1.0 Introduction
Shrimps and prawns are of considerable economic importance, contributing significantly to food security, foreign exchange earnings, and employment generation [1]. The presence of various prawn species in Nigeria’s fresh and brackish waters has been well documented [2]. Despite Nigeria’s capacity for aquaculture development, the country currently produces and exports these fisheries resources in relatively small quantities—approximately 12,000 tonnes annually [3]. Globally, the demand for shrimp and prawns continues to rise at an estimated rate of about three percent per year, driven primarily by increased consumption in the United States, Europe, and Japan [4]. In a related study conducted in the Lower River Benue, Nigeria, [5] examined the reproductive biology of Atya gabonensis and Macrobrachium felicinum, focusing on fecundity, egg diameter, sex ratio, and gonadosomatic index (GSI). The findings provided valuable insights into their reproductive performance and potential for sustainable aquaculture development in Nigerian inland waters. In another study of controlled breeding by (6) investigated reproduction under laboratory conditions. The embryonic development monitoring indicated an incubation period of approximately 12 ± 1 days, with hatching occurring over an average duration of 21.00 ± 1.94 hours, producing larvae with a mean size of 2.30 ± 0.90 mm. According to [7], there is a pressing need for Nigeria to increase its shrimp and prawn production to meet the growing demand at both local and international markets. [8] emphasized that to prevent overexploitation of natural populations, the commercial culture of prawns represents the most sustainable alternative. At present, Macrobrachium rosenbergii remains the only freshwater prawn species widely utilized in commercial aquaculture, primarily because its biology, breeding behavior, and farming techniques are well established compared to other indigenous species.
The reproductive biology of freshwater shrimps and prawns holds essential implications for both ecological understanding and development of aquaculture.Atya gabonensis and Macrobrachium macrobrachion are two species of interest in West Africa due to their economic value, although both belong to the palaemonid group, they display distinct reproductive indices that reflect their ecological adaptations and potential for cultivation.The aim of this work is to determine some aspects of the reproductive indices of Atya gabonensis and Macrobrachium macrobrachion in Lower River Benue, Makurdi at Wadata.
2.0 Materials and Methods
2.1 Study Area
The study was carried out along the lower River Benue axis, specifically at the Wadata Market area in Makurdi, Benue State, Nigeria. The River Benue is one of the major inland water bodies in the country, supporting diverse aquatic fauna and serving as an important source of livelihood for local fishing communities.
2.2 Sample Collection
A total of one hundred and twenty-three (123) specimens of Macrobrachium macrobrachion and one hundred and ninety-five (195) specimens of Atya gabonensis were collected. The prawns were hand-picked from beneath rocks and crevices under the water with the assistance of local fishermen who performed free diving. Specimens of M. macrobrachion were also captured using unbaited, locally made non-return basket traps constructed from bamboo and other traditional materials. All samples were transported live to the Fisheries Laboratory of the University of Agriculture, Makurdi, in ice boxes containing aerated water to minimize stress and mortality. Species identification was conducted to the taxonomic level using standard identification keys provided by [9] and [10].
2.3 Sex Determination
Sex determination was carried out based on distinct morphological characteristics unique to each sex. The diagnostic features used included body size differences, the presence or absence of a reproductive chamber, and the occurrence of nubs on the abdominal segments, as described by [11] and [12]. These morphological markers provided reliable means of distinguishing males from females during examination.
2.4 Sex Ratio
The sex ratio was established by counting and recording the number of male and female prawns collected each month throughout the study period. The proportion of males to females was then calculated to assess the population structure and possible variations in sex distribution over time.
2.5 Gonadosomatic Index (GSI)
The body weights of the samples with their corresponding gonad weights was taken and used to calculate the gonadosomatic index (GSI). This was determined as: GSI = W/B x 100 (13)
Where W=weight of the gonads
B = weight of the fish (g)
2.6 Fecundity Estimation and Oocyte/Egg Diameter
Ripe eggs were carefully removed from mature female prawns, weighed, and preserved in Gilson’s fluid for subsequent analysis. The preserved ovaries were repeatedly washed with distilled water to remove traces of the preservative. Thereafter, the eggs were separated from the ovarian tissue and placed on filter paper to eliminate excess moisture. Clumped eggs were gently teased apart and air-dried at room temperature. The diameters of individual oocytes were measured using an electronic microscope equipped with a calibrated ocular micrometer. Fecundity was estimated using both volumetric and gravimetric methods to determine the total number of eggs contained in each gravid prawn.
2.6.1 Volumetric Method
A total of one hundred (100) eggs were counted, and the volume of water displaced by this sample was measured. The total volume of water displaced by all eggs, including the subsample, was also determined. Fecundity was then estimated using the formula described by Cailliet et al. (1986):
: X/n = V/v (Cailliet et al., 1986) Where X = unknown number of eggs in the total sample collected n = number counted in sample V = total displaced volume v = volume of the sample.
