Reproductive Strategy of Chrysichthys nigrodigitatus (Lacepede, 1803) in a Natural Environment in the Nkam River, Littoral Cameroon

Laboratory of Applied Ichthyology and Hydrobiology (LAIH), School of Wood, Water and Natural Resources (SWWNR), Faculty of Agronomy and Agricultural Sciences (FAAS), !e University of Dschang, P.O. Box 786, Ebolowa Antenna, Dschang, Cameroon Laboratory of Applied Ichthyology and Hydrobiology (LAIH), Department of Animal Productions, Faculty of Agronomy and Agricultural Sciences (FAAS), !e University of Dschang, P.O. Box 222, Dschang, Cameroon Laboratory for Aquaculture and Demography of Fisheries Resources (LADFR), Departement of Aquaculture, Institute of Fisheries and Aquatic Sciences of Yabassi (IFAS), !e University of Douala, P.O. Box 2701, Douala, Cameroon


Introduction
Fish is an extremely nutritious food, a vital source of protein and essential nutrients, especially for many low-income people around the world [1]. According to FAO [2], world fish supply reached a record of 20 kg per capita, in favour of strong growth in aquaculture, which now provides half of the fish for human consumption. is growth is correlated with the progressive enhancement of native species in certain regions of the world [2].
In Africa, and particularly in Cameroon, as reported by Cacot [3], fish farming has remained focusing on three species of fish: Clarias gariepinus, Oreochromis niloticus, and Cyprinus carpio. At the present state of knowledge, very few of these farmed species have reached a large-scale development potential. is situation would certainly be at the origin of the intensification of research towards new species better adapted to the constraints of African aquaculture. is is particularly the case for studies on some African carp and African catfish: Labeo senegalensis by Montchowi et al. [4], Labeo parvus by Montchowui et al. [5] and Montchowui et al. [6] and Chrysichthys auratus [7,8] in Benin and Egypt, Labeo coubie in Nigéria by Ikpi and Okey, and Labeobarbus batesii and Clarias jaensis in the Mbô Floodplain in West Cameroon by Tiogué et al. [10], Tiogué [11], Tiogue [12], Tomedi et al. [13], Zango et al. [14]. e ichthyological fauna of Cameroon's freshwater is of exceptional interest. Nearly 60 species of fish have recently been identified by Ajonina and Tomedi [15] in the Nkam River, amongst which the most abundant and much appreciated species was Chrysichthys nigrodigitatus, locally called "mayèpè." is fish species is the subject of many traditional recipes in households and restaurants, according to Ajonina and Tomedi [15], the consequence of which is the decrease of wild stocks. erefore, studies on the biology of this species is essential to better develop its breeding captivated. Aspects of the biology of C. nigrodigitatus have already been the subject of several studies in West Africa, more particularly in Ivory Coast, where captive breeding trials have been successfully tested by Djéhi [16]. In Cameroon, there is very little information on its reproductive cycle. Hence, the general objective of this study was to evaluate some elements of the reproductive cycle of C. nigrodigitatus in view of its domestication, and more specifically, it was to determine in this species the breeding period, sex ratio, and fecundity of this species depending on the endogenous and exogenous factors of the Nkam River.

Study Zone.
e study was conducted from October 2015 to August 2016, in the Nkam River in Yabassi Township, District of Yabassi, Department of Nkam and Littoral Region of Cameroon ( Figure 1). It is located between 9°50′ and 10°10′ of LN and between 4°20′ and 4°40 LE, with mean altitude of 15 to 20 m [17]. e climate is subequatorial with a tropical tendency of two seasons: a dry season that runs from November to March and a rainy season that runs from April to the end of October. e hottest month is January and the coldest is August. e mean annual rainfall is 2927 mm; the maximum rainfall is between July and August. Monthly temperatures range from 25°C to 35°C and are favourable for aquaculture [18]. e Nkam River results from the merger of two rivers: the "Small Nkam" (Department of Haut-Nkam) and the "Ngoung" (Department of Menoua). It flows from from Nkongsoung (Moungo Department), passing through Yabassi, into the Wouri River at Akwa North (Wouri Department) [19].

Animal Material.
A total of 154 specimens of Chrysichthys nigrodigitatus of mean total weight of 829.96 ± 829.58 g and mean total length of 367 ± 156 mm were collected from the local fishermen of Nkam River. Sampling was done monthly and fish samples were hand-harvested using 3-and 4-finger gillnets. After the first 6 months of collection, noting that this capture technique was selective, it was replaced by "bamboo China" structures installed in the various points of the selected station, to have a wide range of the size of the individuals.

