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Comparative study of algal community in African sharptooth catfish (Clarias gariepinus, Burchell 1822) and Nile tilapia (Oreochromis niloticus, Linnaeus 1758) farming ponds

Comparative study of algal community in African sharptooth catfish (Clarias gariepinus, Burchell... INTRODUCTIONAlgal growth in a water body is a complex activity characterised by the influences of environmental and biological factors, both internal and external to the water body (Wang et al., 2016). Their spatial and temporal distributions, as well as their multiplication and proliferation, are thereby controlled by light, transparency, nutrient availability and grazing by animal communities (Cunha & Calijuri, 2011; Loiselle et al., 2007; Zongo & Boussim, 2015). Thus, through grazing, fish are strongly involved in the growth and proliferation of algae species in the aquatic food web (Janjua & Gerdeaux, 2011; Kitchell et al., 2000; Rand & Stewart, 1998).In the aquatic food web, phytoplankton and algae are the first link in the web. They are eaten by consumers like zooplankton, crustaceans and fish that are exploited in fishing activities (Durand & Levêque, 1980; Mollo & Noury, 2013). Algae constitute then a valuable nutritional source as they are excellent sources of vitamins, proteins, carbohydrates, trace minerals and other bioactive compounds (Kumar et al., 2008; Norambuena et al., 2015). As a major part of the algae community in aquatic habitats, phytoplankton provides many valuable phytonutrients and biologically active ingredients, which are directly available for fish (Napiórkowska‐Krzebietke, 2017). Thus, phytoplankton and algae are the main sources of food for larvae and adults of herbivorous filter‐feeder fish species like Nile tilapia (Oreochromis niloticus) and omnivorous fish species like African sharptooth catfish (Clarias gariepinus; de Graaf & Janssen, 1996; Kestemont et al., 1989). In wild conditions, herbivores consume primarily plant and decayed organic matter, while omnivores consume almost any food source, whether plant or animal (Prabu et al., 2017). Indeed, in natural conditions, the Nile tilapia diet is mainly composed of Chlorophyceae, Cyanophyceae and Euglenophyceae (Kestemont et al., 1989). However, in addition to phytoplankton and macrophytes, the diet of African sharptooth catfish is composed of zooplankton, insects, fish and detritus (Tesfahun, 2018).In fish farming systems, feeds and faeces are known to be the main wastes during fish production. Consequently, important amount of nutrients composed of phosphorus and nitrogen, which are essential for phytoplankton and algae production, is loaded into the environment from these wastes. The released compounds are the major constituents of the main ingredients used in formulated feeds to achieve good growth of fish (Kibria et al., 1997; Lazzari et al., 2008). Fish production in ponds requires large amounts of formulated feeds that inevitably introduce nutrients into water bodies, promoting algal growth and even conducts to algal blooms if eutrophication occurs (Morin, 2007; Wang et al., 2016; Wurtsbaugh et al., 2019).While fee and faeces contribute to increasing the amount of nutrients in the water and thus algal growth, herbivorous and omnivorous fish species contribute to the regulation of algal biomass in their habitats by grazing. This is certainly the case for several fish farming stations, such as the one at the research station of Nazi BONI University in Burkina Faso. Indeed, during African sharptooth catfish and Nile tilapia farming in separated open ponds in this station, fish fee and faeces led to an important growth of microalgae in the farming ponds as well as in the discharged water. Different works have been carried out on the diversity of microalgae in natural environments (Zongo et al., 2019, 2011, 2008). However, algal species that occur in fish farming ponds are not well studied in Burkina Faso. The knowledge of the algal communities that colonise the fish rearing ponds could contribute to characterising, on one hand, the algae consumed by the fish and, on the other hand, the quality of the rearing water. Furthermore, data on algae species and biomass in fish farming ponds and diet could be used in the management of fish farming ponds to thereby enhance fish production.Therefore, the present study characterises algal species in rearing ponds of African sharptooth catfish and Nile tilapia and their contribution to the diet of these species. Specifically, the study identified algae species in rearing ponds, evacuation canals, decantation ponds, stomachs and intestines of the two species and determined some water physical and chemical parameters.MATERIALS AND METHODSExperimental site and facilitiesThis study was carried out in an aquaculture research station located at the Nazi BONI University (Burkina Faso). This experimental station is working on two fish species: Nile tilapia and African sharptooth catfish. These species are reared in separate ponds, and water is collected in a decantation pond through an evacuation canal coming from each rearing pond (Figure 1).1FIGUREExperimental facilities in the fish farming station.Fish rearing conditionsNile tilapia was reared in two ponds of 4 m3 for 20 individuals per m3 density, whereas African sharptooth catfish was reared in two other ponds of 16 m3 for 50 individuals per m3 density. Every day, water was differently renewed in rearing ponds from 5:00 PM to 9:00 AM. During this period of the day, the renewal rates were once per 2 h in Nile tilapia rearing ponds and once per 4 h in African sharptooth catfish rearing ponds. Therefore, water was renewed 8.50 times for Nile tilapia ponds and 4.25 times for African sharptooth catfish ponds. The permanent water inlet and outlet system ensures a complete renewal of the water in the pond without having to empty it. At 9:00 AM, each rearing pond was purged to remove waste and leftover feed that accumulated at the bottom.In rearing ponds, fish individuals were fed three times per day (8:00 AM, 12:00 AM and 4:00 PM) with formulated fee containing 40% protein and 12% lipid for African sharptooth catfish and 35% protein and 10% lipid for Nile tilapia. Fishes in evacuation canals and decantation pond were not fed with any formulated food. They were fed on the feeds naturally produced in their environment including algae.Sampling and identification of algae species from farming watersWater samples were collected in seven points using 120 ml bottle (Zongo & Boussim, 2015; Zongo et al., 2011). The six sampling points were composed of two rearing ponds and one evacuation canal for each fish species. Another sample was collected in the decantation pond (the seventh point) where water from evacuation canals is accumulated. Samples were collected every 15 days for 45 days. On each sampling day, samples were collected in the morning at 9:00 AM according to Leboulanger et al. (2002) and Müller et al. (2001).After sampling, algae species were identified using "sedimentation after fixation" with formalin at 5% as the method described by Laplace‐Treyture et al. (2009). This method allows to take into account nannoplankton species during the identification process, which have great biological importance because of their very high number in water habitats (Bourrelly, 1990). From the 120 ml water sample, a sub‐sample of 50 ml was taken and kept for 24 h for the sedimentation of organisms to be observed under a light microscope (Zongo et al., 2011). After sedimentation, the supernatant was removed with a micropipette, and the precipitate was used for the identification of the algal species under a light microscope (MOTIC, BA 200). Photographs of species were taken for algal species identification, using standard works such as Couté and Rousselin (1975), Compère (1976), Iltis (1980), Bourrelly (1990), Bourrelly and Couté (1986), Kadiri (1993), Ling and Tyler (2000), Nevo and Wasser (2000), Ouattara et al. (2000), Jonh et al. (2002), Wehr et al. (2015) and Zongo et al. (2011). Identified species were verified using AlgaeBase (Guiry & Guiry, 2020).Sampling and identification of algae species from fish stomachs and intestinesTo characterise algae species consumed by African sharptooth catfish and Nile tilapia in the farming pond, fish individuals were sampled from the four rearing ponds and the decantation pond. This sampling was carried out on the same day of water samples collection. Afterwards, they were euthanised with an overdose (200 mg/L) of benzocaïne (Sigma–Aldrich), to find out algae species in the stomachs and intestines (Figure 2). Thus, 18 individuals of African sharptooth catfish and 18 individuals of Nile tilapia were sampled, and their zootechnic parameters were measured (Table 1).2FIGUREIsolation of African sharptooth catfish and Nile tilapia stomachs and intestines for algal collection.1TABLENumber and zootechnic parameters of sampled fish for contents analysis.Sampling pointsOreochromis niloticus (Nile tilapia)Clarias gariepinus (African sharptooth catfish)NMBW (g)MTL (cm)MSL (cm)NMBW (g)MTL (cm)MSL (cm)Rearing pond12115.4 ± 19.818.8 ± 0.815.2 ± 1.112450.3 ± 52.341.4 ± 1.735.5 ± 3.0Decantation pond6119.9 ± 12.419.2 ± 0.315.4 ± 0.26460.2 ± 102.443.9 ± 0.738.0 ± 2.0Note: Value represent mean ± standard deviation.Abbreviations: MBW, mean body weight; MSL, mean standard length; MTL: mean total length; N, individual number.Before opening fish bodies to take out the stomach and intestine, they were washed with technical ethanol (70%) to reduce algae that occur on the bodies. Thereafter, the stomach and intestine were taken out for their contents analysis. Then, the content of each stomach and intestine was diluted 10 times and fixed with 5% formalin solution and kept in a conservation bottle (Al‐Harbi and Uddin 2003). Finally, algae species were identified using the same method as described previously.Measurement of physicochemical water quality parametersDuring the experimental period, parameters such as water transparency, pH, temperature, electric conductivity, dissolved oxygen (DO) and nitrite concentration were measured in the sampling points two times per day. Water transparency was measured with a Secchi disk, while pH, temperature, electric conductivity and DO were measured by using an electronic multiparameter (HACH SAS). A