Impacts of sampling design on the abundance estimation of Portunus trituberculatus using crab pots

Abstract

Fishery-independent surveys can provide high-quality data and support fishery assessment and management. Optimization of sampling design is crucial to increase the quality of fishery surveys. Crab pots are important fishing gears used to catch crabs. We analyzed the impacts of sampling design of crab pots on the abundance of Portunus trituberculatus in the Changjiang (Yangtze) River estuary to the Hangzhou Bay and its adjacent waters in East China Sea. The crab pots were cylindrical, 240 mm in height and 600 mm in diameter of the iron ring. Our sampling designs (including fixed-station sampling, simple random sampling, stratified fixed-station sampling, and stratified random sampling), three number of stations (9, 16, and 24), and three numbers of crab pots (500, 1 000, and 3 000) were simulated and compared with the “true” abundance that obtained from bottom trawl surveys in the study area in 2007. The scenarios with 16 stations were set in stratification as a control group for comparison with unstratified designs. Results show that simple random sampling can obtain more stable results than fixed-station sampling in the abundance estimation of P. trituberculatus. In addition, stratified sampling resulted in more accurate abundance than unstratified sampling. The accuracy of the simulated results improved with the increase of the number of stations. No remarkable differences in the results were found among the scenarios of different number of crab pots at each station. However, resource-intensive areas exerted great impacts on simulation results. Thus, prior information or pre-survey results about resource abundance and density distribution are necessary. This study may serve as a reference for future sampling designs of crab pots of P. trituberculatus and other species.