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

Abstract

In cetaceans, determination of diet through the examination of stomach content or feces is subject to difficulties and potential biases. Application of stable isotope analysis of tissues to diet assessment overcomes some of these caveats but, because tissue isotopic turnover is slow, resultant values are only indicative of long-term feeding and may be misleading if recent shifts in prey consumption have occurred. Determination of stable isotope values in feces stand as a potential alternative source for resolving short-term diet. However, values may be potentially biased by the action of digestive enzymes and bacteria. Here we investigate whether the fecal stable isotope values of freshly dead fin whales are consistent with those of the main food found in their stomachs (krill) as well as with those of other potentially consumed prey from the same ecosystem. Results show that stable isotope values of krill remain unaltered despite their transit along the digestive tract and, therefore, values in feces are reliable indicators of the consumption of this prey. Also, the stable isotope values of feces which under visual inspection appeared to only contain fish remains revealed that contribution of krill in the digested food was substantial, as the δ13C and δ15N values were too low to justify an exclusively piscivorous diet. This demonstrates that results from macroscopic gross analysis of feces may be misleading because less digestible components, such as krill, may be overrepresented. We conclude that stable isotope values of feces contribute significant information to other techniques for assessment of short-term diet reconstruction