spectrophotometer (HACH SAS) was used to determine nitrite (N‐NO2−) concentration in water.Data processingCollected data on algae, were first used to determine algae diversity through ecological indexes. Thus, Shannon–Wiener and Piélou equitability indexes were used to determine algal diversity in sampling points and fish stomach and intestine (Table 2). Jaccard similarity index was used to determine similarities in algal communities between sampling points.2TABLECalculated ecological indexes for identified algae species.IndexFormulaParametersShannon–Weaver diversityH′=−∑i=1npiln(pi)$H^{\prime}=-\sum\limits_{i=1}^{n}pi\,\ln(pi)$n: total number of speciespi: frequency of the taxon "i" in the given communitypi=niN$pi=\frac{ni}{N}$ni: total number of individuals of each speciesN: total number of individuals0 < pi < 1Piélou equitability (E)E=H′Hmax′=H′lnS${\rm{E}}=\frac{H^{\prime}}{H^{\prime}_{max}}=\frac{H^{\prime}}{lnS}$H: Shannon–Weaver diversityS: total number of speciesJaccardCj=j(a+b−j)$C_{j}=\frac{j}{(a+b-j)}$j: number of common species at two sampling pointsa: total number of species in sampling point Ab: total number of species in sampling point BRESULTSAlgae species composition and diversityIn the whole samples obtained from fish ponds, evacuation canals, decantation pond and from fish stomachs and intestines, 73 algal species were recorded. These species belong to 47 genera, 34 families, 21 orders, 11 classes and six phyla. Among the six phyla, Chlorophyta was the most represented with 32 species (43.84% of total recorded species) and Miozoa was the least represented phylum with three species (4.11% of total recorded species; Table 3).3TABLEAlgal species recorded in fish farming water points and in stomachs and intestines of African sharptooth catfish and Nile tilapia.PhylumClassOrderFamilyGenusSpeciesBacillariophytaBacillariophyceaeLicmophoralesLicmophoraceaeLicmophoraLicmophora communis (Heiberg) Grunow 1881BacillariophytaBacillariophyceaeNaviculalesNaviculaceaeNaviculaNavicula sp.BacillariophytaBacillariophyceaeNaviculalesPinnulariaceaePinnulariaPinnularia interrupta ManguinBacillariophytaMediophyceaeStephanodiscalesStephanodiscaceaeStephanodiscusStephanodiscus sp. Ehrenberg 1845CharophytaZygnematophyceaeDesmidialesClosteriaceaeClosteriumClosterium acutum Grönblad 1935CharophytaZygnematophyceaeDesmidialesDesmidiaceaeCosmariumCosmarium angulosum Brébisson 1856CharophytaZygnematophyceaeDesmidialesDesmidiaceaeCosmariumCosmarium contractum O.Kirchner 1878CharophytaZygnematophyceaeDesmidialesDesmidiaceaeCosmariumCosmarium granatum Brébisson ex Ralfs 1848CharophytaZygnematophyceaeDesmidialesDesmidiaceaeCosmariumCosmarium sp.1CharophytaZygnematophyceaeDesmidialesDesmidiaceaeCosmariumCosmarium sp.2CharophytaZygnematophyceaeZygnematalesZygnemataceaeMougeotiaMougeotia sp.CharophytaZygnematophyceaeZygnematalesZygnemataceaeSpirogyraSpirogyra sp.CharophytaZygnematophyceaeZygnematalesZygnemataceaeZygnemaZygnema sp.ChlorophytaChlorophyceaeChaetophoralesUronemataceaeUronemaUronema sp.ChlorophytaChlorophyceaeChlamydomonadalesChlamydomonadaceaeChlamydomonasChlamydomonas sp.ChlorophytaChlorophyceaeChlamydomonadalesVolvocaceaeEudorinaEudorina elegans Ehrenberg 1832ChlorophytaChlorophyceaeChlamydomonadalesVolvocaceaePandorinaPandorina morum (O.F.Müller) Bory 1826ChlorophytaChlorophyceaeOedogonialesOedogoniaceaeOedogoniumOedogonium sp.ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeCoelastrumCoelastrum astroideum De Notaris 1867ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeCoelastrumCoelastrum microporum Nägeli 1855ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus acuminatus (Lagerheim) Chodat 1902ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeAcutodesmusAcutodesmus acutiformis (Schröder) Tsarenko & D.M.John 2011ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeTetradesmusTetradesmus obliquus (Turpin) M.J.Wynne 2016ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus bicaudatus (Hansgirg) Chodat 1926ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus carinatus (Lemmermann) Chodat 1913ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus ecornis Chodat 1926ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus ellipticus Corda 1835ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeDesmodesmusDesmodesmus intermedius (Chodat) E.Hegewald 2000ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeDesmodesmusDesmodesmus opoliensis (P.G.Richter) E.Hegewald 2000ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus perforates Massjuk 1962ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeDesmodesmusDesmodesmus communis (E.Hegewald) E.Hegewald 2000ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus sp.ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus subspicatus Chodat 1926ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus tibiscensis Uherkovich 1960ChlorophytaChlorophyceaeSphaeroplealesSchroederiaceaeSchroederiaSchroederia setigera (Schröder) Lemmermann 1898ChlorophytaChlorophyceaeSphaeroplealesSelenastraceaeMonoraphidiumMonoraphidium convolutum (Corda) Komárková‐Legnerová 1969ChlorophytaChlorophyceaeSphaeroplealesSelenastraceaeKirchneriellaKirchneriella lunaris (Kirchner) Möbius 1894ChlorophytaChlorophyceaeSphaeroplealesSelenastraceaeMessastrumMessastrum gracile (Reinsch) T.S.Garcia 2016ChlorophytaTrebouxiophyceaeChlorellalesChlorellaceaeChlorellaChlorella vulgaris Beijerinck 1890ChlorophytaTrebouxiophyceaeChlorellalesOocystaceaeOocystisOocystis borgei J.W.Snow 1903ChlorophytaTrebouxiophyceaeChlorellalesOocystaceaeOocystisOocystis sp.1ChlorophytaTrebouxiophyceaeChlorellalesOocystaceaeOocystisOocystis sp.2ChlorophytaTrebouxiophyceaeMicrothamnialesMicrothamniaceaeMicrothamnionMicrothamnion kuetzingianum Nägeli ex Kützing 1849ChlorophytaTrebouxiophyceaeTrebouxiophyceae ordo incertae sedisTrebouxiophyceae incertae sedisCrucigeniaCrucigenia quadrata Morren 1830ChlorophytaUlvophyceaeUlotrichalesUlotrichaceaeUlothrixUlothrix zonata (F.Weber & Mohr) Kützing 1833CyanobacteriaCyanophyceaeChroococcalesChroococcaceaeChroococcusChroococcus turgidus (Kützing) Forti 1907CyanobacteriaCyanophyceaeChroococcalesGomphosphaeriaceaeGomphosphaeriaGomphosphaeria sp.CyanobacteriaCyanophyceaeChroococcalesMicrocystaceaeMicrocystisMicrocystis aeruginosa (Kützing) Kützing 1846CyanobacteriaCyanophyceaeChroococcalesMicrocystaceaeMicrocystisMicrocystis flosaquae (Wittrock) Kirchner 1898CyanobacteriaCyanophyceaeNostocalesRivulariaceaeRivulariaRivularia sp.CyanobacteriaCyanophyceaeOscillatorialesMicrocoleaceaeArthrospiraArthrospira platensis Gomont 1892CyanobacteriaCyanophyceaeOscillatorialesOscillatoriaceaeOscillatoriaOscillatoria sp.CyanobacteriaCyanophyceaeOscillatorialesOscillatoriaceaePhormidiumPhormidium autumnale Gomont 1892CyanobacteriaCyanophyceaeSynechococcalesCoelosphaeriaceaeWoronichiniaWoronichinia delicatula (Skuja) Komárek & Hindák 1988CyanobacteriaCyanophyceaeSynechococcalesMerismopediaceaeAphanocapsaAphanocapsa conferta Komárková‐Legnerová & Cronberg 1994CyanobacteriaCyanophyceaeSynechococcalesMerismopediaceaeAphanocapsaAphanocapsa elachista West & G.S.West 1894CyanobacteriaCyanophyceaeSynechococcalesPseudanabaenaceaePseudanabaenaPseudanabaena galeata Böcher 1949CyanobacteriaCyanophyceaeSynechococcalesPseudanabaenaceaePseudanabaenaPseudanabaena limnetica (Lemmermann) Komárek 1974EuglenozoaEuglenophyceaeEuglenidaEuglenidaeEuglenaEuglena oblonga F.Schmitz 1884EuglenozoaEuglenophyceaeEuglenidaEuglenidaeEuglenaformisEuglenaformis proxima (P.A.Dangeard) M.S.Bennett & Triemer 2014EuglenozoaEuglenophyceaeEuglenidaEuglenidaeEuglenaEuglena sanguinea Ehrenberg 1832EuglenozoaEuglenophyceaeEuglenidaEuglenidaeEuglenaEuglena sp.EuglenozoaEuglenophyceaeEuglenidaEuglenidaeEuglenaEuglena viridis (O.F.Müller) Ehrenberg 1830EuglenozoaEuglenophyceaeEuglenidaPhacidaeLepocinclisLepocinclis fusca (G.A.Klebs) Kosmala & Zakryś 2005EuglenozoaEuglenophyceaeEuglenidaPhacidaeLepocinclisLepocinclis fusiformis (H.J.Carter) Lemmermann 1901EuglenozoaEuglenophyceaeEuglenidaPhacidaePhacusPhacus angulatus Pochmann 1942EuglenozoaEuglenophyceaeEuglenidaPhacidaePhacusPhacus orbicularis Hübner 1886EuglenozoaPeranemeaNatomonadidaAstasiidaeAstasiaAstasia inflata Dujardin 1841EuglenozoaPeranemeaNatomonadidaAstasiidaeAstasiaAstasia skadowskii Korshikov 1928EuglenozoaStavomonadeaPetalomonadidaScytomonadidaePetalomonasPetalomonas angusta (Klebs) Lemmermann 1910MiozoaDinophyceaePeridinialesPeridiniaceaeApocalathiumApocalathium aciculiferum (Lemmermann) Craveiro, Daugbjerg, Moestrup & Calado 2016MiozoaDinophyceaePeridinialesPeridiniaceaePeridiniumPeridinium sp.MiozoaDinophyceaePeridinialesProtoperidiniaceaeDiplopsalopsisDiplopsalopsis bomba J.D.Dodge & S.Toriumi 1993Considering the sampling points, the highest species richness was obtained in the decantation pond with 54 species belonging to 27 families. However, the lowest species richness (nine species belonging to eight families) was observed in the stomach and intestine of the African sharptooth catfish individuals from the rearing ponds. A comparison of algal species richness in rearing ponds of the two fish species shows that they have the same species richness (Figure 3). More algal species were recorded in water samples than those from fish stomachs and intestines (Table 4).3FIGURERecorded algae‐specific richness and the number of families according to sampling point Note: Bars represent the species‐specific richness, and the line represents the number of families.4TABLEAlgal species recorded in each sampling point from fish farming waters and stomach and intestine of African sharptooth catfish and Nile tilapia.Algae speciesSampling pointsRearing pondsEvacuation canalsDecantation pondWaterFish stomach and intestineWaterWaterFish stomach and intestineAfrican sharptooth catfishNile tilapiaAfrican sharptooth catfishNile tilapiaAfrican sharptooth catfishNile tilapiaAfrican sharptooth catfishNile tilapiaAcutodesmus acutiformisXXXAphanocapsa confertaXXXAphanocapsa elachistaXXApocalathium aciculiferumXArthrospira platensisXAstasia inflataXXXXAstasia skadowskiiXXChlamydomonas sp.XXChlorella vulgarisXXXXXChroococcus turgidusXXXXXClosterium acutumXXXCoelastrum astroideumXXXCoelastrum microporumXXXCosmarium angulosumXXXXXXCosmarium contractumXXXXXCosmarium granatumXXCosmarium sp.1XXXXXCosmarium sp.2XXXCrucigenia quadrataXDesmodesmus communisXXXXXXXXXDesmodesmus intermediusXXXXDesmodesmus opoliensisXXDiplopsalopsis