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VAN DER WINDEN J, DIRKSEN S †, DOODEMAN D, HOGEWEG N, VAN HORSSEN P, KELDER L, TULP I & POOT M (2019) Common Terns Sterna hirundo in the IJsselmeer area: nest site selection and breeding success in a low-dynamic wetland. LIMOSA 92 (2): 49-64.

In 1932 the Zuiderzee, the estuary of the river IJssel, was closed by a dam. This changed the brackish dynamic wetland into one of the largest fresh water lakes in Europe, lake IJsselmeer-Markermeer. Previously dynamic sand islands became vegetated and the fish community changed dramatically. Formerly migratory fish populations were replaced by a resident freshwater stock, with only Smelt Osmerus eperlanus remaining as an abundant pelagic species. Hard substrate levees border the lake, and there is lack of streams and natural water fluctuations. Despite these changes the Common Tern breeding population in the area increased since 1950 to a fluctuating population of 3000-7000 pairs. In the last decades the Smelt population has crashed, strongly reducing an important food source for breeding piscivorous birds. However, the number of potential breeding sites for Common Terns increased since 1990 due to the creation of new sand islands that provide nesting habitat. These nesting habitats remain suitable for only three years, unless they are intensively managed to halt vegetation succession. Due to a lack of natural dynamics, today only intensive management or continuous creation of new islands can provide Common Terns with sufficient breeding habitat in this freshwater lake system. In a 13 year monitoring study, breeding success of the terns strongly fluctuated among years, ranging between almost zero to around 0.8 fledged chicks per breeding pair. This breeding output correlates significantly with Smelt stocks, indicating a dependence on this fish species as the main prey for chicks. Since 2012 the commercial Smelt fishing was stopped, but the average breeding success remains too low to maintain the population at a stable level. A new management policy aims to create new aquatic habitats, such as shallow areas with abundant aquatic plants, which should lead to more diverse fish communities. An example is the newly created archipelago of the Marker Wadden. It is important for Common Terns to also restore brackish areas in the lake and to intensify vegetation management on previously created islands

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limosa 92.2 2019
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