Comparative analysis of diet of two sympatric species of Tilapia in Ayamé man-made lake (Côte d’Ivoire)

H Shep*, K M Konan, M Ouattara, A Ouattara and G Gourène

Laboratoire d’Environnement et de Biologie Aquatique, Université NANGUI ABROGOUA, 02 BP 801 Abidjan 02, Côte d'Ivoire
konanmexmin@hotmail.fr   /   kmemin@yahoo.fr
* Direction de l’aquaculture et des pêches (Ministère des Ressources Animales et Halieutiques),
Treichville, Rue des Pêcheurs BPV 19 Abidjan, Côte d'Ivoire

Abstract

Tilapia zillii and T. guinensis, occurring in brackish and fresh waters of West Africa, respectively, are sympatric in artificial environment of the Ayamé Lake. The present study analyzed the stomach contents of 243 specimens of both species collected in Ayamé Lake(122 forTilapia zillii and 121 for Tilapia guineensis)in order to determine their food composition and interspecific diet overlap. Food items identified from stomach contents and food overlap were analysed using the Index of Food Preponderance (IFP) and the Morisita index modified by Horn.

The two species feed on a wide variety of items preys from animal and plant origin. T. zillii consumed 13 prey items while T. guinennsis feed on 16, among which 13items were common in their diets. The insect diptera and the macrophytes were the most important item in the diet of both species. Molluscs, coleoptera and zooplankton alsoconstituted some significant items. The food overlap analysis showed that T. zillii and T. guineensis share a wide range of prey types witha generalized feeding behaviour. Despite the wide food overlap, the competition for food is probably minimal, because the two fish species eat a wide range of foods and also because of the abundance of main preys.

Key words: artificial lake, diet composition, diet overlap, Tilapia guineensis, Tilapia zillii

Introduction

Cichlid species are widely distributed in most east and west Afro-tropical hydrosystems (Daget 1988) and belong to the commercially important inland fishes of Africa (Fagade 1971). T. zillii is a well known species from West Africa river system through Chad basin to the Nile (Trewavas 1982, Negassa and Getahun 2004). The natural distribution of the species included Lake Albert, Lake Turkana, Israel and Jordan Valley (Trewavas 1982). T. zillii has been introduced throughout the world for production and vegetation control purposes (Spataru 1978). T. guineensis is a euryhaline species found in estuaries and lagoons of West Africa (Philippart and Ruwet 1982, Campbell 1987). This species has gained an increasing interest for aquaculture purposes, particularly in the estuaries and extensive lagoon systems which constitute its natural range (Akinwumi 2003, Akinrotimi 2006). T. zillii and T. guineensis occur in brackish and fresh waters of West Africa. T. zillii frequents the upper waters whereas T. guineensis is generally found in the lower parts and in the lagoons (Philippart and Ruwet 1982, Agnèse et al 1998). The artificial environment of the Ayamé Lake has favoured the coexistence of T. zillii and T. guineensis (Thys van den Audenaerde 1970). In other respects, a large population of hybrids of Tilapia species as results of hybridization among T. zillii and T. guineensis was found in Ayamé Lake (Pouyaud 1994).According to Pouyaud and Agnèse (1995), both species were genetically very close.Fisheries data from the lake revealed the predominance (52%) of tilapia species in the different landings sites around the Lake (Vanga et al 2002). Living in the same environment, these species likely share resources as well as reproduction habitats in Ayamé Lake. Sabagh and Carvalho-e-Silva (2008) indicated that the resource partition in a community is essential for the understanding of species interactions. In other respects, diets are a fundamental aspect of each niche, and it seems reasonable to assume that the structure of a community is based mostly on the way that food is shared among coexisting species (Sihand Christensen 2001).

 In Africa, the diets of tilapias species have been subject of several studies (e.g. Fryer et al 1955, Fagade 1971, 1978, Lauzanne 1988,Abdel-malek 1972,Akinwumi 2003, Winemiller and Kelso-Winemiller 2003,Negassa and Getahum 2004, Oso et al 2006,Shalloof and Khalifa 2009, Agbabiaka 2012). Particularly, T. zillii was considered as an opportunistic bottom feeder with a high proportion of the food consisting of plants parts (Nwadiaro 1984,Akinwumi 2003, Negassa and Getahun 2004), insects larvae, zooplankton, small crustacean and molluscs (Lauzanne 1988, Agbabiaka 2012) while T. guineensis mainly feed on algal filaments, diatoms, sand grain and unidentified organic material (Fagade 1971).

