The study aims at evaluating the quality of the water in the Ouémé River basin. Data were collected from October 2014 through September 2015. Physico-chemical parameters were assessed and the concentrations of dissolved heavy metals were determined. The results show both physical and chemical pollutions of the water according to a nonlinear pollution gradient. The middle basin up to the height of the city of Kaboua appears to be the most polluted segment. The Delta region, near the coast, is the second most polluted segment, and the remaining parts of the basin are the least polluted. Pollutant concentrations are highest during the low flow season. Cluster analysis and Principal Component Analysis (PCA) of sampled sites reveals that Atchakpa-Rejet and Kaboua have high concentrations of nitrite, nitrate, ammonium, orthophosphate, conductivity, TDS, pH, calcium, magnesium, and total hardness. Kpassa, Bétérou, and Atchakpa-Bethel have high dissolved oxygen and high transparency values. All other sites have high alkalinity. During the low flow season, Cd, Pb, Zn and Cu were detected at concentrations exceeding ecological standards in most of the sites. The Water Quality Index (WQI) ranged from 52.7 to 80.9, meaning that water in that river basin can be classified either as marginal quality, moderately polluted, or good quality for aquatic life. Overall, water in the Oueme River basin is under high and increasing influence of human activities.

Ostracoda species are indicators of their current and past environment (paleoenvironment). The study aims to evaluate the acute and chronic sensitivities of a freshwater ostracod species (Eucypris sp.) to agricultural pesticides (a cypermethrin-based insecticide and a glyphosate herbicide-based formulation). Lethal concentrations (LC₅₀) of each pesticide for the species at 24 and 48 h were determined. The chronic exposure allowed assessing the effects of low concentrations of both pesticides; firstly, on the parthenogenetic reproduction of Eucypris sp., and, secondly, on its population growth. Then, individuals of Eucypris sp. were exposed to 0.536 ppb and 1.072 ppb of cypermethrin and 4.51 ppm and 9.03 ppm of glyphosate. These concentrations are respectively the 10%, and the 20% of the 48-h LC₅₀ (median lethal concentration) of both pesticides for the species. The estimated 24-h LC₅₀ of cypermethrin was 7.287 ppb. At 48-h, it was 5.361 ppb. For glyphosate, the 24-h LC₅₀ was 50.521 ppm, while at 48-h it was 45.149 ppm. After 10 days of exposure to low concentrations of cypermehrin, only 30% of females reproduced parthenogenetically with 10% and 20% of LC₅₀-48-h. For the control treatment, reproduction in 80% of females was observed. Eucypris sp. population growth after 28 days of exposure to low concentrations of cypermethrin showed significant retardation. Regarding glyphosate chronic exposure, 60%, 50%, and 90% of individuals were able to reproduce at 10% of LC₅₀-48-h, 20% LC₅₀-48-h, and the control treatment, respectively. The population growth was also affected by the tested low concentrations of glyphosate. The study showed high sensitivity of Eucypris sp. to cypermethrin compared to glyphosate. However, low concentrations of both pesticides affected the species at individual and population level.

This study aimed to compare the growth performance and economic profitability of rearing Oreochromis niloticus in floating cages using three feeding strategies (5 days a week, 6 days a week, and 7 days a week) coupled with three stocking densities (20, 40, and 60 fish·m⁻³). Male monosex fish measuring 6.44 ± 0.18 cm and weighing 3.42 ± 0.7 g were used. The controls were weighed every 15 days from the 35th day of rearing until 185 days to assess zootechnical parameters. BioMar feed (Efico Cromis 832F) was used during the study, and the feed ration was adjusted for each treatment every 2 weeks. At the end of the study, the average weights varied from 293.09 g to 468.41 g for the conditions of 40 fish·m⁻³ with a 5/7-day diet and 20 fish·m⁻³ with a 7/7-day diet, respectively. The yield per m² values were between 7.11 kg and 23.17 kg, respectively. On the other hand, the economic profitability was better with the condition of 40 fish.m⁻³ with daily feeding. These results suggest that in the Toho-Todougba lagoon complex, a stocking density between 40 and 60 fish·m⁻³ coupled with daily feeding (three times a day) promotes the optimization of fish production and profitability of fish farming.

Context: Macrobrachium vollenhoveni (Herklots), a freshwater prawn native to Africa, is crucial for aquaculture and subsistence fishing. However, limited data exist on its reproductive biology in Benin’s water bodies. Aims: This research aimed to explore the breeding season, size at sexual maturity, fecundity and reproductive output of M. vollenhoveni. Methods: Prawns were sampled monthly from May 2015 to October 2016. In total, 1567 females of M. vollenhoveni were collected from the lower Ouémé (864 prawns) and the lower Mono (703 prawns). Key results: Findings showed that gonadal maturation starts in March, with peaks in July. Then spawning occurs from July to September, and even October. In both environments, ovigerous females (prawns carrying fertilised eggs in incubation) were frequently captured from July to October, with a maximum proportion in August and September. The spawning period for the two prawn populations in both sampled areas matched with an increase in water volume, salinity and pH. The size at sexual maturity was 8.48-cm total length (TL) for the lower Ouémé population, whereas in lower Mono, it was 9.00 cm TL. Absolute fecundity ranged from 3925 to 31,408 eggs in lower Mono and from 4871 to 25,902 eggs in lower Ouémé. The reproductive output was 12.87% for Mono and 11.94% for Ouémé. Conclusions and implications: This research is the first one on the reproductive biology of M. vollenhoveni in Benin and provides vital data for sustainable fishery management of this prawn species in the watersheds throughout the country.

This study was carried out to assess the chronic effects of both an insecticide (spirotetramat) and a herbicide (2,4-Dichlorophenoxyacetic acid) on two phytoplankton species: Monoraphidium griffithii and Coscinodiscus sp. The effects of the chemicals on an exponentially-growing population of the two species were evaluated using single compounds and a compound mixture. The experiments lasted for 27 days. The results show a decrease in the populations’ growth for both species after the application of single pesticides and the mixture. 2,4-D applied alone and the mixture of both chemicals have affected the organisms the most. Both pesticides have induced an increase in chlorophyll-a content in M. griffithii while no effect was observed on chlorophyll-a in Coscinodiscus sp. A significant effect was recorded in the cell size of both species with both pesticides. The two spirotetramat tested concentrations have induced an increase in cell size, while the opposite effect was obtained for the (2,4-D) herbicide concentrations. The mixture of compounds induced an additive effect on the cell size of both species. These results indicate the risk which the two chemicals represent in a natural environment. They are agents that can have chronic impairments at the physiology or behavior level of such phytoplankton species as M. griffithii or Coscinodiscus sp. Such impairments might consequently affect the health of the ecosystem.