Artikel: effecten visserij minimaal 9 jaar aanwezig

Het onderstaande artikel is een samenvatting van een artikel verschenen in het internationaal hoog aangeschreven wetenschappelijk tijdschrift Journal of Applied ecology. Het onderzoek is uitgevoerd door een groep wetenschappers afkomstig van de Rijksuniversiteit Groningen, het Nederlands Instituut voor Onderzoek der Zee (NIOZ) en Rijkswaterstaat, zoals bekend een onafhankelijke overheidsinstelling. Conclusies:

1) Op platen waar in 1988 gevist is waren tot in 1998 (het laatste meetjaar) duidelijk minder mossels, kokkels en andere schelpdieren aanwezig.

2) De samenstelling van de wadbodem is in gebieden waar gevist is verandert. De hoeveelheid slik is afgenomen, en het gehalte aan zand is sterk gestegen. Kortom: het wad is geen wad meer.

3) Eindconclusie: door het opzuigen van de bovenste laag van het wad verstoren de kokkelvissers de samenstelling van het sediment, waardoor er veel minder schelpdieren in kunnen groeien. De vissers gooien hiermee hun eigen glazen in: de weggeviste kokkels komen immers niet meer terug. Kortom: kokkelvisserij is duidelijk niet duurzaam.

---

Journal of Applied Ecology (2001) 38, 976-990

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

Theunis Piersma*†, Anita Koolhaas*, Anne Dekinga*, Jan J. Beukema*, Rob Dekker* and Karel Essink‡

Abstract

1. 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 km² 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.

*Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, The Netherlands,

†Centre for Ecological and Evolutionary Studies (CEES), University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands, and

‡Rijkswaterstaat, National Institute for Coastal and Marine Management (RIKZ), PO Box 207, 9750 AE Haren, The Netherlands