Stocking effectiveness of hatchery-raised swimming crabs (Portunus trituberculatus) released into Liaodong Bay

We conducted a quantitative analysis to assess the stocking effectiveness of swimming crabs (Portunus trituberculatus) using survey data collected in June, July and August 2018. We compared survey results before and after the release of hatchery-raised P. trituberculatus into the waters of Liaodong Bay, China. The wild population was relatively rare, only 2.80 ind./h prior to release of cultured crabs in early to mid-June 2018. Capture rates increased to 16.60 ind./h, 2.20 ind./h, 13.33 ind./h in mid-July, mid-August, and late August, respectively after the release of 36.789 4 million seedlings P. trituberculatus. The changes of resource density and spatial distribution of P. trituberculatus in Liaodong Bay were analyzed. The reproductive capacity of the wild population was assessed using intrinsic rate of increase before and after the release, and the proportion of released individuals in this population was calculated. The percentage of one year-aged group of P. trituberculatus was calculated by virtual population analysis, based on body size distribution. The recapture rate and economic benefits were estimated to quantitatively assess the effect of the release. The results showed that the supplemented stock accounted for 30.10% of the total catch, and one-year old P. trituberculatus accounted for 94.54% of the total catch in 2018. The total yield of P. trituberculatus was 1 890 tons in 2018, and the proliferation release recapture yield was 538 tons with the recapture rate of 9.36%. The input-output ratio was 1∶12.02. Sea sweep method estimated the P. trituberculatus stock biomass to be 2 056 tons, which is close to the actual yield of the harvest survey. It was confirmed that the biomass of the P. trituberculatus can be estimated by using the sea-sweep method in Liaodong Bay. It is suggested that the stock of P. trituberculatus in Liaodong Bay can be substantially increased to promote the restoration of the three-spotted crab population and increase the income of the fishermen.