Pacific oyster ( Crassostrea gigas) spatfall prediction at Hiroshima, Japan, 1977

Abstract

The spatfall prediction techniques used in Hiroshima Bay, Japan, are described. Data are given for the 1977 season. Plankton hauls from seven stations showed successive high peaks of small, medium and large larvae during early August. Small larvae (< 150 μm diameter) peaked at an average of 1,782 larvae per tonne of sea water (m 3) on 10 August; medium larvae (151–250 μm) at 1,258/m 3 peaked on 12 August as did large settling larvae (251–300 μm) at 350/m 3. A small spatfall occurred on 14, 16 and 18 August when an average of 65, 64 and 65 spat per shell, respectively, attached on collectors during 2 days' immersion. A second, much bigger peak of small larvae built up from 16 August to reach an average maximum of 4,702 larvae/m 3 on 24 August. Medium larvae peaked at 754 larvae/m 3 on 28 August. A red tide then appeared consisting of vast numbers of a dinoflagellate tentatively identified as Gymnodinium simplex, and the anticipated main spatfall did not occur. The effects of red tides are discussed. By 17 September water conditions had improved and an estimated 50–100 spat were reported attached per shell on the collectors. This is about half the 200 spat per shell considered by the industry to be a good set.