Food Habits of Bryconaethiops boulengeri Pellegrin , 1900 ( Characiformes : Alestidae ) of Djiri River Tributary of the Right Bank of Congo River

Diet of 300 specimens of Bryconaethiops boulengeri from Djiri River (Congo Brazzaville) caught with cash nets was studied according to the size of fish, sampling stations and hydrological season. Sampling focused on twelve annual withdrawals made during three years. The relative importance index combining numerical and weight percentages of occurrence was calculated and also the sex ratio was evaluated. Bryconaethiops boulengeri consumes terrestrial and aquatic insects and everything that falls into the water (birds feathers, plant debris, fruits, etc.). The percentage of emptiness is 9.66% of the three sampling stations selected; no significant difference in diet was observed whatever the season.


Introduction
Bryconaethiops boulengeri is an Alestidae widespread throughout the Congo Basin [1], mostly found in Alima River [2] and in Léfini River [3].Moreover, Bryconaethiops boulengeri, Bryconaethiops microstoma, Micralestes stormsi and Synodontis nigriventris are the most abundant species found in the Djiri River [4].However, very little information is available on the biology, ecology, mainly their food habits.It should be noted that a major study of fishes ecology and ethology conducted in Lake Tumba and Ikela region provided qualitative data on food habits of B. boulengeri [5].This present study gives information on nutrition, diet change according to the environment, season and the size of Bryconaethiops boulengeri.

Study Area
Djiri River is located in the 9 th district of Brazzaville rises in the southern hiller Mbé and the high hills that extend to the Batékés plates.Length is about 50 km covering an area of 853 km 2 , of 850 mm flow by year, the deficit flow is 960 mm, a flow rate of 27 l/s/km 2 and a flow coefficient of 47%.Djiri River flowing in the direction NW-SE main tributaries Kouala-Kouala, Bamba and Souo on its left bank and Bilolo and Bitatolo on its right bank [6].The sampling area is located between 04.18117 to 04.13095South latitude 15.31177 to 015.32192 east longitude (Figure 1).

Sampling and Analyzes
Fishes were caught in the lower reach of Djiri River during monthly fishing trips from February 2008 to January 2011.They were immediately fixed in formalin 10% and then transferred the next day in 5% formalin for preservation.Each specimen was weighted to the nearest milligram and measured (standard length).
The stomach contents were then diluted in water and examined under a binocular microscope.Different food taxa were sorted and identified, when their state of degradation permitted, according to Lausanne protocol [7].

Percentage of Emptiness
The percentage of emptiness was estimated as the following formula.
N v : number of empty stomachs.N t : total number stomachs examined.

Intestinal Coefficient
The digestive tract has been described from 300 individuals whose are between.
Intestinal coefficient (C) is calculated following the formula [8].

Percentage of Occurrence
The percentage of occurrence was calculated according to the following formula [9].
n i = number of stomachs containing a category of preys.N T = total number of full stomachs examined.

Preponderance Index
To quantify the relative importance of prey, the following indices were calculated: percentage of occurrence and weight percentage [10].To avoid biases related to the use of these indices, the preponderance index [11] that incorporates these two indices was estimated with the following formula.-Ip < 10: accessory prey.-10 < Ip < 25: secondary prey.
A hierarchical cluster analysis (single linkage cluster analysis) was used to highlight the similarities between trophic stations and classes size.Classes size were determined according to the rule of Sturge [12].The diets are considered substantially similar if the Schoener index α is greater than or equal to 0.6 [13].

Results
The digestive tract is formed of a thick-walled esophagus, followed by a U shaped stomach surrounded by pyloric caeca.There are an average of 8 pyloric caeca and intestine is curled (Figure 2).
There was a significant linear relationship between the length of the intestine and the standard length of the fish (r = 0.84, p < 0.05).Intestinal coefficient of 300 individuals analyzed varies between 0.71 and 1.29 with an average of 0.95 ± 0.48 (Figure 3).The intestine of Bryconaethiops boulengeri is so short.

General Profile of the Diet
Of the three hundred Bryconaethiops boulengeri stomachs examined, 29 were empty, a percentage of 9.66% of emptiness.Seventeen categories of food divided into five groups were identified (Table 1): insects, arachnids, crustaceans, nemathelminths, macrophytes and others (fish scales, birds feathers, plastic bags).The weight index shows that preferential foods B. boulengeri are coleoptera (32.7%)  followed by odonata larvae (26.97%) and ephemeroptera (25.9%).The analysis of stomach contents showed that Bryconaethiops boulengeri consumes mainly insects (coleoptera, ephemeroptera and odonata larvae) with a preponderance index of 90.19 followed by macrophytes (dicot leaves and plant debris) with a preponderance index of 8.17.