bombaXEudorina elegansXXXXXXXEuglena oblongaXXXXEuglena sanguineaXEuglena sp.XXXXXEuglena viridisXXXEuglenaformis proximaXXXXGomphosphaeria sp.XXXKirchneriella lunarisXXXXLepocinclis fuscaXLepocinclis fusiformisXLicmophora communisXMessastrum gracileXXXMicrocystis aeruginosaXXXXXXXXMicrocystis flosaquaeXXXXMicrothamnion kuetzingianumXXMonoraphidium convolutumXMougeotia sp.XXXXXXXXXNavicula sp.XXXXXXXXXOedogonium sp.XXXXXXXXOocystis borgeiXXOocystis sp.1XXXXXOocystis sp.2XXXXOscillatoria sp.XXXXXPandorina morumXXXXXXXXPeridinium sp.XXPetalomonas angustaXPhacus angulatusXXXXXPhacus orbicularisXXXXXPhormidium autumnaleXXPinnularia interruptaXXXPseudanabaena galeataXXXPseudanabaena limneticaXXRivularia sp.XScenedesmus acuminatusXXXXScenedesmus bicaudatusXXXXScenedesmus carinatusXXXScenedesmus ecornisXXXXXXScenedesmus ellipticusXXXScenedesmus perforatusXScenedesmus sp.XXXScenedesmus subspicatusXScenedesmus tibiscensisXXSchroederia setigeraXSpirogyra sp.XXXXXXStephanodiscus sp.XXXXXXXXXTetradesmus obliquusXXXXXUlothrix zonataXXXXXXUronema sp.XXXXWoronichinia delicatulaXXZygnema sp.XXXXXXXXSpecies richness414113232824531729Considering the contents of fish stomachs and intestines, Nile tilapia is found to consume more algae than African sharptooth catfish in rearing ponds. Nevertheless, individuals of African sharptooth catfish consume more algae in decantation pond than those in rearing ponds.Thirteen species were recorded in at least six different samples from water sampling points and in the stomachs and intestines of both fish species (Figure 4). In all samples, three algae families were mostly represented with at least five different species per family. Scenedesmaceae was found to be the dominant family with 16 species. This family was followed by Desmidiaceae and Euglenidae with five species each. In rearing ponds of the African sharptooth catfish, the algal community was dominated by Scenedesmaceae, Desmidiaceae and Zygnemataceae with eight, three and three species, respectively. The other families were represented by only two or one species. In rearing ponds of Nile tilapia, the algal community was mostly represented by Scenedesmaceae with 13 species. In the evacuation canal from the rearing ponds of African sharptooth catfish, the algal community was dominated by Scenedesmaceae and Zygnemataceae with four and three species, respectively. The other families in this canal were represented by only two or one species. In the evacuation canal from the rearing ponds of Nile tilapia, the algal community was dominated by Desmidiaceae, Scenedesmaceae and Zygnemataceae by five, three and three species, respectively. The other families were also represented by only two or one species in this canal. In the decantation pond, Scenedesmaceae, Desmidiaceae, Euglenidae, Phacidae, Oocystaceae and Zygnemataceae families were mostly represented by respectively 11, five, four, four, three and three species. In African sharptooth catfish stomachs and intestines, families such as Desmidiaceae with four species, Oocystaceae and Scenedesmaceae with two species each were recorded. The other families were represented by only one species. In Nile tilapia stomachs and intestines, dominant families were Scenedesmaceae with eight species, Desmidiaceae and Euglenidea with three species each. Zygnemataceae, Oocystaceae, Astasiideae and Microcystaceae were represented by two species each. The other recorded families were represented by only one species.4FIGUREPhotographs of the most represented species in all sampling points. Note: 1: Navicula sp., 2: Stephanodiscus sp., 3: Oedogonium sp., 4. Scenedesmus ecornis, 5: Desmodesmus communis, 6: Eudorina elegans, 7: Pandorina morum, 8: Ulothrix zonata, 9: Cosmarium angulatum, 10: Spirogyra sp., 11: Mougeotia sp., 12: Zygnema sp., 13: Microsystis aeruginosaRegarding Shannon–Wiener (H') diversity index, stomach and intestine contents of African sharptooth catfish fish in rearing ponds were less diversified than other sampling points (H' = 1.8798). Decantation pond water remains more diversified than other sampling points with a Shannon–Wiener of H' = 3.8412. The Pielou Equitability index (E) for all sampling points ranged from 0.8555 to 0.9629 (Table 5).5TABLEEcological diversity indexes according to sampling point.SourceSampling pointFish speciesShannon–Wiener (H')Pielou (E)WaterDecantation PondNA3.84120.9629Evacuation CanalA. Catfish3.20770.9626N. tilapia2.98410.9390Rearing PondA. Catfish3.32750.8960N. tilapia3.45080.9293Fish stomachs and intestinesDecantation PondA. Catfish2.52010.8895N. tilapia3.06750.9110Rearing PondA. Catfish1.87980.8555N. tilapia3.04850.9250Abbreviations: A. catfish, African sharptooth catfish; NA, not applicable; N. tilapia, Nile tilapia.When comparing the similarity of algal species composition between sampling points by the Jaccard similarity index (Table 6), more similarity was found between the decantation pond and rearing ponds of Nile tilapia (Cj= 0.5574). No similarity was found between the other samples.6TABLESimilarity in algal species composition between the different sampling points through the Jaccard similarity index.DP WACF RP WACF RP FACF EC WACF DP FNT RP WNT RP FNT EC WDP WACF RP W0.4615ACF RP F0.16670.2195ACF EC W0.30160.50000.2759ACF DP F0.29630.26090.73330.2162NT RP W0.53230.4211NT RP F0.44640.28570.4167NT EC W0.36840.33330.38300.5455NT DP F0.43100.45830.43750.42860.47370.3590Abbreviations: ACF, African sharptooth catfish; DP, decantation pond; EC, evacuation canal; F, fish content; NT, Nile tilapia; RP, rearing pond. W, water.Physical and chemical water quality parameters of sampling pointsMeasurement of turbidity showed that water in sampling points was clear. The Secchi disk used for this measurement was visible until the bottom of each site. Temperature and pH were slightly varying from 24.00 to 26.17°C and from 6.8 to 7.4, respectively. However, strong variations were observed for electrical conductivity ranging from 35.10 to 173.90 μs/cm and DO going from 2.25 to 7.68 mg/L. The lowest conductivity was observed in tilapia ponds and their evacuation canals. The decantation pond had the highest DO, followed by both African sharptooth catfish and Nile tilapia rearing ponds. Evacuation canals had the lowest DO. Nitrite concentrations in all sampling points were low (< 0.3 mg/L; Table 7).7TABLEPhysical and chemical parameters of water in sampling points.Sampling pointFish speciesTemp (°C)pHElect. cond. (μm/s)DO. (mg/l)NO2 (mg/l)Decantation PondNA24.00 ± 0.827.40 ± 0.22173.87 ± 113.397.68 ± 0.910.12 ± 0.05Evacuation CanalAfrican sharptooth catfish25.33 ± 0.476.77 ± 0.4585.73 ± 16.792.25 ± 0.370.18 ± 0.09Nile tilapia25.33 ± 0.476.93 ± 0.3435.77 ± 1.636.63 ± 0.750.09 ± 0.07Rearing PondAfrican sharptooth catfish25.5 ± 0.767.05 ± 0.3371.18 ± 26.994.22 ± 1.060.25 ± 0.21Nile tilapia26.17 ± 0.697.18 ± 0.2735.10 ± 5.824.14 ± 0.560.06 ± 0.05Note: Value represents mean ± standard deviation.Abbreviations: D.O., dissolved oxygen; Elect. cond., electrical conductivity; NO2, nitrite concentration; NA, not applicable; Temp, temperature.DISCUSSIONThe aim of this study was to characterise algal species in rearing ponds of African sharptooth catfish and Nile tilapia and their contribution to the diet of these species.Diversity and distribution of algae in the fish farming stationIn the fish farming station, six different phyla of algae were recorded in water from African sharptooth catfish and Nile tilapia ponds, evacuation canals, decantation pond and African sharptooth catfish and Nile tilapia stomach and intestine contents. They were dominated by Chlorophyta with 43.84% of the recorded species, followed by Cyanobacteria and Euglenozoa (17.81% and 16.44%, respectively), Charophyta, Bacillariophyta and Miozoa. This finding agrees with the previous studies conducted in Burkina Faso natural water systems and reservoirs (Zongo & Zongo, 2016; Zongo et al., 2008). Indeed, Chlorophyta is found to be the dominant phylum in waters from the Sudanian and Sahelian zones of Africa (Ouattara et al., 2000; Zongo & Zongo, 2016; Zongo et al., 2008, 2007). Our results show that recorded algal in fish farming ponds are species occurring only in tropical conditions (Africa, America and Asia) as revealed by Zongo and Zongo (2016). Indeed, the study area is characterised by the Sudanian climate, with a dry season from October to May and a rainy season from June to October. The study was conducted in such tropical conditions during the coldest period of the dry season in December and January explaining algal composition in fish ponds. This period of dry season always registers the highest algal abundance in tropical water bodies. Indeed, a study conducted in a tropical soda lake in Ethiopia showed seasonal variations in phytoplankton abundance and biomass with a peak abundance observed during dry periods (Wagaw et al., 2021). Therefore, these findings suggest that in fish farming ponds and decantation pond, algal biomass during the rainy season will be less, compared to the dry season where the study was conducted. This situation in the shifts in algal presence in water habitats can be explained by evapotranspiration highly occurring during the dry season in tropical zones and dilution of water and its nutrient concentrations during the rainy season. Indeed, rainfall conduces to a reduction of nutrient concentrations in a water body (Arreghini et al., 2005; St. Pierre et al., 2021). For photosynthetic activities, these nutrients are required, and rainfall can contribute to reduce algal species richness and abundance during the rainy season through nutrients dilution.The predominance of Chlorophyta is favoured by the low water depths (0.3–0.5 m), the slight turbidity of the water and the ability of light to penetrate it. In this study, results showed that Scenedesmaceae family represented 50% of species in Chlorophyta phylum. This finding is with Iltis (1980) who stated that Scenedesmaceae is one of the commonly encountered families in Sudanian waters.Species of Cyanophyta and Euglenophyta phyla were relatively present in fish ponds, evacuation canals and decantation pond. They contain species endowing with an ecological plasticity and colonising extremely varied environments like stagnant waters and those rich in organic matter (Leitao & Coute, 2005; Sheath & Wehr, 2003). In Nile tilapia and African