Aquaponics is a sustainable food production system combining aquaculture with hydroponics to simultaneously produce fish and economic crops. Disease infestation is a critical challenge in aquaponics due to the limited available safe curative methods. Biological control and natural fungicides can be crucial to sustainable control of diseases in aquaponics. Therefore, we examined the use of entomopathogenic fungi (Isaria fumosorosea and Lecanicillium attenuatum), mycoparasitic fungus (Trichoderma virens ) and the risks associated with the use of natural (clove oil and lecithin) and synthetic (tebuconazole) fungicides on a biofilter and Nile tilapia, Oreochromis niloticus , in aquaponics. Our study identified that T. virens , besides its biocontrol property, can improve the growth of basil plants in aquaponics at a concentration of 1 x 107 spores per ml. The foliar application of clove oil (eugenol), lecithin, and tebuconazole at recommended dosages, spray-drifted, and were detected in aquaponics water at a percentage runoff rate of 0. 3 %, 2.3 % and 0.3 % respectively. In the biofilter, tebuconazole and clove oil at the maximum runoff concentration showed no significant effects on the nitrification processes during a 96 hr exposure period. In contrast, lecithin altered the ammonium and nitrite levels by substantially increasing ammonium-nitrogen levels from an initial 5 mg L-1 at the 1st hour to ~13 mg L-1 at the 6th-hour post application. These runoff concentrations were further evaluated on the physiology of Nile tilapia in a 28-day semi-acute toxicity test. The tebuconazole-treated group showed a significant effect on hematological (haemoglobin, red blood cell, MCH, etc.), biochemical (total protein, albumin, globulin, etc.), and antioxidative (glutathione peroxidase and glutathione reductase) parameters. Eugenol, on the other hand, showed no significant effects on the fish physiology, indicating its suitability for all aquaponics systems. The use of lecithin and tebuconazole should only be limited to decoupled aquaponics due to their effects on the biofilter and fish.
Materials and Methods
For ethical concerns regarding the unmeasured effects of fungicides on fish, three experiments were conducted separately. The first, a pre-requisite experiment, was conducted on monitoring the runoff of the active ingredients of applied fungicides in the water of decoupled aquaponics over 72 hr timepoints. Using the concentrations detected in the aquaponics water in the first experiment, we exposed a matured biofilter to the maximum concentrations of the active experiment to investigate their effects on nitrification and nitrifying bacteria. In the third experiment, Oreochromis niloticus were exposed to the maximum concentrations in a semi-acute toxicity test over 28 days to determine subacute toxicity from the runoff concentrations.