Natural stable isotopes as indicators of the relative contribution of soy protein and fish meal to tissue growth in Pacific white shrimp (Litopenaeus vannamei) fed compound diets

Reduction of fishmeal usage in feeds and replacement with plant ingredients is a critical element of long term sustainable expansion of shrimp aquaculture. The present study set out to investigate the potential use of differing natural stable isotopes in plant- and fish-sourced dietary components in the assessment of their relative utilisation in compound diets. Pacific white shrimp, Litopenaeus vannamei postlarvae (2.0 ±0.6 mg ww) and juveniles (414 ±11 mg ww) were fed on eight experimental isoenergetic and isonitrogenous diets at each of two protein levels (46 and 23%). Diets were formulated with either fishmeal or soy protein isolate as the only protein source, or in ratios of 50:50 and 10:90 based on nitrogen content. Contrasting isotopic values (δ15N/14N and δ13C/12C) of the fishmeal and soy protein allowed estimation of the relative contribution of both ingredients to muscle tissue growth using an isotopic mixing model. The dietary effects on the nitrogen and carbon turnover rates and on the isotopic diet-consumer discrimination factors (Δ15N and Δ13C), were also assessed. Nitrogen and carbon half lives in muscle tissue were estimated from turnover rates and growth after using an exponential model of isotopic change. In all cases, analysis of postlarval and juvenile muscle tissue indicated a biased nutritional contribution from fishmeal, so that in postlarval shrimp fed on the 46% protein diet containing a 50:50 ratio of the two ingredients, 69% of nitrogen and 59% of carbon in muscle tissue originated from fishmeal; while in animals fed diets containing the 10:90 ratio of the two ingredients, the relative contributions of these elements from fishmeal were 32 and 24% respectively. Juvenile shrimps fed on the same diet, incorporated 73% of nitrogen form fishmeal and 27% from soy protein, and 65 and 35% of the respective contributions of carbon. The mixed diets with higher soy inclusion supported growth equivalent to that of the control and fishmeal-based diets, possibly due to the disproportionate contribution of fishmeal to growth. Muscle tissue of shrimps fed either on 100% fishmeal- or soy-based diets showed contrasting Δ15N and Δ13C, the former being positively correlated to soy protein inclusion level. The magnitude of these values further increased when shrimps were fed on diets containing the lower protein level. The results demonstrate the potential for application of natural stable isotope analysis of diets and tissue in evaluation of plant-source ingredients and the optimisation of their use in diets containing very low levels of fishmeal