# Effects of clam rake harvesting on the intertidal
Ruditapes habitat of the English Channel

Florence Beck, Jean-Philippe Pezy, Alexandrine Baffreau, Jean-Claude Dauvin

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Florence Beck, Jean-Philippe Pezy, Alexandrine Baffreau, Jean-Claude Dauvin. Effects of clam rake harvesting on the intertidal Ruditapes habitat of the English Channel. ICES Journal of Marine Science, 2015, 72 (9), pp.2663 - 2673. 10.1093/icesjms/fsv137. hal-01540832

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ICES Journal of Marine Science (2015), 72(9), 2663–2673. doi:10.1093/icesjms/fsv137

Original Article

# Effects of clam rake harvesting on the intertidal *Ruditapes* habitat of the English Channel

Florence Beck<fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Jean-Philippe Pezy<fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Alexandrine Baffreau<fnref n="1" />,<fnref n="2" />,<fnref n="3" />, and Jean-Claude Dauvin<fnref n="1" />,<fnref n="2" />,<fnref n="3" />*

Beck, F., Pezy, J-P., Baffreau, A., and Dauvin, J-C. Effects of clam rake harvesting on the intertidal *Ruditapes* habitat of the English Channel. – *ICES Journal of Marine Science*, 72: 2663–2673.

Received 15 January 2015; revised 9 July 2015; accepted 15 July 2015; advance access publication 12 August 2015.

Recreational and professional clam fishing was an important activity on the extensive intertidal zone of the western Cotentin coast (western English Channel). A variety of fishing gear was used to harvest the target species: the European clam *Ruditapes decussatus* (Linnaeus, 1758) and the introduced Manila clam *R. philippinarum* (Adams and Reeve, 1850). In this study, we studied the effect of rake harvesting during the spring tides of February–March 2014, following an experimental design with a control station and three stations impacted by rake harvesting in three sediment types: sandy, gravelly, and mixed gravelly rocky habitats. No significant sediment and macrofauna changes occurred at the three sites after rake harvesting. Nevertheless, the number of clams decreased significantly after raking on the gravelly habitat, whereas in the other two habitats, sediment transport in this high-energy hydrodynamic environment was able to transfer clams and other macrofauna species across the fishing sites, thus minimizing the effects of rake harvesting. Therefore, although the effect of rake harvesting appeared limited during winter, the regional impact (high fishing pressure along 60 km of coast) and increase of recreational fishers during summer needed to be studied in the future.

**Keywords:** macrofauna, raking fishing, *Ruditapes decussatus*, *Ruditapes philippinarum*, sediment, size classes.

Introduction

Different categories of fishers could be found on the intertidal zone of the French Atlantic coast, involved in commercial and recreational activities. Moreover, recreational fishing practised mainly for family consumption was a cultural activity and was now an essential part of coastal tourism, especially along the extensive intertidal zone of the west coast of the Cotentin (western English Channel). This leisure activity appeared increasingly practised because of the evolution of social conditions, leading to an increase of free time (Laspougeas, 2007). Nevertheless, the efficiency of the harvest differed according to the experience of the fishers, the equipment used, the location on the intertidal zone, and the target species (molluscs, crab, and shrimps; Sécula, 2011).

Among the bivalves, three target species were actively harvested along the west coast of the Cotentin: the Warty venus *Venus verrucosa* (Linnaeus, 1758), which was accessible only during spring tide (Navon and Dauvin, 2013), the European clam *Ruditapes*

*decussatus* (Linnaeus, 1758), and the Manila clam *R. philippinarum* (Adams and Reeve, 1850). This latter species was introduced in the 1970s to be cultivated in several zones on the French side of the English Channel and on the Atlantic seaboard (Grizel, 1983). As a result, the Manila clam now supplanted the native European clam in many estuaries and lagoons (Bidegain and Juanes, 2013), and had become an important resource for recreational and professional fishers on the western coast of Cotentin. Owing to the increase of the resource, there was an increased pressure on this target species, especially when harvesting was carried out with clam rakes not only by professional but also by recreational fishers.

Different sedimentary types were present on the study site, which influenced the distribution of fishers. There was no mud or muddy sand habitat along the intertidal zone of the western part of the Cotentin, which was characterized by high-energy hydrodynamic environment. The same clam species lived in abundance at other locations along the French Atlantic coast, such as in the Gulf of

<footnote n="1">¹Laboratoire de Morphodynamique Continentale et Côtière, Université de Caen Basse Normandie, UMR CNRS 6143 M2C, 24 rue des Tilleuls, 14000 Caen, France</footnote>

<footnote n="2">²UMR CNRS 6143 M2C, 24 rue des Tilleuls, 14000 Caen, France</footnote>

<footnote n="3">³CREC, Station Marine de l'Université de Caen-Basse Normandie, BP 49, 54, rue du Docteur Charcot, 14530 Luc-sur-Mer, France</footnote>

<footnote>*Corresponding author: tel: + 33 3 21 56 57 22; fax: + 33 3 21 56 57 57; e-mail: jean-claude.dauvin@unicaen.fr</footnote>
Morbihan bordering the northern part of the Bay of Biscay (D'Hardivillé et al., 2008) as well as in the Arcachon basin farther south (Dang et al., 2010; Caill-Milly, 2012).

In the intertidal zone of Blainville-sur-Mer (Figure 1), the most intense fishing pressure was localized in the sandy habitat (“Synergie Mer Et Littoral” and J.-C. Dauvin, pers. obs.). This was because the sandy habitat was more easily accessible (being higher on the intertidal zone) than both other habitats. Moreover, very few fishers were observed in the gravelly habitat owing to the difficulty of raking (indurate bottom), with clams being more deeply embedded in the sediment (less accessible) than on the deep gravelly site where professional fishers were currently active.

The question that emerges was: what was the impact of using the clam rake on clam populations, i.e. the effects on the abundance and size class of clam populations, and the effect on the macrofauna biodiversity associated with the *Ruditapes* habitat.

The aim of the present study was to estimate the short-term impact of rake harvesting of *Ruditapes* target species during February–March 2014 spring tides on: (i) the sediment structure; (ii) the clam population; and (iii) the associated macrobenthic species. The study was carried out during the 2014 winter–spring tides on the west coast of Cotentin at Blainville-sur-Mer (France) (western English Channel). Three sediment types (sandy, gravelly,

and mixed gravelly rocky habitats) were selected and a 'before-after-control impact' or BACI design was adopted to study the impact using a control station and three stations impacted by rake harvesting.

