Algae for Tilapia
Contents
Feeding
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] Tilapia (Sarotherodon melanotheron) shift from visually feeding on zooplankton when juveniles to mostly filter feeding on phytoplankton when adults.[5] 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.[6] Filtration rates of algae increases linearly as water temperature increases from 17°C to 32°C.[7]
Algal growth
In phytoplankton-based recirculating systems, algae may rather be limited by dissolved organic carbon availability rather than by nitrogen or phosphorus.[8]
Selective feeding
Tilapia feed selectively on large algae, mainly cyanobacteria and diatoms.[9] Blue tilapias (4.3 to 18.7 cm long) selectively feed on particles larger than 25 μm.[10] Nile tilapia is particularly effective in filtering the larger particle size taxa.[11] Larger phytoplankton are being filtered proportionally more than the smaller phytoplankton, and cyanobacteria more than green algae (Scenedesmus, Ankistrodesmus, Tetraedron).[12] Average digestion rate of Microcystis aeruginosa (cyanobacteria producing neurotoxins and hepatotoxins) by Tilapia fingerlings is 58 to 78% at 25℃.[13] Ingestion rates are higher on Anabaena cylindrica (larger filamentous cyanobacteria that exists as plankton) than on Microcystis aeruginosa.[14] Anabaena flos-aquae is much better assimilated (83%) than green algae. Gross growth efficiency of fish fed Anabaena flos-aquae was 46% in comparison to fish fed Chlamydomonas (22%) and Ankistrodesmus falcatus; 24%.[15] Tilapia grow much better on Anabaena than on Clamydomonas (green algae) [16] Tilapia (galilaea) favourably consume Peridinium cinctum (30-70 μm armoured dinoflagellates).[17] Feeding rates of Tilapia galilaea and Tilapia aurea reach maximum values for zooplankton and Peridinium cinctum.[18] Tilapia aurea (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.[19]
Spirulina
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[20], at a slightly lower rate than fish fed fishmeal. Fish fed Spirulina had higher protein, polar lipid, linoleic acid and γ-linolenic acid contents and lower ash and omega-3 contents than fish fed the commercial diet.[21] 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.[22] Tilapia prefer Spirulina over Navicula (a diatom).[23] In Tilapia aurea fed Spirulina platensis, food conversion was 2.0.[24] Dietary Spirulina incorporation increases antioxidant activity in tilapia.[25]
Toxin producing cyanobacteria
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 (initially) 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).[26] Accumulation of microcystis increases lipoxidation (a biomarker of oxygen-mediated toxicity) in the liver.[27] Microcystin concentrations in the liver and intestine may gradually decrease as a result of the capacity in Tilapia to depurate and excrete mycrocystins into the bile and surrounding water, to avoid toxicity.[28]
Periphyton
Periphyton is a mixture of algae, cyanobacteria, heterotrophic microbes and detritus attached to submerged surfaces embedded in a slimy mucopolysaccharide matrix.[29] Surface-grazing on periphyton is greater than on Microcystis aeruginosa. For tilapia, filter-feeding may be a relatively unimportant method of ingesting algae.[30] Tilapia mossambica feeds almost exclusively on periphytic detrital aggregate. (Assimilation efficiencies: organic matter 63%, protein 77%, carbohydrate 63%) [31] Feed conversion ratio of dry matter periphyton may be 2.81. Fish growth may be sustained on periphyton despite 55% (dry matter) ash content.[32] 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.[33] Tilpia are equiped with a 'stomach bypass' to be able to bypass or regurgitate unwanted materials.[34]
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.[35] Tilapia (Sarotherodon niloticus) grow better on fishmeal (fish meal may contain 74% protein and 8% lipids.[36]) 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).[37] Protein synthesis (with normal sulfur and carbon content) by green algae during the night may match protein synthesis during the day (in Dunaliella tertiolecta).[38] Protein derived from algae does not promote adequate growth in Rainbow trout.[39] Fish fed 5% ulva meal (Green algae; Ulva rigida) showed increased growth, feed conversion ratio and protein efficiency ratio.[40] Ulva meal 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.[41] Green algae meal (Hydrodictyon reticulatum) may replace meal to the extend of 25% without negatively affecting growth of Oreochromis niloticus and Tilapia zillii fingerlings.[42] Spirulina maxima meal protein can replace up to 40% of the fish meal protein in Oreochromis mossambicus fry diets without negatively affecting growth.[43]
