free Algae
Use of SPF algae concentrates ensures that no dangerous contaminating organisms are introduced with the feed. It is much more economical to treat small volumes intensively (microfiltration and UV) to prevent introductions of pathogens and parasites, so even small facilities can afford to produce pathogen-free seed.
The demand for increased production from bivalve hatcheries continues to grow and the use of microalgae concentrates can be expected to increase as hatchery operators use it to overcome bottlenecks in the expansion of the bivalve industry.
— Eric Henry and Tim Reed
Eric Henry is a research scientist and Tim Reed is founder and President of Reed Mariculture, Inc. in California, one of the world's largest producers of marine microalgae concentrates. The company also produces zooplankton, copepods and weaning feeds. For more information contact Eric Henry at email@example.com or Tim Reed at firstname.lastname@example.org
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