Length-weight models and condition factors of fishes from Okpara Stream, Oueme River, Northern-Benin

Length frequency distributions, length-weight models and condition factors of 21 dominant fish species o f the Okpara stream (Oueme River) were examined in Northern-Benin in order to evaluate the well-being of these fish taxa. Samplings were made monthly from December 2015 to May 2017 with seines, gill nets hawks and a total of 9,302 individuals were collected. Fish abundance ranged from 53 individuals for Mormyrus rume to 2,818 for Hemichromis fasciatus. Standard length (SL) varied from 1.2 cm (Coptodon guineensis) to 51.8 cm (Chrysichthys nigrodigitatus). Length-weight regressions equations showed significant (p < 0.05) correlation coefficients (r) ranging between 0.4664 and 0.9949 with slopes (b) between 2.2262 and 3.7703 corresponding to isometric, positive allometric and negative allometric growths displayed by 4, 8 and 9 species, respectively. Condition factors K varied between 0.17 29.38 and species with higher well-being were Oreochromis niloticus with K=29.38, Hyperopisius bebe (K=20.14), Coptodon zillii (K=19.25), Mormyrus rume (K=17.13) and Sarotherodon galilaeus multifasciatus (K=16.16). A sustainable exploitation of these fishes requires an ecosystem restoration scheme including habitat protection, species conservation and an ecological follow-up of the Okpara stream. Keywords— Allometry, condition factor, ecosystem restoration, Northern-Benin, Oueme River.


INTRODUCTION
In tropical Africa, running waters such as rivers and streams dwell a huge ichthyodiversity that is intensively exploited and constitutes an important component of the artisanal fisheries (Adite et al., 2005). However, because of habitat disturbances in most African aquatic ecosystems , the growth factors of most fish species were reduced with depletion of the fish production FAO, 2018). In Northern Bénin, since 1969, the Okpara stream, a tributary of the Oueme River, is under various degradation pressures such as the permanent withdrawal of water by SONEB, a Company that exploits the water to satisfy domestic needs (Zogo et al., 2008;Sidi Imorou et al., 2019a, b). Also, the use of chemical fertilizers and pesticides for agriculture, the overfishing, the water retrieval for irrigations, the invasion of Echhornia crassipes, a floating plant and the introduction of Oreochromis niloticus, an invasive non-native fish, were some other major threats recorded on the Okpara stream.
Notwithstanding the fisheries importance of the Okpara stream in Northern-Benin, nothing is known about the growth trends of the fish fauna of this running water. Size frequency distribution, condition factors and length-weight models evaluate not only the well-being of the fishes, but also give insight on the productivity level and the "ecological health" of the ecosystems (Adeyemi, 2010;Abowei, 2010;Bolarinwa, 2016). Indeed, as reported by Adite et al. (2017), water quality alterations and intrusions of contaminants in the water body will negatively affect the growth and condition of fishes. Successful fisheries management require knowledge on length-weight model and condition factors that reflect the quality of the environment (Le Cren, 1951;Pauly, 1993;Ecoutin and Albaret 2003;Fiogbe et al., 2003;Abowei, 2010; Ayandiran and Fawole, 2014).
The current study seeks to document length-weight patterns and condition factors of 21 dominant fish species of the Okpara stream of the Oueme River in Northern Benin in  (Kora, 2006). The Okpara stream appeared to be the main source of revenues for the surrounding populations. Indeed, intense commercial and subsistence multi-species fisheries occurred on this stream and were practiced by grassroots and migrant fishermen. The stream also provides water for irrigated agriculture. In addition, a dam was constructed on the stream to supply the surrounding populations with drinking water (Zogo et al., 2008; Sidi Imorou et al., 2019a, b).

Sampling sites
For this study, five (05) sampling stations were selected (Sid i Imorou et al., 2019a, b). Station 1 is located upstream in Perere Township. Station 2 is also situated at Okpara upstream in Gadela village at Parakou Township, about 2 km from SONEB dam (Sidi Imorou et al., 2019a, b). Station 3 is located at Kpassa village, around SONEB dam that serves as a source of drinking water for the surrounding populations. Station 4 is located around Okpara downstream in Yarimaro u village at Tchaourou Township. Site 5 is also located around Okpara downstream, but at Sui village of Tchaourou Township ( Fig. 1). At each fish collection station, two habitats, the aquatic vegetation and the open water were sampled.

Fish collection
Fish samplings were done once a month for eighteen (18) (Froese and Pauly, 2018). In the laboratory, each individual was measured for total length (TL) and standard length (SL) to the nearest 0.1 cm and weighted to the nearest 0.1g using respectively an ichthyometer and an electronic scale (Sidi Imorou et al., 2019 a, b). The specimens were then preserved in 10% formalin and latter in 70% ethanol (Murphy and Willis, 1996).

