Stable isotope analysis of fecal material provides insight into the diet of fin whales

Abstract

In cetaceans, stable isotope analysis of tissues overcomes some of the potential biases encountered in studies of diet based on stomach content or fecal analysis. However, stable isotope ratios of tissues are only indicative of long-term feeding and may be misleading when recent shifts in prey consumption have occurred. Stable isotope ratios of feces stand as a potential alternative source for resolving short-term diet, but may be biased by digestive enzymes and bacteria. We investigate whether fecal stable isotope ratios of freshly dead fin whales are consistent with those of the main food found in their stomachs (krill) and with those of other potential prey. Results show that stable isotope ratios of krill remain unaltered after their transit along the digestive tract and, therefore, values in feces are reliable indicators of this prey consumption. In addition, the low isotope ratios of feces that visually appeared to contain only fish remains revealed a substantial contribution of krill in the digested food. This demonstrates that macroscopic gross fecal analysis may be misleading because less digestible components, like fish bones, may be overrepresented. We conclude that stable isotope ratios of feces contribute significant information to other techniques for short-term diet reconstruction.