Impact of (a) the number of main catch vessel (NMV), (b) the power of main catch vessel (PMV), (c) the number of trawl vessel (NTV), (d) the power of trawl vessel (PTV), (e) the release number of larval crab (RELE), and (f) the sea surface temperature (SST) on the catch of Portunus trituberculatus in the northern East China sea.