Long-term trends in juvenile flatfish indicate a dramatic reduction in nursery function of the Balgzand intertidal, Dutch Wadden Sea

Abstract

Since 1975, juvenile flatfish (plaice and flounder) populations have been monitored at the Balgzand intertidal and, based on this data series (1975-2007), the interannual patterns in preda- tion pressure were quantified. Temporal patterns in abundance have changed greatly, especially for plaice. Up to the early 1980s, 3 year classes (0-, I- and II-group) were present and growing up in the area, but from the late 1980s onwards, densities of the I- and II-group plaice dropped from a few hun- dred individuals per 1000 m 2 to levels close to zero. It appears that the Balgzand intertidal has lost its nursery function for I- and II-group plaice, although feeding conditions have remained the same or even slightly increased since the late 1970s. The absence of I- and II-group flatfish in the intertidal cannot be explained at present; however, processes operating offshore are most likely involved. As a consequence, the annual predation pressure by juvenile flatfish upon the intertidal macrozoobenthos dropped by 94%, declining from an average of approximately 5 to 0.25 g ash free dry mass m -2 . Such a decline in top-down predation may not only have directly influenced the macrozoobenthic commu- nity, but may also have indirectly affected food availability and accessibility for other top predators (e.g. shorebirds).