Diurnal Changes in Vertical Distributions of Anchovy Eggs and Larvge in the Western Wakasa Bay

Abstract

Diurnal Changes in Vertical Distributions of Anchovy Engraulis japonica eggs and larvae were studied, in connection with feeding habits of the larvae, in the western Wakasa Bay, Japan Sea, in July and August 1982.
Eggs of the earliest stage of development are found in abundance in the water of about 50m depty. rising up to 0-25m depth and sinking down to about 50m depth during the late stage fo development. While the larvae of less than 4mm BL are confined to waters between 25m and 50 m depth all day, diurnal vertical migration of rather large larvae occur. Namely, the larvae fo more than 6mm BL are present in surface waters in daytime, but by night move downward to about 25m depth.
Main food components of 5-7 mm larvae are copepod nauplii and copepodites, while Paracalanus parvus and Oithona spp. form the principal food for the larvae fo more than 7mm BL.
As the feeding time of anchovy larvae is estimated as daytime and the larvae of more than 7 mm BL have a well expanded gas bladder, it seems possible that the diurnal vertical migration is due to theri feeding habits and functions of gas bladder, developing into active feeding migration as larval growth procceeds.