A SHORT NOTE ON THE FEEDING OF THE MACKEREL IN THE MIDDLE JAPAN SEA DURING ITS OVER-WINTERING PERIOD

Abstract

Examinations were made on the stomach content of the mackerel Pneumatophorus japonicus(HOUTTUYN)caught by purse seine boats off the southern districts of Niigata Prefecture during the period from late February to late March, 1957. The fish were determined to be under the overwintering demersal period or phase. The results are brlefly summarized as follows : 1. The stomach content was generally small in weight and the majority of the specimens had empty stomachs during the period of the low water temperature which lasted from late Februaty to middle March. Occaslonally much-fed individual or individuals were observed, their diets being composed in the main of large-sized Enphausia pacifica and of small-sized fishes and squids in less frequency. The poor feeding during this period was considered to be due to the scarcity in food animals available to the mackerel, not to the suspension of feeding activity, if any, in the fish by some physiological causes. 2. From middle March onwards, however, the stomach content increased in weight and its specific composition also markedly changed. Several kinds of remarkably fatty copepods and amphipods were abundaatly contained in addition to euphausiids. This fact was supposed to be due to the corresponding change and enrichment in the zooplankton population of the mackerel habitat caused by the shoreward drift and thriving of representative cold-water plankton crustaceans of the Japan Sea such as Calanus plumchrus. Cal. cristatus, Metridia lucens and Parathemisto oblivia. 3. It may be of special interest to note that the mackerel fished in late March voraciously fed on large amounts of the V copepodite-stage larvae of Calanus cristatus, one of the most distinguished copepod species of the arctic or subarctic elements.