RESUMO
Carbon and nitrogen stable isotope ratios (i.e., δ13 C and δ15 N values) in soft tissues of cetaceans are reported to change on decomposition. Care should be taken when determining the values using samples collected from dead-stranded cetaceans. We have developed a new analytical method to exclude the effect of decomposition on δ13 C and δ15 N values using muscles from narrow-ridged finless porpoises (Neophocaena asiaeorientalis). METHODS: The muscles from 53 fresh (bycatch and live-stranded individuals) and 213 dead-stranded individuals were analyzed by isotope ratio mass spectrometry combined with an elemental analyzer. The carbon and nitrogen content was compared between fresh and dead-stranded samples to investigate any decrease in the content with decomposition. In dead-stranded samples, the effect of changes in the carbon and nitrogen content and the C:N ratio (carbon/nitrogen) on δ13 C and δ15 N values was predicted by the generalized additive model (GAM). RESULTS: The carbon and nitrogen content (mean ± SE) in the muscles of fresh samples was 0.462 ± 0.0019 mg/mg and 0.147 ± 0.0005 mg/mg, respectively, while the content of dead-stranded samples was lower: carbon: 0.441 ± 0.0033 mg/mg and nitrogen: 0.135 ± 0.0012 mg/mg. When using the GAM, knowledge of the carbon and nitrogen content and the C:N ratio improved the prediction precision. CONCLUSIONS: Prediction by the GAM showed changes in δ13 C and δ15 N values with a change in carbon and nitrogen content. The effect of decomposition can be excluded in statistical analysis of stranded samples by using samples with carbon and nitrogen content and C:N ratio equivalent to those of fresh samples or by using the carbon and nitrogen content and C:N ratio as an explanatory variable in the GAM.
