底拖网渔具的采样设计对三疣梭子蟹资源量指数估算的影响

在渔业资源调查中，采样方案直接影响资源量估算的准确性，对采样方案进行优化可以提高调查数据的质量。基于2007年使用单拖网渔船在长江口——杭州湾及其邻近海域调查获得的数据，模拟出该海域三疣梭子蟹的资源数量及各季节的分布情况，作为“真值”，模拟计算不同采样方案调查获得的三疣梭子蟹资源量并与“真值”进行比较，评估不同方案的采样效果。本文利用计算机模拟了定点采样、简单随机采样、分层定点采样和分层随机采样四种采样方法，其中，定点采样和简单随机采样分别设置了9个、16个和24个采样站位，分层定点采样和分层随机采样设置了16个采样站位。三疣梭子蟹对拖网渔具的反应距离受年龄、季节和底质等条件的影响，因此，设置了1.5m，3m和5m三种反应距离。结果表明：分层采样设计优于不分层采样，简单随机采样优于定点采样。模拟结果的准确度随着采样站位数量的增加而提高，例如，定点采样方案中，9个站位时相对估算误差（REE）的最大值为163.43%，最小值为49.40%，而24个站位时REE的最大值和最小值分别是38.62%和4.15%。随着反应距离的增加，REE值和相对偏差的绝对值（RAB）逐渐降低。三疣梭子蟹的资源密集区和资源密度方差大的季节对模拟结果具有显著影响，因此，掌握该区域的资源密度情况或者进行预调查都能使采样设计方案更加合理。本研究可为底拖网渔具对三疣梭子蟹及其他物种的采样设计提供参考。

Impacts of the sampling design on the abundance index estimation of Portunus trituberculatus using bottom trawl

In the survey of fishery resources, the sampling design will directly impact the accuracy of the estimation of the abundance. Therefore, it is necessary to optimize the sampling design to increase the quality of fishery surveys.The distribution and abundance of fisheries resource estimated based on the bottom trawl survey data in the Changjiang River(Yangtze River) Estuary-Hangzhou Bay and its adjacent waters in 2007 were used to simulate the"true" situation. Then the abundance index of Portunus trituberculatus were calculated and compared with its true index to evaluate the impacts of different sampling designs on the abundance estimation. Four sampling methods(including fixed-station sampling, simple random sampling, stratified fixed-station sampling, and stratified random sampling) were simulated. Three numbers of stations(9, 16 and 24) were assumed for the scenarios of fixed-station sampling and simple random sampling without stratification. While 16 stations were assumed for the scenarios with stratification. Three reaction distances(1.5 m, 3 m and 5 m) of P. trituberculatus to the bottom line of trawl were also assumed to adapt to the movement ability of the P. trituberculatus for different ages, seasons and substrate conditions. Generally speaking, compared with unstratified sampling design, the stratified sampling design resulted in more accurate abundance estimation of P. trituberculatus, and simple random sampling design is better than fixed-station sampling design. The accuracy of the simulated results was improved with the increase of the station number. The maximum relative estimation error(REE) was 163.43% and the minimum was 49.40% for the fixed-station sampling scenario with 9 stations, while 38.62% and 4.15% for 24 stations. With the increase of reaction distance, the relative absolute bias(RAB) and REE gradually decreased.Resource-intensive area and the seasons with high density variances have significant impacts on simulation results. Thus, it will be helpful if there are prior information or pre-survey results about density distribution. The current study can provide reference for the future sampling design of bottom trawl of P. trituberculatus and other species.