BIODIVERSITY AND FISH REPRODUCTIVE ASPECTS OF THE SÔ RIVER IN SOUTHERN BENIN.

Sô River, located in southern Benin, like other rivers in the country, is subject to human pressure. The aim of this study was to characterize the ichtyofaunal diversity and to evaluate the reproductive aspects of the main species. In total, 11 stations have been sampled on the Sô River. Physico-chemical parameters were taken monthly in the morning between 6 am and 11 am and the fish samples were collected twice a month at each station from June to October 2015. The length-weight relationships, condition factor K, gonado-somatic index (GSI) and the sex ratio were evaluated. The physico-chemical parameters showed a

The fishing sub-sector in Benin represents 15% of the total labor force and 25% of the primary sector. This business sector directly or indirectly provides over 600,000 jobs. Inland fisheries contribute about 75% of national fish production and account for nearly 31% of national consumption of animal protein (Aholoukpe, 2017, Achoh et al., 2018. However, the widespread use of non-regulatory fishing gear, techniques and fishing methods is leading to the degradation of aquatic ecosystems. Fishing is one of the causes of the degradation of river biodiversity (Lederoun et al., 2016. The intensification of the exploitation of freshwater and brackish water populations by permanently growing local populations constitutes a major risk of regression and disappearance of species (Kouakou et al., 2016, Aholoukpe, 2017. In addition, water pollution contributes to lower catches and fish size (Noumon et al., 2015). The analysis of the sustainable management of inland water resources shows that the use of small-scale fishing gear contributes to the progressive depletion of natural resources of water bodies (Aholoukpe, 2017). Thus, the resources taken from these waters decrease when the quality of the continental waters, or even the environment, deteriorates (Agblonon Houelome, et al., 2017). According to Ainan (2014), this situation can generate among the fishermen population many economic, social and health problems such as falling incomes, unemployment, poverty and malnutrition.
In Benin, the municipality of Sô-Ava, which is a lacustrine community, is one of the most vulnerable to this degradation of aquatic ecosystems because aquatic resources are the basic source of income for its population (Koudenoukpo et al., 2017). Indeed, the aquatic resources available there play a role in the local economy, but also at national and international levels (Sossou-Agbo, 2014).
It is therefore necessary to work for the preservation and/or restoration of aquatic ecosystems. But a good conservation policy requires a good knowledge of the different aquatic ecosystems, their components and the factors that influence their functioning and their balance (Houndonougbo et al., 2013, Amoussou et al., 2017, Djidohokpin et al. 2017).
Among the rivers in the commune of Sô-Ava, the Sô River is the only one that has been the subject of very few studies (Kiossa 2012, Guillou 2013and Hamani 2015. Like the other rivers of the town, it is therefore appropriate to make investigations on this river to better understand its extent of degradation. The purpose of the study was therefore to characterize the ichthyological diversity and reproductive aspects of some dominant species of the Sô River.

Measurement of physico-chemical parameters
Physico-chemical measurements were taken in the mornings between 6 am and 11 am, once a month at each of the eleven (11) stations during the months of June, July, August, September and October of the year 2015. During these measurements, the weather was every time assessed (state of the sky). The physico-chemical parameters measured were air and water temperature, river depth, water clarity, dissolved oxygen, pH, conductivity, Total dissolved solids (TDS), and salinity of the water.

Fish collection and monitoring of fishing gear
Fish collection was done by station twice a month. Fishermen's catches were sorted by species, counted and weighed. In each case, a sample of the catches was taken for morphometric and reproductive parameters. The number of individuals collected was a function of the relative abundance of the catches.

Measurement of Morphometric and Reproductive Parameters
In the laboratory, the collected species were first identified. The order of the taxa was presented according to the CLOFFA standard (Check-List of the Freshwater Fishes of Africa (Lévêque et al., 1990and 1992, Paugy et al., 2003. Morphometric data were taken by species and individuals. These included total length (TL), standard length (SL), total weight (TW), stage of gonad maturation, and gonad weight.
The stages of sexual maturity (1 to 5) were determined after macroscopic observation of the fish ovaries or testis. The maturity scale used is that of Legendre and Ecoutin (1989) modified by Tidiani Koné, 1999 (Table 1).

