Cockles

Small, short-lived invertebrates

Large, long-lived invertebrates

Intertidal community effects

Birds

During the process of bait collection, by hand, mechanical digging or boulder turning, many animals and plants other than those being sought will be damaged and their population levels reduced. Species populations will be affected immediately by the disturbance at the time of bait collection, but their recovery will also be dependent upon the longer-term habitat damage caused (see below).

There have been several studies on the impact of hand digging for worms on other populations of common sediment shore invertebrates (Cadee 1977, Cadman 1989, Cryer et al. 1987, Dyrynda and Lewis 1994, Farrell 1998, Heiligenberg 1987, McLusky et al. 1983). The process of digging for bait causes the death of many other marine invertebrates, by physical damage, burial and smothering or exposure to desiccation and predation. Eel grasses Zostera species and Sabella (polychaete worm) beds may also be uprooted by bait digging at the extreme low water mark (Dyrynda and Lewis 1994) and mussel beds loosened, potentially leading to their erosion and loss in bad weather.

Jackson and James (1979) suggest that intensification of digging for bait worms on the North Norfolk coast in the 1950s and '60s resulted in a decline in cockle Cerastoderma edule populations. Undisturbed cockle beds were not affected. The cockle cannot regain its normal position at the surface of the sediment if deeply buried in overturned spoil. There have been conflicts between cockle fishermen and bait diggers in the Burry Inlet, South Wales, where Shackley et al. (1995) demonstrated that the effects reported from North Norfolk also occurred. These authors reported over 90% mortality of cockles in areas affected by baitdigging, with large older cockles most likely to die. Recolonisation was occurring three months after bait digging, but the cockle population structure still showed differences from undisturbed areas. Farrell (1998) also described a reduction in numbers of cockles in experimentally-dug areas of Chichester Harbour.

Smaller, more numerous invertebrates are also affected. Cadee (1977) recorded an 85% decline in the polychaete Heteromastus filiformis after digging. Heiligenberg (1987) examined the effects of both hand and mechanical digging in the Dutch Wadden Sea. Hand digging 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 was removed for every 1 g of Arenicola.

Recovery of these invertebrate populations is fairly swift, through migration into the dug areas. McLusky et al. (1983) found a reduction of 80-100% for the surface-living Hydrobia ulvae and nearly 100% for Macoma after hand digging, but normal populations in test plots after 15 days. Speed of recovery of infauna on mounds and in trenches will vary. Several species prefer the soft substratum found in trenches, but some species avoid both features, so a return to normal may not occur until the habitat has been restored. This is most rapid where trenches have been back-filled during bait digging. Complete recovery of most common species will take place after the successful settlement and recruitment of juveniles to the population (in less than one year).

Long-lived, infrequently recruiting species such as large bivalves (e.g. Mya arenaria), acorn worms Saccoglossus sp. or burrowing echinoderms will take much longer to become re-established after removal or destruction during digging (e.g. Beukema 1995, Dyrynda and Lewis 1994). Some of these species are also very vulnerable to bait digging disturbance because of their fragile nature. Farrell (1998) describes the complete loss of the large sedentary worm Amphitrite johnstoni and Harmathoe imbricata (which is its commensal – living in the same burrow) from areas dug experimentally in Chichester Harbour. Numbers were still extremely low compared with the control undisturbed site a year after digging. A population of the heart urchin Echinocardium cordatum was badly affected by a short period of heavy bait digging at Newton Haven, a small pocket beach in Northumberland, before a byelaw to control this activity was introduced (Fowler 1992). In most cases these long-lived species will also occur below the low water mark, with only a small proportion of the population being damaged. However, the activity may still have a detrimental effect on the nature conservation importance of the site if it has been designated because of the presence of such species in the intertidal where they are accessible for scientific study and monitoring (Olive 1984).

In a few cases where bait digging takes place in very sensitive areas, the whole sediment community may be affected. In the Menai Strait very rich infaunal populations had previously been recorded in areas which were then intensively dug for king ragworms. These diverse populations were no longer present in the disturbed areas. Management of bait digging was proposed to enable recovery of these sites and eventual recolonisation by the original full range of fauna. Eelgrass Zostera spp. beds and saltmarsh habitats are also damaged by bait digging, which loosens and uproots plants and may result in the beds being washed away.

There have also been instances of bait digging for ragworm taking place within mussel beds on sediment areas. The physical disturbance of the beds can result in the mats of mussels breaking up and being washed away in poor weather. Mussels provide a habitat for a wide range of species, which may also be lost, and are important feeding grounds for birds, as well as being of commercial importance. Where shell fish beds or estuarine areas are covered by a Several or Regulating Order, bait digging may be controlled to prevent damage to commercial species.

Farrell (1998) describes an increase in numbers of the common winkle Littorina littorea following bait digging in Chichester Harbour. Large flints exposed by digging provided a suitable habitat for this species which moved into the dug area.

Bird disturbance is one of the most serious impacts of bait digging in British estuaries in winter. Davidson and Rothwell (1993) review this in detail, and include a case study by Townshend and O’Connor (1993) describing the effects of disturbance by baitdiggers in the Lindisfarne National Nature Reserve. During the peak demand for bait (for the winter beach fishing season), the use of intertidal areas for bait digging coincides with the presence of internationally important populations of over-wintering and migrating wildfowl and waders. These birds need to feed continually when the tide is out in order to survive the cold winter and migrate successfully back to their breeding grounds. The presence of numerous bait diggers on the shore frequently has the effect of driving off feeding or roosting birds (Evans and Clark 1993). Bait diggers will even work shores during low water spring tides at night, when birds would also normally be feeding. Because the relationship between bird density and food availability is complex it is not known if British estuaries are at their carrying capacity for wintering wildfowl. However, it is generally accepted that a precautionary approach should be adopted when considering the impacts of disturbance and habitat loss on birds. The basis of this approach is to assume that the use of alternative sites following displacement from preferred habitat may lead to decreased food exploitation and/or increased energy expenditure, thus resulting in harmful impacts on bird populations. A review by Cayford (1993) may help to predict the effects of disturbance on the foraging efficiency, competition and dispersion of waders.

Possibly a secondary problem for bird populations is the reduction in food species caused by bait digging. The significant loss of invertebrate biomass during bait digging affects non-target species as well as the bait worms. No attempt has made to assess the significance of this reduction in abundance of food species in bait-dug areas on bird populations.

The habitat damage caused by bait-digging is also a factor affecting the feeding opportunities of birds on the shore. The basins and trenches left by hand and mechanical bait digging remain filled with water at low tide. Most birds will not use these flooded areas, tending to search for food on the exposed sand and mud flats. Bait digging therefore reduces the area available for feeding birds even when bait diggers are not present on the shore.

Monitoring has been carried out into the effect of bait diggers upon bird populations in Budle Bay, the sanctuary area of the Lindisfarne National Nature Reserve, Northumberland. Concern was initially voiced over the disturbance caused by relatively small numbers of bait diggers in this area in the early 1980s. An agreement was reached with angling groups to close Budle Bay to bait digging for two years, then to reopen a section of the area for a two year trial period and examine bird numbers in the Bay during this time. During the period of closure bird numbers and bait density increased in the Bay. When intensive bait digging (with up to 120 persons at one time) again took place in 1984 and 1985, bird numbers fell significantly. It was apparent that this disturbance was incompatible with the aims of the sanctuary area and Budle Bay was finally closed to bait diggers in 1986. Numbers of birds using the area have since risen considerably.

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