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Buyo Hydroelectric Dam Lake – Côte d’Ivoire: What Impacts on Ichthyological Spawning Grounds?

Yao Kouassi Anderson, N’Dri Olga Rosemonde, Bodji Iridjé Marcelle, N’Zi Konan Gervais*, Koné Tidiani

Laboratory of Natural Environments and Biodiversity conservation, Department of Biosciences, Félix Houphouët-Boigny University, 22 BP 582 Abidjan 22, Côte d’Ivoire

Abstract: The various management projects of hydroelectric dam on the Sassandra River have considerably modified the hydrological regime of Lake Buyo in recent years. In such a context, it seems appropriate to know the influence of water level fluctuations on aquatic organisms, in order to protect the biodiversity of the environment. The present study was conducted from June 2018 to May 2019 in Buyo Lake. The experimental fisheries carried out in-situ in the five sampling stations brought to light the dominance of the species Coptodon zillii which represents 87% of the spawners. Then, the monthly fish nest count showed that the highest nest densities were obtained at the mean depths between 192.2 and 196.2 m. This study haves shown that the increase in water level during prolonged closures of the spillway gates resulted in the disappearance of 35% of laying nests. In addition, the withdrawal of following to the prolonged opening of the gates caused the drying of 75% of laying nests in the spawning grounds of Lake Buyo.

Keywords: Buyo Lake, hydrological regime, fish, spawning grounds, dam impacts, Côte d’Ivoire

Received January 05, 2023; Revised February 11, 2023; Accepted February 19, 2023

Citation: Yao Kouassi Anderson, N’Dri Olga Rosemonde, Bodji Iridjé Marcelle, N’Zi Konan Gervais, Koné Tidiani, Buyo Hydroelectric Dam Lake – Côte d’Ivoire: What Impacts on Ichthyological Spawning Grounds?. American Journal of Environmental Protection, 11 (1). 1-6. January 2023. doi:10.12691/env-11-1-1

1. Introduction

Lake Buyo was created by the construction of a hydroelectric dam in southwestern Côte d'Ivoire, at the confluence of the Sassandra and N'Zo rivers [1]. This man-lake has a diverse fauna [2] with spawning grounds including several fish species in its portion located inside Taï National Park (TNP) [3]. In 2017, a second hydroelectric dam built downstream of Lake Buyo was impounded in the town of Soubré. The latter's water reservoir is supplied by water from Lake Buyo. Since then, water is released sporadically and frequently to meet the requirements of the hydroelectric facilities. Unfortunately, these numerous solicitations of the waters of the lake, related to the opening and closing of the gates of the hydroelectric dams could compromise the sustainability and reproductive success of the fish in the environment. It is well known that hydrological variations strongly influence the functioning of aquatic ecosystems [4]. Indeed, water level fluctuations condition reproductive success [5]. Thus, the drying or flooding of breeding areas could compromise egg incubation and survival of young stages [6]). In Lake Buyo, this phenomenon could affect species that build their nests in shallow shoreline environments [7]. The present study aims to show the impact of hydrological variations on fish spawning grounds in Buyo Lake from June 2018 to May 2019.

2. Material and Methods

2.1. Study Area

Buyo Lake is located in the southwest of Côte d'Ivoire, between 06°54' and 07°31' west longitude and 01°14' and 07°03' latitude. The hydrograph of Lake is characterized by a period of high water (maximum flooded area of 74200 ha) and a period of low water (minimum flooded area of 31350 ha) [8]. From June 2018 to May 2019, the high-water period (196.5 m to 200 m) extended between August and February and the low water period (184 m to 196 m) between March to July [9]. The study area selected corresponds to the portion of Buyo Lake located in the N'zo Reserve of Taï National Park. Five (5) sampling stations were selected. Among them, two (2) are located in the floodplains (S4; S5), two (2) in the bays of the lake (S1; S3) and one (1) station located in the water plain (S2) (Figure 1).

