青海湖裸鲤种群数量变动的初步分析

PRELIMINARY ANALYSIS OF POPULATION DYNAMICS ON THE NAKED CARP (GYMNOCYPRIS PRZEWALSKII PRZEWAISKII (KESSLER))IN QINHAI HU

青海湖裸鲤是青海湖唯一的捕捞对象。开发后的种群数量变动主要根据1975-1977年该湖渔获物资料及过去研究的裸鲤生物学特性。计算了该种群的若干特征值,应用Beverton-Holt数学模式计算不同捕捞努力量或用不同网目的拖网对裸鲤种群在一生中总个体数、总生物量,和对裸鲤种群一生中能捕捞的总个体数及总渔获重量的影响,以及裸鲤的最大持续产量和现有种群数量。评价了裸鲤资源利用现状,提出有关生产的一些初步意见。

The naked carp (Gymnocypris przewalskii przewalskii (Kessler) ) is the only fisheryproduct in Qinghai Hu and investigation on the resource of this fish is rarely taken. Thetotal catches for the whole Lake was nearly 140,000 metric tons during the period from1958--1976, but both the size of individuals and the yield per year have decreased. It isstill jn question whether the resource of population has declined or the present fishing ismoderate. The study of population of naked carps was carried out during 1975--1977.The general status of resource dynamics is investigated by the use of Beverton--Holt's mathmatical model. By analysing the results of catches of naked carp it shows that the total mortalitycoefficient is 0.68; the natual mortality coefficient is 0. 18; and the fishing mortalitycoefficient is 0. 50. The six data (Pn/R, Pw/R, Yn/R, Yw/R, Wy, Ly) are calculatedunder conditions of different fishing mortality coefficient. They are supposed atdifferent initial fished age (tc). The eumetric catches curve is ploted and the maximumbiomass age is estimated. According to actual measurement and calculation results, it is summarized as follows: 1. The naked carp is a stable population of which the status of development of fisheriesagrees with general law of exploited lake. The average length and average weight of fishedindividual in exploited lake declined in general. In the survey of the latest three yearslength and weight show gradually stable without declination any longer. The populationnumber and fishery yield are relatively stable at present, it is considered that the resourcehas not declined. 2. The results of calculation show that under conditions of initial fished age of 10.0and fishing mortality coefficient of 1.25, naked carps have a standing population of50,000 metric tons, its maximum sustained yield is 4,791 metric tons/year. But thepresent yield (4,200 metric tons/year), obtained under conditions of initial fished age of7. 0 and fishing mortality coefficient of 0. 50. If initial fished age being raised to 10 yearsold (i. e. enlarging the mesh size), the catches by weight may be expected to increase by10 per cent, but it still has certain distance to attain maximum, unless both initial fishedage and fishing effort should be increased, the total yield can reach to maximum. In doingso, it is certain to increase the capital of investment, but the yield and the economicaleffect increase only a little. Thus, compared with intensifying fishing effort, whethereconmically or in fish growth, enlarging mesh size seems profitable. Owing to the resultsmentioned above, we consider it is suitable to maintain present mesh size and keep a yieldof 4, 700 metric tons/year. 3. For ensuring recruitment parent spawners, it must be protected on a basis ofregulations and restrictions. 4. Mathmatical model is used for estimation maximum sustained yield of standing fishpopulation in approximation. The subsequent test of practice and further study is needed.