## Material and methods

### Sampling site

Located in the Normano-Breton Gulf, the wide intertidal zone in Blainville-sur-Mer was situated on the west coast of the Cotentin Peninsula (Figure 1); it was composed of a mixture of sandy, gravelly, sandy-rocky, and rocky areas. The upper part of the foreshore of Blainville-sur-Mer was constituted by sand, then to the sea there was a succession of coarse sand and rocky habitat. The site of Blainville-sur-Mer presented a dominance of gravelly habitat. The shoreline here was composed mainly of sandy dunes undergoing intense erosion (Robin and Levoy, 2007; Robin et al., 2009) and sandy intertidal dunes and flats subject to rapid displacements owing to high-energy hydrodynamics in a megatidal environment (tidal range >12 m during equinoctial spring tide) (Levoy et al., 1997, 2013; Montreuil et al., 2014). The autumn–winter of 2013–2014 was also characterized by a very large number of severe storms (http://www.meteofrance.com). The sampling was carried

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Location of the sampling stations in the Blainville-sur-Mer intertidal zone on the western coast of Cotentin: (a) general map of the western English Channel. (b) Central part of the western coast of Cotentin. (c) Location of the three sampling habitats (S: sandy habitat; G: gravelly habitat, and DG: deep gravelly habitat).</figcaption></figure>
out at between 1 and 2 km from the tide line on the intertidal zone which was accessible at each mean spring tide.

The sampling was carried out on three sites exhibiting three sediment types (i.e. S: for sand habitat, G: for gravelly habitat, and DG: deep gravelly habitat in a mixed gravelly and rocky habitat) in which professional fishers use the clam rake to capture the target species *R. decussatus* and *R. philippinarum* (Figure 2). Nevertheless, recreational fishers harvested clams mainly in the sandy habitat (the easiest zone for fishing), using an assortment of gear included professional clam rake. For the S and DG sediment type, sampling had been made on 18 February (before raking) and 18 March (after raking) and for the G sediment type on 3 March (before raking) and 31 March (after raking). For each sediment types, four stations of 10 m² (3.16 m × 3.16 m) were selected at random A, B, and C (replicates) corresponded to stations where rake fishing (0.15 m depth) was carried out (on all the surfaces of the station) 1 month after the first period of observation, whereas T corresponded to a control station at which rake fishing was carried out only at the end of the experimental period (Figure 3). The target clam species were identified and their maximum length was measured, using an automatic calliper with a precision of 0.05 mm.

## Intertidal clam fisheries

The width of the professional clam rake was 16.5 cm, the length of the seven teeth was 7 cm, and the distance between two teeth is 2.8 cm (Figure 2). The number of professional fishers working in the Basse-Normandie region was 95 in 2014 and 105 in 2015 for a mean of 100 (Xavier Tetard, Comité Régional des Pêches Maritimes et des Elevages Marins de Basse Normandie, pers. comm.). About 50% of the professional fishers harvested the *Ruditapes* clam (size>40 mm) during the April–October period when the tide co-efficient was higher than 70 (114 d in 2014); they could fish a

maximum of 20 kg d⁻¹ fisher⁻¹. So, a maximum weight of 114 t could be collected along the western coast of Cotentin by professional fishers. The recreational fishers could be estimated to be 500 per day mainly during three summer tides (49 d when the tide coefficient was higher than 70 in 2014). The maximum number of clams per fishers was 100 for a mean wet weight of an individual estimated to be 25 g (J.-C. Dauvin, pers. obs.) for a maximum weight of 2.5 kg d⁻¹ fisher⁻¹. A maximum weight of 61 t could be collected along the western coast of Cotentin by recreational fishers. The grand total should be 175 t.

In France, the *Ruditapes* professional fisheries extended since the Basse-Normandie to the Aquitaine regions. The two main places of production are the Bay of Arcachon and the Gulf of Morbihan, respectively, in the south and the north of the Bay of Biscay, with an estimation of production of 578 t in 2010 and 614 t in 2011 which represented ~50% of the French Atlantic-English Channel production by professional fishers (Caill-Milly, 2012). For 2008, the French production of this area was estimated to be 1200 t (Caill-Milly, 2012). So, the part of the west coast of Cotentin professional fishery represented ~10% of the French production.

## Infauna sampling

The infauna were sampled, before and 1 month after raking, over an area of ~1/32 m² with a 0.20 m diameter, stainless steel hand corer with a haphazard strategy (Navon and Dauvin, 2013). The depth of sampling was ~0.15 m. Eight replicates were performed at each station, representing a total sampled area of 0.25 m², which was the value recommended by the French National REBENT programme for the intertidal zone (Réseau Benthique, http://www.rebent.org/; Figure 3). Core samples were passed through a 1-mm mesh sieve, fixed with formaldehyde 10%, and coloured with Pink Bengal dye to facilitate sorting. Then, the species were sorted and identified in the laboratory and stored in 70% alcohol.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Rake used on the western coast of the Cotentin by the professional fishers.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Sampling strategy for the three sedimentary habitats (DG: deep gravelly habitat; S: sandy habitat; G: gravelly habitat; A, B, and C: rake fishing station; T: control station; 1, 2, 3, 4, 5, 6, 7, 8: core replicates).</figcaption></figure>

## Sediment parameters

To examine changes in sediment particle size composition for each habitat before and after raking, one core sample of sediment was collected haphazardly from each station in the three sediment types. Sediment was collected with a shovel removing the surface to a depth of 0.15 m. In the laboratory, the sample was desalted with freshwater by successive washing for removing any trace of salt causing crystallization in the grain aggregates. When the sediment contained a significant proportion of very fine particles (<63 µm), these were first separated from the rest of the sample. Then, the sample was dried in an oven at 60°C for 24 or 48 h and passed through a column of 32 sieves with mesh sizes selected based on the Wentworth classification (1922), using a vibrating sieve (60 amplitudes per min) during 15 min. Then, three main categories of sediment were defined: silt–clay <63 µm; sand: 63 and 500 µm, and coarse sand and gravel >500 µm.

