Effects of carbohydrate addition on production in extensive shrimp culture systems

Abstract

One indoor and one on-farm trial were conducted to evaluate the effect of control of carbon/nitrogen ratio (C/N ratio) by addition of carbohydrate to the water column in extensive types of shrimp culture systems. In the indoor experiment, 25% and 40% dietary protein (‘P25’ and ‘P40’) with or without carbohydrate source addition (‘P25+CH’ and ‘P40+CH’) were compared in fiber reinforced plastic tanks of 1200-l capacity stocked with 6 Penaeus monodon juveniles (0.357±0.01 g) m −2. In the on-farm trial, 25% dietary protein with carbohydrate (‘P25+CH’) and 40% dietary protein (‘P40’) were compared in 250-m 2 earthen ponds stocked with 6 post-larvae of P. monodon m −2. Tapioca flour was used as carbohydrate source and applied to the water column followed by the first feeding during the day in both experiments. The addition of carbohydrate significantly ( P<0.001) reduced the total ammonia nitrogen (TAN) in the water and sediment in both experiments. It significantly ( P<0.05) increased the total heterotrophic bacterial (THB) population both in water column and sediment. In the indoor experiments, lower specific growth rate (SGR) and higher feed conversion ratio (FCR) values were recorded in the ‘P25’ treatment compared to shrimps in other treatments ( P<0.05). Higher shrimp yield was recorded in ‘P25+CH’ (64.43 g m −2) when compared to ‘P40’ (44.79 g m −2) ( P<0.001) in the on-farm trial. The FCR value was lower ( P<0.05) in the ‘P25+CH’ treatment than in the ‘P40’ treatment. The nitrogen retention (%) and protein efficiency ratio (PER) were higher ( P<0.001) in the ‘P25+CH’ treatment when compared to other treatments in both experiments. Survival of the shrimps was not affected by treatment ( P>0.05). In the on-farm trial the benefit cost ratio was higher in ‘P25+CH’ treatment than ‘P40’ (1.3 against 0.2) and the profit increased 400% in‘P25+CH’ treatment. A 35% reduction of feed cost and 54% increase in the revenue from shrimp was recorded in the ‘P25+CH’ treatment when compared to the ‘P40’. Control of C/N ratio by the addition of a carbohydrate source to the pond water column benefited the extensive shrimp culture practices in three ways (1) increased heterotrophic bacterial growth supplying bacterial protein to augment the shrimp production, (2) reduced demand for supplemental feed protein and subsequent reduction in feed cost and (3) reduced toxic inorganic nitrogen levels in the pond as well as effluents.