Nevertheless, feeding is one of the most important biological factors and its abundance and variety influence the structure and composition of fish populations (Aranha et al 2000). In addition, diet analysis of fishes allows us to understand their feeding strategy, their intra-or interspecific potential interaction (competition and predation) and indirectly indicate community energy flow (Ramirez-Luna et al 2008).

The present study aimed to understand the coexistence of the two species in a man-made lake. Food traits are discussed in the relation to the interspecific competition and dietary overlap of these fishes.

Material and methods

Study area

The Ayamé Lake (0° 20’ S and 36° 5’ E) is an artificial freshwater lake situated in the south-east region of Côte d’Ivoire. The lake was built in 1959 in the river Bia and has an area comprised between 87 and 194 km² with a mean depth of 30 m. The lake level is subject to fluctuations depending on local rainfall and evaporation (Reizer 1967).The Ayamé Lake is deep and open water characterised by muddy substrate and a low transparency with an annual mean Secchi disk of 110 cm (Kouamélan et al 1999). Fishes were sampled at two main sites in the lake (Bakro and Ayamé).

Figure 1. Map of the Bia river showing the different sampling sites (•).

 Sampling and stomach contents analysis

A total of 243 specimens of T. zillii, T. guineensis were collected over twenty four months period in two sites (Figure 1) from August 1995 to September 1997 using two batteries of gill-nets with mesh size 10, 12, 15, 20, 25, 30, 35, 40 and 50 mm. Distribution per species is as follow: 122 for T. zillii and 121 for T. guineensis. Standard length (SL) of fish caught was measured to the nearest 0.1 cm using a measuring board. The standard length ranged from 60 to 200 mm for T. zillii, 100 to 170 mm for T. guineensis. The stomach was then removed and preserved in pillbox containing 5% formol for further examination in laboratory.

In the laboratory, the stomach contents of each specimen were placed in a Petri disk and aggregates were dispersed with a few drops of water and filtered through 1000, 500 and 100 µm mesh size before microscopic examination. The different prey taxa were sorted, counted and weighed to the nearest 0.0001g. For each stomach, the food items were identified to the lowest possible taxonomic level using descriptions and keys from various sources (Lindley 1970, 1975, Dejoux 1974, Elouard and Levêque 1977, Caratini 1985).

Data analysis

Calculation of indexes

The food of the two cichlids was assessed essentially by the relative frequency (RF) and relative dominance (RD) (King 1989, 1991, 1994) according to the formulae:

Where f_i = frequency of item i; F_i = frequency of the nth item (sum of all f_i); d_i = frequency of item i as dominant dietary; D_i = frequency of nth dominant item (sum of all d_i). The RF and RD of all dietaries are then summed up to 100%. The RF is a modification of the occurrence method (Hyslop 1980) commonly used in fish dietary studies. The RF method over-emphasizes the importance of the numerous small items while RD method over-emphasizes the importance of large items. An index of food preponderance (IFP) was calculated as the mean of % RF and % RD. Items with IFP ≥ 10% were considered as primary dietaries while those with IFP values comprised between 1 to 10 % were considered as secondary. Items with IFP less than 1% were classified as incidental.

For food habits variation in relation to size, three classes (juveniles, sub-adults and adults) were determined as follows:

• The smallest sexual mature individual was determined for each species;

• The size at first sexual maturity was determined by fitting the fraction of mature female to a logistic function by non-linear regression according to the procedure described in Duponchelle and Panfili (1998). The average size at first maturation (L50) was determined and considered as the standard length at which 50% of the females are in an advanced stage of the first sexual cycle during the reproductive season (Duponchelle and Legendre 2000).

• All specimens with size less than the size of the smallest sexual mature individual are considered as juveniles. From the size of the smallest sexual mature individual to L50, individuals are considered as sub-adults while specimens with sizes more than L50 are adults.

   Calculation of food overlapp

Food overlap between species has been calculated, using the overlap measure of Morisita (1959) as modified by Horn (1966).

Where S is the total number of food categories and X_i and Y_i are the proportion of total diet of species X and Y taken from a given category of food i.

Feeding strategy

To assess the feeding strategy along the studied period, the modified Costello (1990) graphical method (Amundsen et al. 1996) was used. In this method, the prey-specific abundance (%Pi) (y – axis) was plotted against the frequency of occurrence (F) (x - axis). The prey-specific abundance (Pi) has been expressed as: 

Where Si is the number of prey i and Sti is the total number of prey in the stomachs containing prey i.