## Statistical analyses

Statistical treatments were chosen according to those selected by Kaiser *et al.* (2001) to test the effect of intertidal soft-sediment benthic communities by cockle *Cerastoderma edule* raking in the River Dee Estuary, UK.

The percentage particle size of sediment was analysed with a two-way ANOVA (treatment and habitat). Before each ANOVA, Shapiro–Wilk normality test and Bartlett's test for homogeneity of variance were performed to confirm if the assumptions of ANOVA were met and if it was unnecessary to transform the data. Tukey's *Honestly Significant Difference* (HSD) test was used to

determine the differences between before and after raking and between the three sediment types.

For the biological parameters, a Shapiro–Wilk normality test and a Bartlett test of homogeneity of variance were performed to verify the conditions of ANOVA application. Then, ANOVAs were performed to assess the effect of clam rake harvesting on the abundances of *Ruditapes*, size structure of *Ruditapes* spp., and *R. philippinarum*, and to assess the effect of clam rake harvesting on benthic abundance and taxonomic richness. Then, a log (x + 1) transformation (for stabilizing the variance) was used on the abundance matrix (data for each station of the three sediment types were pooled before undertaking further analyses), before calculating the Bray–Curtis similarities using Primer 6 (Clarke and Gorley, 2006). A denrogram was created, with group average expressed in the cluster mode. Then, a non-parametric multidimensional scaling (MDS) ordination, using the Bray–Curtis similarity measure, was applied on the abundance of the taxa, after a log (x + 1) transformation with the objective of examining intertidal infauna structure. The SIMilarity PERcentages (SIMPER) routine was used to establish which species contributed the most to the observed differences in the data. ANOSIM (Analysis Of SIMilarities; Clarke, 1993) was also conducted to test for significant differences in intertidal infauna composition in response to raking (one-way analysis).

## Results

### Impact on the sediment

For sandy habitat (S), the median sediment grain size is between 0.78 and 1.03 mm before the first rake fishing, and between 0.59 and
<table><caption>**Table 1.** Mean percentage (± 95% confidence interval) silt-clay (<0.063 mm particle size), sand (0.063–0.5 mm particle size), and gravel (>0.5 mm particle size) content of the sediment.</caption><thead><tr><th rowspan="2"></th><th colspan="3">Sandy habitat</th><th colspan="3">Gravelly habitat</th><th colspan="3">Deep gravelly habitat</th></tr><tr><th>Silt and clay</th><th>Sand</th><th>Gravel</th><th>Silt and clay</th><th>Sand</th><th>Gravel</th><th>Silt and clay</th><th>Sand</th><th>Gravel</th></tr></thead><tbody><tr><td>Control plots B</td><td>0.53<sup>a</sup></td><td>27.11<sup>d</sup></td><td>72.35<sup>f</sup></td><td>0.89<sup>a</sup></td><td>12.12<sup>b</sup></td><td>86.98<sup>c</sup></td><td>0.61<sup>a</sup></td><td>7.57<sup>b</sup></td><td>91.82<sup>c</sup></td></tr><tr><td>Control plots A</td><td>0.31<sup>a</sup></td><td>40.19<sup>e</sup></td><td>59.50<sup>g</sup></td><td>0.83<sup>a</sup></td><td>8.23<sup>b</sup></td><td>90.94<sup>c</sup></td><td>0.57<sup>a</sup></td><td>8.18<sup>b</sup></td><td>91.25<sup>c</sup></td></tr><tr><td>Before raking</td><td>0.45 ± 0.17<sup>a</sup></td><td>23.98 ± 2.65<sup>d</sup></td><td>75.57 ± 2.54<sup>f</sup></td><td>0.7 ± 0.10<sup>a</sup></td><td>15.59 ± 4.99<sup>b</sup></td><td>83.71 ± 5.07<sup>c</sup></td><td>0.77 ± 0.04<sup>a</sup></td><td>8.89 ± 3.62<sup>b</sup></td><td>90.34 ± 3.58<sup>c</sup></td></tr><tr><td>After raking</td><td>0.52 ± 0.49<sup>a</sup></td><td>36.84 ± 5.15<sup>e</sup></td><td>62.64 ± 5.20<sup>g</sup></td><td>0.49 ± 0.17<sup>a</sup></td><td>5.18 ± 2.30<sup>b</sup></td><td>94.33 ± 2.46<sup>c</sup></td><td>0.64 ± 0.91<sup>a</sup></td><td>9.92 ± 4.94<sup>b</sup></td><td>89.44 ± 5.85<sup>c</sup></td></tr></tbody></table>

Means sharing the same superscript letters did not differ significantly (Tukey's HSD test; *p* > 0.05).

B: first day; A: after 1 month.

0.67 mm 1 month later. For G, the median sediment grain size is between 4.07 and 6.73 mm before the first rake fishing and between 6.72 and 12.05 1 month later. Finally, for DG, the median sediment grain size is between 1.52 and 2.15 mm before the first rake fishing, and between 1.33 and 2.18 mm 1 month later. Thus, the mean median grain size increases from S to G, whereas DG showed interme-diated values typical of mixed sediment between sand and gravel.

The three sediment types differed significantly (ANOVA, $F_{2,21} = 49.95$; *p* < 0.001; Table 1). Nevertheless, DG and G habitats appeared similar for their silt-clay, sand, and gravel composition. The disturbance of sediment (by raking) did not induce siltation, and the percentage of silt-clay does not differ significantly between Control plots and the three types of sediment (Table 1). The S habitat was significantly different from both habitat types for sand and gravel composition. After raking, we observed a signifi-cant difference in the sediment composition (ANOVA, $F_{1,22} = 11.1$; *p* = 0.0018); nevertheless, the same significant difference was observed in the Control plot (Table 1), showing that the differences were not caused only by raking.

In summary, 1 month after fishing rake, there was a sediment change only for the S habitat, equally in the raking and the control areas because of change in gravel and sand proportion (Table 1). These changes could be attributed to the sediment transport during winter 2014.

