Effects of stocking density on the performance of lettuce ( Lactuca sativa ) in small‐scale lettuce‐Nile tilapia ( Oreochromis niloticus L.) aquaponic system

Aquaponics incorporates aquaculture with hydroponics, whose principles and operations largely remain untested. Currently, there is a knowledge gap on the performance of aquaponic systems used for fish production under different stocking densities. This may hinder adoption and generally retard the development of aquaponic culture systems for fish and vegetation production in Kenya. The objective of the current study was to determine the effect of fish stocking density on the growth performance of lettuce ( Lactuca sativa ) and Nile tilapia ( Oreochromis niloticus ) reared in a coupled aquaponic system. We used three fish stocking densities of 150, 300 and 450 fish m −3 , each replicated five times. For each treatment or fish stocking density, 16 lettuces were planted m −2 as the planting density. We monitored the following water quality parameters to determine the effects of fish stocking density on water quality; pH, total ammonium–nitrogen (TAN), nitrates( NO 3 − ) $( {{\rm{NO}}_3^ - } )$ ,nitrites ( NO 2 − ) ${\rm{nitrites}}\,( {{\rm{NO}}_2^ - } )$ . In the different stocking densities (450, 300 and 150 fish m −3 ), the water quality parameters ranged from dissolved oxygen: 4–7 mg L −1 ; pH: 6.3–7.3; TAN: 0.32–0.57 mg L −1 ;NO 3 − ${\rm{NO}}_3^ - $ : 0.13–0.36 mg L −1 andNO 2 − ${\rm{NO}}_2^ - $ : 0.020–0.046 mg L −1 . The final weight of the Nile tilapia fingerlings was 25.2 ± 4.2, 32.0 ± 3.8 and 42.6 ± 3.1 g for 450, 300 and 150 fish m −3 , respectively. The biomass of lettuce was statistically different ( p  < 0.005) and increased with increasing stocking density as 166.4 ± 9.8, 276.8 ± 23.2 and 304.6 ± 23.2 g for 150, 300 and 450 fish m −3 , respectively. Adopting the intermediate stocking density of 300 fish m −3 , which gave the best performance of the aquaponic system, is therefore highly recommended.