Speaking during his presentation at Aquaculture Europe 2014 in Spain, Mr Monsees explained how nitrogen can build up in a aquaculture system due to fish waste and uneaten feed.
Previous studies have shown nitrate to increase the plasma NO3 in pike perch and a reduction of growth in some other species but, no tests have been done with tilapia.
In order to investigate the effects on tilapia, Mr Monsees exposed juvenile Nile tilapia to five different nitrate concentrations of 0, 10, 100, 500 and 1000 mg L-1 NO3-N at a water temperature of 27.3 °C and monitored specific growth rate (SGR), feed conversion ratio (FCR), hepatosomatic index (HSI), gonadosomatic index (GSI) and blood for concentrations of hemoglobin, methemoglobin, plasma NO3 and as well as plasma NO2.
In the highest nitrate treatment, the mortality of three fish (62.5 per cent survival rate) occurred.
SGR in this highest treatment also decreased significantly by 29 per cent and the FCR was seen to be reduced (56 per cent).
Haemoglobin concentration was seen to decrease in the highest treatment group and HSI increased.
Plasma NO2 - and methemoglobin also increased with the increasing nitrate concentration, revealing 767 μM NO2 - (± 8.2) and 44 per cent methhemoglobin (± 9.3 per cent) at 1000 mg L-1 NO3-N.
A low growth rate in the highest nitrate treatment could be due to less haemoglobin transporting oxygen round the body. It is therefore likely that metaemoglbinemia was caused by the nitrate toxicity.
Overall, the results of the study showed that high nitrate concentrations can impact growth, health and development of juvenile tilapia and are hence relevant in RAS production.
Nutrient management is therefore very important for intensive juvenile tilapia production and NO3 should be kept under 500mg/per liter as no severe affects were seen under this level.
October 2014
