2.6.1. Preserved algae
2.6.2. Micro-encapsulated diets
2.6.3. Yeast-based diets
The high costs associated with algal production, the risks for contamination, and temporal variations in the algal food value still pose problems for any aquaculture operation depending on the mass-cultures of unicellular algae. In order to overcome or reduce the problems and limitations associated with algal cultures, various investigators have attempted to replace algae by using artificial diets either as a supplement or as the main food source. Different approaches are being applied to reduce the need for on-site algal production, including the use of preserved algae, micro-encapsulated diets, and yeast-based feeds.
To date, the requirement for live algae in the mass-production of prey-organisms has been largely reduced. In this way, bakers yeast, marine yeasts and lipid-enriched yeast diets are now routinely used as a sole diet or in combination with the alga Chlorella for rearing the rotifer B. plicatilis (see Chapter 3). In addition, considerable progress has been made in the replacement of live algae in the larval rearing of commercially important shrimp species. Partial replacement of live algae using micro-encapsulated and yeast-based diets is now routine in hatcheries for penaeid shrimp. Complete substitution of live algae by a commercial micro-encapsulated diet has been accomplished recently for the production of various penaeid species using seawater filtered to 5 µm, eliminating the algae but not the bacteria, which apparently contribute important micronutrients (and possibly immunostimulants). In marine fish hatcheries, the tendency is to apply a clear water technique instead of a green water technique. However, the omission of algae in the larval tanks, which requires optimization of feeding strategies and zootechnical aspects, still often results in less predictable culture performance. Despite extensive research efforts, the use of artificial diets in the culture of bivalve molluscs is still very limited. The advantages and disadvantages of each of the three classes of replacement diets for live algae are briefly discussed below.
A possible alternative to on-site algal culture could be the distribution of preserved algae that are produced at relatively low cost in a large facility under optimal climatological conditions and using the most cost-effective production systems. Centrifugation of algae into a paste form and subsequent refrigeration until required is widely applied in North America by oyster hatcheries using remote setting techniques. However, the limited shelf-life and/or the high prices of the presently available algal pastes (US$ 200 or more per kg dry weight) have discouraged many growers from using them. Recently, the development of preservation techniques has extended the shelf-life of Thalassiosira pseudonana concentrates from about 10 days to more than one year, which makes it possible to utilize excess and off-season algal production. Outdoor pond production on a large scale has lead to the bulk availability of a limited number of algal meals, such as spray-dried Spirulina and a spray-dried extract of Dunaliella salina. The latter may be used as a supplement to live algae to improve the growth of bivalve larvae.
In addition, recent techniques have been developed for the large scale production of marine micro-algae under heterotrophic growth conditions, by utilizing organic carbon instead of light as an energy source. Heterotrophic algal cultures can attain up to 1,000 times higher densities than photoautotrophic cultures and can be preserved by spray-drying. Projected costs of producing algae within industrial fermentors vary from US$ 5 to 25 per kg (Gladue, 1991). Unfortunately, heterotrophic mass-production techniques have only been realized for a few algal species, and most of the species that are known to be of high nutritional value (e.g. Chaetoceros, Isochrysis, Skeletonema, Thalassiosira, Monochrysis) are not capable of growing in the dark. Furthermore, heterotrophic conditions may result in a drastic change in the gross composition and reduced (n-3) HUFA content as compared to light-grown algae. Nevertheless, further developments in this rather new technology may improve the biochemical composition and the range of dried algae available in the future.
Through micro-encapsulation techniques dietary ingredients can be encapsulated within digestible capsules and delivered to suspension-feeders without losses of nutrients to the aqueous medium. Possible problems arising from the use of microparticulate feeds include settling, clumping and bacterial degradation of the particles, leaching of nutrients, and low digestibility of the cell wall material. In this regard, low susceptibility to bacterial attack and high digestibility for the filter-feeder may be conflicting requirements for a capsule wall.
Because of their suitable particle size and high stability in the water column yeasts can easily be removed from suspension and ingested by filter-feeding organisms. Furthermore, as opposed to most of the other alternatives to live algae, yeasts can be mass-produced at a relatively low cost. The potential of yeasts as a food in aquaculture has been proven by their successful application in the rearing of rotifers and some species of penaeid shrimp. However, a limited nutritional value of yeasts was reported for various species of filter-feeders and attributed to their nutritionally deficient composition and/or undigestible cell wall. Despite this, the nutritional value and digestibility of yeast-based diets can be improved through the addition of limiting essential nutrients and the chemical treatment of the yeast cell wall, respectively. In this way, about 50% of the algae can be substituted by yeast-based diets with minimal effects on the growth of juvenile hard clam, Mercenaria mercenaria (Coutteau et al., 1994).