抄録
Fisheries managements are to maintain populations at a level at which the maximum sustainable yield can be provided. For this purpose it is necessary to clarify the reproductive mechanism but unfortunately it is almost unknown in many kinds of fishes. To overcome this difficulty a rapid method developed and explained here is considered very useful to obtain the optimum coefficient of fishing mortality.
Yield isopleth diagram is not adequate for regulation measures, because it is only a technique for determining the catch of given recruits. In the rapid method, however, reproductivity is introduced and is as described below. The number of spawning adults, A, will be calculated, therefore rates of decrease of A to that of the virgin stock can be obtained. Thus the contours of the rates can be drawn, overlapped on the yield isopleth diagram. Usually the rate in maximum sustainable yield is approximately 1/2. Regulation measures such as fishing efforts and mesh size of net are to take care of the steps to approach the 1/2 contour line of rate. If the actual rate would be below 1/3, the status of the population is in an unfavourable phase for fishing. Rates between 1/2 to 1/3 should be considered as the cautious phase.
As for chrysophrys major in the East China Sea and the Yellow Sea, the present rate of decrease of spawning adults is much less than 1/3. The extreme depletion of the population can be explained by this unfavourable phase. The most effectual maximum sustainable yield can be achieved by adopting adequate regulations as follows; 1/4 of present fishing effort and 100mm mesh-size net instead of the 54mm at present.