Traitements de donnees et analyses statistiques Data processing and statistical analysis
The physico-chemical parameters were calculated by the Proc means procedure of SAS (Statistical Analysis System, 2013). The correlation between the physico-chemical parameters was calculated by the SAS Proc corr procedure (2013). The General Linear Model (GLM) procedure was used to test the station effect on the physicochemical parameters and the means were compared using paired student's t-test.
The weight-length relation was established according to the classical formula of Le Cren (1951).
where TW and TL represent total body weight in grams and total length in centimeters, respectively.
The factors a and b are allometric coefficients. The factor b is the relative growth coefficient of the weight at height. The coefficient b is close to 3 and varies with genetic and physiological factors. For b = 3, the growth is said to be isometric and allometric for b different from 3. A value of b greater than 3 indicates a better growth in weight than in length whereas a value of b less than 3 indicates that the fish grows more in length than in weight. The Student's ttest was performed to compare the values of the coefficient b with the value 3. The factor a is a coefficient related to the ecological factors and varies according to the time of the catch, the health status of the fish and the degree of development of the gonads.

Physico-chemical parameters
The means of the physicochemical parameters are presented in Table 1. The variation of the temperatures as a function of the stations was not observed (P> 0.05). The average transparency obtained at the Kinto-oudjra station arm 2 was significantly higher than that observed at the Ahomey-glon, Ahomey-lokpo, Ahomey-ounmey, Sindomey, Vekky and Dogodo (P <0.05). The transparency of the water at Ahomey-ounmey was also significantly higher than those obtained at Kinto oudjra (P <0.05). The water depth values at Dogodo were significantly higher than those of all other stations (P <0.05). The Kinto-oudjra depth values were very significantly higher than those obtained at Ahomey-glon, Ahomey lokpo, Ahomey-ounmey, Sindomey, Vekky, Kinto-oudjra arms 1 and 2 (P <0.05). The pH of the Ahomey-glon water was significantly lower than the pH at Sindomey and Vekky (P <0.05). As for the dissolved oxygen and the saturation rate, no significant difference was recorded between the stations (P> 0.05). On the other hand, the salinity values obtained at Dogodo were significantly higher than those obtained at Kinto-oudjras arms 1 and 2, at Zoungomey and at Ahomey-lokpo (P <0.05). Similarly, the salinity values obtained at Vekky were significantly higher than those obtained at Ahomey-lokpo (P <0.05). The mean value of TDS at Kinto-oudjra arms 1 and 2 was significantly lower than the values obtained at Dogodo and Vekky (P <0.05). Table 2 presents the list of species found in the Sô River. During this study, 29 species of fish belonging to 16 families and 21 genera were identified. The diversity of species by station varied from 5 in Sindomey to 21 in Zoungo.

Growth of dominant species
The analysis of the growth coefficients of the abundant species revealed that there was no significant difference between the "b" values obtained for the four species and the value "3" ( Table 3). The dominant species showed then isometric growth in the Sô River. Table 4 shows that the condition factor varied from one species to another with higher values in Eleotris vittata and lower in Sarotherodon melanotheron. There was no difference between the K values of C. guineensis and S. melanotheron (p ˃ 0.05). In contrast, the condition coefficient of E. vittata was different from those of C. guineensis and S. melanotheron (P <0.05).

Sex ratio and stage of sexual maturity
The sex distribution of the dominant species identified in the catches showed an unequal distribution of the numbers by sex of each species ( Table 5). The distribution of the Sarotherodon melanotheron and Coptodon guineensis specimens obtained and processed by size class showed that the size of sexual maturity of these species was between 3 cm and 10 cm (Table 6 and 7).

