1. Algae
Algae have been used in animal and human diets since very early times. Filamentous
algae are usually considered as ‘macrophytes’ since they often form floating masses that
can be easily harvested, although many consist of microscopic, individual filaments
of algal cells. Algae also form a component of periphyton, which not only provides
natural food for fish and other aquatic animals but is actively promoted by fishers and
aquaculturists as a means of increasing productivity. This topic is not dealt with in
this section, since periphyton is not solely comprised of algae and certainly cannot be
regarded as macroalgae. However, some ancillary information on this topic is provided
in Annex 2 to assist with further reading. Marine ‘seaweeds’ are macro-algae that have
defined and characteristic structures.
Microalgal biotechnology only really began to develop in the middle of the last
century but it has numerous commercial applications. Algal products can be used
to enhance the nutritional value of food and animal feed owing to their chemical
composition; they play a crucial role in aquaculture. Macroscopic marine algae
(seaweeds) for human consumption, especially nori (Porphyra spp.), wakame (Undaria
pinnatifida), and kombu (Laminaria japonica), are widely cultivated algal crops. The
most widespread application of microalgal culture has been in artificial food chains
supporting the husbandry of marine animals, including finfish, crustaceans, and
molluscs.
The culture of seaweed is a growing worldwide industry, producing 14.5 million
tonnes (wet weight) worth US$7.54 billion in 2007 (FAO, 2009). The use of aquatic
macrophytes in treating sewage effluents has also shown potential. In recent years,
macroalgae have been increasingly used as animal fodder supplements and for the
production of alginate, which is used as a binder in feeds for farm animals. Laboratory
investigations have also been carried out to evaluate both algae and macroalgae as
possible alternative protein sources for farmed fish because of their high protein content
and productivity.
Microalgae and macroalgae are also used as components in polyculture systems
and in remediation; although these topics are not covered in this paper, information
on bioremediation is contained in many publications, including Msuya and Neori
(2002), Zhou et al. (2006) and Marinho-Soriano (2007). Red seaweed (Gracilaria spp.)
and green seaweed (Ulva spp.) have been found to suitable species for bioremediation.
The use of algae in integrated aquaculture has also been recently reviewed by Turan
(2009).
1.1 Classification
The classification of algae is complex and somewhat controversial, especially concerning
the blue-green algae (Cyanobacteria), which are sometimes known as blue-green
bacteria or Cyanophyta and sometimes included in the Chlorophyta. These topics are
not covered in detail this document. However, the following provides a taxonomical
outline of algae.
Archaeplastida
• Chlorophyta (green algae)
• Rhodophyta (red algae)
• Glaucophyta
Rhizaria, Excavata
• Chlorarachniophytes
, Use of algae and aquatic macrophytes as feed in small-scale aquaculture – A review
• Euglenids
Chromista, Alveolata
• Heterokonts
• Bacillariophyceae (diatoms)
• Axodine
• Bolidomonas
• Eustigmatophyceae
• Phaeophyceae (brown algae)
• Chrysophyceae (golden algae)
• Raphidophyceae
• Synurophyceae
• Xanthophyceae (yellow-green algae)
• Cryptophyta
• Dinoflagellates
• Haptophyta
The following sections discuss the characteristics and use of both ‘true’ algae and the
Cyanophyta, hereinafter referred to as ‘blue-green algae’).
1.2 Characteristics
Filamentous algae and seaweeds have an extremely wide panorama of environmental
requirements, which vary according to species and location. Ecologically, algae are
the most widespread of the photosynthetic plants, constituting the bulk of carbon
assimilation through microscopic cells in marine and freshwater.
The environmental requirements of algae are not discussed in detail in this document.
However, the most important parameters regulating algal growth are nutrient quantity
and quality, light, pH, turbulence, salinity and temperature. Macronutrients (nitrate,
phosphate and silicate) and micronutrients (various trace metals and the vitamins
thiamine (B1), cyanocobalamin (B12) and biotin) are required for algal growth (Reddy
et al., 2005). Light intensity plays an important role, but the requirements greatly
vary with the depth and density of the algal culture. The pH range for most cultured
algal species is between 7 and 9; the optimum range is 8.2–8.7. Marine phytoplankton
are extremely tolerant to changes in salinity. In artificial culture, most grow best at
a salinity that is lower than that of their native habitat. Salinities of 20–24 ppt are
found to be optimal. Lapointe and Connell (1989) suggested that the growth of the
green filamentous alga Cladophora was limited by both nitrogen and phosphorus, but
the former was the primary factor. Hall and Payne (1997) found that another green
filamentous alga, Hydrodictyon reticulatum, had a relatively low requirement for
dissolved inorganic nitrogen in comparison with other species. Rafiqul, Jalal and Alam
(2005) found that the optimum environment for Spirulina platensis under laboratory
conditions was 32 ºC, 2 500 lux and pH 9.0. Further information on the environmental
requirements of algae cultured for use in aquaculture hatcheries is contained in Lavens
and Sorgeloos (1996). The environmental requirements of cultured seaweeds are
discussed by McHugh (2002, 2003).
A brief description of some of the filamentous algae and seaweeds that have been used
for feeding fish, as listed in Tables 1.1–1.3, is provided in the following subsections.
1.2.1 Filamentous algae
Filamentous algae are commonly referred to as ‘pond scum’ or ‘pond moss’ and
form greenish mats upon the water surface. These stringy, fast-growing algae can
cover a pond with slimy, lime-green clumps or mats in a short period of time, usually
beginning their growth along the edges or bottom of the pond and ‘mushrooming’ to
the surface. Individual filaments are a series of cells joined end to end which give the
