Ichthyofauna of Okpara Stream, a Tributary of Oueme River, Benin, West-Africa

In tropical Africa, and particularly in Benin, running waters dwell a high and valuable fish fauna that are almost unknown, but under severe degradation. The current study inventoried the fish biodiversity of the Okpara stream, the main tributary of the Oueme River (the largest in Benin), in order to fill the gap of ichthyological data and to improve habitat protection, species conservation and valorization. During 18 months, fish samplings were made monthly with various fishing gears on five (5) locations. A total of 53 fish species belonging to 30 genera, 14 families and 7 orders were recorded among a total of 9552 individual fish collected. The most representative families were Mormyridae with 9 species and Cichlidae with 8 species, then follow Mockokidae, Clariidae, Cyprinidae and Alestidae with 6, 6, 6, 4 species, respectively. Families such as Anabantidae, Bagridae, Claroteidae, Malapteruridae, Polypteridae and Schilbeidae were less represented and individually included 2 species. The Hepsetidae and Aplocheilidae were represented by one (1) species, respectively the African pike, Hepsetus odoe and Epiplatys bifasciatus. A holistic scheme of ecosystem management and species conservation requires a complete knowledge on the fish community structure, eco-toxicology and species ecological status.


Introduction
Notwithstanding the economic and commercial importance of the fish resources in Africa, and particularly in the Sub-Saharan Africa, the fish biodiversity remains poorly known in most inland waters (rivers, streams, lakes, lagoons, floodplains etc.), and even in marine coastal systems (Aboua et al., 2015;Sanogo et al., 2015). Meanwhile, these aquatic habitats are being seriously degraded by multiple uses that cause profound modifications of the environmental quality leading to habitat fragmentations and an increase of endangered and threatened species. As reported by the International Union for Conservation of Nature (IUCN, 2010), about 21% of known African fish species are threatened.
In West Africa, and particularly in Benin, fishery is among the foremost ancestral activities that provides sustainable and substantial financial resources to grassroots (Adite, 1995;Laleye, 2003;Gbaguidi et al., 2016). Annual fish production was estimated at about 40 metric tons (Direction des Pêches, 1999) from which about 70% accounted for the Oueme River (the longest river in Benin) and its associated lakes, floodplains and tributaries. Among the tributaries of the Oueme River, the Okpara stream is the biggest one that provides an important commercial fish resources for the Northern Region of Benin. Despite the fisheries importance in the Okpara stream and the multiple cases of habitat degradations occurring in this geographic region, little is known about the fish biodiversity. Indeed, since a half of century (year 1969), the Okpara stream is subjected to a permanent water withdrawal by a Water Company (SONEB) that withdraws and treats the stream water to satisfy domestic needs (Zogo et al., 2008). Likewise, water withdrawal for irrigations, the use of chemical fertilizers and pesticides for agriculture, the overfishing caused by the utilization of devastating fishing gears were the major threats recorded on this stream. Currently, the introduction of the invasive non-native fish species, the Nile Tilapia, Oreochromis niloticus, and the invasion of a floating plant, the water hyacinth (Echhornia crassipes), constitute an ecological disaster that seriously affect the environmental quality of the stream and leading to habitat fragmentations, species replacement with an increase of the number of threatened fish species (Gourene et al., 1999;Laleye et al., 2004;Adite et al., 2013). As reported by MEPN (2009), more than 43 fish species are currently threatened in Benin. Consequently, habitat restoration and protection, and species conservation and valorization require a perfect knowledge of the fish diversity and their status in order warrant a sustainable exploitation.
The current study aimed to assess the fish biodiversity of the Okpara stream, and data obtained from this survey will serve as documentation to contribute to habitat protection, species conservation and valorization and an ecological follow-up of this stream.

Materiel and Methods Study area
The study area is the Okpara stream, the longest tributary of the Oueme River. This stream belongs to the northern hydrographic system and goes through two biggest regions of Benin, Borgou and Zou provinces. The Okpara stream is located between 8°14'-9°45' North and 2°35'-3°25' East and extended on about 200 Km. The Oueme River originated from the Southwest of Nikki city at an altitude of 450 m (Laleye et al., 2004). The climate is tropical with three main seasons: the dry season (November -April), the wet season (May -August) and the flood period (September -October). The annual average temperature is 26.6 °C and lower temperatures were recorded in December and January. The annual mean rainfall is about 1200 mm with a peak (1300 mm) recorded in July, August or September (Kora, 2006;INSAE, 2004). Most of the soils are ferruginous and alluvial (from deposits of sediments left by the stream) (Dossou-Yovo, 2009; Ogouwale, 2013). In general, the soil is covered by a wooded savanna characterized by the presence of Parkia biglobosa, Khaya senegalensis and Vitellaria paradoxa. Also, the vegetation included marshy meadows, bamboo and fallow bushes. Multi-species and commercial fisheries occur in the Okpara stream that appears to be the main source of fish resources for the surrounding populations and even for most cities of the northern region of Benin. The Okpara stream provides water for irrigated agriculture and supply the surrounding populations with drinking water from a dam built by the Benin water company, the SONEB.

