Impacts on habitat

Digging for bait or other infaunal species disturbs the sediment, which is removed from its original position, overturned and exposed to air and wave or current action. Transport of fine sediment and previously buried contaminants takes place at the sediment surface. Stones and shell buried in the sediment are exposed (Anderson and McLusky 1981, Anderson and Myer 1986, Farrell 1998). The effect is not confined to the areas excavated, but usually extends to an equal area covered by the excavated spoil. If the displaced mounds of sediment are subsequently returned to the trenches by the bait digger (the process of back- or in-filling), then the effect of disturbance is reduced and recovery hastened. Recovery of dug areas takes place most quickly (within three weeks) where holes and trenches are back filled (McLusky et al. 1983), and in the most wave-exposed areas. Sheltered sediment shores exposed only to small amounts of wave action take longer to recover.

Some of the more detailed studies on recovery of sediments were carried out in the Firth of Forth (Anderson and McLusky 1981, McLusky et al. 1983). These studied the recovery of areas where bait digging had been simulated. A series of holes were dug, with the mounds produced from the spoil left alongside (the method used by many amateur bait diggers), and some long trenches excavated and infilled (copying the more experienced diggers). These were monitored over a period of 30 days, with microtopography, sediments in suspension and surface sediments being studied. A similar study (Anderson and Meyer 1986) studied surface and suspended sediments after clam digging in Maine, USA. Coates (1983) and Johnson (1984) have also studied the recovery of bait dug areas in the Menai Strait.

The immediate effect of bait digging is to change the sediment stratigraphy. In undisturbed conditions, bioturbation of sediments (primarily by feeding lugworms) usually produces a layer of well-mixed sand 10 cm deep, which overlies a bed of shell or stone. The sediment may be anoxic at or below this layer, with contaminants often retained in this anoxic layer. Digging moves the coarse material and anoxic sediments to the surface, where they are exposed to the action of waves and currents and quickly oxidised, releasing pollutants (see below).

Where no back-filling takes place, the mounds of spoil are exposed to increased wave and current erosion and winnowing out of the finer sediments. The basins collect organic material (drift seaweed) and fine sediments from suspension. The result is the formation of a soft, organically enriched and anoxic layer at the bottom of the basin, which also holds water permanently. The holes initially fill in much more swiftly than the mounds erode, but the latter disappear well before the basins fill completely. Back-filled trenches recover much more quickly, but some stones and shell dug up will still be left on the surface.

Overall recovery rates will depend on the energy of the site. Thus coarse sandy beaches with wave action will lose the signs of digging much more quickly than sheltered sites with poorly sorted sediments. Storms will speed up the disappearance of bait dug areas. In the very sheltered conditions of the Menai Strait, where bait digging results in the movement of underlying boulder clay to the surface, Johnson (1984) recorded that some experimental plots were still visible one year after having been dug. In contrast, on the more exposed, muddy sand shore of Red Wharf Bay, Anglesey, unfilled holes and mounds took from 25 to 30 days to completely disappear. This is an insufficient period to enable shores to recover between the peaks of collection at each low water spring tide. (Bait diggers are now using pumps in Red Wharf Bay, and the signs of this activity are lost overnight (Mr Sharp pers. comm.))

In addition to these physical effects, bait digging can cause changes to the chemical content of sediments. Howell (1985) records that increased levels of heavy metals were found in surface sediments and invertebrates following intensive bait digging in Budle Bay, where 50 diggers were estimated to turn over about 62.5 t of sediment containing 3 kg of lead and 40 g of cadmium on each tide. The exposure and subsequent oxidisation of deep sediments by digging enables these heavy metals, which are bound to sediment particles in reduced (anoxic) conditions, to become bioavailable. Cadmium is also concentrated in the anoxic layers by the activity of lugworms; their removal therefore exacerbates this problem.

Bait digging can also cause the destruction of mussel bed and eelgrass habitats on sediment areas.

These changes to the intertidal habitat also affect populations of other intertidal invertebrates.

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