Figure 1. Location of study area sites in Buyo Lake from June 2018 to May 2019

2.2. Data Collection and Analyzing

The hydrological data were obtained from the Compagnie Ivoirienne d'Electricité (CIE) of Buyo. These data concerned the lake's water retention level, inflow, and outflow from the gates for each month from June 2018 to y 2019. Other parameters such as water temperature was measured with Waterproof DO 330 multiparameter; water depth was measured with a weighted, graduated stick. The condition of the riparian vegetation in relation to the water level and the different habitat types were visually described. Recession and flooding distances were measured with a tape measure. Fish nests were located and counted monthly using a quadrat [10]. For this purpose, a total area equivalent to three times that of a 10 m × 3 m rectangle was explored monthly per site. Nest density (d) was calculated as follows:

In order to identify the species breeding in these nests, a hawk net test fishery was conducted after each nest counting session. Five hawk fishing sessionsts were made in-situ at the nest count plots. Captured fish were identified according to [11] and then dissected. The macroscopic identification scale was used to determine the sexual maturation stages of each specimen [12,13].

2.3. Statistical Treatment

The Mann-Whitney test was used to compare the spatio-temporal variation of nest densities according to water levels as well as the monthly variation of physicochemical parameters between low and high water. In addition, the Kruskal-Wallis test was used to compare nest density between stations and between months. Spearman's rank correlation coefficient was used to determine the correlation between nest density and environmental parameters. All analyses were performed using Statistica 7.1 software.

3. Results

3.1. Identification of Fish Spawning Grounds and Breeding Species

A total of 458 fish specimens were recorded in the spawning areas. Out of which, 376 spawners or mature individuals (82.09%) were recorded. Among the spawners, the highest numerical abundance was recorded for Coptodon zillii (87%), followed by Hemichromis fasciatus (5%) and Oreochromis niloticus (3%). The other species (Sarotherodon melanotheron, S. Galilaeus and C. guineesis) contributed 5% of the spawners (Figure 2).

Figure 2. Proportion of spawners captured on Buyo Lake fish spawning grounds from June 2018 to May 2019

3.2. Identification of Fish spawning Grounds and Breeding Species

Figure 3 showed the variation in nest density at the five sampling stations. In the bays of lake, nests density varied from 1.68 (S3) to 2.44 (S1) nests / m². At the stations located in the floodplains, density of nests varied from 1.5 (S4) to 1.47 nests / m2 (S5). In open water, station S2, any nests were recorded. Statistical analyses showed no significant difference (Kruskal-Wallis test, p > 0.05) in nest density between bay and floodplain stations.

Figure 3. Spatial distribution of nest density in Buyo Lake from June 2018 to May 2019

3.3. Relationship between Nest Density and Water Level

The monthly variation in nest density and water level (Figure 4 and Figure 5) showed that the average fish nest density recorded in the study area was 2 nests/m². Highest nest densities (3 and 4 nests / m²) were respectively recorded between July and August and, between March and April. During these two periods, water level recorded varies between 192.2 and 196.2 m. The lowest densities (0 to 1.5 nests/m²) of nests were recorded between September and January, with a water level between 198 and 200 m. For water level between 190 and 192 m, nest densities fluctuate between 1 and 2 nests / m². Between different recorded water levels, Kruskal Wallis test showed significant differences in nest density (Kruskal Wallis test, p < 0.05).

Figure 4. Monthly variation in nest density and water level (WL) in Buyo Dam Lake from June 2018 to May 2019

Figure 5. Average nest density at different water levels of Lake Buyo from June 2018 to May 2019

3.4. Relationship between Nest Density and Seasons

During the flood period (wet season) from June to August, water inflow varied from 753 to 861 m³/s and nest density from 2.41 to 3.54 nests/m² with highest value in August (Figure 6). During the low flow period (dry season) from December to May, water inflow ranged from 131 to 165 m³/s, nest density ranged from 3.05 to 3.2 nests / m² and the highest values of nest densities was recorded in April. Statistical tests comparing lake inflow during flood and water level decrease periods with fish nest density showed no significant difference (Mann-Whitney U-test; p > 0.05).

