Using hydrogen isotopes of freshwater fish tissue as a tracer of provenance

Hydrogen isotope (H-2) measurements of consumer tissues in aquatic food webs are useful tracers of diet and provenance and may be combined with C-13 and N-15 analyses to evaluate complex trophic relationships in aquatic systems. However, H-2 measurements of organic tissues are complicated by analytical issues (e.g., H exchangeability, lack of matrix-equivalent calibration standards, and lipid effects) and physiological mechanisms, such as H isotopic exchange with ambient water during protein synthesis and the influence of metabolic water. In this study, H-2 (and N-15) values were obtained from fish muscle samples from Lake Winnipeg, Canada, 2007-2010, and were assessed for the effects of species, feeding habits, and ambient water H-2 values. After lipid removal, we used comparative equilibration to calibrate muscle H-2 values to nonexchangeable H-2 equivalents and controlled for H isotopic exchange between sample and laboratory ambient water vapor. We then examined the data for evidence of trophic H-2 enrichment by comparing H-2 values with N-15 values. Our results showed a significant logarithmic correlation between fork length and H-2 values, and no strong relationships between N-15 and H-2. This suggests the so-called apparent trophic compounding effect and the influence of metabolic water into tissue H were the potential mechanisms for H-2 enrichment. We evaluated the importance of water in controlling H-2 values of fish tissues and, consequently, the potential of H isotopes as a tracer of provenance by taking account of confounding variables such as body size and trophic effects. The H-2 values of fish appear to be a good tracer for tracking provenance, and we present a protocol for the use of H isotopes in aquatic ecosystems, which should be applicable to a broad range of marine and freshwater fish species. We advise assessing size effects or working with fish of relatively similar mass when inferring fish movements using H-2 measurements.