Action: Extract aggregates from a vessel that is moving rather than static
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- One study examined the effects of dredging from a vessel that is moving rather than static on subtidal benthic invertebrate populations. The study was in the English Channel (UK).
COMMUNITY RESPONSE (1 STUDY)
- Overall species richness/diversity (1 study): One site comparison study in the English Channel found that a site where aggregate extraction was undertaken using a moving trailer suction hopper dredger had similar invertebrate species richness and lower diversity compared to a site where extraction occurred using a static suction hopper dredger.
POPULATION RESPONSE (1 STUDY)
- Overall abundance (1 study): One site comparison study in the English Channel found that a site where aggregate extraction was undertaken using a moving trailer suction hopper dredger had higher abundance of invertebrates compared to a site where extraction occurred using a static suction hopper dredger.
'Aggregates’ is the collective term for sand, gravel and crushed rock. They are used as raw materials for the construction industry as well as for beach replenishment schemes. Aggregate extraction can impact subtidal benthic invertebrates through physical removal, loss and alteration of seabed and habitat, and direct physical damage from the machinery used (De Groot 1996). Two methods are commonly practised: anchor suction hopper dredging, a static type of extraction (De Groot 1996), and trailer suction hopper dredging which takes place from a moving vessel (Birchenough et al. 2010; Boyd & Rees 2003). In some areas, such as in the UK, both methods are used (Boyd & Rees 2003). Although both methods cause severe disturbance in seabed invertebrate communities (Desprez 2000, Sardá et al. 2000), the differences between dredged and surrounding undredged areas are more significant after static dredging (Boyd & Rees 2003). Trailer dredging is thought to reduce the intensity, and therefore the impact, of dredging in any one area by leaving small pockets of areas unaffected and from which recolonization and recovery may occur (Szymelfenig et al. 2006). Evidence for related interventions is summarised under “Threat: Energy production and mining – Mining and quarrying”.
Birchenough S.N., Boyd S.E., Vanstaen K., Coggan R.A. & Limpenny D.S. (2010) Mapping an aggregate extraction site off the Eastern English Channel: a methodology in support of monitoring and management. Estuarine, Coastal and Shelf Science, 87, 420–430.
Boyd S.E. & Rees H.L. (2003) An examination of the spatial scale of impact on the marine benthos arising from marine aggregate extraction in the central English Channel. Estuarine, Coastal and Shelf Science, 57, 1–16.
De Groot S.J. (1996) The physical impact of marine aggregate extraction in the North Sea. ICES Journal of Marine Science, 53, 1051–1053.
Desprez M. (2000) Physical and biological impact of marine aggregate extraction along the French coast of the Eastern English Channel: short-and long-term post-dredging restoration. ICES Journal of Marine Science, 57, 1428–1438.
Sardá R., Pinedo S., Gremare A. & Taboada S. (2000) Changes in the dynamics of shallow sandy-bottom assemblages due to sand extraction in the Catalan Western Mediterranean Sea. ICES Journal of Marine Science, 57, 1446–1453.
Szymelfenig M., Kotwicki L. & Graca B. (2006) Benthic re-colonization in post-dredging pits in the Puck Bay (Southern Baltic Sea). Estuarine, Coastal and Shelf Science, 68, 489–498.
Supporting evidence from individual studies
A site comparison study in 2000 of two soft seabed areas in the central English Channel, UK (Boyd & Rees 2003) found that using moving trailer rather than static suction hopper dredgers during aggregate extraction appeared to result in a similar number of invertebrate species, and a lower species diversity, but a higher abundance. Data were not statistically tested. The number of species at trailer- and static-dredged sites were similar (trailer: 20; static: 21). Species diversity was lower at the trailer dredged site than at the static dredged site (data presented as diversity indices). However, abundance of invertebrates was higher at the trailer dredged site (1,617 individuals/sample) compared to the static dredged site (103). In June 2000, sediment samples were collected using a sediment grab (0.1 m2) from two sites at 18–25 m depths. One site had been dredged since 1968 by static suction, while the other had been dredged since 1989 by trailer suction. Invertebrates >0.5 mm were identified and counted from three to four samples/site.