Extraction of maerl, either from beds where live thalli are present or where the maerl is dead or semi-fossilised, has been carried out in Europe for hundreds of years. Initially, the quantities extracted were small, being dug by hand from intertidal banks, but in the 1970s c. 600,000 tonnes of maerl was extracted per annum in France alone (Briand, 1991). Amounts have declined to c. 500,000 tonnes p.a. since then. Live maerl extraction is obviously very problematic with regard to growth rates for replacement. Dead maerl extraction is liable to lead to muddy plumes and excessive sediment load in water that later settles out and smothers surrounding communities. 'Commercial dredging of maerl deposits is particularly destructive since this removes the productive surface layer and dumps sediment on any plants which escape dredging, inhibiting habitat recovery' (Hall-Spencer, 1994).

In the Fal the Cornish Calcified Seaweed Co. has extracted dead maerl since 1975. Only the dead maerl is taken, and the most serious danger to the important St Mawes bank has therefore been thought to be the settling out of the dredge plume (Anon., 1993). The company has attempted to minimise damage by dredging only on the ebb tide so that the plume was taken out to sea. Reports on the maerl beds made over the last 15 years (e.g. Farnham & Bishop, 1985) have indicated that the flora and fauna are very diverse. However, direct comparisons of the flora with that of maerl in Galway Bay (Rostron, 1988; see summary table) show that the Fal beds are less species-rich than those in Galway Bay. It is not known whether this is related specifically to effects of dredging. Perrins et al. (1995) reported that between 1982 and 1992 the proportion of dead maerl on the St Mawes bank increased significantly, from 12% to 23%.

Hardiman et al. (1976) attempted to assess the effects of maerl dredging in the Fal by taking core samples. They found black anaerobic mud under the living maerl, the amount of mud increasing towards the main river channel. They apparently advocated the removal of maerl as it provided a poor settlement ground for oysters!

A report prepared for IFREMER on the maerl beds at Brest, Brittany (Augris & Berthou, 1990), suggested that due to the very slow rate of growth, maerl beds develop very slowly. The biological equilibrium is precarious - effectively, maerl extraction is the exploitation of a non-renewable resource as the slow rate of growth implies a slow rate of accumulation. Grall & Glémarec (1997) compared various indices of biological health for exploited and control maerl beds at the isles of Glénan, and found few significant differences except for a reduction in the number of individuals of each species counted in samples.

The positioning of cages over a maerl biotope is likely to lead to fish faeces and partly consumed food pellets contaminating the maerl bed and resulting in anaerobiosis due to the oxygen demand of the decomposing material. The detrital rain from the cages could act in a similar way to terrigenous silt, reducing light penetration through the water column and smothering the maerl surface so that the stabilizing epiphytic algae could no longer establish themselves. As a minimum impact the increase in nutrient levels might produce local eutrophication effects.

SNH reported in Marine Scene (Autumn 1996) that part of Loch Ailort was surveyed to establish a location where the development of a mussel farm would not affect the maerl beds present in the area. Monitoring of a salmon farm anchored over a maerl bed in Shetland has shown a buildup over a 10-year period of Beggiotoa and anoxic conditions (J. Hall-Spencer, pers. comm.).

In Ardmore Bay, Kilkieran Bay, Co. Galway, fish cages are anchored over maerl beds in one area. Current speed seems to be sufficient to clear detrital material and the maerl has not suffered obvious damage (B. O'Connor, pers. comm.). However, at a sheltered site at Mweenish Island, also in Co. Galway, Maggs & Guiry (1987a) noted that maerl under fish cages was covered with Beggiotoa and fungi.

In the Galician rias, Spain, mussel rafts have affected maerl beds (J. Hall-Spencer, pers. comm.). Mussel faeces and pseudofaeces rain down onto the maerl surface, altering sediment structure and compromising the ability of maerl thalli to photosynthesise and grow - work is ongoing under the BIOMAERL programme to evaluate this damage.

The removal of the living maerl thalli from the biotope surface, the loss of the stabilising algae and the disruption of the structure of both the physical habitat and the community structure occur. These major changes have been reported from areas where scallops are dredged from maerl beds (Hily et al., 1992; Hall-Spencer, 1995a, 1998).

The effects of scallop (Pecten maximus) dredging in the upper Firth of Clyde, where maerl beds are rare, has been evaluated by Hall-Spencer (1995a, 1998), using video and direct observation. Passage of the dredges destroyed large animals and algae and raised particulate sediments into the water, which later settled over a large area, stressing filter feeders and reducing photosynthesis. Dredge teeth penetrated 10 cm into the maerl, crushing maerl fragments and killing them by burial. Four months after dredging there were less than half as many live maerl thalli as in control undredged areas. There was evidence that the community structure was altered in favour of opportunistic species such as scavengers. Overall, the effect of scallop dredging on maerl beds was very serious, with the effects on living maerl compromising habitat integrity and future recovery.

In the rade de Brest the maerl beds support populations of the black scallop Chlamys varia, which are locally abundant and are intensively fished during the winter months. The dredging activity has been reported to result in severe disruption to the maerl bed and associated flora and fauna (Hily & Le Fol, 1990).

One of the biggests threats to live and dead maerl beds is suction dredging for large burrowing bivalves such as Ensis and Venerupis species, which are marketed in Spain (D. McKay, pers. comm.). Suction dredging not only has major impacts on the target species, but causes structural damage to the community from which they are being extracted. The detrimental effects on maerl beds are expected to include impacts of resuspended sediment settling out over the maerl and reducing photosynthesis.

Along the west coast of Scotland, sublittoral harvesting of Venerupis has occurred in the North Sound, Arisaig, and Ensis has been harvested at various locations including Shetland and Orkney (D. McKay, pers. comm.). Suction dredging for these species causes disruption of the substratum to considerable depths, creating holes up to 2 m across and 1 m deep in sandy substrata. Comparable studies have not been made in maerl habitats, however.

In order to renew or enlarge navigational channels, extensive dredging may take place. This involves removing the seabed, which results in the suspension of the fine silt and clay fractions of the sediment. This fine sediment may be deposited by the inshore currents either locally or at a considerable distance from the dredging operation. The additional sediment load will increase local turbidity and may also settle on maerl beds, burying the calcareous thalli, smothering other algae and animals, possibly destroying the physical stability of the habitat as well as the ecology of the biotope. Seabed removal where a maerl bed is present will of course result in the removal of the maerl itself. If the underlying substratum is altered, it is unlikely that maerl will be able to re-establish itself at that site, given the probable method of reproduction of the species involved.

No case studies are known.

The results of these activities would be similar to those mentioned above, such as removal of the seabed, redistribution of mud, and destroying the biotope stability and viability.

No case studies are known.

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