Diet and Food Assimilation by Channel Catfish and Malaysian Prawns in Polyculture as Determined by Stomach Content Analysis and Stable Carbon Isotope Ratios ¹

Stable carbon isotope ratios (∂ 3 C) and stomach contents were used to determine the diet of channel catfish (Ictalurus punctatus) and freshwater Malaysian prawns (Macrobrachium rosen‐bergii) in polyculture. Catfish stomach contents were dominated by formulated rations. Macrophyte fragments and catfish ration were predominate in prawn stomachs. Catfish ∂ 13 C ratios became lighter from June to October, which paralleled the change in ∂ 13 C ratio of the formulated feeds fed to the fish. Prawn ∂ 13 C ratios became heavier from June to October with a change in diet from seston to aquatic macrophytes and formulated catfish feed. Formulated feeds contributed 68‐99% of catfish growth, and “natural” pond biota, principally insects, the remainder. Prawns depended to a greater extent on the autotrophic food web for their diet. Seston and macrophytes contributed 18‐75% of prawn growth, and formulated feeds the remainder. The prawns fed on seston at small sizes (<7 g) and consumed more aquatic macrophytes and formulated catfish ration at larger sizes (<7 g).