Impacts on other species

Impacts on habitat

Impacts on other shore users

Dredging causes the complete removal of all lugworms in the dredge tracks (Heiligenberg 1987), but dredges usually only operate within a very large area of intertidal sand flat, and are likely to leave considerable areas untouched. Beukema’s (1995) study of a 1 km² area found that near doubling of annual lugworm mortality rate occurred, resulting in a gradual and substantial decline of local lugworm stock from more than twice the overall mean at the start of the four year digging period.

This activity removes a very large amount of the invertebrate biomass, in comparison with hand digging. Heiligenberg (1987) examined the effects of both hand and mechanical digging in the Dutch Wadden Sea. Hand digging (reviewed above) caused a significant reduction in many of the common species, including Scoloplos armiger, Nereis diversicolor, Heteromastus and, of course, Arenicola (50% removal). A total of 1.9 g of other benthic animals were removed for every 1g of Arenicola. Mechanical digging has a much more serious effect, with complete removal of Arenicola and up to an 80 or 90% loss of the Baltic tellin Macoma baltica, Scoloplos and Heteromastus. Using this method, for every gram of lugworm taken, 9 to 13.4 g of other invertebrates are removed from the area. Beukema (1995) found that recovery of the benthos took several years, mainly because of the slow re-establishment of a soft shell clam Mya arenaria population with a normal size and age structure.

The impact on feeding birds of the introduction of mechanical bait dredging requires consideration. Bird disturbance at low water should not be an important factor, if the dredging barges are left unattended at this time. On the other hand, this activity removes a very large amount of the invertebrate biomass, in comparison with hand digging, and the habitat damage will reduce the feeding areas available to shore birds.

Mechanical dredging for lugworm has a similar effect on the sediment habitat as that caused by hand digging. Dredging in the Wadden Sea, where the dredged sediment is strained through a sieve with water jets, leaves gullies 40 cm deep and one metre wide, bordered on each side by a 1.5 metre wide ridge a few cm high (Heiligenberg, 1987). This is similar to, but more severe an effect than caused by a hand-dug trench with no back filling. Fines are released, and any contaminants in the sediments also become available for uptake by marine organisms. Monitoring of the fauna of dredged sites in the Netherlands was carried out for six months, suggesting that the relief of the sediment surface may have enabled relocation of the dredged areas throughout this time (the author does not record the rate of physical recovery of the sediment surface over that period). Dredged tracks in Essex (pers. obs, 1989) tend to fill with water and accumulate seaweed, as seen for bait-dug holes, and previously buried shell rejected from the sieves is scattered over the surface. The area of effect can be greater than occurs during normal levels of bait digging.

Hugget (1992) notes that a dredger can make three 250 m x 1 m trenches per tide, and working just once a day could damage at least ½ hectare of mudflats per week. Working a 5-day week for 30 weeks of the year, a single dredger might mobilise and redeposit 90,000 tons of sediment per year.

Most likely to be affected if the dredge tracks interfere with recreational activities, commercial fisheries, or archaeological sites.

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