The influence of metabolic effects on stable hydrogen isotopes in tissues of aquatic organisms

A steady-state mass-balance model describing controls on the stable hydrogen isotopic ratios (H-2) of tissues in fish was previously developed but physiological effects related to fish size and growth had not been tested. Here, we assessed the influence of size (or growth rate) on tissue H-2 composition of a fish species (Poecilia reticulata) and the incorporation of metabolic products derived from dietary lipids (water, NADH). Sampled tissues were obtained from individuals that grew at different rates while raised on an isotopically homogeneous commercial diet (lipid-free fraction, H-2=-95 +/- 2 parts per thousand; and dietary lipids,-198 +/- 11 parts per thousand) under different controlled water hydrogen isotopic composition (H-2=-128 +/- 3 parts per thousand;+17 +/- 5 parts per thousand; and+202 +/- 5 parts per thousand). Our findings suggested that fish growth rate was correlated positively with the degree of incorporation of metabolic products from dietary lipids that, in turn, influenced both fish tissue protein and lipid H-2 values. We conclude that H-2 measurements of lipids (and, subsequently, of body water) in fish could become a physiological tool that provides insights into fish growth rates.