2.6.2 Sub-sampling Dry Gravimetric Method
All eggs from each mature prawn were first weighed to obtain the total egg mass. Three subsamples, each consisting of 200 eggs randomly selected from the total, were then weighed individually. The mean weight of these subsamples was calculated, and fecundity was estimated using the following expression:
This gravimetric approach provided a reliable estimation of the total number of eggs produced per gravid prawn.
2.7 Data Analysis
Microsoft Excel (2007) was used for Data analysis.
3.0 Results
Table 1: Explains the mean parameters of reproductive indices in prawn from Lower River Benue. The result depicts a significant difference for egg weight only in Sept 2021 and October 2021 no significant difference was seen in Aug 2021,Nov 2021, Dec 2021 and Jan 2022, egg diameter followed the same trend as egg weight showing significant difference only in Sept and October 2021 in the same vein gonad weight showed a significant difference for Sept and the rest of the months but did not differ significantly with October 2021,GSI followed the same trend as gonad weight, condition factor showed a significant difference only in Aug 2021 and October 2021 the rest of the months showed no significant difference.
Table 2: presents the mean seasonal reproductive indices of Macrobrachium macrobrachion from the Lower River Benue. The results show that egg weight differed significantly between the wet and dry seasons, with a higher mean value recorded during the wet season (0.01 ± 0.00 g). Similarly, egg size exhibited a significant seasonal variation, being greater in the wet season (7.37 ± 3.69 mm) than in the dry season. Gonad weight followed the same trend, with a higher mean value of 0.03 ± 0.01 g observed in the wet season. The gonadosomatic index (GSI) also showed a significant difference between seasons, with a higher mean value (0.08 ± 0.00) during the wet season. Furthermore, the condition factor varied significantly, with prawns in the wet season exhibiting a higher mean value (1.33 ± 0.07) compared to those in the dry season. These results indicate that reproductive activity in M. macrobrachion is more pronounced during the wet season, reflecting favorable environmental and physiological conditions for breeding.
Table 4: presents the mean seasonal reproductive indices of prawns from the Lower River Benue. The results show that egg weight differed significantly between the wet and dry seasons, with a higher mean value recorded during the wet season (0.01 ± 0.00 g). Similarly, egg size exhibited a significant seasonal variation, being greater in the wet season (5.93 ± 2.98 mm) than in the dry season. Gonad weight followed the same pattern, with a higher mean value (0.02 ± 0.01 g) observed during the wet season. The gonadosomatic index (GSI) also varied significantly between the two seasons, showing a higher mean value (0.06 ± 0.00) in the wet season. In contrast, the condition factor displayed an opposite trend, being significantly higher during the dry season (2.14 ± 0.06) than in the wet season. These findings suggest that while reproductive parameters such as egg and gonad development are enhanced during the wet season, overall body condition tends to improve during the dry season, possibly due to environmental or feeding differences.
Table 5:Explains the sex distribution of prawns from Lower River Benue. The result showed that the male M. macrobrachion had a count of 101 and recorded 46.80% and the female 69 and a percentage of 35.60 and a ratio of 1:0.68 of male to female A. gabonensis had a count of115, a percentage of 53.20 the female a count of 125, a percentage of 64.40 and a ratio of 0.9:1. The overall ratio obtained depicts that male were slightly more than the female.
Table 6: Shows the mean monthly sex distribution of prawns from Lower River Benue. The result showed that male and female prawns recorded 0 catch in Aug. Sept had a count of 18 and 56.20% 14 counts and a percentage of 43.80 recording a ratio of 1:0.7, Oct recorded 32 counts for male 50.80%,31 counts for female,49.20 percent and a ratio of 1:0.97 Nov recorded 59 count for male and 63.40% .34 count for female 36.60% with a ratio of 1:0.58 December with a count of 34 and 47% for male,37 count for female 52.10% and a ratio of 0.92:1.Jan had a count of 29 and a percentage of 49.20 for male while the female recorded 30 count and a percentage of 50 and a ratio of 0.97:1.The overall ratio showed male to be higher than the female.
Table 7 shows the mean monthly sex distribution of M. macrbrachion from Lower River Benue. The result showed that male and female M. macrobrachion recorded 0 catch in Aug. Sept had a count of 14 and 56% for male,11 counts and a percentage of 44 for female recording a ratio of 1:0.79 Oct recorded 28 counts for male 53.80%,24 counts for female,46.20 percent and a ratio of 1:0.86 Nov recorded 18 count for male and 85.70% .3 count for female 14.3% with a ratio of 1:0.17 December with a count of 10 and 62.50% for male,6 count for female 37.50% and a ratio of 1:0.60.Jan had a count of 3 and a percentage of 33.3 for male while the female recorded 6 count and a percentage of 66.70 and a ratio of 0.5:1.The overall ratio also showed male to be higher than the female.
