Algae for Tilapia

Tilapia start feeding shortly before dawn and feed continually until about dusk. They do not feed during the night.[1][2] Planktonic plants and -animals make up the bulk of the diet under natural conditions.[3] Tilapia (Oreochromis niloticus) may effectively control algal blooms in eutrophic waters.[4] Blue-green algae are common components of the Tilapia diet. In the stomach of Tilapia nilotica the cells of blue-green algae are lysed by high concentrations of acid (pH 1.4–1.9). After lysis, cell contents are digested in the intestine (by pepsinogen, a pancreatic α-amylase, trypsin, chymotrypsin and esterase activity). Acid is secreted in relation to feeding. Acid is not secreted by stressed fish.[5] Filtration rates of algae increases linearly as water temperature increases from 17°C to 32°C.[6]

Selective feeding

Anabaena cylindrica © snkonsta

Uroglenopsis © John DuFresene

Spirulina platensis © http://mcc.nies.go.jp/

Ceratium © Y. Tsukii

Peridinium cynctum © Jacek

Keratella © Michael Plewka

Tilapia feed selectively on large algae, mainly cyanobacteria and diatoms.[7] Blue tilapias (4.3 to 18.7 cm long) selectively feed on particles larger than 25 μm.[8] Nile tilapia is particularly effective in filtering the larger particle size taxa.[9] Larger phytoplankton are being filtered proportionally more than the smaller phytoplankton, and cyanobacteria more than green algae (Scenedesmus, Ankistrodesmus, Tetraedron).[10] Ingestion rates are higher on Anabaena cylindrica (larger filamentous cyanobacteria that exists as plankton) than on Microcystis aeruginosa (freshwater cyanobacteria that produce neurotoxins and hepatotoxins).[11] Tilapia (galilaea) favourably consume Peridinium cinctum (30-70 μm armoured dinoflagellates).[12] Blue tilapia favour Uroglenopsis sp. (cells forming hollow spherical colonies, > 400 μm diameter), Ceratium sp.(dinoflagellate, 310 μm long, 58 μm wide) and Keratella sp. (100 μm long 'wheel animals') over (small-sized algae) Rhodomonas sp., Chrysochromulina sp., Chlamydomonas sp., Cyclotella sp. and (zooplankter) Diaptomus sp.[13]

Larval tilapia that were fed solely raw Spirulina (blue-green alga/cyanobacteria) at a feeding rate of 30% (on a dry basis) of bodyweight in the first 3 weeks, 10% in weeks 4–6, and 3% in weeks 7–10, kept growing without any abnormality.[14] Larval Tilapia (Oreochromis niloticus < 3.4 cm) prefer consuming Spirulina platensis over Euglena gracilis (flagellate protists), and is also more readily assimilated. Tilapia prefer both species over Chlorella vulgaris (green algae, 45% dry matter protein), which is hardly ingested by larval tilapia.[15] In Tilapia aurea fed Spirulina platensis, food conversion was 2.0.[16] Dietary Spirulina incorporation increases antioxidant activity in tilapia.[17]

Various cyanobacteria may produce toxins (eg Microcystis aeruginosa). Toxin producing cyanobacteria do not (negatively) affect growth, feed conversion efficiency, health or mortality in Oreochromis niloticus. However, microcystins do accumulate in muscle tissue (and particularly in the liver), which may exceed the upper limit of the tolerable daily intake of microcystins suggested by the WHO (0.04 μg/kg body weight/d).[18]

Periphyton

Surface-grazing on periphyton (a slimy matrix/mixture of algae, cyanobacteria, heterotrophic microbes and detritus attached to submerged surfaces) is greater than on Microcystis aeruginosa. For tilapia, filter-feeding may be a relatively unimportant method of ingesting algae.[19] Tilapia mossambica feeds almost exclusively on periphytic detrital aggregate. (Assimilation efficiencies: organic matter 63%, protein 77%, carbohydrate 63%) [20] The fresh microbial mat was 81% digestible by Nile tilapia, comparing favorably with commercial catfish feed in digestibility by Nile tilapia. The dried form was significantly less digestible.[21] Tilpia are equiped with a 'stomach bypass' to be able to bypass or regurgitate unwanted materials.[22]

Algae meal

Under most unnatural feeding conditions tilapia are unable to sufficiently ingest high volumes of algae. They may need constant grazing to fulfill their nutrient requirements.[23] Tilapia (Sarotherodon niloticus) grow better on fishmeal than on a 25% protein green algae meal (Cladophora glomerata). Weight gain decreased as the level of algal protein increased as replacement of fish meal. Protein digestibility was highest on a 5:1 ratio (fishmeal : green algae meal).[24] Protein synthesis (with normal sulfur and carbon content) by green algae during the night may match protein synthesis during the day (in Dunaliella tertiolecta).[25] Protein derived from algae does not promote adequate growth in Rainbow trout.[26] Green algae ulva meal (Ulva rigida) may replace soy bean meal to the extend of 20% without negatively affecting growth of male larval tilapia. Feed conversion ratio increased with increasing ulva meal content.[27] Green algae meal (Hydrodictyon reticulatum) may replace meal to the extend of 25% without negatively affecting growth of Oreochromis niloticus and Tilapia zillii fingerlings.[28] Spirulina maxima meal protein can replace up to 40% of the fish meal protein in Oreochromis mossambicus fry diets without negatively affecting growth.[29]