Feeding Habits of Adult Masu Salmon, Oncorhynchus masou in Coastal Waters near Oshika Peninsula, Honshu, Japan

Abstract

Adult masu salmon were collected with set nets or purchased in fish markets near Oshika peninsula (the Pacific coast of northern Honshu) from 1983 to 1989. Their stomach contents were observed and the food composition was analyzed using 3 different indices (number, weight, frequency of occurrence of each kind of prey) . The Index of Relative Importance (IRI) was also used to compare the accuracy of the 3 indices. Results were analyzed in relation to the month of catch size classes of masu salmon. Sand lance, Ammodytes personatus, anchovy Eugraulis japonica, juvenile of Alaska pollack, Theragra chalcogramma, sardine, Sardinops melanostictus among others constituted the main food independently of season and size but masu salmon also preyed on large-sized plankton such as Themisto japonica and Euphausia japonica. Overall, sand lance was a major contributor to food in weight and occurrence between March and June whereas T. japonica was the major contributor to food in number in April and May. Large-sized masu salmon depended more heavily on fish as food than small-sized masu salmon. Furthermore, the size and composition of prey differed between adult and juvenile masu salmon. Adult masu salmon took 1-age sand lance, large-sized T. japonica, and E. japonica, and pelagic fish such as sardine. Juvenile masu salmon took 0-age sand lances, small-sized T. japonica, Thysanoessa longipes (Euphausiasia), and brackish water fishes such as ice goby, Leucopsarion petersi. These results indicate that masu salmon change habitat and food-size selectivity as they grow.