## Abundances of *Ruditapes* target species

For the three habitats, there was a high local variability of clam abundances between stations, especially for S. The total clam (*Ruditapes* spp.) abundance varied from 133 individuals per 10 m² at station A in G to only 4 individuals per 10 m² at station A in S (Figure 4). Before raking, the mean total clam abundance ± standard variation was higher in G (105 ± 25 ind. 10 m²) than in DG habitat (66 ± 23 ind. 10 m²) and S (22 ± 20 ind. 10 m²; ANOVA, $F_{2,15} = 7.32$; *p* = 0.008; Figure 4). The harvesting of clams at the control station in the three sediment types 1 month after the first raking confirmed the high scatter in abundances of the clams. Between both rakings, there was a significant decrease in the number of clams in only one type of substrate: G (Table 2).

The Manila clam *R. philippinarum* represented between 74 and 100% of the harvested clams according to the different stations with 98% for S, 87% for G, and 95% for DG of the total number of clams sampled in the three habitats. We observed only a significantly difference of *R. philippinarum* after raking in G like the total clam abundance (Table 2).

## Size structure of *Ruditapes* spp.

The smallest Manila clam was 8.27 mm in size, and the largest 55.14 mm, whereas the European clam showed minimum and

maximum sizes of 23.55 and 51.93 mm, respectively. As the minimum marketable size of clam was set at 40 mm in the Manche department (local decision of the French fishing organization), two groups of clams *Ruditapes* spp. could be defined: small clams with a length of <40 mm and larger clams with a length of >40 mm (Figure 5). In habitat G, 62% of the individuals showed a size of >40 mm, whereas this proportion was 41% in habitat DG and only 24% in habitat S.

For S, the mean size structures of the clams were similar between harvesting dates, i.e. 35.94 and 36.90 mm. Similarly, for DG, the mean size structures of the clams between harvesting dates were comparable: 38.18 and 38.85 mm (Figure 5). And for G, most of the small clams disappeared between harvesting dates (smaller mean size of 14–16 mm on 3 March as against 34–36 mm on 31 March), the harvested individuals were mainly composed of large individuals >40 mm, with a mean size of 40.30 mm on 3 March and 41.90 mm on 31 March (Figure 4). Clam size on S did not change significantly after harvesting (ANOVA, $F_{1,139} = 0.01$; *p* = 0.927), whereas after harvesting, clam size increased for G habitat (ANOVA, $F_{1,378} = 4.16$; *p* = 0.042) and DG habitat (ANOVA, $F_{1,333} = 6.95$; *p* = 0.009).

## General composition of the macrofauna

A total of 83 taxa was found in the three habitats sampled during winter. The taxonomic diversity was dominated by the polychaetes, which account for 46 taxa, followed by the molluscs with 18 taxa, the arthropods with 13 taxa, the cnidarians and the echinoderms with two taxa each, whereas the Nemertean, Syllidae, and Sipunculida were represented by a single taxon each but were not identified to a specific level.

The polychaetes were numerically predominant, representing between 51 and 96% of the total abundances (Table 3). Sipunculida and Nemertea are also among the dominant taxa (Table 3). For each habitat, a pair of taxa was found to dominate: the polychaetes *Ophelia rathkei* (McIntosh, 1908) and Syllidae for S, the Sipunculida *Golfingia vulgaris* (de Blainville, 1827) and the polychaete *Notomastus latericeus* (Sars, 1851) for G, and finally the polychaetes *Malacoceros girardi* (Quatrefages, 1843) and Syllidae for DG (Table 3). The total number of individuals collected per 2 m² was 781 in S (the poorest habitat), 803 in DG, and 2265 in G (the richest habitat). The 10 dominant taxa represented 93, 83, and 77% of the total number of the macrofauna collected in S, G, and DG, respectively.

## Impact of rake harvesting on the macrofauna

Two cores yielded no individuals: the first was sampled from habitat DG at station A on 18 March and the second from habitat S at station A on 18 February (Table 4). A large range of abundances was observed between replicates at the same station for both sampling dates (before rake clam harvesting and 1 month after). In S, the number of individuals per replicate varied from 0 to 53 on 18
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Numbers of clams for each station (10 m²) for the three habitats (S: sandy habitat; G: gravelly habitat, and DG: deep gravelly habitat) before (black) and after (gey) raking with three replicates (A, B, and C) and a control station (T) (not raked on the first date). Replicates with asterisks (**) are significantly different (p < 0.01).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Table 2.** ANOVA analysis on the total clam (*R. philippinarum* and *R. decussatus*) and *R. philippinarum* from three sediment types (G: gravelly habitat; DG: deep gravelly habitat; S: sandy habitat) before and after raking.</figcaption></figure>

<table><thead><tr><th></th><th>Sediment type</th><th>Abundance before raking</th><th>Abundance after raking</th><th>F</th><th>P-value</th></tr></thead><tbody><tr><td rowspan="3">Total clam abundance</td><td>G</td><td>326</td><td>54</td><td>F<sub>2,4</sub> = 35.72</td><td>0.004</td></tr><tr><td>DG</td><td>213</td><td>122</td><td>F<sub>2,4</sub> = 2.452</td><td>0.192</td></tr><tr><td>S</td><td>78</td><td>62</td><td>F<sub>2,4</sub> = 0.098</td><td>0.77</td></tr><tr><td rowspan="3">R. philippinarum</td><td>G</td><td>314</td><td>48</td><td>F<sub>2,4</sub> = 55.01</td><td>0.002</td></tr><tr><td>DG</td><td>199</td><td>114</td><td>F<sub>2,4</sub> = 2.891</td><td>0.164</td></tr><tr><td>S</td><td>67</td><td>54</td><td>F<sub>2,4</sub> = 0.1</td><td>0.767</td></tr></tbody></table>

February and from 2 to 44 on 18 March, whereas for G, it varied from 5 to 90 on 3 March and from 6 to 66 on 31 March. Finally, for DG, it varied strongly from 2 to 74 on 18 February and from 0 to 45 on 18 March.

