两种经验方法估算中国明对虾自然死亡的比较

为掌握不同自然死亡估算方法对资源变动规律和群体结构特征的影响,以放流中国明对虾渔业为例,采用2种基于生长参数的经验公式估算自然死亡系数,并与已报道的基于渔获量数据得到的结果进行比较,分析3种自然死亡系数随时间的变化规律及对资源的性比结构影响。结果显示:估算方法理论及依据的数据资料不同对自然死亡系数的估算结果影响显著,利用叶昌臣等基于渔获量数据得到的自然死亡系数进行放流后至捕捞结束全时段的模拟,存在低估放流初期个体自然死亡的现象;利用Chen等提出的经验公式估算的自然死亡系数,存在高估放流初期幼体自然死亡的可能,至放流个体生长一周年时性比达4.44:1;利用Gislason等提出的方法估算的自然死亡系数,存在低估放流初期和高估稳定生长期自然死亡的可能,至放流个体生长一周年时性比达2.22:1。在开捕时的BPR和捕捞结束时的累计YPR,基于叶昌臣等估算的自然死亡系数得到的值分别为23.81和22.02,分别是利用Gislason等经验公式得到的值的2.42倍和2.87倍,是利用Chen等提出方法得到的资源量的76.25倍和102.50倍。研究表明,选择渔业资源自然死亡估算方法应以最谨慎的方法进行审查和对比,利用经验方法进行自然死亡系数估算时,为提高估算的准确性、科学性和得到结果具有生物学意义,应引入性别因子(或系数)。

Comparison of two empirical estimates of natural mortality for Chinese shrimp

Chinese shrimp (Fenneropenaeus chinensis) in stocking program was used an example, to illustrate the uncertainty associated with empirically estimated natural mortality on the dynamics and structure of fish populations. Two empirical formulas based on the growth parameters were used to estimate the natural mortality rates which were then compared to the natural mortality estimated based on catch data. The differences in three natural mortalities over time and their impacts on sex ratio were evaluated. Our analyses showed that estimation methods could greatly affect the estimation of natural mortality. Although the mating mortality was included in the natural mortality obtained based on the catch data (Ye et al., 1987), the M of young fish has been underestimated at early releasing stages. The M was overestimated with the empirical formula proposed by Chen & Watanabe (1989), and the sex ratio was 4.44:1 at the end of one-year old China shrimp. The M was underestimated at the early releasing and overestimated at the steadily growth stage with the Gislason's method, and the sex ratio was 2.22:1 at the same time. At the beginning and end of fishing, the amounts of resources and catches based on Ye et al method were over 2.42 and 2.87 times the amounts estimated using the Gislason's empirical formula, and about 76.25 and 102.50 times of the values estimated with Chen and Watanabe's method. These data suggest that the empirical methods selected to estimate the natural mortality need to be based on the most rigorous criterion, and when calculating the natural mortality gender-specifically by empirical methods, sex parameter should be included to ensure that the results are biologically meaningful.