Watershed source determination of fishes is used to assess stock and health, and to determine sources of metals and other contaminants in fish populations. Food web trophic position and watershed source assignment is typically done using carbon and nitrogen isotopes. Research conducted on four fish species shows that, while H isotopes have been posited as a better alternative, metabolic effects and isotope exchange may complicate interpretations.

Recent studies suggest that stable hydrogen isotope ratios (δ²H) of fish can be used to track their watershed origin and the relative contributions of source material. Both applications assume no metabolic or trophic effect on tissue δ²H. However, a study of the local variation of δ²H values in four fish species of contrasting size and feeding habits in an isotopically homogenous reservoir (Flix, Spain) showed that size and feeding habits are factors that should be controlled when tracing origins of fish or their dependence on nutrient inputs using δ²H measurements.

Muscle δ²H values differed greatly among fish species: European catfish, common carp, rudd and roach. The influence of fish size and trophic position affected the δ²H values. Possible mechanisms that drive such variation may be a metabolic effect and /or the compounding effect of ambient water δ²H on exchangeable H at each trophic stage.

Source: Soto, D.X., L.I. Wassenaar, K.A. Hobson and J. Catalan. 2011. Effects of size and diet on stable hydrogen isotope values (δD) in fish: implications for tracing origins of individuals and their food sources. Canadian Journal of Fisheries and Aquatic Sciences 68:2011-2019 DOI: doi:10.1139/F2011-112

Contact: David Soto, 306-975-5757, Aquatic Ecosystem Impacts Research Division; Keith Hobson, 306-975-4102, Wildlife Research