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 , the interannual patterns in predation 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 hundred 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 community, but may also have indirectly affected food availability and accessibility for other top predators (e.g. shorebirds).

Shifts in nursery habitat utilization by 0-group plaice in the western Dutch Wadden Sea

“…Despite performing tidally movements onto and off the intertidal to deeper areas associated to feeding (Kuipers, 1973;van der Veer & Bergman, 1986), migrations were considered of minor importance up till August and no severe decline in abundances were seen before September (Zijlstra et al, 1982;van der Veer, 1986;van der Veer & Witte, 1993). In agreement with recent observations for the Balgzand, a well-studied tidal flat area in western part of the Wadden Sea (van der Veer et al, 2011), this study shows that 0-group plaice also disappears from several other intertidal areas already in early summer and the high abundances found in subtidal and tidal channels suggest that deeper habitats nowadays can contain and support large populations of young-of-the-year plaice. The fact that these patterns were also observed in 2010, when half of the sampling stations were revisited (van der Veer & Witte, unpubl.…”

“…In the 1980s and 1990s, large juvenile plaice in the southern North Sea gradually moved to deeper water earlier in life (van Keeken et al, 2007) and this offshore shift seems to have continued in recent years (Poos et al, 2013). Similar shifts in spatiotemporal distribution were also observed in the inshore waters of the western Wadden Sea: large juvenile (Iand II-group) plaice have almost completely disappeared from the tidal flats in spring and summer (van der Veer et al, 2011) and strongly declined in abundance in deeper parts (> 2 m depth) of the area in autumn (Tulp et al, 2008). Abundance of young-of-the-year (0-group) plaice inside the Wadden Sea, has also declined over the same period as suggested by the long term trend in fyke catches at the entrance of the Dutch Wadden Sea (van der Veer et al, 2011;.…”

“…Similar shifts in spatiotemporal distribution were also observed in the inshore waters of the western Wadden Sea: large juvenile (Iand II-group) plaice have almost completely disappeared from the tidal flats in spring and summer (van der Veer et al, 2011) and strongly declined in abundance in deeper parts (> 2 m depth) of the area in autumn (Tulp et al, 2008). Abundance of young-of-the-year (0-group) plaice inside the Wadden Sea, has also declined over the same period as suggested by the long term trend in fyke catches at the entrance of the Dutch Wadden Sea (van der Veer et al, 2011;. These observations have led to the postulation that the role of the western Wadden Sea as juvenile plaice nursery might have declined (van der Veer et al, 2011;.…”

“…Abundance of young-of-the-year (0-group) plaice inside the Wadden Sea, has also declined over the same period as suggested by the long term trend in fyke catches at the entrance of the Dutch Wadden Sea (van der Veer et al, 2011;. These observations have led to the postulation that the role of the western Wadden Sea as juvenile plaice nursery might have declined (van der Veer et al, 2011;. Although these studies have shown unequivocal long-term shifts in juvenile plaice abundance and/or spatial distribution, their lack of simultaneous focus on spatial (restricted to the Balgzand tidal flat area, van der Veer et al, 2011) and temporal (restricted to autumn, Tulp et al, 2008) scales hampers the possibility of examining in more detail the fine-scale dynamics of putative habitat shifts.…”

“…However, field assessments of these relationships are seldom done due to the difficulty in quantifying the abundance of appropriate prey items (Stoner et al, 2001; but see van der Veer & Witte, 1993). Benthic macroinvertebrates are an important food source (Gibson, 1994;de Vlas 1979;Amara et al, 2001) and juvenile flatfish can feed not only on whole prey items but also on regenerating body parts such as bivalve siphon tips and polychaete tails tips, heads and tentacles (Kuipers, 1977;Ansell et al, 1999;van der Veer et al, 2011). Using qualitative information from 0-group plaice diet, combined with macrozoobenthic biomass data from specific surveys in intertidal and subtidal areas of the western Dutch Wadden Sea we conclude that, although subtidal areas present much higher total macrobenthic biomass, the amount that is actually available for 0-group plaice does not differ significantly between areas, which suggests that food per se may not explain the redistribution patterns of 0-group plaice in early summer.…”