On the Fluctuation of Daily Catch in Shirasu Fishery by the Small Drag Net along the Coast of Tosa Bay

Abstract

The daily catch of shirasu fishery in the coastal region of the Tosa Bay fluctuates irregularly day by day, while the seasonal change of species or size composition in catch is almost stationary every year. As the histogram of C(t) which is the cubic root of daily catch shows some normal distribution, time series analysis was carried out depending upon the stantionary and ergodic property of C(t).
According to the inspections, it is inferred that C(t) fluctuation is based on the long term period of a month or more and is modified by some characteristic period of a few days or a week. And under the assumption that C(t) may be determined by the abunadance of fishing resource and the complex system which is composed of some factors and is specific to the fishery ground, the author attempted to analyze C(t) fluctuation by considering the ordinary differental equation of nonhomogeneous type. It was found that the two comparative spectra which are S(ƒ) that is got from the real C(t) fluctuatoon and Sc(ƒ) from this system analysis are in close correspondence.