Data analysis
Total length (TL), standard length (SL) and weight (W) data of each fish individual were recorded in Excel 2017 spreadsheet along with sampling dates and sampling sites. The size structures (frequency histograms) of dominant fishes were constructed for the whole population. Le Cren's (1951) model was used to establish the length-weight relationship of fishes: Where, W is the total weight (g), TL the total length (cm), a, the intercept and b the allometry coefficient (Le Cren, 1951 In the current fisheries study, size distribution, length-weight models and condition factors has been performed for the first 21 dominant species of abundance more than fifty (50) individuals, belonging to 11 families , 15 genus, and totalizing about 9,305 specimens (Table1).
In particular, the growth pattern of some large and tolerant fishes such Sarotherodon galilaeus multifasciatus and Clarias gariepinus agreed with those reported by Tossavi (2011) in Lake Toho and by Adjibade (2013) in the Oueme River where these species exhibited an isometric growth and a positive allometric growth, respectively. Nevertheless, C. gariepinus exhibited a negative allometric growth in Ikpoba River in Nigeria (Omatsuli et al., 2017). Unlike the currrent findings, the large fishes, Oreochromis niloticus and Chrysichthys nigrodigitatus exhibited a negative allometric growth in the Mono River in Benin (Adite and Fiogbe, 2013) and in the Tai national Park of Ivory Coast (Kamelan et al., 2014) probably because of habitat disturbances . Also, unlike the isometric growth exhibited by the dominant species H. fasciatus in this study, Ecoutin and Albaret (2003), Kamelan et al. (2014) and Hazoume et al. (2017) reported positive allometric growth in the Ebrie Lagoon, in the Tai national Park and in the Sô River, respectively, indicating that this predator showed a relatively high well-being, probably because of suitable habitat conditions coupled with an efficient predation strategy. As repored by Bagenal (1978), Adite et al. (2017) and Lederoun et al. (2018), habitat conditions and stochasticity, food resource availabilities , unbalanced ecosystems and fish species tolerance to critical habitat factors could favor the differential growth pattern recorded.
Seasonally, because flooding brought substantial food resources to fishes, the slopes were higher and ranged between 2.225 (Ctenopoma petherici) and 3.578 (Hepsetus odoe) with twelve (12) species showing positive allometric growth against only three (3) species , Ctenopoma petherici, Brycinus macrolepidotus and Enteremius callipterus that exhibited significant (p<0.05) negative allometric growth. In contrast, the dry and wet seasons with relatively moderate food resources, comprised high percentage of species , 65% and 54%, respectively that exhibited negative allometric growth.
In this fisheries survey, condition factors (K) were moderate and ranged between 0.17 for Epiplatys bifasciatus and 29.38 for the invasive Nile tilapia, Oreochromis niloticus. In general the condition factors recorded in this study were higher than those reported by Kamelan et al. (2014) for Hemichromis fasciatus, Chrysichthys nigrodigitatus, Chromidotilapia guntheri, Brycinus macrolepidotus, Hepsetus odoe and Mormyrus rume in the Tai national Park of Ivory Coast. Likewise, values of K for Hepsetus odoe and Ctenopoma petherici in this study were higher than those reported by Adjibade (2013) in the traditional fishponds ("whedo") of the Oueme River. Also, the introduced invasive cichlid, Oreochromis niloticus displayed a higher well-being in the Okpara stream compared to individuals reported by Omatsuli et al. (2017) in Ikpoba River in Nigeria and those reported by Adite et al. (2017) in Lake Toho in the Southern Benin, probably because of differential habitat conditions . Nevertheless, in the Okpara stream, the common African catfish, C. gariepinus, showed lower K=14.30 compared to individual reported by Adjibade (2013) in the traditional fish ponds ("whedo") known as highly productive (Imorou Toko et al., 2013). According to Richter (2007) and Abowei (2009), combined factors such as seasonal variabilities, habitat conditions, food availability , ontogeny and sexual stage of maturation could act to affect the level of K and the well-being of the fishes (Le Cren, 1951; Pauly, 1993; Ayandiran and Fawole 2014).

V.
CONCLUSIONS Though under multiple degradation factors, some dominant fishes of the Okpara stream showed higher well-bein g indicated by an isometric growth and a positive allometric growth for more than half of the sub-community. However, the nine (9) less tolerant species that showed a negative allometric growth reflected the impacts of habitat disturbances and fragmentations. A sustainable exploitation of this fish fauna requires an ecosystem restoration plan including habitat protection, species conservation, an ecomanagement of the fishseries and a periodic bio-ecological follow-up of the Okpara stream.