Effects of stocking density on the growth and flesh quality of rainbow trout (Oncorhynchus mykiss) reared in a low-tech aquaponic system

In the present study, we evaluated the effects of two stocking densities (low - ALD, 3.81 kg m−3 vs. high - AHD, 7.26 kg m−3) on the growth, health, and flesh quality of rainbow trout (Oncorhynchus mykiss) and the yield and microbiological quality of lettuce (Lactuca sativa) produced in a low-tech aquaponic system compared to hydroponic cultivation (HYP). Nine experimental units (three replications per treatment) were utilised. A total of 123 rainbow trout (initial body weight: 142 ± 35 g) were randomly distributed in six 500-L tanks (3 per stocking density) and monitored during a 117 day trial period. The final weight (331 g on average), specific growth rate (0.73% d−1), feed conversion ratio (1.58), and mortality (3%) of the fish did not differ between stocking densities. The morphometric indices, slaughter results, and flesh quality were not affected. Similarly, the quantity of lettuce produced during two consecutive cycles was similar among treatments (2.4 kg m−2 on average). At harvest, microbial contamination (total viable count, E. coli, Enterobacteriaceae, Pseudomonas, mould, and yeasts) was similar in the fish skin and lettuce produced in aquaponic systems with different stocking densities, as well as in the lettuce produced in aquaponic and hydroponic systems. In conclusion, rainbow trout and lettuce productions were successful in the tested aquaponic system, whereas stocking density did not affect fish growth or flesh quality.