Assessment of errors in cetacean diet analysis: in vitro digestion of otoliths

Research into the feeding habits of cetaceans has traditionally relied on examining samples taken From the digestive tracts of commercially caught or stranded and by-caught animals. This depends on the identification and measurement of hard parts such as fish otoliths. Otoliths are often partially digested in the stomach, making quantitative estimates of fish size from otolith size biased.

As a preliminary attempt to quantify the size reduction of otoliths found in the stomachs of small cetaceans, an in vitro digestion experiment was carried out. Otoliths of herring, whiting, poor cod and haddock were digested at 37 degrees C, pH = 3.6, and measured every 60 min.

Linear regressions relating otolith size to digestion time and original size generally provided an adequate description of the time course of size reduction. The parameter that changed most rapidly due to digestion was otolith thickness. Differences in digestion rates between species reflect the general robustness and shape of the otoliths.

A simulation using actual dietary data showed that the overall diet composition was not radically changed by accounting for otolith digestion, although the apparent contribution of herring increased as the assumed digestion time is increased, because herring otoliths were the most susceptible to digestion.