Sampling sites
For this study, five (05) sampling locations were selected ( Figure 1). These sites were chosen according to localities, accessibility, fisheries importance and levels of sites degradation.

Evaluation of water characteristics
The quality of the different habitats (aquatic vegetation, open water) was assessed in situ at each sampling site. The depth was measured to the nearest 1 cm using a graduated rope attached to a water sampler. The temperature and the dissolved oxygen were measured respectively to the nearest 0.1 °C and 0.1 mg.l -1 using a digital multi-probe (HANNA model 9150 waterproof). pHs were measured to the nearest 0.1 using a pH meter "model 3150 waterproof". Turbidities were measured to the nearest 1 cm using a Secchi disc.

Fish collection
Fish samplings were done once a month for eighteen (18)  In addition, fish samplings were directly made in the fishermen artisanal captures. Thus, one third of each fisherman catches was sampled, but including all uncommon or rare species (Okpeicha, 2011). Fishing gears such as gillnets, seines, cast nets, hooks, and traps were used by the fishermen to collect the fishes. After collection, the fish samples were first identified in situ using fish identification references such as  1996).

Water Characteristics
In Okpara stream, temperatures were relatively moderate and ranged between 25 to 30.1 o C, the depths varied from 17 to 1080 cm and transparencies were low and ranged between 10 and 78.1 cm. The dissolved oxygen concentrations ranged between 0.44 and 5.66 mg/l and the percentage of saturation were moderate to low and varied from 6.37 to 75.3%. The water was acid or alkaline with pHs ranging between 6.4 to 8.1. As results, and based on the physicochemical features recorded, the Okpara stream display a relatively adequate water quality that is suitable for a high primary production and the prominence of the fish fauna.

Fish species composition
During our study, a total of 9552 fish individual were sampled. Overall, fifty three (53) fish species belonging to 30 genera and 14 families (  (8) families gathered not more than 15 species. Also, with regards to seasons, the dry period (November -April), and the wet season (May -August) exhibited the highest species richness with 46 and 40 species recorded, respectively. During the flood period (September -October), the water volume and pulse was very high and dynamic reducing fish concentration and catch per unit of effort (CPUE) ( Table 3).    (Hazoume, 2017) and absent in the Okpara stream from this current assessment is due to the connection of the Sô with Lake Nokoué, a brackish water from which these estuarine and marine species migrate. Likewise, a less species richness (48 fish species) was recorded by Moritz (2010) in the Iguidi running water, a Benin South-East small forest stream, also connected to the Oueme River. Spatially, the study showed high species richness at Okpara up stream while the downstream exhibited a relatively less fish fauna. The great variability of the ichtyofauna from site to site probably indicated differential degradation levels with regards to space. Indeed, Kpassa village (Site 3), a collecting site under severe degradation due to the construction of a dam, showed a relatively reduced fish fauna (32 species) compared to Gadela village (Site 2) that harbored a high number of fish species (41 species). Probably, the water retrieval may cause habitat losses and fragmentations leading to a modification of the fish community structure with an increase of endangered and threatened fish species.
With regards to season, the dry and wet periods concentrated a high number of fishes, 46 and 40 species, respectively. The withdrawal of water by the water company (SONEB) during the dry season could cause fish concentrations, making them more available and more vulnerable to fishing gears. In contrast, the flood season, with its huge water volume and dynamic hydrological regime, reduced the vulnerability of fishes to fishing gears and thus displayed a poor species number (21 species). Moreover, local migrations of fishes for spawning during this flooding period could lessen access to some fish species. In addition, the permanent use of chemical fertilizers and pesticides for adjacent agriculture pollutes and degrades the Okpara stream water quality and the nutritional values of the fishes through deposits of heavy metals in the fish flesh. Indeed, from the Northern Benin agriculture, a huge quantity of fertilizers and pesticides are drained in the stream by runoff and contaminates the fishes. Consequently, a study on the eco-toxicology of Okpara stream is required to assess the contaminations and overall quality of exploited, commercialized and ingested fishes in order to guaranty the health of grassroots and consumers of the Okpara stream fish resources.

Conclusion
The current ichthyological exploration on the Okpara stream indicated that this Oueme River tributary exhibited a high fish species richness (53 species) compared to other associated streams. However, the overfishing, the use of detrimental fishing gears, the use of chemical fertilizers and pesticides and the presence of a dam constructed by the Benin water company (SONEB), constitute some major threats for the Okpara stream fish biodiversity and even for the whole Oueme River. A study on the fish community structure and on the eco-toxicology of this running water is required to better assess dynamics, ecological status and contaminations of the Okpara stream ichthyofauna and to contribute to species management and sustainable exploitation.