Assimilation and isotopic discrimination of hydrogen in tilapia: implications for studying animal diet with δ2H

Hydrogen isotope (delta H-2) analysis has become a valuable tool in the study of animal migration; however, the biochemical framework required for ecologists to confidently apply delta H-2 to quantify resource use has yet to be adequately resolved. In contrast to carbon (delta C-13) and nitrogen (delta N-15) isotopes where food is the only source of these elements, there are two distinct sources of hydrogen available to consumers for tissue synthesis: food and water. To effectively use tissue delta H-2 values as a tracer of food and water resources, two fundamental questions need to be examined in animals that inhabit different environments: (1) What proportion of hydrogen in tissues is derived from sources of food vs. water? and (2) What is the magnitude and degree of variation in total delta H-2 discrimination (Delta H-2(net)) between consumer tissues and these sources? We completed a 3 9 3 controlled feeding experiment on Nile tilapia (Oreochromis niloticus) in which we varied the delta H-2 of tank water and dietary macromolecules to examine these questions in two tissues commonly analyzed by ecologists: muscle and liver. We found that the proportion of hydrogen in tilapia tissue derived from tank water was similar for muscle (similar to 23%) and liver (similar to 25%). We then used linear regression and an isotope mixing model based on accompanying delta C-13 data to estimate the proportion of hydrogen in muscle and liver tissue derived from dietary protein (34-44%), cornmeal (21-27%), corn syrup (4-5%), and lipids (<= 1%). With this information and the delta H-2 values of water, protein, carbohydrates, and lipids supplied to fish in each treatment, we calculated Delta H-2(net) values of -47 parts per thousand +/- 5 parts per thousand for muscle and -41 parts per thousand +/- 5 parts per thousand for liver. Our experiment is the first to quantify the relative proportion of hydrogen from different dietary macromolecules used by an omnivore to synthesize its tissues. Such information is needed to further refine the use of d 2. analysis as a dietary tracer for aquatic animals.