Coastal saltpans as foraging grounds for migrating shorebirds: an experimentally drained fish pond in Portugal

Worldwide, low-salinity ponds of coastal saltpans are often drained for artisanal fishing, at which time shorebirds feed opportunistically on the drained ponds. This case study sought to examine how shorebirds exploit this food supply after experimentally draining a large pond (9.1 ha) used for artisanal fishing at the Tagus estuary, Portugal. Shorebirds rapidly increased after draining (reaching 156 birds ha(-1)), and the pond provided a suitable foraging ground for a high density of shorebirds during 6-12 days after the study site was drained, supporting up to 6% of the total population using the estuary. The benthic invertebrate community was dominated by polychaete worms, and their high density (2215.0 +/- 329.9 ind m(-2)) apparently explained the relatively large percentage of feeding birds in the pond. Most shorebirds fed on polychaete worms at high intake rates throughout the experiment, probably because they increased their searching effort. Bird predation contributed to the food supply decrease during the first 3-6 days, and desiccation was presumably the major factor in 8-15 days. The drainage of low-salinity ponds by artisanal fishing is a traditional activity in coastal saltpans that might play a significant role in providing suitable foraging grounds for migrating shorebirds.