Discussion:-
The average values obtained for the air and water temperatures (27.3 ± 1.89 ° C, 27.6 ± 1.87 ° C) are close to the average values obtained by Koudenoupko et al. (2017). Also, the water temperature recorded during the study period fits perfectly into the temperature range that provides ideal living conditions for fish species (Zinsou et al., 2016). Indeed, in tropical environments, particularly in southern Benin, the temperature of water that can be tolerated by most species is between 23 ° C and 31 ° C (Nounmon et al., 2015). The significant variation in ambient temperatures and water depending on the months could be explained by the effect of the seasons on temperature. The highest temperatures were obtained in June and October for air temperature and only in June for the temperature of the water. Thus, the high sunshine during the dry season (July, August) would favor the warming of the water and thus the rise of the temperatures whereas the abundance of rains in the rainy season (June, September, October) causes the fall of the temperatures. Temperature variations play a large role in egg development, sex determinism in fry and in adult reproduction (Zinsou et al., 2016). The superiority of the water temperature values of the Sô River to those of the ambient temperature was observed during the months of June, July and August, where the values obtained for conductivity and suspended solids were also very high. This situation could be related to the high levels of suspended solids which can be considered as a form of pollution responsible for a warming of the water (Makhoukh et al., 2011). This high water turbidity reduces habitat quality for fish and causes mineralization of water (Agblonon Houolome et al., 2017). The high temperature of the water during this period could therefore be related to the high content of water in suspended solids. In addition, suspended elements can also clog fish branch systems or cause irritation, and silt deposits significantly alter the quality of substrates in breeding areas (Zinsou et al., 2016). The high average value of the obtained conductivity (2055.5 ± 3047.32 μs / cm) explains the high content of suspended solids and reveals the anthropogenic pollution. In fact, the drainage of runoff water during the rains in the river and the leaching of plant protection products and chemical fertilizers used for crops in the upstream zones of the ecosystem contribute to an important part to the enrichment of the water body. Agblonon Houelome et al. (2016) emphasized in particular that the position of fields in relation to rivers causes, by the leaching phenomenon, the loading of water by agricultural discharges that sediment in rivers. . The oxygen saturation observed in the Sô River (54.5 ± 22.99%) in this study differs from the 13.9 ± 5.4% recorded by Kiossa (2012) on the same river during the same season period. The average value of transparency obtained in this study was overall low. This situation could be explained by the absence of flooding, which favors dilution during the study period. The high transparency measured at the Kinto-oudjra station arm 2 is related to its position in a non-urbanized area protected from any human activity likely to influence the physicochemical parameters of the water and the embankmnts are well covered by abundant vegetation. In addition, it is located on the arm of the river leading to Adjohoun and in direct connection with the Ouémé River. The average depth of water obtained (3.7 ± 1.24 m) is close to 3.6 ± 0.9 m obtained by Kiossa (2012). This is certainly due to the 1413 exploitation of sand at the bottom of the Sô River by local residents for construction and for commercialization because a variation in the depth according to the stations was observed (Guillou, 2013).
The results of the inventory (29 species belonging to 16 families and 21 genera) are similar to those of Kiossa (2012) which identified 38 species grouped in 19 families and that of Guillou (2013) which identified 38 species of fish belonging to 23 families. The difference between the number of species identified during our work and those of Kiossa (2012) and Guillou (2013) can be explained by the absence of flood during the sampling period. The abundance of Sarotherodon melanotheron and Coptodon guineensis and their distribution at all stations can be explained by the ability of these Cichlids to live in environments of variable salinity, their mode of reproduction and feeding (Aholoukpe, 2017) Djidohokpin et al. (2017) in the river Tovè. The difference between the results obtained in this study and those results can be explained by the fact that S. melanotheron has during the month of June or July (rainy season), the environmental conditions favorable to its development. In addition, the relatively high values of K for the other species could be in the same trend. The S. melanotheron, C. guineensis, E. vittata and B. niger species were able to find suitable conditions in the Sô River during June, July and August.
The Gonado-Somatic indices (GSIs) obtained in this study are consistent with the values of tropical fish GSIs. The low values obtained for S. melanotheron versus C. guineensis can be explained by the nature of the oral incubator of S. melanotheron. This situation is confirmed by the study of Kidé et al. (2015). They reported in their work on the reproduction of Sarotherodon melanotheronheudelotii that the male is responsible for the incubation of the eggs and larvae that lasts 22 to 25 days and that during these periods, the males stop feeding and become inactive. The studies of Keremah and Ndah (2013), on aspects of the reproductive biology of Coptodon guineensis in the laboratory confirmed that this species is a substrate spawner, that incubation of eggs lasted only 2 days and that parental care was performed by both parents.
The size class of sexual maturity obtained in this work (3 to 10 cm) for Sarotherodon melanotheron and Coptodon guineensis is lower than the size of first maturity obtained by Hazounme et al. (2017) for Sarotherodon melanotheron. Sanogo et al. (2012) reported that in male subjects the smallest emission size of soft roe is 7.4 cm against 13.2 cm in females for oocytes. In addition, these authors reported that the comparison between experimental and artisanal catches revealed a notable difference. Male and female individuals have the respective sizes of first sexual maturity of 13.6 cm and 14.6 cm for catches obtained in artisanal fisheries. In experimental fishing, the 90 mm male individuals were already at first sexual maturity. It can be deduced that Sarotherodon melanotheron and Coptodon guineensis are confronted with a phenomenon of dwarfism, certainly due to the fishing pressure to which they are subjected on the Sô River.

Conclusion:-
The species inventory identified 29 species grouped into 16 families and 21 genera. The dominant species in the ichthyofauna stand were Coptodon guineensis and Sarotherodon melanotheron and incidentally Brevimyrus niger and Eleotris vittata. The study of the condition factors of these species and the length-weight relationships showed that these species have been able to adapt to the ecological conditions of the watercourse and the anthropogenic 1414 pressures on the ecosystem. Evaluation of the gonado-somatic indices of Coptodon guineensis and Sarotherodon melanotheron revealed that the reproductive physiology of these species has allowed them to grow in the river.

Declarations: -Conflict of interest: The authors declare no conflict of interest
Ethical approval: This Manuscript does not report studies involving human participants, human data or human tissue. The use of fish and sample collection procedures were in accordance with the ethical standards in Benin.