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Monitoring of impact (and recovery) on 3 marine aggregate extraction sites in the eastern channel
- Liam Wyns
- Marc Roche
- Florian Barette
- Annelies De Backer
Based on data from Multibeam Echosounders (MBES) and Van Veen grab samples, near-field effects of marine aggregate extraction by trailing suction hopper dredgers in the Belgian part of the North Sea were assessed on a decadal scale. The combined approach allowed to investigate and compare seabed and macrobenthic community characteristics for three extraction areas with similar ecological settings, but with a different geological context and each subjected to a different extraction regime. MBES measurements detected slight alterations of the seabed for areas exposed to a continuous, low extraction regime (monthly average volume = 17 to 83 × 10³ m³). However, no significant changes in sediment composition nor the macrobenthic community could be attributed to this low extraction regime. High and continuous extraction in the most intensely extracted area (monthly average volume = 164 × 10³ m³) increased surface heterogeneity and created a local depression, hereby exposing clay and gravel from the underlying geological layer. In this area, the highest environmental impact was observed, as the physical changes in the seabed triggered a shift towards a more heterogeneous, transitional macrobenthic community including opportunistic species and species typically associated with muddy sands. Together with the species already present, this resulted in a local increase in macrobenthos density, species richness and biomass. A high but periodic extraction without screening activity on the most offshore located extraction area (monthly average volume = 230 × 10³ m³, averaged for those months where extraction took place) led to a redistribution of the medium to fine sand fraction and a winnowing of coarse sediment and shell fragments. The decreased median grain size induced a shift in the macrobenthic community from a typical medium to coarse sand Hesionura elongata community towards medium to fine sand representatives of the Nephtys cirrosa community, although the overall macrobenthic density and biomass in this extraction area remained stable. Based on these results, we conclude that extraction regime and local geological context are important factors driving the near-field environmental impact of marine aggregate extraction on tidal sandbanks.
Compare the recovery time of benthos and demersal fish in commercial (10 years of extensive extraction) and experimental sites (2 years of intensive extraction) located in the eastern Channel.
Conservation managers need to be able to assess and prioritize issues that may affect their target habitats and species. In the Baie de Somme, France, conservation issues affecting overwintering shorebirds include hunting pressure, cockle fishing, recreational disturbance, Spartina encroachment, and changing sediment levels. We used an individual-based model to predict the effect of these issues on the survival of three shorebird species: dunlin Calidris alpina, oystercatcher Haematopus ostralegus and curlew Numenius arquata. In the model, removing hunting from the mudflats in the eastern part of the estuary had the greatest positive effect on shorebird survival. Oystercatcher survival decreased markedly when stocks of large cockles were reduced to <250 m-2 or numbers of fishermen per day were doubled. Short-term disturbance events, such as walkers, had more effect on shorebird survival than long-term events, such as fishermen. Dunlin, as a protected species, were able to feed outside the Réserve Naturelle and were unaffected by disturbance within the Réserve. Oystercatcher survival decreased when the number of disturbance events within the Réserve exceeded one h-1, and curlew survival when disturbance events exceeded six h-1. Spartina encroachment caused dunlin survival to decline steadily as feeding habitat was lost. Dunlin were also the species most affected by changes in sediment levels, likely to occur through either sedimentation or sea level rise.
In coarse sediments of the eastern Channel, an appropriate extraction strategy can minimize the degradation of the biodiversity and function of the marine environment : - a low extraction intensity (< 1h/ha/year) limits the impact on benthos and fish communities, and accelerates the functional restoration of the disturbed area by promoting the bentic recolonisation; - a spatial and temporal zoning (fallow areas) favours the opportunistic recolonisation by epifaunal species which are preys for commercial species such as cod, gurnards and black seabream; - this zoning is an opportunity to increase the local number of habitats and biodiversity.
A low intensity of extraction can : - reduce the impact on the main fish population parameters (species richness, , abundance, biomass) ; - provide feeding areas for sole, cod and black seabream (movement into more suitable habitats) ; - increase habitat diversity favouring resilient species of economical value (sole, red mullet) ; -have potential benefits for some OSPAR threatened or protected species (cod) and habitats (Sabellaria reefs) ; - limit the propagation of the invasive mollusk Crepidula.
This document is a comprehensive survey of the scientific knowledge acquired in late 2012 on the environmental impacts of marine aggregate extraction. The synthesis completes international knowledge (ICES WGEXT, COST Action 638 "MAGGNET") with local investigation in Normandy on two sites of the eastern English Channel in the framework of the 2003-2011 SIEGMA programme.
2 G.I.S SIEGMA Stakeholder consortium on the impact of marine gravel extraction End users (fishermen, mining companies) Scientists Authorities, policymakers Supported by private, regional, national and european fundings Objectives : Complete international knowledge on the impact Test exploitation methods to minimize impact and favour restoration