The water quality, incubation period and fecundity of the prawn Macrobrachium vollenhovenii

The physicochemical and biophysical characteristics of the natural habitat of the African river prawn (Macrobrachium vollenhovenii) in the lower Volta River, Ghana, were surveyed as a prelude to assessing the optimal water quality requirement for hatching and larval rearing in captivity. Additionally, the fecundity of gravid females and incubation period were determined. The physicochemical and biophysical assessments were done at two sampling zones along the Volta River estuary. Gravid broodstock were collected from artisanal fishers, transported to a laboratory and maintained at 8 ppt until hatching. These females were used to estimate fecundity and determine incubation periods for M. vollenhovenii. A preliminary larval rearing trial was conducted at salinities of 0, 4, 8, 12, 16 and 20 ppt to determine the optimum range for growth and survival. The results from the study indicated that prawns were found from the littoral to the mid zones of the river where there were submerged aquatic plants. In terms of the water quality in the natural habitat of the prawn, the temperature, pH and dissolved oxygen levels were similar at the two zones (p > 0.05) but there were significant differences in salinity (p = 0.001), total dissolved solids (p = 0.001) and alkalinity (p = 0.003) levels at Ada Foah and Agave Afedome. The duration of egg incubation in M. vollenhovenii was 18 days, with eggs changing from orange at spawning to brown between the 9th and 14th day and grey between the 14th and 18th day before hatching. Fecundity ranged between 2,400 and 26,400 larvae per female. Although, hatching was successful, larval development could not proceed beyond the first larval stage owing to imbalances in the ionic ratios of minerals in the brackish water. Further studies are recommended to resolve the water quality challenges to ensure successful larval culture in captivity.