Mineral nutrient content of tomato plants in the aquaponics system as affected by the fish density

Aquaponics is a technique combining fish rearing with the hydroponic cultivation of plants, especially vegetables, in a closed system based on a virtuous recycling of water and nutrients, according to the principles of the circular economy. In an aquaponic unit, wastewater from aquaculture, after being filtered mechanically in order to remove solid waste, passes through a bio-filter, where nitrifying bacteria convert ammonia into nitrate, then is used as a nutrient solution by the plants, and finally comes back to the fish tank, purified. Aquaponics is acknowledged as an innovative sustainable food production system with high potentiality for facing problems like population rise, climate change, soil degradation, water scarcity and food security, especially in developing countries. The main strength of this production system is that two different kinds of products are simultaneously obtained, allowing the virtuous use of resources and then the sustainability of the food chain. The present study reports the results of the production of Nile tilapia (Oreochromis aureus Steindachner, 1864) and tomato (Lycopersicon esculentum L. Newton) in the classical aquaponic system (1 loop) with different fish density with emphasising the growth and mineral nutrient content of tomato plant. The experiment was conducted at the Faculty of Agriculture of Ankara University (Turkey), using in-door, small-scale aquaponic systems with coupled system. Ninety-six tilapia (O. aureus) juveniles around 7 g of individual weight were stocked at different ratio: 25 kg/m3 (Group I), 35 kg/ m3 (Group II) and 50 kg/m3 (Group III) and fed with 45% raw protein feed at the level of 2% body weight for 126 days. Fish density affected the fish growth with better values in the highest fish density, however, the overall calculation of the condition factor did not show any significant difference due to fish density. Water quality parameters measured fluctuated in all experimental groups during the experiment. Total plant biomass was low due to the insufficient lighting of the in-door aquaponics system and the low level of water potassium. An increase in fish stocking rate caused significant increases in tomato plant leaves growth. The mineral nutrient content in the plant was altered by the fish stocking rate. The mineral nutrient content of the leaves of the plant was at the sufficiency rate with the exception of potassium when compared to tomato plants grown in soil.