POPULATION DYNAMICS OF THE BASTARD HALIBUT, PARALICHTHYS OLIVACEUS (T. & S.) AND MANAGEMENT OF ITS RESOURCE
Zhu Xinhua(Institute of Oceanology, Academia Sinica, Qingdao 266071)
Although information on fishing effort and yearly catch of the bastard halibut, Paralich-thys olivaceus (T. S.) is not available yet, length-based analysis and age determination of samples collected in 1980-1986 in the Yellow Sea and Bohai Sea were carried out to obtain the age-special strength and mortality traits of the population off the northern China Sea. The main results are summarized as follows:1. On the level of Cπ=0.50, the similarity cluster diagram shows that there are two major inhabitating groups based on changes of the monthly age peaps: (1) on overwinter group (December to March), dominated by age Ⅰ (80.89%), and (2) a breeding group (April to May), dominated by age Ⅲ (33.21%). These two groups outnumbered those in the other seasons. Few fishes older than 4 years old were found in the coastal waters where age 0-I comprised 75.53% from June to November. Spatially, the age structure in Jiaozhou Bay waters is of more complicated pattern than that in the Huanghe River estuary with respect to function as a nursery ground.2. The correlation among the life history parameters and natural mortality coefficient (M) was estimated with multiple regression to modify the formula presented by Gunderson et al. (1988) asM =-0.2302 + 1.7439WGSI + 1.8×10-3L∞ + 0.5352k F1,2 =1.50; F = 30.12; sy = 0.1172; r = 0.9215 where, WGSI is the index of gonad maturity (gonad/body weight), and L∞, k are von Berta-lanffy growth parameters. Several methods were employed to test the correlation among the di- fferent factors selected. As a result, the mean natural mortality coefficient (M) was found to be 0.1553 with variation coefficient of 30.96%, and the total mortality coefficient (Z) was found to be equal to 0.6839 (8.05% variation).3. To demonstrate the substantial effects of different values of M, k, W∞ the biomass curves of a single cohort for both sexes were plotted (Figure 2), with given initial recruit of 1000 fish. It has been shown that female biomass was greater (1231.63 kg at critical age of 6.8199 a) than male's (725.23 kg at the age of 5.9970 a). Maximum cohort production and the critical age decreased significantly with increase of Z. At the present level (F - 0.5286), the fisheries resource would be inadequately exploited for 2.9668 a and 120.59 kg. The yield isopleth diagram (Figure 3) also displayed that the value of 1492 g for maximum sustainable yield per recruit (MSY), can be obtained at fishing mortality coefficient F=1.0 and age of first capture of 6.0 a. Under present conditions, (F= 0.5286, tc = 1.0289 a), MSY of 583 g indicates overf ishing.4. For optimum MSY, regulatory measures should be implemented to: (a) produce more proper size juveniles as soon as possible so that a large number of them can be released for resoure enhancement; (b) limit the harvesting length (larger than 301 mm) for 50% of the sexual mature and mesh size to more than 77mm: (c) totally ban fishing in breeding (April-May) and nursery (August-October) grounds during these months in the coast waters of northern China Seas.