Carrying capacity for shorebirds during migratory seasons at the Jiuduansha Wetland, Yangtze River Estuary, China

The carrying capacity of food resources for migrating shorebirds was estimated at a stopover site in the Yangtze River Estuary during the two migratory seasons(spring and autumn).From March to May and September to November 2005,the macrobenthos resources of the Jiuduansha Wetland were investigated,and most of the macrobenthos species in the newlyformed shoal were found to be appropriate food for shorebirds.Biomass measurements showed that the total food resource was about 4541.20 kg AFDW(Ash-Free Dry Weight)in spring and about 2279.64 kg AFDW in autumn.Calculations were also done in the available habitats(intertidal bare mudflat and Scirpus×mariqueter/Scirpus triqueter zones)for the shorebirds.The food resources in the available areas were about 3429.03 kg AFDWin spring and about 1700.92 kgAFDWin autumn.Based on the classification(by lean weight,basic metabolic rate and body length)of the shorebird community,and using the energy depletion model,it was theorized that all of the food resources in the Jiuduansha Wetland could support about 3.5 million shorebirds during spring season and 1.75 million shorebirds during autumn season.The shorebird carrying capacities in terms of the available food were about 2.6 million and 1.3 million birds during the two respective migration seasons.Considering the effect of intake rate,the potential carrying capacity was about 0.13–0.26 million shorebirds in the study area.The main factor restricting use of the area by shorebirds was the scarcity of available habitats for roosting at high tide rather than availability of food supply.We recommend restoring some wading pools in the dense Phragmites australis and Spartina alterniflora zones for shorebirds to roost in,to improve shorebirds’utilization efficiency of the resources in the Jiuduansha Wetland.