On the first day of the experiment before raking, the abundances of the benthic communities in the three sediment types were significantly different (Figure 6 and Table 5, ANOSIM). One month after raking, there were again significant differences in the benthic

community among the three sediment types (Figure 6 and Table 5). Similarly, the total number of individuals and the number of taxa at the three raking stations between both dates did not show significant difference, for S (ANOVA abundance, $F_{1,4} = 1.57$; $p = 0.26$; taxonomic richness, $F_{1,4} = 3.99$; $p = 0.093$), for G (ANOVA abundance, $F_{1,4} = 3.15$; $p = 0.13$; taxonomic richness, $F_{1,4} = 2.07$; $p = 0.20$), and DG (ANOVA abundance, $F_{1,4} = 0.04$; $p = 0.85$; taxonomic richness, $F_{1,4} = 1.92$; $p = 0.21$).
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Size clams before raking (black) and after raking (grey), for the three substrates. Size classes are expressed in mm. The vertical line indicates the size of the marketable clams (40 mm), above the vertical line: the average size.</figcaption></figure>

<table><caption>**Table 3.** Ten dominant taxa in the three sediment types classified in decreasing rank; abundances are given as the number of individuals collected for a total of 2 m² (S: sandy habitat; G: gravelly habitat; DG: deep gravelly habitat).</caption><thead><tr><th>Sandy habitat</th><th>Abundance/2 m²</th><th>Gravelly habitat</th><th>Abundance/2 m²</th><th>Deep gravelly habitat</th><th>Abundance/2 m²</th></tr></thead><tbody><tr><td>Ophelia rathkei</td><td>449</td><td>Notomastus latericeus</td><td>648</td><td>Malacoceros girardi</td><td>114</td></tr><tr><td>Syllidae</td><td>110</td><td>Golfingia (Golfingia) vulgaris vulgaris</td><td>299</td><td>Syllidae</td><td>96</td></tr><tr><td>Saccocirrus papillocercus</td><td>3</td><td>Cirratulus cirratus</td><td>282</td><td>Saccocirrus papillocercus</td><td>86</td></tr><tr><td>Goniadella bobretzkii</td><td>29</td><td>Lanice conchilega</td><td>169</td><td>Cirratulus cirratus</td><td>66</td></tr><tr><td>Ruditapes philippinarum</td><td>22</td><td>Hilbigneris gracilis</td><td>136</td><td>Notomastus latericeus</td><td>64</td></tr><tr><td>Nemertea</td><td>20</td><td>Sagartia troglodytes</td><td>133</td><td>Aonides oxycephala</td><td>49</td></tr><tr><td>Nephtys cirrosa</td><td>19</td><td>Actiniaria</td><td>67</td><td>Schistomeringos neglecta</td><td>48</td></tr><tr><td>Archiannelida</td><td>15</td><td>Aonides oxycephala</td><td>56</td><td>Caulleriella sp.</td><td>44</td></tr><tr><td>Golfingia (Golfingia) vulgaris vulgaris</td><td>15</td><td>Venerupis corrugata</td><td>46</td><td>Golfingia (Golfingia) vulgaris vulgaris</td><td>30</td></tr><tr><td>Schistomeringos neglecta</td><td>9</td><td>Maldanidae</td><td>39</td><td>Sphaeroma serratum</td><td>24</td></tr></tbody></table>

SIMPER illustrated the biological meaning of the clustering before and after raking (Figure 6), by displaying the group similarity and identifying the species contributing most of the dissimilarity between groups. Group S was represented by sand species, such as *O. rathkei*, *Syllidae*, *Goniadella bobretzkii* (Annenkova, 1929),

