Population Fluctuation of the Small Filefish, Rudarius ercodes, in the Zostera Bed in Odawa Bay

Abstract

Seasonal and annual population fluctuations of Rudarius ercodes in the Zostera bed in Odawa Bay were studied. A recruitment group of small sized fish occurred every summer and grew into large sized fish by spring of the following year. The number of catch per haul of each recruitment group was used as an index of relative population density. From the seasonal changes of this index, three peaks were observed in September-October, May-June, and August-September at the head of the bay and in October-November, April-May, and August-October at the mouth of the bay. In winter only a few individuals stayed at the monuth and none at the head of the bay, although they were distributed in both areas in other seasons. From the change of the number per haul of three recruitment groups at each peak, it was found that the great annual population fluctuation at the first peak decreased gradually and the population level approached about 100 individuals per haul at the third peak. These facts suggest the possiblilty of movement in and out of the bay and some process of population regulation.