sharptooth catfish rearing ponds, the leftover distributed feed as well as fish faeces sediment at the bottom of the ponds. The remaining wastes constitute an important source of degradable organic matter, favourable to the development of algal species such as Microcystis spp., Oscillatoria spp., Phormidium spp., Euglena spp., Phacus spp. and Lepocinclis spp. encountered in the present study. However, because of the daily renewal of the water in the ponds, these organic matters are continuously eliminated through the evacuation canals and transported to the decantation pond. The high content of organic matter and nutrient in the decantation pond is proved by the high electrical conductivity in this pond. indeed, this electrical conductivity is determined by the amount of dissolved matter (Meride & Ayenew, 2016). Removal of wastes allows for controlling excessive nutrient pollution in rearing ponds. Thus, in the decantation pond, organic matter is continually being sedimented and contributes strongly to the primary productivity in the habitat. Algae and zooplankton from the primary production are continuously consumed by Nile tilapia and African sharptooth catfish as well as amphibians living in harmony in this habitat. Indeed, in fish rearing ponds and decantation pond, sedimented organic matters are converted to nutrients by microorganisms and used by algae species for growing and multiplication through photosynthesis and reproduction as reported by Yaakob et al. (2021). However, in evacuation canals, organic matters are only transported, and few nutrients can be used by algae species. That could explain the low species diversity of algae in general, and particularly of Euglenoids and Cyanobacteria in evacuation canals, compared to fish rearing ponds and decantation pond. Because of the intolerance of Nile tilapia to low oxygen concentrations, water renewal is higher in Nile tilapia ponds, compared to African sharptooth catfish ponds. Consequently, water from Nile tilapia ponds contain less Euglenoids than African sharptooth catfish ponds.The high Shannon–Wiener (H between 1.88 and 3.84) and Pielou (E between 0.86 and 0.96) index values at all samples indicate that these sites were diverse in algal species. Pielou index that trends towards 1 shows that the abundance of algae species is relatively similar across sampling points, meaning that these populations are in balance (Triplet, 2020). However, the Jaccard similarity index shows small values between 0.17 and 0.55, meaning that the compositions of algae in sampling points are not similar.Impact of fish breeding on water variables and algae compositionAfrican sharptooth catfish and Nile tilapia are fed every day with formulated feed responding to their nutritional needs for growth and reproduction. The catabolism of feeds in fish as well as uneaten feeds release huge amounts of nutrients into the water, thus involving a proliferation of phytoplankton communities in waters (Tucker & Lloyd, 1984). Inversely in the meantime, phytoplanktonic species absorb nutrients through their development and contribute to the natural purification of water, thereby helping to avoid eutrophication. Thus, in aquaculture, phytoplankton plays a significant role in stabilising the whole pond ecosystem and in minimising the fluctuations in water quality (Priyadarshani et al., 2012). However, long‐term high nutrient loading coming from feeds and fish faeces contributes to eutrophication and algal species proliferation. Once the algae blooms become too large, they can significantly reduce sunlight penetration and then favour the growth and development of blue‐green algae (Brunson et al., 1994). If fish farming contributes to the production of algal biomass (Tucker & Lloyd 1984), algal blooms in ponds affect the production of fish by (a) shallow thermal and chemical stratification of the water; (b) wide diel fluctuations in pH, dissolved carbon dioxide and oxygen concentrations; (c) water quality deterioration after sudden massive phytoplankton death and (d) production of compounds that give undesirable tastes and odours to fish. Blooms, as regularly observed in decantation pond that receive nutrient‐enriched water, can lead to a lack of aeration and high consumption of oxygen, low penetration of light and finally death of fish.In fish farming ponds, fish feeds and algae biomass that are both consumed by fish contribute to the food web and the regulation of algal biomass. The consumption of algae by the African sharptooth catfish and Nile tilapia species is necessary for controlling and attenuating algal blooms in farming ponds.Contribution of algae to Nile tilapia and African sharptooth catfish dietsAmong the recorded six phyla in sampling points, Miozoa was the only one that was not detected in fish stomachs and intestines. The dominance of the other five phyla in fish stomachs and intestines follows the successive dominance obtained in waters of sampling points. Chlorophyta is the predominant phylum followed by Cyanobacteria, Euglenozoa, Charophyta and Bacillariophyta. These results suggest that grazing from fish is mainly oriented towards the most present algal community and species. However, species richness (S) and Shannon index (H) attained high values in fish ponds (S = 73, H = 2.98–3.84) than in fish stomachs and intestines (S = 41, H = 1.88–3.07). This situation may be related to a preference for fish for some algae species (Neya et al., 2018).Algae are the first link in the aquatic food web and are thus an important source of feed for fish populations (Chader & Touzi, 2001; Iltis, 1973). The nutritional characteristic of an algae species for an organism depends on its cell size, digestibility, production of toxic compounds and biochemical composition (Priyadarshani et al., 2012). Depending on their diet, fish species use algae differently. Differences in algal‐specific richness were observed between African sharptooth catfish ponds, Nile tilapia ponds and decantation pond. In fish stomachs and intestines, the specific composition of algae was also species‐dependent. Indeed, stomach and intestine of some individuals of African sharptooth catfish were empty and free of algae, while they were found in all stomachs and intestines of Nile tilapia individuals. The stomach contents of Nile tilapia individuals were more diverse in algal species than African sharptooth catfish in rearing ponds as well as in decantation pond. These differences are related to the diet of these two species. Indeed, the diet of Nile tilapias is generally omnivorous with a microphagous or herbivorous tendency, whereas African sharptooth catfish is omnivorous with a carnivorous tendency (Lévêque & Paugy, 2006). In natural conditions, Temesgen et al. (2022) found that phytoplankton was the most consumed feed by Nile tilapia. In addition, through the filtration process, Nile tilapia can considerably reduce Cynaobacteria population by 53% and green algae by 28% (Turker et al., 2003). According to these authors, the number of filtered particles increases with particle size from the smaller Tetraedron and Chlorella to the larger four‐cell stacks of Scenedesmus with the green algal water sources and from the smaller Merismopedia to larger Microcystis with cyanobacterial water sources.Species of some families such as Scenedesmaceae, Microcystaceae, Naviculaceae, Zygnemataceae, Euglenidae and Ulothricaceae were mostly founded in the stomachs and intestines of both fish species. The algal species of these families are the most encountered in the sampling points and thus the most accessible by fishes.Algae by their nutritional composition contribute to the good growth of fish species. As algae are found to be low‐cost and commercially available (Henry, 2012), species that mostly contribute to fish diet can be selected and cultivated onsite for fish feeding during fish farming. However, the presence of Cyanobacteria species can be a source of a nuisance as some of them (e.g., Microcystis) produce toxins (Turker et al., 2003).CONCLUSIONFrom the present study, 73 algal species belonging to 47 genera, 34 families, 21 orders, 11 classes and six phyla were recorded in African sharptooth catfish and Nile tilapia rearing ponds, evacuation canals, decantation pond and in stomachs and intestines of both species. In sampling points, the development and proliferation of algae species were favoured by the leftover feeds and fish faeces, which sediment at the bottom of ponds and constitute an important degradable organic matter.In the sampling points, Scenedesmaceae, Desmidiaceae, Euglenidae and Zygnemataceae were found to be the most represented families. Algal species differentially contribute to the feeding and diet of African sharptooth catfish and Nile tilapia, as fish species have different types of diets. Indeed, Nile tilapia, which is microphagous, consumes more algae than African sharptooth catfish, which is omnivorous with a carnivorous tendency. Nevertheless, as some algal species in fish ponds are consumed by both fish species, they can be selected and cultivated for biomass production for fish feeding. Thus, algae species from Scenedesmaceae family can be used for biomass production in subsistence and small‐scale fish farming to improve fish productivity.AUTHOR CONTRIBUTIONSSaïdou Santi contributed to acquisition, analysis and interpretation of data. He made substantial contribution in the conception, design and drafting the manuscript. Benjamin Poda made substantial contributions to collection of data. Bilassé Zongo contributed to identification of algae species. He made substantial contribution on acquisition, analysis and interpretation of data. He was also involved in the conception, design and drafting the manuscript. Aboubacar Toguyeni critically revised the manuscript content and gave the final approval for the submission of the version to be published. All authors have read and approved the final version of the manuscript.ACKNOWLEDGEMENTSWe are grateful to Aquaculture and Aquatic Biodiversity Research Unit of Université Nazi BONI (UNB) that provided all chemicals and equipment to conduct this study.CONFLICT OF INTERESTThe authors declare that there are no conflicts of interest related to the use of the data in this study.ETHICS STATEMENTThe experiments were carried out according to the European animal welfare recommendations.DATA AVAILABILITY STATEMENTData are not available online.PEER REVIEWThe peer review history for this article is available at: https://publons.com/publon/10.1002/aff2.99REFERENCESAl‐Harbi, A.H. & Uddin, N. (2003) Quantitative and qualitative studies on bacterial flora of hybrid tilapia (Oreochromis niloticus × O. aureus) cultured in earthen ponds in Saudi Arabia. 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Comparative study of algal community in African sharptooth catfish (Clarias gariepinus, Burchell 1822) and Nile tilapia (Oreochromis niloticus, Linnaeus 1758) farming ponds

