The international Wadden Sea is an important flatfish nursery. Information from the
Dutch Wadden Sea indicates that the flatfish nursery function of the area has been affected during
the last decades. Increased seawater temperature has affected settling, habitat suitability for and
growth performance of the various flatfish species. Settling of plaice, flounder and to a lesser
extent sole larvae occurs earlier nowadays. In the 1960s, 0-, I-, II- and III-group plaice were present,
but since 2000, II-group has disappeared and densities of I-group have decreased. For juvenile
flounder, II-group almost disappeared, and for dab, a decline in densities of all age groups
was observed from the 1990s onwards. Summer temperatures exceed the optimum for the coldwater
species (plaice, flounder and dab) with increasing frequency, level and duration. Only for 0-
group sole, the period with optimal growth conditions has become longer and has resulted in
increased growth. Mortality rates in 0-group plaice have increased, coinciding with an increase in
water temperatures and an increase in the abundance of predators. The decrease in density of
juvenile plaice and dab in the Wadden Sea has not affected recruitment to North Sea stocks, suggesting
that other areas have taken over part of the nursery function. The predicted increase in
seawater temperature in the next decades will continue to improve the conditions for sole. The
temperature tolerance of plaice and dab and to a lesser extent flounder will further reduce their
scope for growth and may ultimately result in their disappearance from the Wadden Sea.
