Long‐term indirect effects of mechanical cockle‐dredging on intertidal bivalve stocks in the Wadden Sea

Abstract

Summary1. There is world-wide concern about the effects of bottom-dredging on benthic communities in soft sediments. In autumn 1988, almost a third of the 50-km 2 intertidal system around the island of Griend in the western Dutch Wadden Sea was suction-dredged for edible cockles Cerastoderma edule and this study assessed subsequent effects. An adjacent area not directly touched by this fishery and an area from which the mussel Mytilus edulis beds were removed, served as reference areas. 2. Sediment characteristics, together with the total stock size and settlement densities of Cerastoderma , Baltic tellin Macoma balthica and soft-shelled clam Mya arenaria , were documented during 11 successive autumns before (August-September 1988) and after (August-September 1989-98) the suction-dredging event in fished and unfished areas. Four other areas in the Dutch Wadden Sea, where changes in densities of juvenile bivalves from 1992 to 1998 were measured, served as additional reference locations. 3. Between 1988 and 1994, median sediment grain size increased while silt was lost from sediments near Griend that were dredged for cockles. The initial sediment characteristics were re-attained by 1996. 4. After the removal of all Mytilus and most Cerastoderma , the abundance of Macoma declined for 8 years. From 1989 to 1998, stocks of Cerastoderma , Macoma and Mytilus did not recover to the 1988 levels, with the loss of Cerastoderma and Macoma being most pronounced in the area dredged for cockles. Declines of bivalve stocks were caused by particularly low rates of settlement in fished areas until 1996, i.e. 8 years after the dredging. 5. A comparison of settlement in the short (1992-94) and medium term (1996-98) after cockle-dredging in several fished and unfished areas spread over the entire Dutch Wadden Sea, showed a significant negative effect of dredging on subsequent settlement of Cerastoderma . Macoma also declined, but not significantly. 6. We conclude that suction-dredging of Cerastoderma had long-lasting negative effects on recruitment of bivalves, particularly the target species, in sandy parts of the Wadden Sea basin. Initially, sediment reworking by suction-dredging (especially during autumn storms) probably caused losses of fine silts. Negative feedback processes appeared to follow that prevented the accumulation of fine-grained sediments conducive to bivalve settlement.

Transitions in sandflat biota since the 1930s: effects of sea-level rise, eutrophication and biological globalization in the tidal bay Königshafen, northern Wadden Sea

“…Most changes in the Wadden Sea have been attributed to eutrophication (e.g. van Beusekom 2005), to the loss of apex predators (Lotze 2005), fisheries (Piersma et al 2001), artificial shores (Reise 2005), alien invaders (Buschbaum et al 2012) and to climate change (Beukema and Dekker 2005;Diederich et al 2005;Loebl et al 2006;Nehls et al 2006, Witte et al 2010. Here, we add a long-term study on sandflat benthos to further unravel small-scale responses to large-scale causes in a coastal ecosystem.…”

When enough is not enough: shorebirds and shellfishing

“…We varied only the size of the shellfish beds since this was sufficient to make our main point. Importantly, this also most closely replicates the potentially serious long-term impact of large-scale harvesting techniques, notably suction dredging and trawling, which render intertidal flats unsuitable for cockles (Piersma et al 2001) and mussels (Stillman et al 2001), respectively. The data for shellfish abundance, bird numbers and ambient temperature for each system were taken either from a typical autumn and winter or from the average of several seasons ( …”

Predation and habitat modification synergistically interact to control bivalve recruitment on intertidal mudflats

“…Over the last decades, large-scale declines of bivalve stocks have occurred in North-America, Australia and Europe (including the Wadden Sea) due to mechanical dredging for commercially exploitable species like oysters, mussels and cockles (Schulte et al, 2009;Eriksson et al, 2010). Apart from the physical removal of adult bivalves, mechanical dredging also removes hard substrates (e.g., dead shells, tubeworm structures) and destabilizes the sediment (Ferns et al, 2000;Piersma et al, 2001). Our results show that recruitment of native bivalves in the Wadden Sea is negatively affected by these activities.…”

“…Even after intertidal mechanical dredging was banned in 2004, the functioning of these Marine Protected Areas (MPAs) has remained threatened as recovery of cockles has been slow and unpredictable (Piersma et al, 2001;van Gils et al, 2006), and mussel beds have recovered only partly to this day (Eriksson et al, 2010). In this study, we therefore investigated the importance of predation, habitat modification and their interplay in explaining bivalve recruitment dynamics in the intertidal of the Wadden Sea.…”