Assay Conduct and Data Collection.
e collected fish were kept alive in 20-liter buckets and transported to the Laboratory for Aquaculture and Demography of Fisheries Resources (LADFR) of the Institute of Fisheries and Aquatic Sciences of Yabassi (IFAS), where they were identified according to the identification key of Stiassny et al. [20]. Each individual was then recorded and sexed with the naked eye. e total and standard lengths (tL and sL, respectively) were measured with a 1 mm ichthyometer. e total weight (tW) and the weight after evisceration (evW) were obtained using a Sartorius Competence electronic scale at the 10th of g. After dissection of each fish using the scissors of a dissection kit, the gonads (Figure 2(a)) and the liver were removed with a razor blade, drained, and weighed at 0.01 g on a sensitive electronic scale brand TATINA model 1479V (Figure 3(a)). Based on the sexual maturity scale of the ovaries of C. nigrodigitatus established by Otémé [21], only the ovaries of 29 females at stage V of sexual maturity were selected for the estimation of the fecundity: three samples of 1 g of ovary were collected in the rostral, middle, and caudal regions (Figure 2(b)) and weighed. One gram of ovary was put in Gilson's liquid (pure ethanol 98°C (60 ml) + water (920 ml) + acetic acid (9 ml) + mercury chloride (20 g)) to dissociate the oocytes and thus facilitate counting (Figure 3). e oocytes of each sample were counted under a stereoscopic binocular magnifying glass (magnification ×10).

Collection of Physical Parameters of Water.
One station representing the fishing location was selected in the upstream part of the Nkam River. Due to a lack of chemical parameter collection equipment, only the following physical parameters were determined: the transparency, the depth, the temperature, and the rainfall were measured monthly, respectively, using a Secchi disk to within 1 cm, a gallows graduated to the nearest mm, a mercury thermometer, and a rain gauge.

Breeding Period.
e breeding period was determined by calculating the following indices as used by Saâdia [22]: × 100, where gW � gonads weight and evW � eviscerated weight of fish (ii) Hepatosomatic index (HSI): HSI (%) � (lW/evW) × 100, where lW � liver weight and evW � eviscerated weight of fish (iii) K factor has been calculated using the formula of Ricker [23]: K � (tW/tL 3 ) × 100, where tW and tL are, respectively, the total weight and the total length of the fish

Sex Ratio (S/R).
e sex ratio according to Ragheb [8] is translated by the following relation: S/R � number of males/number of females.

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International Journal of Zoology 2.5.3. Fecundity. Fecundity was evaluated according to Ragheb [8]: (i) Absolute fecundity (aF) was determined on a sample of 29 females at stage V of sexual maturity. It is defined by the formula: af � total weight of oocytes × number of oocytes in 1 g of ovary (ii) Relative fecundity (rF) was calculated from the aF. It is translated by: rF � total number of oocytes/weight of the female (Kg) 2.6. Statistical Analysis. Descriptive statistics, the χ2 comparison test, the Pearson correlation test, and the one-way ANOVA test were used to process the data for the parameters studied at 5% level of probability. is was done using the SPSS16.0 statistical software. Figure 4 shows the monthly evolution of the gonadosomatic index  International Journal of Zoology (GSI) in C. nigrodigitatus by sex. It seems that regardless of the sex considered, the GSI has evolved in the same way with values significantly (P < 0.05) high from April, and the peak is observed in June. Figure 5 portrays the monthly evolution of the hepatosomatic index (HSI) in C. nigrodigitatus by sex. In females, three (3) Figure 6 illustrates the monthly variation of condition factor K in C. nigrodigitatus by sex in the Nkam River. It shows that the mean K factor of C. nigrodigitatus recorded between October 2015 and August 2016 in the Nkam River was 1.09 ± 0.07%. Except for the month of December, there was no significant difference between the sexes and the months (P > 0.05). However, between May and July, the K factor remained slightly below 1 (0.97 ± 0.01) in females.

Correlation between Reproduction and Physical
Parameters. Correlations between reproduction parameters (GSI, his, and condition factor K) and environmental parameters as presented in Table 1 and Figure 7 revealed a positive and significant correlation between rainfall and gonadosomatic index (r � 0.74, P < 0.01) on the one hand and a negative and significant correlation between this same parameter and the condition factor K (r � − 0.73, P < 0.05) on the other hand. Negative but insignificant correlations were observed between GSI, transparency, and temperature. Only the precipitation correlated positively with HSI, but not significantly (P > 0.05).

