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 km², of 850 mm flow by year, the deficit flow is 960 mm, a flow rate of 27 l/s/km² 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 04.13095 South latitude 15.31177 to 015.32192 east longitude (Figure 1).

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] .

The percentage of emptiness was estimated as the following formula.

V = N v N t × 100

N_v: number of empty stomachs.

N_t: total number stomachs examined.

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

C l = L i L s

Li: intestinal length.

Ls: standard length.

The percentage of occurrence was calculated according to the following formula [9] .

% O c = n i N T × 100

n_i = number of stomachs containing a category of preys.

N_T = total number of full stomachs examined.

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.

I p = % O c × % P ∑ ​ ( % O c × % P ) × 100

Ip: preponderance index.

%P: weight percentage.

Different preys categories are classified According to the values of preponderance index.

- Ip < 10: accessory prey.

- 10 < Ip < 25: secondary prey.

- 25 < Ip < 50: important prey.

- Ip > 50: main 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] .

N C = 1 + 10 log 10 N 3

I = L S max − L S min N C

NC: Number 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] .

α = 1 − 0.5 ( ∑ i = 1 n | P x i − P y i | )

Pxi: proportion 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).

The diets are considered substantially similar if the Schoener index α is greater than or equal to 0.6 [13] .

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.

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.

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 season. 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).

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] .

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.

Class 1 (n = 1): 39 ≤ LS < 47.5 mm; Class 2 (n = 13): 47.5≤ LS < 56 mm; Class 3 (n = 49): 56 ≤ LS < 64.5 mm; Class 4 (n = 69): 64.5 ≤ LS < 73 mm; Class 5 (n = 62): 73 ≤ LS < 81.5 mm; Class 6 (n = 46): 81.5 ≤ LS < 90 mm; Class 7 (n = 24): 90 ≤ LS < 98.5 mm; Class 8 (n = 5): 98.5 ≤ LS < 107 mm; Class 9 (n = 1): 107 ≤ LS < 115 5 mm.

The hierarchical cluster analysis performed on the basis of different weight calculated in each class size food index, allows us to consider four groups of classes size (Figure 6). Group 1 consists of specimens of the 7^th class; group 2 consists of specimens of the sixth class, group 3 specimens consisting of the 4^th, 5^th and 8^th classes, and finally the group consisting of 4 specimens of the second and third classes.

Insects are the main prey of Bryconaethiops boulengeri in all groups of classes size. The Macrophytes are secondary preys in class 4 and class 5. All other items are the some accessories preys either group. Furthermore, no significant difference was observed between individuals of different size, the Schoener index is greater than 0.60 between the four groups. However, the similarity dendrogram regime reveals that individuals in the group 3 (class 4, class 5 and class 8) consume more macrophytes than other groups, this result shows that the young fishes of group 4 (class 2 and class 3) feed almost exclusively insects and they incorporate other foods into their diet with age (size superior to 64.5 mm).