Size of Mysids Eaten by Juvenile Flounder Paralichthys olivaceus and Diurnal Change of its Feeding Activity

Abstract

Juvenile flounder Paralichthys olivaceus in the Sea of Japan mainly fed on mysids. Five mysid species were identified in the 197 stomachs of the flounder examined, and they contained 1673 specimens of mysids. Acanthomysis robusta was most numerous (93.5%). Other species were Neomysis japonica (5.3%), A. nakazatoi (0.9%), Nipponomysis perminuta (0.2%). Ni. toriumii (0.1%). Total length of broken mysid in stomach was estimated using the regression equations between total length and the six part lengths i.e. carapace length, carapace width, length of 6th abdominal segment, exopod length of uropod, endopod length of uropod and telson length. Maxi-mum size of A. robusta in stomach increased with size of juvenile flounder. Mysid eaten by the flounder less than 20mm in body length was 5.4mm, and ones consumed by the20-25mm and 25-30mm flounder became rapidly larger to 7.4mm and 9.7mm, respectively. Juveniles less than 35mm mainly fed on small mysids less than 5mm, whereas ones greater than 35mm actively fed on large mysids (>30% of mysid number in stomachs).
The percentage of mysid weight in stomachs to body weight of flounder less than 30mm was peaked (4.9%) in the evening, whereas the percentage dropped to 0% after midnight. A similar diurnal feeding pattern was found for juveniles greater than 30mm.