Results

Overall food composition

The results of the food items recorded are represented in table1. Tilapia zillii and T. guineensis feed on a wide trophic spectrum of food constituted by 16 and 13 items preys, respectively.The main food item of T. zillii were macrophytes (IFP = 15.90 %) and insects diptera (IFP = 11.62 %). The secondary preys (IFP = 1 – 10%) of this species were composed by molluscs, coleoptera, zooplankton, hymenoptera, ephemeroptera, mud, trichoptera and animal debris. The most common food item in the stomach of T. guineensis is the insect diptera (IFP = 13 %). Secondarily, T. guineensis fed on macropthytes, molluscs, coleoptera, mud, zooplankton, hymenoptera, trichoptera, ephemeroptera, animal debris and sand.

Food similarity between the two species (C_λ = 0.98) was superior than 0.60.  A wide diet overlap between the both Tilapia species in Ayamé Lake was observed. T. zillii and T. guineensis feed on the same resources.

Food in relation to fish size

The figure 2 presented the preponderance index of the major food items, for the various size classes. All stages (juveniles, subadults and adults) of both species mainly fed on insects (IFP = 22 - 43.75 %), macrophytes (IFP = 16 – 33 %) and mollusks (IFP = 12 – 33 %). Zooplankton in juveniles of T. zillii (IFP = 11 %) and other preys (sand, animal debris, mud and undetermined preys) in juvenile (IFP = 11.52 %) and adults (IFP = 16.59 %) of T. guineensis also constituted some primaries preys. Only the feeding pattern of T. guineensis was significantly different between size classes (total χ²: 23.12, ddl = 4, P = 0.000104).

Figure 2. Food variation in relation to fish size of Tilapia zillii and Tilapia guineensis in the Ayamé Lake.

Seasonal variation in diet composition

The food composition in relation to season is presented in table2. In both seasons, insects (IFP = 37.69 – 44.62%) and mollusks (IFP = 14 – 18 %) constituted primaries preys for the two fish species. These preys were taken innearlythe same quantitiesby the studied fish. The specimens of Tilapia guineensis also mainly feed on fish (12 %) and other preys (IFP = 15 %) in wet season while in dry season these are the macrophytes and other preys which are also mainly consumed. In both seasons, the other primaries preys in the stomach of T. zillii are macrophytes. Zooplankton and arachnids were secondary and incidental preys, respectively, in the studied fish in all seasons.

The feeding pattern was significantly different between the studied species in wet season (total χ² : 43.65, ddl = 13, P <  0.0001) and dry season (total χ² : 58.242, ddl = 13, P = 0.000092).

Feeding strategy

Analysis of feeding strategy, based on the Amundsen’s method (figure 3), showed that for the two species, almost all the preys were located below the prey importance axis indicating that these fishes exploited a broad niche with a generalized feeding behaviour. Considering the prey importance, Tilapia zillii’s diet is mostly dominated by the items 14 (Macrophytes), 1 (Diptera), 11 (Molluscs) and 5 (Coleoptera). These preys were broadly consumed by all specimens of this species. The remaining items were eaten occasionally and in relative small amounts. For T. guineensis, diet was mostly based on 1, 14, 11 and 12 (Zooplankton) which tend to be dominant preys. The other items are occasionally and rarely consumed.

Figure 3. Feeding strategy of individuals of Tilapia zillii and Tilapia guineensis in Ayamé Lake (see table 1 for item number).