Monthly Variation of Sex
Ratio. e monthly variation of sex ratio in C. nigrodigitatus (Table 2) indicates that the mean sex ratio during the study was in favour of females (1 : 1.5). is sex ratio value was statistically not significantly (P < 0.05) different from the theoretical sex ratio (1 : 1). Sex ratio in favour of males was observed only in June. e standard sex ratio (1 : 1) was recorded only in May and December. However, except for November, a sex ratio (1 : 4) significantly (P < 0.05) different from the theoretical sex ratio (1: 1) was recorded.

Fecundity.
e absolute fecundity was 1374 ± 1022 oocytes for females with a mean weight of 107.3 ± 111.9 g and a mean total length of 211 ± 52.8 mm and the relative fecundity of 14 ± 3 oocytes/kg for females.  Figure 8 shows the relationship between absolute fecundity and total weight and between this same parameter and the total length of C. nigrodigitatus females. It shows that a very strong and significantly positive correlation exists between absolute fecundity and total weight on the one hand (r � 0.97, P < 0.01) (Figure 8(a)) and between this same parameter and the total length (r � 0.98, P < 0.01) on the other hand (Figure 8(b)).

Breeding Period.
e breeding season of C. nigrodigitatus coincides with the continuous rainy season, which lasts approximately six months and is characterized by a single spawning season in June as reported elsewhere [11,12,24,25].       International Journal of Zoology e reproduction of C. nigrodigitatus is of the asynchronous type as reported elsewhere [11,12], thus making this species a partial or heterochronous breeder, or fish with multiple spawning or split-spawning. e peak breeding period observed between April and June is similar to that recorded by Ekamen [26] and Offem et al. [27] in the Cross River in Nigeria in the same species and by Dia [28] and Dossou [7], respectively, in Ivory Coast and in Benin. ese results, however, are different from those obtained by Otémé [29] and Djéhi [16], who under breeding conditions observed maximum laying in September and October; and those reported by Ragheb [8] which show that the spawning season of C. auratus extends from October to June. According to Dia [28], laying occurred between May and July and lasted until November when temperatures had varied between 25°C and 27°C. Hem [30] and Albaret [31] also observed C. nigrodigitatus spawning during the rainy season, adding that temperature is a major factor in this phenomenon, associated with salinity and floods. e geographical position, the duration of the rainy season, and the specific characteristics of the water courses may vary the extent and the period of reproduction in C. nigrodigitatus as recorded by Ottémé [29]. A positive correlation has been recorded between the gonadosomatic and the hepatosomatic indices, whereas logically these two weight indices vary in opposite directions. Indeed, the fish would draw the reserves of the liver (in particular, the vitellogenin in the females) to manufacture the sexual products as discussed elsewhere [11,12,32]. It should probably be due to the fact that some females emit their eggs in a split way; and the partially emptied ovaries do not return to complete rest but continue the vitellogenesis of oocytes that will be emitted later. ese results are similar to those of Bouaziz et al. [33] in the Merlu, Merluccius rnerluccíu Linnaeus, 1758. e K condition factor of C. nigrodigitatus obtained in this study was higher compared to that obtained by Ezenwa et al. [34] and Dossou [7], respectively, recorded in the Imo River in Nigeria (0.78) and small water body of the Ouémé basin in Benin (0.64), showing that the samples of the Nkam River were of good overweight. ese results are close to 0.96, 0.97, and 0.99, respectively, reported in the Bandagry Lagoon in Nigeria by Ezenwa et al. [34] and the river cross in Nigeria by Ekamen [26] and Offem et al. [27] in the same species. As a result, the Nkam River may have very favourable natural conditions for the development of C.
nigrodigitatus. However, the slight drop in K-factor values (0.94 ± 0.06) noted between May and August may be related to female oviposition weight loss. Indeed, according to Kraïem [35], the fall in values of the relative condition coefficient (Kr < 1) indicates the postspawning period qualified by energy expenditure and weight loss.