Qualitative and Quantitative Aspects of Diet
Food items found in the stomachs of fish one spread over animal and plant fractions (Table 1).Animal fraction contains 17 items divided into six classes: insects, arachnids, shellfish, nematodes, fish, birds feathers.The fraction consists in plant leaf, fruit and stems of dicotyledonous.The classification of preys by calculating the preponderance index (Ip) has classified insects (90.19%) in the category of main prey and macrophytes (8.17%) as accidental prey.

Study of the Diet According to Season
Insects are the main preys eaten by Bryconaethiops boulengeri whatever the season, with a preponderance index 83.60% in the rainy season against 86.61% in the dry season.Macrophytes are secondary preys during two seasons with a preponderance index of 10.80% in the rainy season and 12.70% in the dry sea-son.Beetles are important preys in the rainy season followed by Ephemeroptera and Odonata larvaes that are secondary preys (Table 2).In the dry season, Ephemeroptera and Odonata larvae are important preys followed by beetles that are secondary preys.Schoener index between the two seasons is 0.89.The study of diet depending on the hydrological season shows no significant difference between the dry season and the rainy season (Schoener index = 0.89).This diet similarity between the two seasons could be related to the fact that Djiri River did not have a strong period of floods and low flows.
The affinity of prey consumed between the two seasons dendrogram shows no discrimination regime between the two seasons (Figure 4).Two groups of preys are discriminated, prey to high consumption (Coleoptera, Odonata larvae and that of Ephemeroptera) forming group 1 and those of low consumption (dicot leaves, plant debris and other Hymenoptera).

Study Diet According to Stations
Insects are the main prey of Bryconaethiops boulengeri, whatever the station with a preponderance index of 84.44% at station 1, 94.34% to 83.35% and station 2 to station 3. Macrophytes are secondary prey in station 3 with 16.6% of weight and accessory prey at station 1 and 2 respectively with a preponderance index of 9.03% and 2.58% (Table 3).
Among insects, Coleoptera, the Ephemeroptera and Odonata larvae are most consumed in the three stations (Table 4).
An isolated sequence provided by the station 1, it has relatively low affinities of diet with the stations 2 and 3.The hierarchical cluster analysis performed on the basis of index calculated in each different station isolates the feed station 1 to the other two stations (Figure 5).There is no significant variation in diet among the three sampling stations.
Schoener index is equal to 0.87 between the first and second biotope of 0.88 between second and third biotope and 0.86 between the first and third biotope.
This observation confirms that the diet of a species is substantially the same throughout its range [10].

Study of the Diet According to Size
Specimens examined were between 39.6 and 114.6 mm standard length.Nine classes size were determined according to the Sturge rule (Table 5).Due to the very low numbers, class 1 was fused to Class 2 and Class 9 is fused to Class 8.   avoided because if the diet remains insectivorous, three main biological groups (Coleoptera, Odonata larvae, Hymenoptera) considered as prey organisms constitute the preponderant food of Bryconaethiops boulengeri at all sizes, at all levels in all seasons and in all stations.

Conclusion
The analysis of 271 digestive tubes of Bryconaethiops boulengeri of Djiri River allows us to conclude that this species is insectivorous, it operates relatively well available food resources (because the coefficient of emptiness is low), it means that, the sustainability of resource, could explain the subjugation of this species in streams tributary of the Congo River.The food always available limits the width of the trophic recess.The speed variation depending on season, size class or station is not significant.It is desirable to extend this study to the reproduction in order to complete data on the bioecology of this species.

∑
Ip: preponderance index.%P: weight percentage.Different preys categories are classified According to the values of preponderance index.
of classes size.I: interval of classes.N: total number of classes.LS max: maximum length standard.LS min: minimum length standard.The index of Schoener was used to assess the degree of similarity between the different seasons, stations and classes size[13]. of prey i consumed by a mature stage (individuals in a season).Pyi: proportion of prey i consumed by a mature stage (individuals of the season).

Figure 3 .
Figure 3. Relationship between intestinal length and standard length of B. boulengeri.

Figure 4 .
Figure 4. Affinity dendrogram of different food items according to season.

Figure 5 .
Figure 5. Affinity dendrogram of different food items according to station.

Figure 6 .
Figure 6.Dendrogram affinity of various food items according to classes size.

Table 2 .
Composition of the dominant items in the diet of B. boulengeri in Djiri River according to season (Ip).

Table 3 .
Composition of food items B. boulengeri in Djiri River according to station (Ip).

Table 4 .
Composition of the dominant food items B. boulengeri in Djiri River according to station (Ip).

Table 5 .
Composition of food items B. boulengeri in Djiri River according to size (Ip).