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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