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Abstract

INTRODUCTIONAlgal growth in a water body is a complex activity characterised by the influences of environmental and biological factors, both internal and external to the water body (Wang et al., 2016). Their spatial and temporal distributions, as well as their multiplication and proliferation, are thereby controlled by light, transparency, nutrient availability and grazing by animal communities (Cunha & Calijuri, 2011; Loiselle et al., 2007; Zongo & Boussim, 2015). Thus, through grazing, fish are strongly involved in the growth and proliferation of algae species in the aquatic food web (Janjua & Gerdeaux, 2011; Kitchell et al., 2000; Rand & Stewart, 1998).In the aquatic food web, phytoplankton and algae are the first link in the web. They are eaten by consumers like zooplankton, crustaceans and fish that are exploited in fishing activities (Durand & Levêque, 1980; Mollo & Noury, 2013). Algae constitute then a valuable nutritional source as they are excellent sources of vitamins, proteins, carbohydrates, trace minerals and other bioactive compounds (Kumar et al., 2008; Norambuena et al., 2015). As a major part of the algae community in aquatic habitats, phytoplankton provides many valuable phytonutrients and biologically active ingredients, which are directly available for fish (Napiórkowska‐Krzebietke, 2017). Thus, phytoplankton and algae are the main sources of food for larvae and adults of herbivorous filter‐feeder fish species like Nile tilapia (Oreochromis niloticus) and omnivorous fish species like African sharptooth catfish (Clarias gariepinus; de Graaf & Janssen, 1996; Kestemont et al., 1989). In wild conditions, herbivores consume primarily plant and decayed organic matter, while omnivores consume almost any food source, whether plant or animal (Prabu et al., 2017). Indeed, in natural conditions, the Nile tilapia diet is mainly composed of Chlorophyceae, Cyanophyceae and Euglenophyceae (Kestemont et al., 1989). However, in addition to phytoplankton and macrophytes, the diet of African sharptooth catfish is composed of zooplankton, insects, fish and detritus (Tesfahun, 2018).In fish farming systems, feeds and faeces are known to be the main wastes during fish production. Consequently, important amount of nutrients composed of phosphorus and nitrogen, which are essential for phytoplankton and algae production, is loaded into the environment from these wastes. The released compounds are the major constituents of the main ingredients used in formulated feeds to achieve good growth of fish (Kibria et al., 1997; Lazzari et al., 2008). Fish production in ponds requires large amounts of formulated feeds that inevitably introduce nutrients into water bodies, promoting algal growth and even conducts to algal blooms if eutrophication occurs (Morin, 2007; Wang et al., 2016; Wurtsbaugh et al., 2019).While fee and faeces contribute to increasing the amount of nutrients in the water and thus algal growth, herbivorous and omnivorous fish species contribute to the regulation of algal biomass in their habitats by grazing. This is certainly the case for several fish farming stations, such as the one at the research station of Nazi BONI University in Burkina Faso. Indeed, during African sharptooth catfish and Nile tilapia farming in separated open ponds in this station, fish fee and faeces led to an important growth of microalgae in the farming ponds as well as in the discharged water. Different works have been carried out on the diversity of microalgae in natural environments (Zongo et al., 2019, 2011, 2008). However, algal species that occur in fish farming ponds are not well studied in Burkina Faso. The knowledge of the algal communities that colonise the fish rearing ponds could contribute to characterising, on one hand, the algae consumed by the fish and, on the other hand, the quality of the rearing water. Furthermore, data on algae species and biomass in fish farming ponds and diet could be used in the management of fish farming ponds to thereby enhance fish production.Therefore, the present study characterises algal species in rearing ponds of African sharptooth catfish and Nile tilapia and their contribution to the diet of these species. Specifically, the study identified algae species in rearing ponds, evacuation canals, decantation ponds, stomachs and intestines of the two species and determined some water physical and chemical parameters.MATERIALS AND METHODSExperimental site and facilitiesThis study was carried out in an aquaculture research station located at the Nazi BONI University (Burkina Faso). This experimental station is working on two fish species: Nile tilapia and African sharptooth catfish. These species are reared in separate ponds, and water is collected in a decantation pond through an evacuation canal coming from each rearing pond (Figure 1).1FIGUREExperimental facilities in the fish farming station.Fish rearing conditionsNile tilapia was reared in two ponds of 4 m3 for 20 individuals per m3 density, whereas African sharptooth catfish was reared in two other ponds of 16 m3 for 50 individuals per m3 density. Every day, water was differently renewed in rearing ponds from 5:00 PM to 9:00 AM. During this period of the day, the renewal rates were once per 2 h in Nile tilapia rearing ponds and once per 4 h in African sharptooth catfish rearing ponds. Therefore, water was renewed 8.50 times for Nile tilapia ponds and 4.25 times for African sharptooth catfish ponds. The permanent water inlet and outlet system ensures a complete renewal of the water in the pond without having to empty it. At 9:00 AM, each rearing pond was purged to remove waste and leftover feed that accumulated at the bottom.In rearing ponds, fish individuals were fed three times per day (8:00 AM, 12:00 AM and 4:00 PM) with formulated fee containing 40% protein and 12% lipid for African sharptooth catfish and 35% protein and 10% lipid for Nile tilapia. Fishes in evacuation canals and decantation pond were not fed with any formulated food. They were fed on the feeds naturally produced in their environment including algae.Sampling and identification of algae species from farming watersWater samples were collected in seven points using 120 ml bottle (Zongo & Boussim, 2015; Zongo et al., 2011). The six sampling points were composed of two rearing ponds and one evacuation canal for each fish species. Another sample was collected in the decantation pond (the seventh point) where water from evacuation canals is accumulated. Samples were collected every 15 days for 45 days. On each sampling day, samples were collected in the morning at 9:00 AM according to Leboulanger et al. (2002) and Müller et al. (2001).After sampling, algae species were identified using "sedimentation after fixation" with formalin at 5% as the method described by Laplace‐Treyture et al. (2009). This method allows to take into account nannoplankton species during the identification process, which have great biological importance because of their very high number in water habitats (Bourrelly, 1990). From the 120 ml water sample, a sub‐sample of 50 ml was taken and kept for 24 h for the sedimentation of organisms to be observed under a light microscope (Zongo et al., 2011). After sedimentation, the supernatant was removed with a micropipette, and the precipitate was used for the identification of the algal species under a light microscope (MOTIC, BA 200). Photographs of species were taken for algal species identification, using standard works such as Couté and Rousselin (1975), Compère (1976), Iltis (1980), Bourrelly (1990), Bourrelly and Couté (1986), Kadiri (1993), Ling and Tyler (2000), Nevo and Wasser (2000), Ouattara et al. (2000), Jonh et al. (2002), Wehr et al. (2015) and Zongo et al. (2011). Identified species were verified using AlgaeBase (Guiry & Guiry, 2020).Sampling and identification of algae species from fish stomachs and intestinesTo characterise algae species consumed by African sharptooth catfish and Nile tilapia in the farming pond, fish individuals were sampled from the four rearing ponds and the decantation pond. This sampling was carried out on the same day of water samples collection. Afterwards, they were euthanised with an overdose (200 mg/L) of benzocaïne (Sigma–Aldrich), to find out algae species in the stomachs and intestines (Figure 2). Thus, 18 individuals of African sharptooth catfish and 18 individuals of Nile tilapia were sampled, and their zootechnic parameters were measured (Table 1).2FIGUREIsolation of African sharptooth catfish and Nile tilapia stomachs and intestines for algal collection.1TABLENumber and zootechnic parameters of sampled fish for contents analysis.Sampling pointsOreochromis niloticus (Nile tilapia)Clarias gariepinus (African sharptooth catfish)NMBW (g)MTL (cm)MSL (cm)NMBW (g)MTL (cm)MSL (cm)Rearing pond12115.4 ± 19.818.8 ± 0.815.2 ± 1.112450.3 ± 52.341.4 ± 1.735.5 ± 3.0Decantation pond6119.9 ± 12.419.2 ± 0.315.4 ± 0.26460.2 ± 102.443.9 ± 0.738.0 ± 2.0Note: Value represent mean ± standard deviation.Abbreviations: MBW, mean body weight; MSL, mean standard length; MTL: mean total length; N, individual number.Before opening fish bodies to take out the stomach and intestine, they were washed with technical ethanol (70%) to reduce algae that occur on the bodies. Thereafter, the stomach and intestine were taken out for their contents analysis. Then, the content of each stomach and intestine was diluted 10 times and fixed with 5% formalin solution and kept in a conservation bottle (Al‐Harbi and Uddin 2003). Finally, algae species were identified using the same method as described previously.Measurement of physicochemical water quality parametersDuring the experimental period, parameters such as