Ontogenetic changes of feeding selectivity in juvenile jack mackerel Trachurus japonicus collected off south-east Kyushu, Japan

:  We examined the stomach and anterior intestine contents of 123 juvenile Trachurus japonicus over a range of body lengths from 10.7 mm to 51.6 mm, which were sampled in the area south-east of Kyushu on 21 April 2000 using a frame net. Numerically, 97.8% of the prey were copepods. Juveniles of a body length class from 10 mm to 12.5 mm most frequently fed on prey with a body width from 150 µm to 200 µm. Juveniles from 12.5 mm to 45 mm and from 45 to 50 mm most frequently ate zooplankton with a body width of 250–350 µm and 350–400 µm, respectively. Generally, the body width of prey increased with the growth of juveniles. We compared the body width frequency of zooplankton collected with plankton nets with that of the prey in the alimentary canals of T. japonicus. Juveniles of body length from 10 mm to 35 mm and more than 35 mm were found to selectively feed on zooplankton with a body width from 250 µm to 350 µm and from 500 µm to 1000 µm, respectively. We conclude that the ability to feed on large prey items is related to the increased behavioral development with growth.