Swimming depth and behaviour of newly recruiting post-larvae of Sicyopterus japonicus (Gobioidei: Sicydiinae) in the estuary of the Ota River, Wakayama, Japan

Recruitment from the sea to freshwater is a turning point in the life history of amphidromous fish. The amphidromous gobioid subfamily Sicydiinae is distributed widely in Indo-Pacific regions. To understand their ecology at recruitment, which is poorly known for this subfamily, underwater observations of recruiting post-larvae of Sicyopterus japonicus were conducted and their swimming depth was measured in the estuary of the Ota River of Wakayama, Japan. During 16 observation periods from April to July in 2006,2007 and 2008, newly recruiting post-larvae were observed swimming in shoals in the middle layer of the lower half of the water column near the river banks in the upper area of study site and in the upper half of the water column near the river mouth. Although the estuary appears to be a temporary transit area for newly recruiting post-larvae, it may be an important place for them to adapt to their later benthic herbivorous life in freshwater.