Recirculating aquaculture systems (RAS) are expected to play an important role in aquaculture’s continued growth. RAS largely conserves water, but substantial volumes of water still leave the system while backwashing mechanical filters and flushing accumulated nitrogenous wastes. While aquaponics is expensive and has limited capacity to absorb such volumes of effluent, field crops can provide the land base to absorb this nutrient-rich water that might otherwise be wasted. Using lettuce (Lactuca sativa) to compare tilapia (Oreochromis niloticus) RAS effluent against synthetic fertilizer and synthetically amended RAS effluent, this study centers on the hypothesis that integrated aquaculture and agriculture minimize waste while maximizing productivity.
A 21-day growth trial was set up with 2 varieties of lettuce (Rouxai; Rex), and 4 levels of fertigation: water control, RAS effluent, soluble 21-5-20 fertilizer control at 100ppm NO3-N, and RAS effluent amended with a lower dose of 21-5-20 fertilizer to reach a combined 100ppm NO3-N. The plants were transplanted 15 days from seeding, with 96 plants in total (n = 12). An earlier analysis of the RAS effluent composition is reported in Table 1. Lettuce head volume was calculated throughout the growth period (Figure 1). Fresh weight, leaf area, chlorophyll, and tissue composition were evaluated at the trial’s termination.
The goal of the trial was to determine the fertilizer value of the RAS effluent alone and the extent to which RAS effluent nutrient credits can offset synthetic fertilizer in a complementary system. At 20 days from transplanting, amended effluent was not different from the synthetic fertilizer control for both varieties (p>0.1). Plants receiving unamended fish effluent were on average twice the size of those receiving only water (p<0.001) but still only half as large as those receiving amended effluent (p<0.001). In all, nutrient credits from RAS effluent are shown to offer substantial fertilizer savings without compromising plant productivity when used in combination with synthetic fertilizer.