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High water exchange in RAS may minimize prevalence of diseases on juvenile Nile tilapia

High water exchange within a culture system may result in good growth and feed utilization. A study in Ghana observed this on Nile tilapia (Oreochromis niloticus) juvenile under moderately high exchange of at least 100 per cent tank water replacement per hour in a recirculating aquaculture system (RAS).

The effects of low, medium and high water exchange rates in RAS on juvenile Nile tilapia digestion efficiency, growth and feed utilization parameter and disease prevalence were investigated.

“This study thus aimed to test the hypothesis that varying water flow rate over defined incremental levels will significantly affect Nile tilapia growth with impaired growth associated with low water exchange regimes, despite the unique tolerance of the species to sub-optimal water quality,” authors Kwasi Adu Obirikorang et al. said in their research paper, “Feed digestion, growth and disease prevalence in Nile tilapia (Oreochromis niloticus) cultured at different water exchange rates in a Recirculating Aquaculture System.”

The significant benefits of high water exchange were demonstrated on the effects on disease prevalence. Fish in medium exchange and low exchange regimes were more susceptible to cataracts.

“The mean prevalence of cataract in the middle exchange and low exchange treatment were 32 per cent and 40 per cent, which were significantly higher and approximately three-and four-fold higher than the prevalence in the high exchange treatment,” it was noted.

The research paper was published on Aquaculture Studies.

While 37 per cent of fish in low exchange developed epidermal lesions, it was only four per cent in high exchange. It was likewise observed that reducing water flow rates increased the prevalence of fin erosions.

“Advanced forms of all the assessed disease conditions were generally associated with the low exchange and medium exchange fish, compared to high exchange group. Although there were incidents of all the assessed disease conditions in the high exchange treatment, all the observed infections were mild and none progressed into advanced forms.”

Water quality indices across all treatments were within the published acceptable ranges for tilapia rearing.

All water quality indices recorded for the various treatments were within published acceptable ranges for tilapia rearing. WQI is calculated using some or all of the parameters of water clarity, dissolved oxygen, oxygen demand, nutrients and bacteria.

The difference in the water flow rates from the main reservoir tank into the culture tanks resulted in significant differences in dissolved oxygen concentrations among different treatments.

The different water flow rates also significantly influenced pH levels.

Growth performance and feed utilization parameters was directly related to the volume of water flow exchange. The difference of the best performing group, high exchange, was not significant over the that of the low and medium exchange.