Proportional up scaling of African catfish (Clarias gariepinus Burchell, 1822) commercial recirculating aquaculture systems disproportionally affects nutrient dynamics

This study reports nutrient dynamics during the “Run-in-”, “Batch-” and “Staggered 1 - 3” production phases of African catfish (Clarias gariepinus Burchell, 1822) in commercial warm water recirculation aquaculture systems (RAS) under three different stocking densities, extensive (max 50 kg m−3; 35 fish tank−1), semi-intensive (max 100 kg m−3; 70 fish tank−1) and intensive (max 200 kg m−3; 140 fish tank−1) at similar system volumes of 13.9, 15.1 and 16.9 m3. The concentrations of the dissolved macronutrients Total Ammonia Nitrogen (TAN, as NH4+-N), nitrite (NO2−-N), nitrate (NO3−-N), Total Dissolved Nitrogen (TDN), Total Oxidized Nitrogen (TON), phosphorus (measured as ortho-phosphate) (P), potassium (K), calcium (Ca) and magnesium (Mg) were monitored with colorimetric analyses and budgeted for potential use for aquaponics. We found, that stocking density and RAS maintenance affected oxygen thresholds, subsequently affecting nutrient concentrations of NO3−-N due to denitrification. K proportionally increased with feed input, while Ca, Mg and P reacted disproportionally, because of precipitation. Highest deviation was observed for TON, especially when oxygen levels dropped <6 mg L−1. Maintenance intervals, feed ratios and resulting oxygen levels changed dissolved nutrient concentrations, with highest concentrations under intensive stocking density, medium feed input and oxygen > 6 mg L−1, and lowest under semi-intensive stocking density, high feed input and oxygen > 6 mg L−1. Calculated nutrient budgets under consideration of stocking density, feed input and water exchange rates demonstrated that the production conditions affected nutrient availability and output. The macronutrient ratios TDN, P, K, Ca and Mg as well as the physical and chemical parameters changed constantly under production conditions, due to daily adjusted feeding and RAS maintenance. This must be considered during process management, in stand-alone commercial catfish aquaculture as well as in integrated systems such as aquaponics.