AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS Aquatic Conserv: Mar. Freshw. Ecosyst. 13: S33–S41 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/aqc.566 Bivalve ¢shing and maerl-bed conservation in France and the UK}retrospect and prospect J.M. HALL-SPENCERa,*, J. GRALLb, P.G. MOOREc and R.J.A. ATKINSONc a School of Biological Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, UK b Institut Universitaire Europeeen! de la Mer, LEMAR UMR-CNRS 6539, Place Copernic, Plouzane, France c University Marine Biological Station, Millport, Isle of Cumbrae, KA28 0EG, UK ABSTRACT 1. Maerl beds are carbonate sediments, built by a surface layer of slow-growing coralline algae, forming structurally fragile habitats. 2. They are of international conservation significance, often supporting a high biodiversity and abundant bivalve molluscs. 3. Experimental fishing for scallops (Pecten maximus) on French and UK grounds has shown that although large epifauna are often killed, many organisms escape harm as they burrow deeply or are small enough to pass through the dredges. 4. Bivalve dredging is currently one of the main threats to European maerl grounds as it reduces their biodiversity and structural complexity and can lead to long-term degradation of the habitat. 5. Protecting maerl grounds is of importance for fisheries since they provide structurally complex feeding areas for juvenile fish (e.g. Atlantic cod - Gadus morhua) and reserves of commercial brood stock (e.g. Ensis spp., P. maximus and Venus verrucosa). 6. We outline improved mechanisms to conserve these ancient and unique biogenic habitats. Copyright # 2003 John Wiley & Sons, Ltd. KEY WORDS: maerl; biodiversity; conservation; scallop dredging; bivalves; Scotland; France INTRODUCTION Maerl beds are sedimentary deposits derived from calcareous red seaweeds (Rhodophyta) that occur in coastal areas worldwide (Bosence, 1983; Littler et al., 1991; Steller and Foster, 1995) but are being degraded by a number of human activities (BIOMAERL, 1999). Their distribution is patchy and restricted by their need for light coupled with an intolerance of desiccation and low salinities (Birkett et al., 1998). Live maerl is limited to depths 532 m in the relatively turbid inshore waters of northern Europe. Maerl growth requires a degree of shelter from wave action, to prevent burial and dispersal into deep water, and enough water movement to prevent smothering with silt (Hall-Spencer, 1998). The special status of maerl grounds *Correspondence to: Dr J.M. Hall-Spencer, School of Biological Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, UK. Copyright # 2003 John Wiley & Sons, Ltd. Received 4 April 2001 Accepted 20 August 2002 S34 J.M. HALL-SPENCER ET AL. derives from their unique character as complex biogenic structures of great antiquity (Adey and McKibbin, 1970; Canals and Ballasteros, 1997; Hall-Spencer, 2001). The complex architecture and extreme longevity of these habitats accounts for their high biodiversity (Maggs, 1983; Grall and Gleemarec,! 1997; Birkett et al., 1998; Hall-Spencer, 1998; BIOMAERL, 1999). In Europe, there are moves to establish Marine Protection Areas (MPAs) following agreements such as the Rio de Janeiro Convention on Biodiversity (1992) and the EC ‘Habitats Directive’ (1992). Notably, the maerl-forming corallines Phymatolithon calcareum and Lithothamnion corallioides are currently the only two algae specified as requiring management under the EC ‘Habitats Directive’ (Annex V). Biogenic reefs, such as maerl beds, are accepted as being vulnerable habitats by France and the UK and have been included within a proposed network of conservation areas termed ‘Natura 2000’ sites (Holt et al., 1998; French Government Website, 2001). Over the past six years, EC-funded research has provided a rounded insight into the ecological conditions, population dynamics and significance of a variety of human impacts on European maerl beds (BIOMAERL, 1999). It is now clear that bivalve dredging is one of the major threats to these habitats. In this paper we outline key findings from our work on maerl beds in the Bay of Brest (NW France) and the Clyde Sea area (SW Scotland) to assess the past and future impacts of bivalve fishing. THE TOPIC AS VIEWED FROM DIFFERENT PERSPECTIVES Conservation of maerl habitats requires urgent attention since there is considerable potential for their accelerated and widespread degradation over the next decade (BIOMAERL, 1999). Here we present ‘devil’s advocate’ arguments that some might use in favour of current practices or in targeting a wider range of maerl-dwelling bivalves. We then highlight the negative aspects of unrestricted exploitation and call for the long-term protection of a range of maerl habitats. ARGUMENTS IN FAVOUR OF BIVALVE EXPLOITATION ON MAERL BEDS The main commercial target of NE Atlantic maerl beds is currently the scallop Pecten maximus. This species can exhibit good recruitment and growth on maerl beds due to low siltation and high rates of water exchange (Thouzeau and Lehay, 