and *G. vulgaris*. Group G was represented by coarse sand species with affinity for silt and clay, such as *N. latericeus*, *Cirratulus cirratus* (O. F. Müller, 1776), *G. vulgaris*, and *Sagartia troglodytes* (Price in Johnston, 1847). Then, Group DG was represented by a mix of coarse sand and sand species, such as *M. girardi*;
<table><caption>**Table 4.** Total number of macrofauna individuals sampled at each station of the three sediment types (S: sandy habitat; G: gravelly habitat; DG: deep gravelly habitat).</caption><thead><tr><th rowspan="2">Replicate</th><th colspan="2">SA</th><th colspan="2">SB</th><th colspan="2">SC</th><th colspan="2">ST</th></tr><tr><th>18 Feb</th><th>18 Mar</th><th>18 Feb</th><th>18 Mar</th><th>18 Feb</th><th>18 Mar</th><th>18 Feb</th><th>18 Mar</th></tr></thead><tbody><tr><td>1</td><td>29</td><td>28</td><td>24</td><td>11</td><td>53</td><td>44</td><td>16</td><td>11</td></tr><tr><td>2</td><td>26</td><td>16</td><td>19</td><td>8</td><td>40</td><td>25</td><td>7</td><td>9</td></tr><tr><td>3</td><td>21</td><td>12</td><td>13</td><td>8</td><td>39</td><td>14</td><td>6</td><td>9</td></tr><tr><td>4</td><td>13</td><td>10</td><td>11</td><td>6</td><td>37</td><td>12</td><td>5</td><td>5</td></tr><tr><td>5</td><td>13</td><td>5</td><td>11</td><td>4</td><td>23</td><td>8</td><td>4</td><td>4</td></tr><tr><td>6</td><td>10</td><td>4</td><td>9</td><td>4</td><td>13</td><td>8</td><td>4</td><td>4</td></tr><tr><td>7</td><td>10</td><td>2</td><td>7</td><td>2</td><td>8</td><td>6</td><td>4</td><td>3</td></tr><tr><td>8</td><td>0</td><td>2</td><td>6</td><td>1</td><td>3</td><td>6</td><td>3</td><td>3</td></tr><tr><td>Total</td><td><strong>122</strong></td><td><strong>79</strong></td><td><strong>100</strong></td><td><strong>44</strong></td><td><strong>216</strong></td><td><strong>123</strong></td><td><strong>49</strong></td><td><strong>48</strong></td></tr><tr><td>Mean</td><td><strong>15.25</strong></td><td><strong>9.88</strong></td><td><strong>12.50</strong></td><td><strong>5.50</strong></td><td><strong>27.00</strong></td><td><strong>15.38</strong></td><td><strong>6.13</strong></td><td><strong>6.00</strong></td></tr><tr><td>SD</td><td><strong>9.53</strong></td><td><strong>8.89</strong></td><td><strong>6.14</strong></td><td><strong>3.38</strong></td><td><strong>17.88</strong></td><td><strong>13.15</strong></td><td><strong>4.19</strong></td><td><strong>3.16</strong></td></tr><tr><td></td><td colspan="2">GA</td><td colspan="2">GB</td><td colspan="2">GC</td><td colspan="2">GT</td></tr><tr><td></td><td>3 Mar</td><td>31 Mar</td><td>3 Mar</td><td>31 Mar</td><td>3 Mar</td><td>31 Mar</td><td>3 Mar</td><td>31 Mar</td></tr><tr><td>1</td><td>74</td><td>21</td><td>60</td><td>40</td><td>73</td><td>43</td><td>90</td><td>66</td></tr><tr><td>2</td><td>48</td><td>19</td><td>46</td><td>37</td><td>51</td><td>37</td><td>90</td><td>65</td></tr><tr><td>3</td><td>47</td><td>18</td><td>40</td><td>29</td><td>46</td><td>24</td><td>72</td><td>63</td></tr><tr><td>4</td><td>31</td><td>15</td><td>36</td><td>28</td><td>44</td><td>23</td><td>62</td><td>55</td></tr><tr><td>5</td><td>24</td><td>13</td><td>32</td><td>27</td><td>43</td><td>20</td><td>56</td><td>50</td></tr><tr><td>6</td><td>24</td><td>13</td><td>30</td><td>25</td><td>41</td><td>12</td><td>29</td><td>34</td></tr><tr><td>7</td><td>24</td><td>9</td><td>25</td><td>25</td><td>28</td><td>11</td><td>22</td><td>27</td></tr><tr><td>8</td><td>19</td><td>6</td><td>25</td><td>18</td><td>26</td><td>9</td><td>5</td><td>20</td></tr><tr><td>Total</td><td><strong>291</strong></td><td><strong>114</strong></td><td><strong>294</strong></td><td><strong>229</strong></td><td><strong>352</strong></td><td><strong>179</strong></td><td><strong>426</strong></td><td><strong>380</strong></td></tr><tr><td>Mean</td><td><strong>36.38</strong></td><td><strong>14.25</strong></td><td><strong>36.75</strong></td><td><strong>28.63</strong></td><td><strong>44.00</strong></td><td><strong>22.38</strong></td><td><strong>53.25</strong></td><td><strong>47.50</strong></td></tr><tr><td>SD</td><td><strong>18.72</strong></td><td><strong>5.09</strong></td><td><strong>11.84</strong></td><td><strong>6.99</strong></td><td><strong>14.56</strong></td><td><strong>12.33</strong></td><td><strong>31.69</strong></td><td><strong>18.17</strong></td></tr><tr><td></td><td colspan="2">DGA</td><td colspan="2">DGB</td><td colspan="2">DGC</td><td colspan="2">DGT</td></tr><tr><td></td><td>18 Feb</td><td>18 Mar</td><td>18 Feb</td><td>18 Mar</td><td>18 Feb</td><td>18 Mar</td><td>18 Feb</td><td>18 Mar</td></tr><tr><td>1</td><td>74</td><td>45</td><td>18</td><td>24</td><td>13</td><td>17</td><td>24</td><td>23</td></tr><tr><td>2</td><td>34</td><td>18</td><td>11</td><td>19</td><td>12</td><td>13</td><td>21</td><td>22</td></tr><tr><td>3</td><td>30</td><td>12</td><td>11</td><td>17</td><td>12</td><td>10</td><td>15</td><td>18</td></tr><tr><td>4</td><td>17</td><td>10</td><td>8</td><td>8</td><td>11</td><td>10</td><td>14</td><td>15</td></tr><tr><td>5</td><td>8</td><td>8</td><td>7</td><td>8</td><td>9</td><td>9</td><td>7</td><td>12</td></tr><tr><td>6</td><td>8</td><td>8</td><td>4</td><td>8</td><td>6</td><td>6</td><td>6</td><td>11</td></tr><tr><td>7</td><td>7</td><td>6</td><td>3</td><td>2</td><td>4</td><td>6</td><td>5</td><td>9</td></tr><tr><td>8</td><td>5</td><td>0</td><td>2</td><td>2</td><td>3</td><td>6</td><td>4</td><td>8</td></tr><tr><td>Total</td><td><strong>183</strong></td><td><strong>107</strong></td><td><strong>64</strong></td><td><strong>88</strong></td><td><strong>70</strong></td><td><strong>77</strong></td><td><strong>96</strong></td><td><strong>118</strong></td></tr><tr><td>Mean</td><td><strong>22.88</strong></td><td><strong>13.38</strong></td><td><strong>8.00</strong></td><td><strong>11.00</strong></td><td><strong>8.75</strong></td><td><strong>9.63</strong></td><td><strong>12.00</strong></td><td><strong>14.75</strong></td></tr><tr><td>SD</td><td><strong>23.41</strong></td><td><strong>13.76</strong></td><td><strong>5.29</strong></td><td><strong>8.09</strong></td><td><strong>3.92</strong></td><td><strong>3.89</strong></td><td><strong>7.67</strong></td><td><strong>5.75</strong></td></tr></tbody></table>

Data are presented before and after raking. T stations correspond to the control stations (not raked on the first date). Mean and standard deviation (SD) are represented. Bold value indicates the total number of individuals recorded in the 8 replicates.

*Saccocirrus papillocercus* Bobretzky, 1872; *N. latericeus*; and *G. vulgaris*.

# Discussion

## Main characteristics of the intertidal habitats

The cumulative grain size curves of the sediments (not presented in the study) and the three main sediment categories showed that the three environments (deep gravelly, sandy, and gravelly habitats) were different; so, the clams lived in a broad ecological niche, ranging from clean sand to mixed gravelly sand. The winter mean densities of the total macrofauna in S, G, and DG were 390, 1135, and 310 individuals per m², respectively. For the three sediment types, the benthic fauna are dominated by polychaetes, and to a lesser extent by sipunculids. In the sandy habitat, *Ophelia rathkei* and Syllidae made up the dominant taxa. This habitat corresponded to the polychaete/amphipod-dominated fine sandy shores (A2.23) of the EUNIS habitat classification (Davies and Moss, 2004). The gravelly habitat was dominated by two species, *N. latericeus* and

*G. vulgaris*. In the deep gravel habitat, the dominant taxa were *M. girardi* and Syllidae. Both habitats corresponded to “Species-rich mixed sediment shores (A2.42)” of the EUNIS habitat classification (Davies and Moss, 2004).

