Size matters: comparing stable isotope ratios of tissue plugs and whole organisms

Stable isotope analysis provides an integrated measure of diet source and trophic position and is becoming an increasingly popular tool in studies of food web structure and function. However, stable isotope analysis can require sacrificing whole organisms, which is not always practical or ethical. As an alternative, biopsy punches cause minimal mortality in fish and provide enough sample for stable isotope analysis. Researchers commonly use smaller tissue samples for stable isotope analysis. Such analyses rely on the untested assumption that the stable isotope ratios from small tissue samples are representative of whole organism stable isotope ratios. Here we test this assumption by comparing stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) from tissue plugs and whole organisms over a range of fish sizes. We show that plugs from fish smaller than approximately 90 mm total length introduce a positive bias in δ 15 N. For larger fish, plugs provide a reliable and accurate means of measuring δ 15 N and δ 13 C, with the caveat that they introduce some variability in δ 13 C. Inclusion of overlying skin did not affect stable isotope measurements in our tissue samples.