Sex Ratio.
e sex ratio of 1M: 1.5F observed in this study is statistically close to the theoretical sex ratio (1 : 1). However, it is biologically in favour of females, thus giving this species a reproductive strategy of "r" type, thus a polygamous matrimonial regime. ese results are similar to the observations made on C. auratus (1M: 2F) and on C. nigrodigitauts (1M: 4F) by Imerbore and Bakare [36] and on C. auratus (1M: 1.18F) by Ragheb [23] in Damietta branch of the River Nile in Egypt. ese observations are contrary to those reported in Lake Asejire by Taiwo and Aransiola [37] and Dossou [7] in C. auratus (1M: 0.96F) and C. nigrodigitatus (1M: 0.93F), respectively. In addition to the spatial and temporal difference, the sex ratio diversification could also be due to the fishing techniques practiced: Indeed, during this study, between May and August, fishermen have replaced the usual gill nets with bamboo from China placed about 3 m on the banks. is fishing gear allows the capture of C. nigrodigitatus by couple (sex ratio of 1M: 1F). Dossou [7] also reported the use of this technique in Benin. However, the sex ratio obtained during the breeding season is close to the theoretical sex ratio (1: 1) just like that observed elsewhere [7,21,27]. us, it seems that the natural approximation of males and females is in relation with the reproduction period.

Fecundity.
e absolute fecundity recorded in this study is very low compared to the 24000 oocytes observed in Kousson on Bandama in Ivoiry Coast by Kouassi [38]. It is also lower than that of 4878 to 87724 oocytes recorded by Otemé [21] in Ebrié Lagoon in individuals weighing from 500 g to 6000 g with an average of 20000 oocytes. However, it is close to 7596 oocytes reported in the South Benin Lagoon Complex by Lalèyè et al. [39]. In the Cross River in Nigeria, Ekamen [26] and Offem et al. [27], respectively, reported a fecundity of 12063 and 25905 oocytes. More recently, in the small water plans of the Ouémé basin in Benin, Dossou [7]  recorded fecundity from 3474 to 28778 with an average of 10783 oocytes in individuals weighing 557.3 g to 2270.1 g. However, these results are nevertheless comparable to those of Fagade and Adébisi [40], which counted 2884 oocytes in Lac Asejire populations. In addition to the fact that fecundity varies from one region to another and from one watercourse to another, it would certainly be linked to environmental factors. It could be strongly related to the weight and length of individuals. According to Otémé [29], the length of C. nigrodigitatus allows a better estimate of its fecundity. Offem et al. [27] added that this fecundity increases with the length and weight of C. nigrodigitauts.
Unlike absolute fecundity, relative fecundity varies very little from one region to another. e absolute fecundity obtained in this study is comparable to that reported by other authors elsewhere: Dossou [7] reported 15 ± 6 oocytes/ g, Otémé [29] 15 oocytes/g, Ekamen [26] 13 oocytes/g, Offem et al. [27] 13 oocytes/g, and Hem [30] 14 oocytes/g in the same species. However, the slight spatial and seasonal variations in the fecundity of C. nigrodigitatus are not to be neglected. According to Offem et al. [27], this could be related to the availability of the food. Gorenka et al. [41] add the duration of the spawning season and accumulated reserves. According to Offem et al. [27], an overweight fish is more likely to produce large numbers of oocytes compared to one with poor conditions.

Conclusion
C. nigrodigitatus breeds between April and October in the middle of the rainy season. Reproduction in C. nigrodigitatus is of the multiple-spike asynchronous type. Gonadosomatic index (GSI) is higher in females throughout the year, and the peak is observed in June in both sexes. e hepatosomatic index (HSI) was higher in females than in males throughout the year. It correlated positively with GSI in females during spawning. Condition factor K was ˃1 indicating that the Nkam River is a favourable environment for the good development of C. nigrodigitatus. Condition factor K correlated negatively with the increase in GSI during the reproduction period of C. nigrodigitatus. e gonadosomatic and hepatosomatic indices were positively correlated with precipitation, in contrast to the K factor, which was strongly and negatively correlated with this physical parameter of the environment. e sex ratio was in favour of females for most of the year and is close to the theoretical sex ratio in December, May, and June, reflecting an "r" type reproductive strategy.
Absolute and relative fecundity increases with weight and length of C. nigrodigitatus, but these parameters seem to be directly influenced by certain environmental factors (food, temperature, salinity, etc.).
In view of these results, C. nigrodigitatus is a potential candidate for aquaculture in Cameroon.

Data Availability
All raw and analyzed data of this work are available from the corresponding author (claudine.tiogue@univ-dschang.org/ Claudinetekou@gmail.com) upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.