water transparency, pH, temperature, electric conductivity, dissolved oxygen (DO) and nitrite concentration were measured in the sampling points two times per day. Water transparency was measured with a Secchi disk, while pH, temperature, electric conductivity and DO were measured by using an electronic multiparameter (HACH SAS). A spectrophotometer (HACH SAS) was used to determine nitrite (N‐NO2−) concentration in water.Data processingCollected data on algae, were first used to determine algae diversity through ecological indexes. Thus, Shannon–Wiener and Piélou equitability indexes were used to determine algal diversity in sampling points and fish stomach and intestine (Table 2). Jaccard similarity index was used to determine similarities in algal communities between sampling points.2TABLECalculated ecological indexes for identified algae species.IndexFormulaParametersShannon–Weaver diversityH′=−∑i=1npiln(pi)$H^{\prime}=-\sum\limits_{i=1}^{n}pi\,\ln(pi)$n: total number of speciespi: frequency of the taxon "i" in the given communitypi=niN$pi=\frac{ni}{N}$ni: total number of individuals of each speciesN: total number of individuals0 < pi < 1Piélou equitability (E)E=H′Hmax′=H′lnS${\rm{E}}=\frac{H^{\prime}}{H^{\prime}_{max}}=\frac{H^{\prime}}{lnS}$H: Shannon–Weaver diversityS: total number of speciesJaccardCj=j(a+b−j)$C_{j}=\frac{j}{(a+b-j)}$j: number of common species at two sampling pointsa: total number of species in sampling point Ab: total number of species in sampling point BRESULTSAlgae species composition and diversityIn the whole samples obtained from fish ponds, evacuation canals, decantation pond and from fish stomachs and intestines, 73 algal species were recorded. These species belong to 47 genera, 34 families, 21 orders, 11 classes and six phyla. Among the six phyla, Chlorophyta was the most represented with 32 species (43.84% of total recorded species) and Miozoa was the least represented phylum with three species (4.11% of total recorded species; Table 3).3TABLEAlgal species recorded in fish farming water points and in stomachs and intestines of African sharptooth catfish and Nile tilapia.PhylumClassOrderFamilyGenusSpeciesBacillariophytaBacillariophyceaeLicmophoralesLicmophoraceaeLicmophoraLicmophora communis (Heiberg) Grunow 1881BacillariophytaBacillariophyceaeNaviculalesNaviculaceaeNaviculaNavicula sp.BacillariophytaBacillariophyceaeNaviculalesPinnulariaceaePinnulariaPinnularia interrupta ManguinBacillariophytaMediophyceaeStephanodiscalesStephanodiscaceaeStephanodiscusStephanodiscus sp. Ehrenberg 1845CharophytaZygnematophyceaeDesmidialesClosteriaceaeClosteriumClosterium acutum Grönblad 1935CharophytaZygnematophyceaeDesmidialesDesmidiaceaeCosmariumCosmarium angulosum Brébisson 1856CharophytaZygnematophyceaeDesmidialesDesmidiaceaeCosmariumCosmarium contractum O.Kirchner 1878CharophytaZygnematophyceaeDesmidialesDesmidiaceaeCosmariumCosmarium granatum Brébisson ex Ralfs 1848CharophytaZygnematophyceaeDesmidialesDesmidiaceaeCosmariumCosmarium sp.1CharophytaZygnematophyceaeDesmidialesDesmidiaceaeCosmariumCosmarium sp.2CharophytaZygnematophyceaeZygnematalesZygnemataceaeMougeotiaMougeotia sp.CharophytaZygnematophyceaeZygnematalesZygnemataceaeSpirogyraSpirogyra sp.CharophytaZygnematophyceaeZygnematalesZygnemataceaeZygnemaZygnema sp.ChlorophytaChlorophyceaeChaetophoralesUronemataceaeUronemaUronema sp.ChlorophytaChlorophyceaeChlamydomonadalesChlamydomonadaceaeChlamydomonasChlamydomonas sp.ChlorophytaChlorophyceaeChlamydomonadalesVolvocaceaeEudorinaEudorina elegans Ehrenberg 1832ChlorophytaChlorophyceaeChlamydomonadalesVolvocaceaePandorinaPandorina morum (O.F.Müller) Bory 1826ChlorophytaChlorophyceaeOedogonialesOedogoniaceaeOedogoniumOedogonium sp.ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeCoelastrumCoelastrum astroideum De Notaris 1867ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeCoelastrumCoelastrum microporum Nägeli 1855ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus acuminatus (Lagerheim) Chodat 1902ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeAcutodesmusAcutodesmus acutiformis (Schröder) Tsarenko & D.M.John 2011ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeTetradesmusTetradesmus obliquus (Turpin) M.J.Wynne 2016ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus bicaudatus (Hansgirg) Chodat 1926ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus carinatus (Lemmermann) Chodat 1913ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus ecornis Chodat 1926ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus ellipticus Corda 1835ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeDesmodesmusDesmodesmus intermedius (Chodat) E.Hegewald 2000ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeDesmodesmusDesmodesmus opoliensis (P.G.Richter) E.Hegewald 2000ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus perforates Massjuk 1962ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeDesmodesmusDesmodesmus communis (E.Hegewald) E.Hegewald 2000ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus sp.ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus subspicatus Chodat 1926ChlorophytaChlorophyceaeSphaeroplealesScenedesmaceaeScenedesmusScenedesmus tibiscensis Uherkovich 1960ChlorophytaChlorophyceaeSphaeroplealesSchroederiaceaeSchroederiaSchroederia setigera (Schröder) Lemmermann 1898ChlorophytaChlorophyceaeSphaeroplealesSelenastraceaeMonoraphidiumMonoraphidium convolutum (Corda) Komárková‐Legnerová 1969ChlorophytaChlorophyceaeSphaeroplealesSelenastraceaeKirchneriellaKirchneriella lunaris (Kirchner) Möbius 1894ChlorophytaChlorophyceaeSphaeroplealesSelenastraceaeMessastrumMessastrum gracile (Reinsch) T.S.Garcia 2016ChlorophytaTrebouxiophyceaeChlorellalesChlorellaceaeChlorellaChlorella vulgaris Beijerinck 1890ChlorophytaTrebouxiophyceaeChlorellalesOocystaceaeOocystisOocystis borgei J.W.Snow 1903ChlorophytaTrebouxiophyceaeChlorellalesOocystaceaeOocystisOocystis sp.1ChlorophytaTrebouxiophyceaeChlorellalesOocystaceaeOocystisOocystis sp.2ChlorophytaTrebouxiophyceaeMicrothamnialesMicrothamniaceaeMicrothamnionMicrothamnion kuetzingianum Nägeli ex Kützing 1849ChlorophytaTrebouxiophyceaeTrebouxiophyceae ordo incertae sedisTrebouxiophyceae incertae sedisCrucigeniaCrucigenia quadrata Morren 1830ChlorophytaUlvophyceaeUlotrichalesUlotrichaceaeUlothrixUlothrix zonata (F.Weber & Mohr) Kützing 1833CyanobacteriaCyanophyceaeChroococcalesChroococcaceaeChroococcusChroococcus turgidus (Kützing) Forti 1907CyanobacteriaCyanophyceaeChroococcalesGomphosphaeriaceaeGomphosphaeriaGomphosphaeria sp.CyanobacteriaCyanophyceaeChroococcalesMicrocystaceaeMicrocystisMicrocystis aeruginosa (Kützing) Kützing 1846CyanobacteriaCyanophyceaeChroococcalesMicrocystaceaeMicrocystisMicrocystis flosaquae (Wittrock) Kirchner 1898CyanobacteriaCyanophyceaeNostocalesRivulariaceaeRivulariaRivularia sp.CyanobacteriaCyanophyceaeOscillatorialesMicrocoleaceaeArthrospiraArthrospira platensis Gomont 1892CyanobacteriaCyanophyceaeOscillatorialesOscillatoriaceaeOscillatoriaOscillatoria sp.CyanobacteriaCyanophyceaeOscillatorialesOscillatoriaceaePhormidiumPhormidium autumnale Gomont 1892CyanobacteriaCyanophyceaeSynechococcalesCoelosphaeriaceaeWoronichiniaWoronichinia delicatula (Skuja) Komárek & Hindák 1988CyanobacteriaCyanophyceaeSynechococcalesMerismopediaceaeAphanocapsaAphanocapsa conferta Komárková‐Legnerová & Cronberg 1994CyanobacteriaCyanophyceaeSynechococcalesMerismopediaceaeAphanocapsaAphanocapsa elachista West & G.S.West 1894CyanobacteriaCyanophyceaeSynechococcalesPseudanabaenaceaePseudanabaenaPseudanabaena galeata Böcher 1949CyanobacteriaCyanophyceaeSynechococcalesPseudanabaenaceaePseudanabaenaPseudanabaena limnetica (Lemmermann) Komárek 1974EuglenozoaEuglenophyceaeEuglenidaEuglenidaeEuglenaEuglena oblonga F.Schmitz 1884EuglenozoaEuglenophyceaeEuglenidaEuglenidaeEuglenaformisEuglenaformis proxima (P.A.Dangeard) M.S.Bennett & Triemer 2014EuglenozoaEuglenophyceaeEuglenidaEuglenidaeEuglenaEuglena sanguinea Ehrenberg 1832EuglenozoaEuglenophyceaeEuglenidaEuglenidaeEuglenaEuglena sp.EuglenozoaEuglenophyceaeEuglenidaEuglenidaeEuglenaEuglena viridis (O.F.Müller) Ehrenberg 1830EuglenozoaEuglenophyceaeEuglenidaPhacidaeLepocinclisLepocinclis fusca (G.A.Klebs) Kosmala & Zakryś 2005EuglenozoaEuglenophyceaeEuglenidaPhacidaeLepocinclisLepocinclis fusiformis (H.J.Carter) Lemmermann 1901EuglenozoaEuglenophyceaeEuglenidaPhacidaePhacusPhacus angulatus Pochmann 1942EuglenozoaEuglenophyceaeEuglenidaPhacidaePhacusPhacus orbicularis Hübner 1886EuglenozoaPeranemeaNatomonadidaAstasiidaeAstasiaAstasia inflata Dujardin 1841EuglenozoaPeranemeaNatomonadidaAstasiidaeAstasiaAstasia skadowskii Korshikov 1928EuglenozoaStavomonadeaPetalomonadidaScytomonadidaePetalomonasPetalomonas angusta (Klebs) Lemmermann 1910MiozoaDinophyceaePeridinialesPeridiniaceaeApocalathiumApocalathium aciculiferum (Lemmermann) Craveiro, Daugbjerg, Moestrup & Calado 2016MiozoaDinophyceaePeridinialesPeridiniaceaePeridiniumPeridinium sp.MiozoaDinophyceaePeridinialesProtoperidiniaceaeDiplopsalopsisDiplopsalopsis bomba J.D.Dodge & S.Toriumi 1993Considering the sampling points, the highest species richness was obtained in the decantation pond with 54 species belonging to 27 families. However, the lowest species richness (nine species belonging to eight families) was observed in the stomach and intestine of the African sharptooth catfish individuals from the rearing ponds. A comparison of algal species richness in rearing ponds of the two fish species shows that they have the same species richness (Figure 3). More algal species were recorded in water samples than those from fish stomachs and intestines (Table 4).3FIGURERecorded algae‐specific richness and the number of families according to sampling point Note: Bars represent the species‐specific richness, and the line represents the number of families.4TABLEAlgal species recorded in each sampling point from fish farming waters and stomach and intestine of African sharptooth catfish and Nile tilapia.Algae speciesSampling pointsRearing pondsEvacuation canalsDecantation pondWaterFish stomach and intestineWaterWaterFish stomach and intestineAfrican sharptooth catfishNile tilapiaAfrican sharptooth catfishNile tilapiaAfrican sharptooth catfishNile tilapiaAfrican sharptooth catfishNile tilapiaAcutodesmus acutiformisXXXAphanocapsa confertaXXXAphanocapsa elachistaXXApocalathium aciculiferumXArthrospira platensisXAstasia inflataXXXXAstasia