Home

Table 5.15. Effects of hydraulic clam dredges on sand substrate habitat: summary of published studies. (S = statistically significant; citations in bold print are peer-reviewed publications.)

No. Reference Location Depth Sediment Effects Recovery Approach
1 Hall et al. (1990) Loch Gairloch, Scotland 7 m Fine sand Shallow trenches (25 cm deep) and large holes; sediment “almost fluidized”; median sediment grain size S higher in fished area; S reductions in numbers of infaunal organisms; no effect on abundance of individual species; some mortality (not assessed) of large polychaetes and crustaceans retained on conveyor belt or returned to sea surface. Complete recovery of physical features and benthic community after 40 days; filling of trenches and holes accelerated by winter storms. Experimental study in unexploited area to evaluate effects of simulated commercial escalator dredging activity; recovery evaluated after 40 days.
2 Kaiser, Edwards et al. (1996b) SE England Intertidal Muddy sand Resuspension and loss of fine sand from sediment surface; S reductions in total number of infaunal species and individuals. Complete recovery of sediments and benthic community within 7 mo. Experimental study; effects of suction dredging for cultivated clams evaluated after 3 hr and 7 mo.
3 MacKenzie, 1982 East of Cape May, New Jersey, USA 37 m Very fine to medium sand Resorting of sediments (coarser at bottom of dredge track); no effect on number of infaunal individuals or species, nor on species composition.   Comparison of actively fished, recently fished, and never fished areas on the continental shelf; dredging conducted with hydraulic cage dredges.
4 Maier et al. 1995 South Carolina, USA Tidal creeks Muddy sand Turbidity plumes; no S effects on abundance of dominant infaunal taxa or total number of individuals. Turbidity plumes persisted for a few hours. Before and after study of commercial escalator dredging effects in four tidal creeks. Turbidity monitored 2 wk before, during, and 2 wk after dredging at one location, and during and immediately after dredging at another. Infaunal samples collected 3 wk before and 2 wk after dredging in a creek that had been commercially dredged 5 yr prior to the study and in a creek that had never been dredged before.
5 Medcof and Caddy 1971 Southern Nova Scotia, Canada 7-12 m Sand and sand-mud Smooth tracks with steep walls, 20 cm deep; sediment cloud. Sediment plume lasted 1 min; dredge tracks still clearly visible after 2-3 days. SCUBA and submersible observations of the effects of individual tows with a cage dredge.
6 Meyer et al. 1981 Long Island, New York, USA 11 m Fine to medium sand, covered by silt layer >20-cm-deep trench; mounds on either side of trench; silt cloud, attraction of predators. Trench nearly indistinct, and predator abundance normal, after 24 hr; silt settled in 4 min. SCUBA observations during and following a single tow with a cage dredge in a closed area; effects evaluated after 24 hr.
7 Pranovi and Giovanardi 1994 Venice Lagoon, Adriatic Sea, Italy 1.5-2 m Sand 8-10 cm deep trench; S decrease in total abundance, biomass, and diversity of benthic macrofauna in fishing ground; no S effects outside fishing ground. After 2 mo, dredge tracks still visible; densities (especially of small species and epibenthic species) in fishing ground recovered, biomass did not. Experimental dredging with a cage dredge (single tows) in previously dredged and undredged areas in coastal lagoon; recovery monitored every 3 wk for 2 mo.
8 Tuck et al. 2000 Sound of Ronay, Outer Hebrides, Scotland 2-5 m Medium to fine sand Steep-sided trenches (30 cm deep); sediments fluidized up to 30 cm; S decrease in number of infaunal species and individuals within a day of dredging; S decrease in proportion of polychaetes and S increase in proportion of amphipods 5 days after dredging; S increases in abundance of some species and S decreases in abundance of other species. Trenches no longer visible but sand still fluidized after 11 wk; species diversity and total abundance recovered within 5 days; proportions of polychaetes and amphipods, and abundances of individual species, returned to pre-dredge levels after 11 wk. Experimental dredging with cage dredge (individual tows at 6 sites) in area closed to commercial dredging, effects evaluated 1 day, 5 days, and 11 wk after dredging.