Discussion

Tilapia species have been reported to be plankton and deposit feeders (Adiase 1969,Fagade 1971, 1978, 1982, Fryer and Iles 1972,Fagade and Olaniyan 1973, Harbott 1975,Pauly 1976, Buddington 1979, Akinwumi 2003, Winemiller and Kelso-Winemiller 2003,Negassa and Getahum 2004, Oso et al 2006,Agbabiaka 2012).The food of the species covers a wide spectrum. The major prey items of Tilapia zillii and Tilapia guineensis in the Ayamé Lake were macrophytes, insects and molluscs parts. On that account, the feeding habits of these species were similar to those reported by Fagade and Olaniyan (1972) in the Lagos lagoon on Tilapia guineensis and Sarotherodon melanotheron and Fagade (1979) on Tilapia guineensis from Lekki lagoon.In addition, a wide variety of invertebrates are taken as food by Tilapia zillii andT. guineensis. According to Amundsen graphical, the studied fishes exhibited a general feeding strategy. Stomach content analysis based on the index of food preponderance showed that in Ayamé Lake, Tilapia zillii, T. guineensisfeed on a wide range of food organisms that makes them euryphagous feeding with a food base comprising both plants and animals.T. zillii has been variously classifiedas plankton feeders, higher plant and algae feeders or macrophagous as well as mud suckers (Fagade 1971,Brown and Colgan 1984, Negassa and Getahun 2004).Nevertheless, food of plant origin was the major component of the diet of T. zillii in this study. The same diet has been reported for T. zillii from Lake Victoria by Welcomme (1979), for fish from Lake Quarun by Abdel-Malek (1972) and from Nile canal by Khallaf and Alne-na-ei (1987).For that reason, T. zillii were brought to Florida (USA) in 1961 by the Florida Game and Freshwater Fish Commission to investigate it`s potential use as a biological weed control (Courtenay and Robins 1973).Because of its diet, covering a wide spectrum of food ranging from various types of plankton to invertebrate, T. zillii can be classified as an omnivorous. This characterization was reported for the same species by Spataru (1978) in Lake Kinneret (Israel).

Examination of the diet of Tilapia guineensis indicatedthat there was high index of food preponderance of Insects in their stomachs. Apart from the major food items, studied fishes also picked a variety of other food items. The wide variety of items occurring in the stomachs of T. zillii (13 items preys), T. guineensis (16 items preys) is an indication that they are non selective in feeding. In fact, 280 phytoplankton taxon (Ouattara et al 2000), 29 macroinvertebrates (24 Insect taxons) with a high abundance of MolluscPotadoma liberiensis (Diomandé and Gourène 2005) and 56 zooplankton taxons (Ouattara et al 2007) were identified in Ayamé Lake.In addition, about 10% of Lake Surface was occupied by the macrophytes Pistia stratiotes (Etien and Arfi 1996). Other macrophytes as Bacopa crenata, Ceratopteris cornuta, Scieria verrucosa, Commelina diffusa, Vigna campestris, Gongronerna latifolium) were present in the lake (Sankaré et al 1986).

The two species are able to use many sources of protein as food. Liem (1980) stated that teleost including cichlids are able to exploit more than one source. This ability to exploit different varieties of food makes T. zillii and T. guineensis as omnivorous fishes. The overlap measure C_λ (0.98) is near to 1. This result indicated a great food overlaps between species (Zaret and Rand 1971). So, competition for food becomes possible. However, direct competition seemed to be avoided to some extent as a result of great food availability in Ayamé Lake. In fact, Ouattara et al (2007) reported that the density of zooplankton in the Ayamé Lake was 116675ind/m3. In addition, the lentic conditions created by the lake are favorable to algal development. According to Spataru (1978), it is an important strategy for survival and an advantage over the species competing for a specific food item. The same observations were reported by Ahmed (2011) for the three sympatric species of Cyprinid fish larvae in Al-Huwaiza marsh (Southern Iraq). The similarity in ecological niche was also accompanied by some discrete differences in the selection of complementary food items. The difference noted in the diversity of supplementary food items of the two species may be an active and immediate response to interspecific competition or other habitat factors. T. zillii and T. guineensis were observed to be omnivorous, with highest frequency of occurrence of insect and macrophytes. Although some cichlids are known to feed entirely on fish scales (Fryer et al 1955). The low occurrence of fish scales in the stomachs of these specimens of tilapia species suggests that this type of prey is not important in the diet of tilapia species in the Ayamé Lake.

In conclusion there is a strong trophic-niche overlap between Tilapia guineensis and Tilapia zillii. However,the occurrence of overlap, even to a high degree,does not necessarily mean that competition is present, if the resource is not limited.

Acknowledgments

This work is a part of Ivorian-Belgian project VLIR / KUL (Flemish Interuniversity Raad) " Évolution de la biodiversité des poissons après la construction d’un barrage: cas de la rivière Bia en Côte d’Ivoire" financed by the General Agency for Development Cooperation Bestuur voor Algemeen Ontwikkelingssamenwerking (ABOS-BADC) of Belgium. We would be grateful to the promoter D.F.E. Thys van den Audenaerde and the co-promoter, late G.G. Teugels.  

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Received 12 June 2013; Accepted 21 August 2013; Published 4 September 2013

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