## Clam resources

There was a wide range of clam abundances between stations within the same habitat type and between the three sampled habitats. It could be estimated that clam abundance in the gravelly habitat is twice as high as in the deep gravelly and sandy habitats. The gravelly habitat also yielded the highest number of marketable clams (62%, as against 41% for the deep gravelly habitat and 24% for the sand habitat).

In this study, the mean winter densities of the Manila clam are 2.2, 10.5, and 7 ind. per m² for S, G, and DG, respectively; results for the European clam were ~1 ind. per m². Between 2004 and 2006, Laspougeas (2007) estimated greater densities on the western coast of the Cotentin, with values of between 8 and
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Two-dimensional MDS ordination of community data found in each of the control (T) and raked plots (A, B, and C). Before raking (B) (stress = 0.04) and After raking (A) (stress = 0.06). S: sandy habitat; G: gravelly habitat; DG: deep gravelly habitat.</figcaption></figure>

<table><caption>**Table 5.** Pairwise comparisons (ANOSIM) of the benthic endofauna community data from three sediment types (S: sandy habitat; G: gravelly habitat; DG: deep gravelly habitat) before and after raking.</caption><thead><tr><th>Sampling day</th><th>Sediment type</th><th>R-statistic</th><th>P-value</th></tr></thead><tbody><tr><td rowspan="3">Before raking</td><td>DG vs. S</td><td>0.99</td><td>&lt;0.01</td></tr><tr><td>DG vs. G</td><td>1</td><td>&lt;0.01</td></tr><tr><td>S vs. G</td><td>1</td><td>&lt;0.01</td></tr><tr><td rowspan="3">After raking</td><td>DG vs. S</td><td>0.969</td><td>&lt;0.01</td></tr><tr><td>DG vs. G</td><td>0.917</td><td>&lt;0.01</td></tr><tr><td>S vs. G</td><td>1</td><td>&lt;0.01</td></tr></tbody></table>

17 individuals per m² for Agon-Coutainville and Bréhal, respectively, which were two localities south of Blainville-sur-Mer. Several campaigns had been carried out to estimate the stocks of clams in areas where professional fisheries are important, such as in the Arcachon basin (Dang et al., 2010; Caill-Milly, 2012): these studies revealed densities of 24–28 clams per m² including both *Ruditapes* species of clams together, but with the Manila clam representing >90%. In addition, along the French Atlantic coast, for the Gulf of Morbihan on the northern part of the Bay of Biscay,

D'Hardivillé et al. (2008) estimated a mean density of 25 Manila clams per m². At Pont-l'Abbé, in Finistère bordering the northern Bay of Biscay, Latrouite (2011) reported a very high density for the Manila clam: 69 individuals per m². Nevertheless, in these Atlantic locations, the sediment was mainly composed of mud and muddy sand, where the stocks of Manila clam appeared greater than those estimated in the sandy and mixed sediments on the western coast of the Cotentin.

In addition, marketable clams harvested in late winter 2014 accounted for ~42% of the stock, the others being represented by individuals that had not yet attained the threshold size for fishing (>40 mm). Some of these had become marketable mater in 2014. Owing to the extended intertidal zone and broad ecological niche of the Manila clams which largely dominated in this coastal area, the clam resource remained important in fishery (Sécula, 2011). Moreover, the large number of young clams indicated that the population was in good condition and could ensure its renewal.

## Impact of the rake fishing on clam populations and macrofauna

The main objective of this study was to study the short-term impact of pebble rake fishing on sediment structure, clam populations, and macrobenthic species.

There were many effects of fishing gear on the subtidal habitats [see the reviews of Jennings and Kaiser (1998) and Kaiser and De Groot (2000)]. Munari et al. (2006) had compared the impact of three different types of rakes (manual rake, hydraulic, and conveyor rake) on the macrobenthos from subtidal flats in the Sacca di Goro (northwestern Adriatic Sea). These authors showed that the conveyor rake had a greater impact on the fauna than the two other types of rake, and encouraged professional operators to adopt a hydraulic rake to fish the three target species *Chamelea gallina* (Linnaeus, 1758), *R. decussatus*, and *R. philippinarum* in this shallow environment. More recently, Constantino et al. (2009) had assessed the impacts of clam dredging and the rate of recovery of benthic communities in different depths on the southern coast of Portugal. For both of the studied dredging depths (6 and 18 m), the impact was low and the recovery rate was high. However, the recovery rate was higher at the shallower depth.

For the intertidal zone, the effects of fishing gear were less documented. Brown and Wilson (1997) studied the influence of commercial digging on worms and clams, as well as on the associated benthic infaunal community (Lowes Cove, Walpole, ME, USA). Two digging intensities were used; polychaetes dominated the macrofauna, whereas the number of individuals of these target groups, and other macrofauna taxa, appeared to decrease after digging. Spencer et al. (1998) showed that suction dredging on a cultivated population of *R. philippinarum* in the estuarine intertidal zone of the River Exe (southwest England) caused an immediate reduction of 80% in the infauna. Recovery of the sediment structure and the macrofauna occurred 1 year after harvesting. Long-term suction dredge of another bivalve, *C. edule* (Linnaeus, 1758), had a negative effect on bivalve recruitment in sandy habitats of the Wadden Sea (Piersma et al., 2001). Studies on the effect of raking on the macrofauna associated with *C. edule* in Liverpool Bay (Kaiser et al., 2001), and on the faunal composition in the San Juan Islands (Washington, DC, USA) carried out by Griffiths et al. (2006), showed a negative impact of harvesting clams on the target and non-target fauna. Recently, Navon and Dauvin (2013) examined the short-term impact of pebble fork fishing on the Warty venus *V. verrucosa* community on the west coast of the Cotentin.
These authors show that there was a decrease in the benthic macrofauna on sandy-gravelly habitats 4 d after using the fork.

Most studies on intertidal zones revealed a negative impact of harvesting on the target species and surrounding macrofauna. Nevertheless, the study of Sousa Leitao and Gaspar (2007), comparing the immediate impact of using two types of gear (harvesting knife and hand dredge) to harvest the edible cockle *C. edule*, showed that the effects were similar and very slight, with both fishing gears affecting only the superficial sediment which probably limited the impact of fishing on the infauna. Negative effect had been observed on the fishing of the clam species *Macoma balthica* (Linnaeus, 1757) and *C. edule* from the Paulina Polder (The Netherlands), probably because of death by smothering owing to the trampling (Rossi et al., 2007).