Table 8: Shows the mean monthly sex distribution of A. gabonensis from Lower River Benue. The result showed that male and female A. gabonensis recorded 0 catch in Aug. Sept had a count of 4 and 57.10% for male,3 counts and a percentage of 42.9 for female recording a ratio of 1:0.795,Oct recorded 4 counts for male 36.4%,7 counts for female,63.60 percent and a ratio of 0.57:1, Nov recorded 41 count for male and 56.90% .31 count for female 43.10% with a ratio of 1:0.76 December with a count of 24 and 43.60% for male,31count for female 56.4% and a ratio of 1:0.92.Jan had a count of 26 and a percentage of 52 for male while the female recorded 24 counts and a percentage of 48.00 and a ratio of 1:0.92.The overall ratio also showed female to be higher than the male.
4.0 Discussion
The results of this study reveal that the gonadosomatic index (GSI) of Atya gabonensis and Macrobrachium macrobrachion was generally low across sampling periods, though peaks were observed in September and October. Seasonal increases in GSI typically indicate the onset of reproductive activity, often linked to favorable environmental conditions such as temperature, rainfall, and food availability as reported by [13-14]. The higher GSI values in September and October suggest that these months may correspond to peak gonadal maturation in the study area, this agrees with reports that stated that tropical prawns often exhibit seasonal breeding patterns influenced by hydrological cycles [15-16]. The low GSI disagrees with the work of [11] for M. vollenhovenii. The low GSI may be because the samples were collected shortly after their breeding season the result of the seasonal GSI also showed a better GSI during the wet season.The low GSI values that differed with season and stage, coupled with stage dependent egg size development, entails that the reproductive indices of A. gabonensis and M. macrobrachion as being shaped more by environmental seasonality and requirements of the embryo than by size. This emphasize the importance of understanding species specific reproductive indices when managing freshwater prawn populations for conservation and aquaculture purposes
For A. gabonensis, egg diameter ranged between 0.3–0.6 mm. Importantly, egg size was not dependent on the female body size but rather on the stage of embryonic development. This pattern is in line with earlier observations in freshwater decapods where egg diameter is more closely linked to developmental stage than maternal size [17-18]. The findings of the result shows that fecundity in A. gabonensis may not be directly inferred from female body size alone, as egg volume and stage specific development contribute significantly to reproductive output. This finding is similar to that of [19] whose range of egg diameter for this species falls within the range established in this work. This may be attributed to the fact that each berried female of this specie carried eggs of different sizes at various stages of development. Seasonal egg diameter was also seen to be higher in wet season than the dry which may be as a result of food availability.
Species of the genus Macrobrachium often exhibit larger egg sizes that increase with female size [21], suggesting that A. gabonensis may adopt a different reproductive strategy. The small but progressively enlarging egg diameters observed in this study reflect an adaptive reproductive trait, possibly related to developmental constraints in freshwater environments where the species inhabits.
In the present study, females of Atya gabonensis were found to be more numerous than males, whereas Macrobrachium macrobrachion exhibited a higher proportion of males compared to females. This observation aligns partially with the findings of [22], who reported a higher number of female Macrobrachium species than males, with a sex ratio of 1:2 (M:F) in Akor River, Ibere Ikwuano, Abia State. Similarly, [24] recorded a sex ratio of 1:2 (M:F) in M. vollenhovenii. However, the results of the present study differ from those of [23] and [24], who observed equal sex ratios in their investigations of Penaeus indicus, Metapenaeus dobsoni, Macrobrachium affinis, and Parapenaeopsis stylifera. The variation in the current findings may be attributed to ecological factors, behavioral differences, or sampling bias. It is possible that more males of A. gabonensis were captured because they are more vulnerable to fishing activities, while females tend to migrate into deeper waters shortly after spawning. According to [25], sex ratios in prawn populations are not static and may fluctuate seasonally or annually within the same habitat due to environmental and biological factors.
5.0 Conclusion
The study demonstrates that the reproductive biology of Atya gabonensis and Macrobrachium macrobrachion is strongly influenced by environmental seasonality, particularly rainfall and food availability during the wet season. Peaks in gonadosomatic index (GSI) observed in September and October indicate that these months are critical for gonadal maturation, supporting the idea of seasonal breeding patterns. Egg size in A. gabonensis was found to depend on embryonic stage rather than female body size, highlighting a reproductive strategy distinct from many Macrobrachium species where egg size typically goes up with female size. This shows the adaptive differences in reproductive pattern between the two genera. Additionally, observed variations in sex ratios point to species-specific behavioral or ecological factors, such as migration and vulnerability to capture, that can alter population structure. Overall, these findings stress the importance of considering seasonal and species-specific reproductive indices in the management and conservation of freshwater prawns, as well as in optimizing their aquaculture potential.
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