Impacts on other species

Impacts on habitat

Impacts on other shore users

The drags are very effective collectors of king ragworm Nereis virens, but quite a large proportion are injured during dragging (up to 50% on occasions in shelly ground). Some of the damaged worms are discarded over the side of the boat. Other worms are likely to be damaged during dragging, but not brought to the surface. Survival of damaged worms may be quite high (they are capable of regeneration) if they are not predated before being able to rebury themselves. Dyrynda (1995) notes that evidence suggests that intensive bait dragging does cause local stock depletion, but that there are no convincing indications of a large-scale decline in stocks across the harbour as a result of this practice. Substantial stock depletion would result in the activity becoming uneconomic, providing a certain amount of self-regulation.

The effect of bait dragging is not considered to be significant for small surface-dwelling or infaunal invertebrates. However, large burrowing invertebrates are more likely to be damaged by the large tines. Species and communities considered by Dyrynda (1995) to be particularly vulnerable include softshell clams Mya spp., peacock worm Sabella pavonina beds, seagrasses Zostera marina beds, saltmarsh (although it seems unlikely that dragging would take place in this habitat), and commercial mussel Mytilus edulis beds. The peacock worm and seagrass beds are of high marine nature conservation importance. Dragging may break up mussel beds, and the mussels may be driven into the underlying ‘mussel mud’ to a depth from which recovery and survival is unlikely. Dyrynda and Lewis (1994) note the concerns of nature conservation bodies over the potential effects of disturbance and changes in prey community structure that may be caused by bait dragging and digging on bird populations.

Dyrynda’s 1995 study carried out intensive dragging during 2.5 hours over a small area of only 0.02 hectares. Surprisingly, this affected only about 10-30% of the surface area. However, the scars caused were very conspicuous in places, consisting of furrows up to 10-20 cm wide flooded with water and showing black anoxic subsurface sediment. Most of the physical disturbance was sub-surface, covering a much larger area where the sediment had been disturbed and softened by the buried tines of the drag. Fine sediment is released during dragging, causing turbidity in the water column until it is redeposited elsewhere. Some cockles were unearthed, but undamaged, burrowing anemones disturbed, and significant quantities of dead shell exposed. Otherwise, no significant differences in sediment composition were detected. However, it is important to note that this activity takes place in some of the most remote and undisturbed areas of the harbour, which would otherwise be almost completely undisturbed by man. The activity also overlaps with areas affected by bait digging, potentially leaving virtually no undisturbed refuge areas within the harbour.

The visual effects of bait dragging are only rarely visible from the shore, meaning that the aesthetic effect of this activity on the appearance of the harbour mudflats is limited. The activity is a cause for concern to mussel fishermen, who relay spat in the harbour for on-growing. Bait dragging is not permitted on leased mussel grounds, but may occur there accidentally or otherwise (mussel beds are thought to harbour particularly large stocks of king ragworms). The activity is also of concern within nature reserves, because of potential impacts on natural habitats and vulnerable species, possibly including bird populations. Bait dragging is also discouraged in the vicinity of the main navigational channels, mooring areas and navigational and berthing installations.

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