Introduction
Aquaponics combines recirculating aquaculture (RAS) with hydroponics, a soilless plant farming method . Plant choice in aquaponics may affect the growth and microbiome of fish because each plant has its specific microbiome, which may alter the microbes in aquaponics. Plant secrete root exudates, which may alter the microbe s in the system. Mint is a plant known for its growth promoter effects on animals, and it may influence the fish growth when grown together with fish in aquaponics . On the other hand, fish in aquaponics excrete ammonia, which is toxic to fish and must be removed from circulating water. Ammonia is converted to nitrites and then to nitrates by microbial action during the process of nitrification. Nitrificati on is an essential process for the performance of RAS and maintains the water quality of aquaponics. The aims of this study were to investigate and the growth of fish and fatty acids contents of rainbow trout (Oncorhynchus mykiss) in aquaponics when grown with mint. The other aims were to investigate microbial communities in the mucous and gut of the rainbow trout and to compare start-up of nitrification in aquaponics and RAS .
Materials and methods
Seven-week experiment was performed with three replicated RAS and aquaponic systems. Twenty rainbow trout (initial weight 55 ± 1.06 g ) were stocked in each fish tank for RAS and aquaponic systems. Thirty mint seedlings were transplanted in each deep water culture rafts . Change in biomass for fish was recorded at the end of the experiment. Microbial communities in fish mucous and gut were analysed at the start and end of the experiment. The fatty acids were extracted from fish muscle from the start (fingerlings) and end samples. Concentrations of total ammonia nitrogen, nitrite, nitrate were recorded during the experiment.
Results
The weight gains (aquaponics = 324.33 ± 31.87 g, RAS = 295.13, ± 9.70 g , p = 0.24 ) and specific growth rates (aquaponics = 2.11 ± 0.11 g, RAS = 2.00, ± 0.06 g, p = 0.21) of rainbow trout were similar in RAS and in the aquaponics’. M int improved the fish growth through reduced feed conversion ratios (Figure 1). Mint reduced the feed consumption of the rainbow trout in aquaponics (Figure 1). However, fish in aquaponics maintained equal weight as fish reared in RAS despite of the lower feed consumption. The omega 3 fatty acids content of rainbow trout in aquaponics were slightly higher than the fish reared in RAS but without statistically significant difference. The retention of omega 3 fatty acids content was speculated in fish because fish maintained fatty acid composition similar to fish in RAS in spite of low feed consumption in aquaponics. M int altered the microbial communities of rainbow trout in mucous and gut compared to fish reared in RAS. Nitrification start-up was faster in aquaponics than in RAS.
Conclusions
Aquaponics with mint improved f ish growth, water quality and initiated earlier start-up of nitrification as compared to RAS . Using mint in aquaponics may improve omega 3 fatty acids content of the fish. Growing fish together with mint in aquaponics may lower the feed consumption of fish compared to RAS which will reduce the cost related to fish feed and minimise the impact of aquaculture feed on aquatic environment.
Acknowledgments : We would like to thank University of Jyväskylä, JAMK University of Applied Sciences , Sisä-Suomen kalatalousryhmä , Maa- ja Vesitekniikan Tuki Ry and Niemi -säätiö for their contribution towards this research work. We also thank Juhani Pirhonen for his contribution to this research especially in conceptualizing nitrification process.