Feeding Patterns of 'Finless Porpoise (Neophocaena asiaeorientalis)' in the Yellow Sea as Indicated by Stable Carbon and Nitrogen Isotope Ratios

Finless porpoise (Neophocaena asiaeorientalis) is a representative marine mammal that has the largest habitat near the Korean coastal area. However, little information on their trophic ecology has been reported in Korea to date. The stable carbon and nitrogen isotope ratios from muscle samples (n = 60) were analyzed for their trophic ecology. The muscle tissues from the stranded or bycaught porpoises in the Yellow Sea were obtained in 2015 (n = 37) and 2016 (n = 23) by Cetacean Research Institute in Korea. The averages of delta C-13 and delta N-15 for all the samples were -17.74 parts per thousand (S.D. = +/- 0.74 parts per thousand) and 12.96 parts per thousand (S.D. = +/- 0.95 parts per thousand), respectively. The mean isotopic values of the porpoise groups obtained from different seasons were -18.13 parts per thousand (+/- 0.56 parts per thousand) and 12.43 parts per thousand (+/- 0.73 parts per thousand) and -17.06 parts per thousand (+/- 0.46 parts per thousand) and 13.87 parts per thousand (+/- 0.46 parts per thousand) for summer (n = 38) and spring (n = 22), respectively, which were significantly different among the two seasons (t-test; p < 0.01, n = 60, respectively). These increasing isotope values from summer to spring reflected a shift in major prey sources. The small increasing in delta N-15 values could be caused partly by different body lengths of the porpoises. However, the most plausible reason for the large shift in delta C-13 value relative to delta N-15 could be a change from pelagic to benthic prey in the different seasons since benthic diets are reported for enriched delta C-13 over the pelagic diets. For better understanding, further validation should be conducted.