Biomedical Engineering Reference
In-Depth Information
aquaculture feed species chosen must be toxin free to avoid any possibility of toxins being
transferred to the animals being grown.
The nutritional quality of the microalgae used as live feeds is a vital factor; different
aquaculture animals or even stages of growth of a particular aquaculture species will have
different requirements (Volkman and Brown, 2006). The best nutritional composition for a
particular situation is often achieved by using a mixture of several microalgal species, even
though this involves the extra cost of maintaining multiple species in culture. Strains
fulfilling these attributes include the prymnesiophyte Pavlova lutheri and the eustigmatophyte
Nannochloropsis oculata , the diatoms Chaetoceros calcitrans, Chaetoceros muelleri,
Skeletonema costatum and Thalassiosira pseudonana , and the prasinophyte Tetraselmis
suecica . Within a local area there is often interest in using local microalgae, particularly in
low technology 'green water' situations, where fish larvae or crustaceans are reared together
with phytoplankton and zooplankton.
Microalgal production may be up to half the cost of the operation of a hatchery. Therefore,
the aquaculture industry generally uses low technology production systems such as plastic
bags (Plate 9.1b) and ponds. There have been numerous attempts to use dried algae (de
Pauw et al ., 1984 ; Borowitzka, 1997 ; Muller-Feuga, 2000 ) as aquaculture feedstocks in
order to eliminate the difficulty and expense of in-house live algal production, but results
have been mixed and progress in this area remains slow. Algal pastes have also been trialled
in aquaculture (McCausland et al ., 1999) and some pastes, such as those from Tetraselmis
spp., are able to be stored for long periods. However, most pastes cannot be stored for more
than a few weeks before they lose their nutritional value (Borowitzka, 1997). As well as live
microalgal feeds, components of the microalgae are used as feed additives in aquaculture. In
particular, pigments such as
-carotene and astaxanthin are used to enhance the colour of
fish and crustaceans (Section 9.8.5).
Microalgae, either in dried form or as a slurry, have been investigated as a possible
feedstock for cattle and other farmed animals with encouraging results. Chowdury and
co-workers (1995) working at the Bangladesh Livestock Research Institute grew a mixed
algal culture ( Chlorella and Scenedesmus ) in shallow ponds and achieved a daily yield of
algal suspension of 95 tonnes, equivalent to 247 kg dry material per hectare. When fed to
cattle as a supplement to a straw diet, the algal suspension seemed to have created a more
favourable environment in the rumen which enhanced microbial growth and thus increased
nutrient availability. A large number of nutritional and toxicological evaluations have
demonstrated the suitability of algal biomass as a feed supplement or substitute for
conventional protein sources (soybean meal, fish meal, rice bran etc.). There is considerable
interest today in the commercial use of microalgae as an animal feed for the poultry industry
(Becker, 2006 ).
9.8.1 Bioactive compounds
It has long been known that many marine organisms produce a variety of chemical
signals, toxins, anti-fouling agents, and biochemical means of protection from harmful
environments (Volkman, 1999; Skulberg, 2004). An understanding of how these agents
work has wide potential for commercial and biomedical applications. For example, many
marine plants use chemical means to protect themselves from grazers and fouling, and
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