One diet containing 9.1 g lysine kg-1 (which is lysine-deficient) and one diet containing 13.1 g lysine kg-1 diet (which is lysine-sufficient) were formulated . These diets are referred to the low lysine (LowL) and balanced lysine (BalL ) diet, respectively. Fish were grown in clear water aquaria or in earthen ponds. The aquaria had a water volume of 110-L and were part of a recirculation aquaculture system (RAS). In each of 4 replicate aquaria per diet, twenty 30-g (± 1.9-g SD) Nile tilapia, Oreochromis niloticus were stocked. The ponds had a surface area of 200 m2 , each with an independent water inlet and drain. In each of 5 replicate ponds per diet, nine hundred 17-g (± 1.6 g) Nile tilapia were stocked. The fish were fed two times daily, receiving with each feeding half of the ration of 24 g kg-0.8 d-1 in RAS and 18 g kg-0.8 d-1 in ponds.
In RAS, fish fed the BalL diet performed better than fish fed the LowL diet, with a higher yield (kg m-3), specific growth rate (% body weight d-1), nitrogen retention efficiency (%), protein efficiency ratio (g weight gain g-1 dietary protein), feed conversion ratio (g feed g-1 body weight gain), protein content and essential ammino acid content (P < 0.05). The percentage survival, feed intake and apparent digestibility were similar between diets (P > 0.05).
In ponds, the fish performed similar for both diets (P > 0.05) for nitrogen utilization efficiency, percentage survival, body composition and essential amino acids, and feed intake. The plankton content in the gut and plankton abundance in the water column were very similar between diets. The Pearson correlation coefficient (r) between plankton abundance and growth in fish fed the LowL diet was .76 and −.96 for phytoplankton and zooplankton, respectively, compared with .50 and .54 in fish fed the BalL diet. The contribution of natural food to nitrogen gain was 30% in fish fed the LowL diet, compared with 21% in fish fed the BalL diet (P < .05).
The present study shows that the food web provided natural foods containing lysine, allowing the fish in ponds to overcome the dietary lysine deficiency. The protein efficiency ratio in ponds was improved 46% thanks to the food web, which was not possible in clear-water tanks.