skadowskiiXXChlamydomonas sp.XXChlorella vulgarisXXXXXChroococcus turgidusXXXXXClosterium acutumXXXCoelastrum astroideumXXXCoelastrum microporumXXXCosmarium angulosumXXXXXXCosmarium contractumXXXXXCosmarium granatumXXCosmarium sp.1XXXXXCosmarium sp.2XXXCrucigenia quadrataXDesmodesmus communisXXXXXXXXXDesmodesmus intermediusXXXXDesmodesmus opoliensisXXDiplopsalopsis bombaXEudorina elegansXXXXXXXEuglena oblongaXXXXEuglena sanguineaXEuglena sp.XXXXXEuglena viridisXXXEuglenaformis proximaXXXXGomphosphaeria sp.XXXKirchneriella lunarisXXXXLepocinclis fuscaXLepocinclis fusiformisXLicmophora communisXMessastrum gracileXXXMicrocystis aeruginosaXXXXXXXXMicrocystis flosaquaeXXXXMicrothamnion kuetzingianumXXMonoraphidium convolutumXMougeotia sp.XXXXXXXXXNavicula sp.XXXXXXXXXOedogonium sp.XXXXXXXXOocystis borgeiXXOocystis sp.1XXXXXOocystis sp.2XXXXOscillatoria sp.XXXXXPandorina morumXXXXXXXXPeridinium sp.XXPetalomonas angustaXPhacus angulatusXXXXXPhacus orbicularisXXXXXPhormidium autumnaleXXPinnularia interruptaXXXPseudanabaena galeataXXXPseudanabaena limneticaXXRivularia sp.XScenedesmus acuminatusXXXXScenedesmus bicaudatusXXXXScenedesmus carinatusXXXScenedesmus ecornisXXXXXXScenedesmus ellipticusXXXScenedesmus perforatusXScenedesmus sp.XXXScenedesmus subspicatusXScenedesmus tibiscensisXXSchroederia setigeraXSpirogyra sp.XXXXXXStephanodiscus sp.XXXXXXXXXTetradesmus obliquusXXXXXUlothrix zonataXXXXXXUronema sp.XXXXWoronichinia delicatulaXXZygnema sp.XXXXXXXXSpecies richness414113232824531729Considering the contents of fish stomachs and intestines, Nile tilapia is found to consume more algae than African sharptooth catfish in rearing ponds. Nevertheless, individuals of African sharptooth catfish consume more algae in decantation pond than those in rearing ponds.Thirteen species were recorded in at least six different samples from water sampling points and in the stomachs and intestines of both fish species (Figure 4). In all samples, three algae families were mostly represented with at least five different species per family. Scenedesmaceae was found to be the dominant family with 16 species. This family was followed by Desmidiaceae and Euglenidae with five species each. In rearing ponds of the African sharptooth catfish, the algal community was dominated by Scenedesmaceae, Desmidiaceae and Zygnemataceae with eight, three and three species, respectively. The other families were represented by only two or one species. In rearing ponds of Nile tilapia, the algal community was mostly represented by Scenedesmaceae with 13 species. In the evacuation canal from the rearing ponds of African sharptooth catfish, the algal community was dominated by Scenedesmaceae and Zygnemataceae with four and three species, respectively. The other families in this canal were represented by only two or one species. In the evacuation canal from the rearing ponds of Nile tilapia, the algal community was dominated by Desmidiaceae, Scenedesmaceae and Zygnemataceae by five, three and three species, respectively. The other families were also represented by only two or one species in this canal. In the decantation pond, Scenedesmaceae, Desmidiaceae, Euglenidae, Phacidae, Oocystaceae and Zygnemataceae families were mostly represented by respectively 11, five, four, four, three and three species. In African sharptooth catfish stomachs and intestines, families such as Desmidiaceae with four species, Oocystaceae and Scenedesmaceae with two species each were recorded. The other families were represented by only one species. In Nile tilapia stomachs and intestines, dominant families were Scenedesmaceae with eight species, Desmidiaceae and Euglenidea with three species each. Zygnemataceae, Oocystaceae, Astasiideae and Microcystaceae were represented by two species each. The other recorded families were represented by only one species.4FIGUREPhotographs of the most represented species in all sampling points. Note: 1: Navicula sp., 2: Stephanodiscus sp., 3: Oedogonium sp., 4. Scenedesmus ecornis, 5: Desmodesmus communis, 6: Eudorina elegans, 7: Pandorina morum, 8: Ulothrix zonata, 9: Cosmarium angulatum, 10: Spirogyra sp., 11: Mougeotia sp., 12: Zygnema sp., 13: Microsystis aeruginosaRegarding Shannon–Wiener (H') diversity index, stomach and intestine contents of African sharptooth catfish fish in rearing ponds were less diversified than other sampling points (H' = 1.8798). Decantation pond water remains more diversified than other sampling points with a Shannon–Wiener of H' = 3.8412. The Pielou Equitability index (E) for all sampling points ranged from 0.8555 to 0.9629 (Table 5).5TABLEEcological diversity indexes according to sampling point.SourceSampling pointFish speciesShannon–Wiener (H')Pielou (E)WaterDecantation PondNA3.84120.9629Evacuation CanalA. Catfish3.20770.9626N. tilapia2.98410.9390Rearing PondA. Catfish3.32750.8960N. tilapia3.45080.9293Fish stomachs and intestinesDecantation PondA. Catfish2.52010.8895N. tilapia3.06750.9110Rearing PondA. Catfish1.87980.8555N. tilapia3.04850.9250Abbreviations: A. catfish, African sharptooth catfish; NA, not applicable; N. tilapia, Nile tilapia.When comparing the similarity of algal species composition between sampling points by the Jaccard similarity index (Table 6), more similarity was found between the decantation pond and rearing ponds of Nile tilapia (Cj= 0.5574). No similarity was found between the other samples.6TABLESimilarity in algal species composition between the different sampling points through the Jaccard similarity index.DP WACF RP WACF RP FACF EC WACF DP FNT RP WNT RP FNT EC WDP WACF RP W0.4615ACF RP F0.16670.2195ACF EC W0.30160.50000.2759ACF DP F0.29630.26090.73330.2162NT RP W0.53230.4211NT RP F0.44640.28570.4167NT EC W0.36840.33330.38300.5455NT DP F0.43100.45830.43750.42860.47370.3590Abbreviations: ACF, African sharptooth catfish; DP, decantation pond; EC, evacuation canal; F, fish content; NT, Nile tilapia; RP, rearing pond. W, water.Physical and chemical water quality parameters of sampling pointsMeasurement of turbidity showed that water in sampling points was clear. The Secchi disk used for this measurement was visible until the bottom of each site. Temperature and pH were slightly varying from 24.00 to 26.17°C and from 6.8 to 7.4, respectively. However, strong variations were observed for electrical conductivity ranging from 35.10 to 173.90 μs/cm and DO going from 2.25 to 7.68 mg/L. The lowest conductivity was observed in tilapia ponds and their evacuation canals. The decantation pond had the highest DO, followed by both African sharptooth catfish and Nile tilapia rearing ponds. Evacuation canals had the lowest DO. Nitrite concentrations in all sampling points were low (< 0.3 mg/L; Table 7).7TABLEPhysical and chemical parameters of water in sampling points.Sampling pointFish speciesTemp (°C)pHElect. cond. (μm/s)DO. (mg/l)NO2 (mg/l)Decantation PondNA24.00 ± 0.827.40 ± 0.22173.87 ± 113.397.68 ± 0.910.12 ± 0.05Evacuation CanalAfrican sharptooth catfish25.33 ± 0.476.77 ± 0.4585.73 ± 16.792.25 ± 0.370.18 ± 0.09Nile tilapia25.33 ± 0.476.93 ± 0.3435.77 ± 1.636.63 ± 0.750.09 ± 0.07Rearing PondAfrican sharptooth catfish25.5 ± 0.767.05 ± 0.3371.18 ± 26.994.22 ± 1.060.25 ± 0.21Nile tilapia26.17 ± 0.697.18 ± 0.2735.10 ± 5.824.14 ± 0.560.06 ± 0.05Note: Value represents mean ± standard deviation.Abbreviations: D.O., dissolved oxygen; Elect. cond., electrical conductivity; NO2, nitrite concentration; NA, not applicable; Temp, temperature.DISCUSSIONThe aim of this study was to characterise algal species in rearing ponds of African sharptooth catfish and Nile tilapia and their contribution to the diet of these species.Diversity and distribution of algae in the fish farming stationIn the fish farming station, six different phyla of algae were recorded in water from African sharptooth catfish and Nile tilapia ponds, evacuation canals, decantation pond and African sharptooth catfish and Nile tilapia stomach and intestine contents. They were dominated by Chlorophyta with 43.84% of the recorded species, followed by Cyanobacteria and Euglenozoa (17.81% and 16.44%, respectively), Charophyta, Bacillariophyta and Miozoa. This finding agrees with the previous studies conducted in Burkina Faso natural water systems and reservoirs (Zongo & Zongo, 2016; Zongo et al., 2008). Indeed, Chlorophyta is found to be the dominant phylum in waters from the Sudanian and Sahelian zones of Africa (Ouattara et al., 2000; Zongo & Zongo, 2016; Zongo et al., 2008, 2007). Our results show that recorded algal in fish farming ponds are species occurring only in tropical conditions (Africa, America and Asia) as revealed by Zongo and Zongo (2016). Indeed, the study area is characterised by the Sudanian climate, with a dry season from October to May and a rainy season from June to October. The study was conducted in such tropical conditions during the coldest period of the dry season in December and January explaining algal composition in fish ponds. This period of dry season always registers the highest algal abundance in tropical water bodies. Indeed, a study conducted in a tropical soda lake in Ethiopia showed seasonal variations in phytoplankton abundance and biomass with a peak abundance observed during dry periods (Wagaw et al., 2021). Therefore, these findings suggest that in fish farming ponds and decantation pond, algal biomass during the rainy season will be less, compared to the dry season where the study was conducted. This situation in the shifts in algal presence in water habitats can be explained by evapotranspiration highly occurring during the dry season in tropical zones and dilution of water and its nutrient concentrations during the rainy season. Indeed, rainfall conduces to a reduction of nutrient concentrations in a water body (Arreghini et al., 2005; St. Pierre et al., 2021). For photosynthetic activities, these nutrients are required, and rainfall can contribute to reduce algal species richness and abundance during the rainy season through nutrients dilution.The predominance of Chlorophyta is favoured by the low water depths (0.3–0.5 m), the slight turbidity of the water and the ability of light to penetrate it. In this study, results showed that Scenedesmaceae family represented 50% of species in Chlorophyta phylum. This finding is with Iltis (1980) who stated that Scenedesmaceae is one of the commonly encountered families in Sudanian