The aim of the study is to investigate the food habits of the Eurasian otter, Lutra lutra, and to examine any effects of the construction of the Busan New Port (BNP) on the prey. The frequencies and bulk estimate ratios of the biological debris, taken from spraints collected seasonally during the period from 2005 to 2011 at 16 areas, were analyzed. As the results, it was demonstrated that otters prey mainly on fish but occasionally on crustaceans and gastropods, etc., and the Mugiliformes was the most preferred fish. Although fish debris was observed throughout the season, it was suggested that the amount of fish eaten by the otter seemed to be associated with the amount of fish inhabiting the study area, judging from the striking similarities in the pattern of seasonal variations in each order of the fish between the frequency from the spraints and the fish catch. On the other hand, the frequencies of most of the fish, crustaceans, and gastropods from the spraints decreased from 2010, indicating the decrease of the amount of the prey by the construction of BNP and the strong possibility of the decrease in the number of otters in the near future. This is supported by the other studies, such as one showing a sudden decrease in the number of spraints since 2010, recent decrease in the fish catch, and the increase of marine pollution at this study area.