Potential of nutrient reutilisation in combined intensive–extensive pond systems

Abstract

The experiments on the intensive–extensive system were carried out between 2008 and 2010 in three ponds (area 310 m², depth 1 m) serving as extensive units, where cages were placed as an intensive units (volume 10 m³) one in each pond. In the intensive units, African catfish (Clarias gariepinus) was cultured and fed with pellet whilst common carp (Cyprinus carpio) was stocked in each extensive unit and raised without any artificial feeding. Three different setups of extensive ponds were tested: the additional artificial plastic substrate for periphyton development equalled to 0, 100 and 200 % of the pond surface area (PP0 %, PP100 % and PP200 %) at feed loading level of 1.2, 1.9 and 2.8 gN m⁻² day⁻¹ in 2008, 2009 and 2010, respectively. The additional net fish yields in the extensive unit were 2.8–6.5 t ha⁻¹ in PP0 %, 5.1–8.1 t ha⁻¹ in PP100 % and 2.1–4.3 t ha⁻¹ in PP200 %. The nitrogen recovery in the additional fish yields of extensive ponds, expressed as the percentage of feed load, was 5.6–6.1, 6.8–10 and 2.1–6.1 % in the treatments PP0 %, PP100 % and PP200 %, respectively. The combined fish production resulted in higher protein utilisation by 22–26 %; even this ratio can be increased by 33–45 % with periphyton application.