On the western coast of Cotentin, the effect of rake fishing on clams showed a significant decrease on clam abundances only in G and for size class structure on G and DG (Figures 4 and 5). There were no significant differences of the taxonomy richness and macrofauna abundances in the three habitats 1 month after raking (Figure 6).

Migration of clams on these main habitats during the interval between sampling (1 month) could be facilitated by the drifting of individuals living near these zones, which were affected by a high-energy hydrodynamic regime. Moreover, the abundances of adult clams fished in S and DG were lower than in G ("Synergie Mer Et Littoral" and J.-C. Dauvin, pers. obs.). In contrast, although clam fishing was more difficult in G, corresponding to areas less intensively worked by professional and recreational fishers, the number of clams in this habitat showed a marked decline after raking (Figures 3 and 4). Therefore, the presence of few clams after raking in G could be explained by (i) a lack of exhaustiveness of our sampling technique; (ii) the transport of sediment and clam; and (iii) large individuals are buried too deeply (>0.15 m) to be harvested during the first campaign. However, the size class histogram for the second campaign showed that small clams (<40 mm) are mainly absent (Figure 4). Consequently, we excluded the second explanation involving little sediment and macrofauna transport in this area with indurate mixed sediment. In contrast, in DG and S, the clam population densities showed no significant decrease between sampling dates and remain high 1 month after raking. This small reduction in the number of clams could be explained by large movements of sediments and organisms—including clams—in these dynamic sandy habitats (Cargnelli et al., 1999).

In fact, sedimentary movements from one campaign to another could be detected in the sandy habitat and in the deep gravelly habitat (J.-C. Dauvin, pers. obs.), mainly resulting in a net supply of sand particles. It was known that, along the intertidal zone of the western coast of the Cotentin, considerable sedimentary transit occurred during a tidal cycle, which might be amplified during storms (Levoy et al., 1997). In this area, tidal currents were parallel with the coast during most of the tidal cycle on account of a strong longshore gradient in water level between the Cotentin embayment and the English Channel (Levoy et al., 2001). These currents were directed northward around high tide and southward at low tide. Moreover, waves generated sand transport over the ebb delta platform of Régneville-sur-Mer, located south of Blainville-sur-Mer (Levoy, 1994), which induced longshore sand transport, then leading to the onshore migration of wave-formed bars (Robin and Levoy, 2007; Robin et al., 2009). During spring tides, it was possible to observe the displacement of intertidal sand dunes with a volume between 25 000 and 30 000 t and crested

higher than 2 m (Robin et al., 2009; Montreuil et al., 2014). This sedimentary process probably occurred in the sampling area because of the proximity of the ebb delta platform of the Blainville-sur-Mer estuary.

Thus, the small decrease in clam numbers in S and DG habitats 1 month after rake fishing could be explained by a large amount of sediment transit and the drifting of organisms between sampling dates. As a result, drifted clams should succeed in penetrating rapidly into the sediment. The influence of hydrodynamic regime on the displacement of bivalves such as *Spisula solida* (Linnaeus, 1758) had already been described by Dolbeth et al. (2006) along the southern coast of Portugal. It had also been shown that Atlantic Ocean storms could move adult individuals of *Spisula solidissima* (Dillwyn, 1817) outside their subtidal sand dune substrate (Cargnelli et al., 1999). Therefore, disturbance and movements of sediment could influence the benthic species distributions. Dernie et al. (2003a, b) found that it was difficult for macrofauna to become established in a highly mobile habitat. Therefore, we suggested that drifting of clams could have an important influence in this intertidal zone and could ensure secondary recruitments which maintained permanent low abundances (<10 individuals per m²) of clams in S and DG habitats.

To summarize, raking in winter tended to reduce clams, but its effect differs from one habitat to another: the most marked effects were in the indurate gravelly habitat. In fact, in this habitat, there was a decrease in the number of individuals of all the combined taxa after raking, probably with very limited or no external input caused by sediment transport, but this decrease was not statistically significant. Similarly, in the sandy and deep gravelly habitats, the effects of rake fishing on benthic fauna were more moderate and usually non-significant. Indeed, in mobile sand in S and DG habitats, there was a balance between decrease of the infauna including clams because of fishing and an input of drifted organisms including clams because of sediment transport. Finally, the effects of rake fishing appeared relatively weak in the studied intertidal zone; our conclusions were similar to those of Sousa Leitao and Gaspar (2007), who compared the effects of two types of fishing gear on the infauna after harvesting the edible cockles *C. edule*. The results of Kaiser et al. (2001) on the effects of hand raking on the macrofauna associated with intertidal cockle, *C. edule*, in the River Dee estuary in the Liverpool Bay (UK) showed that 2 weeks after the raking disturbance, there were also no significant differences in the benthic community among the different treatments.

Thus, although the effect of rake harvesting appeared limited during winter, the regional impact (high fishing pressure along 60 km of coast) and increased recreational fishing activity during summer needs to be studied in the future. Moreover, other sampling campaigns (using four quadrats of 10 m²) were necessary to estimate clams stock on other sites, mainly in the ebb delta of the other estuaries along the western coast of Cotentin, to corroborate the general phenomenon of clam drifting in such dynamic environment. Finally, information about the magnitude of annual recruitment would also be indispensable before forbidding clam rake fishing along the western coast of Cotentin.

## Acknowledgements

This study was carried out in the Framework of the LIFE + programme entitled *Experimentation pour une gestion concertée et durable de la pêche à pied de loisir* and coordinated by the "Agence des Aires Marines Protégées". The authors thank the *Association pour une Pêche à Pied Respectueuse de la Ressource* (APP2R) and
*Synergie Mer Et Littoral (SMEL)*, as well as Q. Bachelet, B. de Boisgelin, L. Camenez, Y. Joncourt, F. Lelong, and E. Schlund for their help during the sampling, C. Thouroude and C. Brunaud for their assistance in sediment analyses, and M. Carpenter for the English revision.

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