Isotopic turnover rate and trophic fractionation of nitrogen in shrimp Litopenaeus vannamei (Boone) by experimental mesocosms: implications for the estimation of the relative contribution of diets

The effect of size and diet on the relative contribution of growth and metabolic turnover to changes in isotopic composition of whiteleg shrimp Litopenaeus vannamei was examined by mesocosm experiments. Trough testing different diet types (natural and commercial formulated foods and combinations) and feeding scenarios (satiation, limited‐fed and starvation), we determined the growth (k) and metabolic (m) turnover, rate of isotopic turnover (t₅₀) and isotopic fractionation of N (Δ¹⁵N) during different growth stages [from postlarvae (PL20's) to large juveniles], under controlled conditions. Results revealed that L. vannamei is a diet sensitive species with growth and metabolic processes controlled by the quantity (feeding rate) and quality (in terms of C:N) of food. Relative to the total isotopic change (k + m), the contribution of growth decreased with size shrimp from PL20's to largest juveniles. Metabolic turnover also decreased, but showed an elevated variability. Coefficient k in limited‐fed and starved specimens was lower than those on satiation; m showed the opposite trend. Δ¹⁵N values averaged from 0.30‰ to 3.5‰ (1.87 ± 0.87‰) in satiated L. vannamei, but increased to 2.59–3.09 and 3.28–4.81‰ in limited‐fed and starved specimens. Δ¹⁵N increased with a diet's C:N, indicating an effect of diet quality on isotopic discrimination and also increased with shrimp size, indicating the influence of the metabolic changes. Variations in Δ¹⁵N need to be considered in nutrition studies to estimate correctly the temporal relationship between an organism's isotope ratio and its diet and to analyse the variations of food availability or preferences along a culture cycle.