waters.Species of Cyanophyta and Euglenophyta phyla were relatively present in fish ponds, evacuation canals and decantation pond. They contain species endowing with an ecological plasticity and colonising extremely varied environments like stagnant waters and those rich in organic matter (Leitao & Coute, 2005; Sheath & Wehr, 2003). In Nile tilapia and African sharptooth catfish rearing ponds, the leftover distributed feed as well as fish faeces sediment at the bottom of the ponds. The remaining wastes constitute an important source of degradable organic matter, favourable to the development of algal species such as Microcystis spp., Oscillatoria spp., Phormidium spp., Euglena spp., Phacus spp. and Lepocinclis spp. encountered in the present study. However, because of the daily renewal of the water in the ponds, these organic matters are continuously eliminated through the evacuation canals and transported to the decantation pond. The high content of organic matter and nutrient in the decantation pond is proved by the high electrical conductivity in this pond. indeed, this electrical conductivity is determined by the amount of dissolved matter (Meride & Ayenew, 2016). Removal of wastes allows for controlling excessive nutrient pollution in rearing ponds. Thus, in the decantation pond, organic matter is continually being sedimented and contributes strongly to the primary productivity in the habitat. Algae and zooplankton from the primary production are continuously consumed by Nile tilapia and African sharptooth catfish as well as amphibians living in harmony in this habitat. Indeed, in fish rearing ponds and decantation pond, sedimented organic matters are converted to nutrients by microorganisms and used by algae species for growing and multiplication through photosynthesis and reproduction as reported by Yaakob et al. (2021). However, in evacuation canals, organic matters are only transported, and few nutrients can be used by algae species. That could explain the low species diversity of algae in general, and particularly of Euglenoids and Cyanobacteria in evacuation canals, compared to fish rearing ponds and decantation pond. Because of the intolerance of Nile tilapia to low oxygen concentrations, water renewal is higher in Nile tilapia ponds, compared to African sharptooth catfish ponds. Consequently, water from Nile tilapia ponds contain less Euglenoids than African sharptooth catfish ponds.The high Shannon–Wiener (H between 1.88 and 3.84) and Pielou (E between 0.86 and 0.96) index values at all samples indicate that these sites were diverse in algal species. Pielou index that trends towards 1 shows that the abundance of algae species is relatively similar across sampling points, meaning that these populations are in balance (Triplet, 2020). However, the Jaccard similarity index shows small values between 0.17 and 0.55, meaning that the compositions of algae in sampling points are not similar.Impact of fish breeding on water variables and algae compositionAfrican sharptooth catfish and Nile tilapia are fed every day with formulated feed responding to their nutritional needs for growth and reproduction. The catabolism of feeds in fish as well as uneaten feeds release huge amounts of nutrients into the water, thus involving a proliferation of phytoplankton communities in waters (Tucker & Lloyd, 1984). Inversely in the meantime, phytoplanktonic species absorb nutrients through their development and contribute to the natural purification of water, thereby helping to avoid eutrophication. Thus, in aquaculture, phytoplankton plays a significant role in stabilising the whole pond ecosystem and in minimising the fluctuations in water quality (Priyadarshani et al., 2012). However, long‐term high nutrient loading coming from feeds and fish faeces contributes to eutrophication and algal species proliferation. Once the algae blooms become too large, they can significantly reduce sunlight penetration and then favour the growth and development of blue‐green algae (Brunson et al., 1994). If fish farming contributes to the production of algal biomass (Tucker & Lloyd 1984), algal blooms in ponds affect the production of fish by (a) shallow thermal and chemical stratification of the water; (b) wide diel fluctuations in pH, dissolved carbon dioxide and oxygen concentrations; (c) water quality deterioration after sudden massive phytoplankton death and (d) production of compounds that give undesirable tastes and odours to fish. Blooms, as regularly observed in decantation pond that receive nutrient‐enriched water, can lead to a lack of aeration and high consumption of oxygen, low penetration of light and finally death of fish.In fish farming ponds, fish feeds and algae biomass that are both consumed by fish contribute to the food web and the regulation of algal biomass. The consumption of algae by the African sharptooth catfish and Nile tilapia species is necessary for controlling and attenuating algal blooms in farming ponds.Contribution of algae to Nile tilapia and African sharptooth catfish dietsAmong the recorded six phyla in sampling points, Miozoa was the only one that was not detected in fish stomachs and intestines. The dominance of the other five phyla in fish stomachs and intestines follows the successive dominance obtained in waters of sampling points. Chlorophyta is the predominant phylum followed by Cyanobacteria, Euglenozoa, Charophyta and Bacillariophyta. These results suggest that grazing from fish is mainly oriented towards the most present algal community and species. However, species richness (S) and Shannon index (H) attained high values in fish ponds (S = 73, H = 2.98–3.84) than in fish stomachs and intestines (S = 41, H = 1.88–3.07). This situation may be related to a preference for fish for some algae species (Neya et al., 2018).Algae are the first link in the aquatic food web and are thus an important source of feed for fish populations (Chader & Touzi, 2001; Iltis, 1973). The nutritional characteristic of an algae species for an organism depends on its cell size, digestibility, production of toxic compounds and biochemical composition (Priyadarshani et al., 2012). Depending on their diet, fish species use algae differently. Differences in algal‐specific richness were observed between African sharptooth catfish ponds, Nile tilapia ponds and decantation pond. In fish stomachs and intestines, the specific composition of algae was also species‐dependent. Indeed, stomach and intestine of some individuals of African sharptooth catfish were empty and free of algae, while they were found in all stomachs and intestines of Nile tilapia individuals. The stomach contents of Nile tilapia individuals were more diverse in algal species than African sharptooth catfish in rearing ponds as well as in decantation pond. These differences are related to the diet of these two species. Indeed, the diet of Nile tilapias is generally omnivorous with a microphagous or herbivorous tendency, whereas African sharptooth catfish is omnivorous with a carnivorous tendency (Lévêque & Paugy, 2006). In natural conditions, Temesgen et al. (2022) found that phytoplankton was the most consumed feed by Nile tilapia. In addition, through the filtration process, Nile tilapia can considerably reduce Cynaobacteria population by 53% and green algae by 28% (Turker et al., 2003). According to these authors, the number of filtered particles increases with particle size from the smaller Tetraedron and Chlorella to the larger four‐cell stacks of Scenedesmus with the green algal water sources and from the smaller Merismopedia to larger Microcystis with cyanobacterial water sources.Species of some families such as Scenedesmaceae, Microcystaceae, Naviculaceae, Zygnemataceae, Euglenidae and Ulothricaceae were mostly founded in the stomachs and intestines of both fish species. The algal species of these families are the most encountered in the sampling points and thus the most accessible by fishes.Algae by their nutritional composition contribute to the good growth of fish species. As algae are found to be low‐cost and commercially available (Henry, 2012), species that mostly contribute to fish diet can be selected and cultivated onsite for fish feeding during fish farming. However, the presence of Cyanobacteria species can be a source of a nuisance as some of them (e.g., Microcystis) produce toxins (Turker et al., 2003).CONCLUSIONFrom the present study, 73 algal species belonging to 47 genera, 34 families, 21 orders, 11 classes and six phyla were recorded in African sharptooth catfish and Nile tilapia rearing ponds, evacuation canals, decantation pond and in stomachs and intestines of both species. In sampling points, the development and proliferation of algae species were favoured by the leftover feeds and fish faeces, which sediment at the bottom of ponds and constitute an important degradable organic matter.In the sampling points, Scenedesmaceae, Desmidiaceae, Euglenidae and Zygnemataceae were found to be the most represented families. Algal species differentially contribute to the feeding and diet of African sharptooth catfish and Nile tilapia, as fish species have different types of diets. Indeed, Nile tilapia, which is microphagous, consumes more algae than African sharptooth catfish, which is omnivorous with a carnivorous tendency. Nevertheless, as some algal species in fish ponds are consumed by both fish species, they can be selected and cultivated for biomass production for fish feeding. Thus, algae species from Scenedesmaceae family can be used for biomass production in subsistence and small‐scale fish farming to improve fish productivity.AUTHOR CONTRIBUTIONSSaïdou Santi contributed to acquisition, analysis and interpretation of data. He made substantial contribution in the conception, design and drafting the manuscript. Benjamin Poda made substantial contributions to collection of data. Bilassé Zongo contributed to identification of algae species. He made substantial contribution on acquisition, analysis and interpretation of data. He was also involved in the conception, design and drafting the manuscript. 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Journal

Aquaculture Fish and Fisheries – Wiley

Published: Apr 1, 2023

Keywords: African sharptooth catfish; algal species; Nile tilapia; rearing ponds

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