Changes in functioning of the largest coastal North Sea flatfish nursery, the Wadden Sea, over the past half century

Abstract

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 cold-water 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.