Figure 6. Variation of nests according to water inflow level in Lake Buyo from June 2018 to May 2019

3.5. Relationship between Nest Density, Opening and Closing of Hydraulic Turbines

The proportions of functional nests and destroyed nests on one hand, and the distance of water retreat from shore on other hand, according to the opening and closing of the hydraulic gates were illustrated by Figure 7 and Figure 8 respectively. When the gates of the dam were open, 75% of the nests were destroyed and the distance of water recession recorded was between 0.5 and 0.75 m. In contrast, when the dam gates were closed, 25% of the nests were destroyed and the distance of water recession varied between 0.25 and 0.55 m.

Figure 7. Photograph of Lake Buyo spawning grounds during gate opening (A = flooded nests) and gate closing (B = Shoreline flooding)

Figure 8. Daily variation in nest density according to Buyo Dam gate opening and closing from June 2018 to May 2019

3.6. Effects of Environmental Factors on Fish Nest Abundance

During low water period, water temperature recorded ranged from 29.8 ± 5.46°C (S1) to 30.3 ± 5.50°C (S5), water depth from 18.4 ± 4.28 cm (S5) to 23.7 ± 4.86 cm (S1), and riparian vegetation from 15 ± 3.87% (S3) to 44 ± 6.63% (S5). During high-water period, water temperature ranged from 29.3 ± 5.41°C (S1) to 30.1 ± 5.48°C (S4), water depth from 20.4 ± 4.51 cm (S5) to 24.2 ± 4.51 cm (S1), and riparian vegetation from 5 ± 2.23% (S1) to 25 ± 5.00% (S4). Mann-Whitney U test showed not significant different between spawning grounds stations across hydrological seasons for water temperature (p ˃ 0.05) and differ significantly for water depth and riparian vegetation (p < 0.05 respectively) (Table 1).

Table 1. Variation (mean ± standard deviation) of physico-chemical parameters according to water levels in spawning grounds in Lake Buyo from June 2018 to May 2019

Monthly observation of potential fish spawning habitat showed that during high water, the area used for spawning ranged from 5.2 m2 to 7.7 m2 and during low water, it varied from 8.8 m2 to 17.3 m2. These values showed a significant difference only for the months of June and October (Mann-Whitney U test, p < 0.05). Concerning the proportion of habitat lost for reproduction, it varied from 33% to 88% during high water, and between 52% to 77% during low water. The Mann-Whitney U test showed no significant difference (p > 0.05) in the proportion of habitat lost with water level (Figure 9).

Spearman's rank correlation coefficient analysis performed on nest density revealed a positive influence with temperature (TP), shoreline vegetation (SV) and shoreline depth (D). A significant negative correlation was observed between nest density and water inflow (WIR), water outflow (WO), hydroelectric power plant turbine outflow (TO) and lake impoundment level (R) (Table 2).

Figure 9. Comparative change in the proportion of lost habitat (LP) and useful spawning area (US) of fishes in Buyo Lake shoreline from June 2018 to May 2019

Table 2. Results of Spearman rank correlation analysis between fish nest density and environmental factors in Buyo Dam Lake from June 2018 to May 2019

4. Discussion

The dominance of Coptodon zillii spawners in the spawning grounds investigated could be explained by the fact that Coptodon zillii builds its nests in the shallow areas of rivers. Furthermore, according to [9], 90% of the shoreline of Buyo Lake belongs to Coptodon zillii. Indeed, according to [14], shoreline nest building is a specific behavior of Cichlidae. Similarly, no nests were recorded at station S2, located in open water. This result could be explained by the fact that potential spawning habitats for most fish are located in shallow environments [15].