1988; Hily et al., 1992; Hall-Spencer and Moore, 2000a, c). Of 242 maerl beds recently surveyed around the UK, only about one third supported P. maximus (MNCR database, 2000). Where scallops do occur on maerl, strong currents and shallow plankton-laden water encourage the growth of large, high quality scallops that provide an attractive target for exploitation (Briggs, 2000). As northern European populations of maerl mainly reproduce through a cycle of fragmentation and growth (Cabioch, 1970; Freiwald et al., 1991), it has been argued that dredging can potentially benefit the spread of live maerl, allowing the colonization of new areas of sediment. However, experimental studies have shown that dredged maerl tends to become crushed and buried leading to high levels of mortality due to lack of light (Hall-Spencer and Moore, 2000a). When maerl grounds are first dredged, debris and non-target invertebrate species such as sponges and Limaria hians nests are gradually removed from the ground and this improves subsequent dredge efficiency (Hall-Spencer and Moore, 2000b). Although large, fragile epibenthos (e.g. the sea urchin Echinus esculentus) are killed, hard-shelled organisms (e.g. the whelk Buccinum undatum) often survive. Scallop dredges are designed to allow young scallops to pass through and many survive to be harvested in future years (Caddy, 2000). Provided there is a consistent supply of spat, dredged areas can yield substantial catches year on year. Schemes to improve spat supply are in place on maerl grounds in the Bay of Brest, which are re-seeded with hatchery-reared juveniles (Hall-Spencer et al., 2001). Copyright # 2003 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 13: S33–S41 (2003) BIVALVE FISHING S35 (a) BRITTANY 80 60 40 mean no. taxa 20 0 BC AC BI AI AC AI AC AI AC AI Summer Autumn Winter Spring 0.1 m2 grab samples (b) SCOTLAND 80 60 40 mean no. taxa 20 0 BC AC BI AI AC AI AC AI AC AI Summer Autumn Winter Spring 0.1 m2 grab samples Figure 1. Mean number of taxa from 1 mm-sieved 0.1 m2 grab samples in control (open columns) and test areas (shaded columns) before and after experimental scallop dredging on maerl in the Bay of Brest (Brittany) and the Clyde Sea (Scotland). X-axis codes starting with ‘B’ denote ‘before fishing’, ‘A’ denotes ‘after fishing’. ‘C’ denotes ‘control plot’ samples, ‘I’ denotes ‘impacted plot samples. Error bars are +S.D. (n ¼ 6). Although current bivalve dredging practices do alter maerl habitats (Hall-Spencer and Moore, 2000a), they do not lead to long-term devastation of target species or the creation of barren grounds. Certain maerl grounds in the Bay of Brest have been dredged for scallops and the infaunal bivalve Venus verrucosa for >40 years, yet these sites remain productive for commercial bivalves and retain high levels of live maerl cover (Hall-Spencer et al., 2001). The sustained live maerl cover in the Bay of Brest may relate to local restrictions which set a limit of one scallop dredge per boat; this probably causes less damage than the sets of 36 dredges that are used routinely by large UK scallop boats. Typical French and UK scallop dredges have ca 10 cm long teeth to extract scallops living recessed within the sediment. The majority of inhabitants in the upper 10 cm of maerl sediment are small macrofauna. Grab sampling over the past two years showed that the number of taxa remained high after dredging maerl in the Bay of Brest and the Clyde Sea area (Figure 1). Small organisms survived in high numbers as many were displaced on a ‘bow wave’ that disturbed the sediment ahead of the advancing gear (see Gilkinson et al., 1998). Copyright # 2003 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 13: S33–S41 (2003) S36 J.M. HALL-SPENCER ET AL. Maerl communities have a degree of resilience to scallop dredging as many organisms often live below the depth raked through by the dredge teeth (Keegan and Koonnecker,. 1973; Hall-Spencer and Atkinson, 1999). Deep-burrowing organisms can represent the majority of infaunal maerl biomass and can survive dredging in high numbers (Hall-Spencer et al., 2001). This work has also revealed that maerl beds represent a huge, presently untapped, source of edible infaunal bivalves. The hydraulic technology needed to exploit these populations is available and supports an extensive fishery in Italian waters (Froglia, 1989). A rapidly developing UK fishery for razor clams (mainly Ensis siliqua) is proving profitable, based on hand-collection on sandy sediments. There are also global markets opening up for the export of other maerl-dwelling bivalves such as Dosinia exoleta, Tapes rhomboides and Venus verrucosa. If such resources could be exploited sustainably, this could bring much needed employment and benefits to rural communities. ARGUMENTS AGAINST BIVALVE EXPLOITATION OF MAERL BEDS Scallop dredging on French and UK maerl beds has significantly reduced the complexity, biodiversity and long-term viability of these habitats (Hily et al., 1992; MacDonald et al., 1996; Hall-Spencer and Moore, 2000a-c).