東シナ海におけるマアジ資源の研究-I

Abstract

In the case of assessment of fish resources, it is essential to know natural and fishing mortality coefficients of the fish resources under consideration. In 1958, the virgin resource of the jack mackerel, Trachurus japonicus (TEMMINCK et SCHLEGEL), was found in the mid-region of the East China Sea, and has been intensely exploited since 1959 (Fig. 2). In consequence the yield of the fish from the East China Sea has markedly been increasing, but recently this trend has stopped and the catch remains on the same level (Fig. 1). In the present paper is shown: (1) the corrected fishing effort (X) in terms of the number of fishing days of the purse seine of one-boat-operation type of 50-100 tons (Table 1). (2) the total mortality coefficient of the fish in each of the recent years which is acquired on the basis of the age composition of the fish caught in one day's operation by purse seine of one-boat-operation type in the East China Sea during the period from 1957 to 1963 (Tables 2 and 3). According to WIDRIG (1954), if a series of pairs of values of fishing effort (X) and total mortality coefficient (Z) is known, equation (3) gives a linear relation between them, as shown in Fig. 3. By application of the method, estimates of natural mortality coefficient (M) and catchability coefficent (q) are obtained at a high level of 0.994 and 0.03154 per 1, 000 fishing days, respectively. The 95% confidence limits for M and q are 0.055-1.933 and -0.00491-0.07797, respectively. The estimate of M aforementioned is greater than that of F, and also correspondent to 1.32 K (“K” represents growth rate in BERTALANFFY's growth equation). Judging from the various facts, the estimate of M in the present paper seems to give an indication of its order of magnitude. The high level of M shows one of the characteristics of the jack mackerel fisheries in the East China Sea.