According to monthly variation in nest density and water level, water level would promote environmental conditions conducive to fish nesting. Indeed, it has been reported that water level fluctuations in lakes would influence fish communities in various ways [16]. Similar results were recorded for Oreochromis niloticus nests in Mutirikwi Lake in southeastern Zimbabwe where the highest densities were recorded during low water periods [17]. Also, the decrease in lake inflow from 1806 to 211 m³ /s is the result of the low nest densities recorded (0 to 0.3 nests / m²).

In addition, when the Buyo dam gates are open, 75% of the nests are dewatered and when the gates are closed, 25% of the remaining nests are destroyed by bank flooding. Thus, the alternating opening and closing of the hydroelectric dam gates strongly disrupts the availability of nesting sites. These changes in hydrology would hinder the reproduction of fish that spawn in shallow areas. In fact, a major flood, followed by a stable water level for several weeks after the eggs are deposited, very often ensures better reproductive success and higher survival of the young in the first stages of life [18]. In contrast, a flood of too short a time duration can lead to desiccation of eggs and reduced survival of young [19].

The influence of water levels on reproductive success of Cichlidae has been recorded in Mutirikwi Lake in southeastern Zimbabwe [17], in 10 irrigation reservoirs in Sri Lanka [20], and in the Comoé dam lake in Burkina Faso[21]. From all these studies it appears that most fish in shallow areas in the tropics have their life cycle depending on water fluctuations. The same observations have been reported by [22].

The extent of the area available for spawning generally increases during the flood season. This is because during the flood season, floodplains and other types of vegetation are inundated. These floodplains are essential for many fish species as they act as both a food source (submerged plants and invertebrates available) and as spawning sites (stable water and diverse habitats). In Buyo Lake, the invasion of riparian Oryza glaberrium (Poacea) species coincides with the onset of rising waters in August. These riparian terrestrial plants represent favorable microhabitats and feed sources for nesting Cichlidae. Such results were observed in Lake Chicamba in Mozambique, where reproduction and survival success of juveniles of Tilapias rendalli and Oreochromis mossambicus were depending on flooding and shoreline vegetation [23].

Spearman's rank correlation coefficient analysis performed on nest density showed a positive relationship with temperature, shoreline vegetation and depth to shore. Among the physicochemical parameters that influence spawning site selection, temperature, depth and nature of shoreline vegetation are essential factors for the setting of nesting processes. According to [24], temperature plays an important role in the selection of spawning areas by a species. In lakes, depth is closely related to slope which plays an important role in the distribution of ichthyofauna. They effectively contribute to hatching success [25]. A negative correlation has been observed between nest density and water flows. Thus, the hydrological regulations established in recent years would have major consequences not only on the Cichlidae family but on all aquatic organisms.

5. Conclusion

This study focused on the effects of hydroelectric dam lake on ichthyological spawning grounds. Some fish species, such as Coptodon zillii, were found to be very abundant in the spawning grounds of Buyo Lake. Water level and the gates regulation influence the spawning process. Hydrological changes would be harmful to the reproduction of fish spawning in shallow water. Among the physicochemical parameters, temperature, depth and the nature of the riparian vegetation are essential factors for the establishment of nesting processes. It is recommended that the Ivorian government must, for the purpose of food self-sufficiency and increased fisheries production, preserve the spawning grounds of fish species in order to renew the stocks. This preservation must be done by regulating the frequency of opening and closing of turbines and also by creating artificial nesting areas for the species.

Acknowledgements

We are grateful to Professor KONE Tidiani, Vice President of the University Jean Lorougnon Guédé of Daloa, who directed this project and allowed us to complete it. Thanks are addressed to the village chiefs, local authorities, CIE agents in Buyo and others participants in this study. We are indebted to the hydrobiology team for their technical assistance with data collecting and subsequent laboratory analysis. This work is part of the research project entitled “Spawning sites of fishes of Lake Buyo in the Taï National Park (Côte d'Ivoire): identification and characterization”. This project received financial support from Strategic Support to Scientific Research Program (PASRES), and by Ivorian Parks and Reserves Office (OIPR) of Côte d’Ivoire.

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