Ecological Studies on Young Jack-Mackerel, Trachurus japonicus (TEMMINCK et SCHLEGEL), with Special Reference to Behavior and Feeding Habit

In early summer, we can find the young jack-mackerel of various stages along the shore faced to Goto-nada, because the spawning of jack-mackerel is continued from Decmber to August in the coastal waters of western Kyûshû. 1. Observations on marine animals which were attracted to the lamp light were made in early summer of 1953 and 1954 on board at the shore regions of northern Goto-nada (Tab. 1), and young jack-mackerels were classified into 4 groups by their fork length, behaviors and feeding habits. Their behaviors are summarized as follows, refering to the pre-reported data which were collected by the larval net. i) Those of 5-30mm in fork length are in pre-juvenile stage. They stay quietly at the sea surface bending their bodies like the hook under the lamp light. They are chiefly caught by the larval net. ii) Those of 30-45mm in fork length are in meso-juvenile stage. Their swimming abilities are stronger than the former, but they are not able to make schools yet. They are captured by the larval net less than the pre-juvenile ones. iii) Those of 50-80mm in fork length are called "Zengo-Azi" (post-juvenile stage). They grew up so stout as to make schools, and usually migrate to the shore regions. 2. Juvenile ones, 5-45mm in fork length, seem to take no food under the lamp light. Perhaps they are feeding at daytime (Tab. 5). When the fishes attain the size of ca. 50mm, they change their feeding habits from the primary predator to the secondary one, and "Zengo-Azi" become to attack on the juvenile which are less than 45mm CTab. 2). It will be natural that, in these fishes which have long spawning term and change their feeding habits during the period of juvenile stage, there are food relation between the later hatched ones and the early hatched ones. 3. Anchovy is one of the most favourable food for the young jack-mackerel when it inhabits at the shore regions. In early summer young jack-mackerels of 50-80mm do not feed on young sardine, but when they attain the size of ca. 90mm they usually attack the latter