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Benthic Spat Collection of Softshell Clams (Mya arenaria Linnaeus, 1758) usingMatsAuthor(s): Bruno Myrand, Lise Chevarie and Réjean TremblaySource: Journal of Shellfish Research, 31(1):39-48. 2012.Published By: National Shellfisheries AssociationDOI: http://dx.doi.org/10.2983/035.031.0105URL: http://www.bioone.org/doi/full/10.2983/035.031.0105

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BENTHIC SPAT COLLECTIONOF SOFTSHELL CLAMS (MYA ARENARIA LINNAEUS, 1758)

USING MATS

BRUNO MYRAND,1*†LISE CHEVARIE

2AND REJEAN TREMBLAY

3

1Centre maricole des Iles-de-la-madeleine, MAPAQ, 107-125 chemin du Parc, Cap-aux-Meules, Quebec,Canada, G4T 1B3; 2Institut des Sciences de la Mer a Rimouski-MAPAQ, 108-125 chemin du Parc,Cap-aux-Meules, Quebec, Canada, G4T 1B3; 3Institut des Sciences de la Mer a Rimouski, Universite duQuebec a Rimouski, 310 allee des Ursulines, C.P. 3300, Rimouski, Quebec, Canada, G5H 3Z4

ABSTRACT Supply of softshell clam (Mya arenaria) seed for culture and restoration usually comes from hatcheries or through

transfers from one site to another. Natural spat collection using Astro-Turf mats installed directly onto the sediments could be

a viable alternative. Here, we describe a series of experiments to examine the performance of mats for benthic collection, and how

to use them efficiently. Interannual variability was studied for an 8-y period (2002 to 2009) at a commercial clam culture site

located in the Havre-aux-Maisons lagoon (Iles-de-la-Madeleine, southern Gulf of St. Lawrence). Spat collection (clams, 2.5–15

mm) ranged from 883–2,422 clams/m2 (overall mean, 1,599 clams/m2). Throughout the years, clams on themats were 7 timesmore

abundant than in the nearby sediments. Their mean shell length varied from 7.3–9.9 mm relative to the year. Mats were deployed

during an extended 5-wk period (early June to early July), and also retrieved during an extended 5-wk period (early September to

early October) with no negative effect on collection success. Indeed, spat abundance was higher when mats were retrieved later in

the season (late September to early October). Spat collection was greater at the mid to upper intertidal level (1,551–2,622 clams >

2.5mm/m2) compared with the lower intertidal (near-subtidal) level (586 clams > 2.5 mm/m2). Indeed, the lowest abundances and

the smallest sizes were observed onmats deployed at the lower intertidal level.Mats were installed side-by-side in groups of 35 (735), with no negative impact on collection success compared with individual mats. Last, spat collection was examined at two

different sites within the lagoon for 4 y. Although the overall mean number of clams was similar at both sites (1,538 clams > 2.5

mm/m2 vs. 1,503 clams > 2.5 mm/m2), collection success varied among sites (951–1,916 clams > 2.5 mm/m2 vs. 552–2,938 clams >

2.5mm/m2) from year to year. Therefore, collection success withmats should be investigated for any new site and for several years.

KEY WORDS: clam, mats, Mya arenaria, spat collection

INTRODUCTION

Commercial culture of softshell clam (Mya arenaria)

began only recently and it is still limited to North America(Buttner et al. 2004, Chevarie & Myrand 2006a). However,enhancement of softshell clam flats has been performed for a

while in the northeastern United States using wild or hatch-ery-reared seed, but only limited information is available(Beal 2005, Calderon et al. 2005, Beal 2006a). Although somefield experiments have been carried out with this species (e.g.,

Beal 1993, Beal et al. 1995, Beal et al. 1999, Beal & Vencile2001, Beal & Kraus 2002, Buttner et al. 2004, Beal 2006a, Beal2006b), much remains unknown about softshell clam culture/

enhancement.The first initiatives to develop the softshell clam culture in

Iles-de-la-Madeleine (southern Gulf of St. Lawrence) began in

1994, and one recurrent constraint has been to secure seedsupply. Hatchery production was not an option in this areabecause there was no interest from the few commercial hatch-

eries in eastern Canada to produce this species, and hatcheryproduction is expensive (Helm & Bourne 2004, Beal 2007b,Calderon et al. 2007). During the early years, seed collectionwas based on suspension collection using scallop bags filled with

Netron, but the results were disappointing. Although clamcollection was possible (up to 1,000 clams per bag) the hugeabundance of associated mussel (Mytilus edulis) spat in the bags

made it of very limited interest for commercial operations as

a result of sorting logistics (Chevarie & Myrand 2006a).Vassiliev et al. (2010) also found large quantities of mussel spatin bags used for clam collection in Maine.

In 2000, seed supply was shifted toward harvesting small

clams (15–40 mm) from a wild population (Chevarie &Myrand2006a). Once again, the results were disappointing because ofpoor growth and high losses after seeding (Chevarie & Myrand

2006b), and this approach was abandoned. In parallel, net tentswere also deployed, as used in Cape Cod, to increase spatsettlement in underlying sediments (Leavitt 1998, Calderon

et al. 2005), but with limited success (Chevarie & Myrand2007). Another option was the use of mats for benthic collection,as proposed by Chandler et al. (2001), who collected 12,000–

15,000 spat/m2 of softshell clams on artificial turf pads placed atthemid intertidal level in the Bay of Fundy. Themean shell lengthof collected young clams was about 2.5–4.0 mm in late October.This approach is fairly simple because it uses mats laid down

directly on the substratum in the intertidal zone.Large numbers of mats would be needed for a commercial-

size culture operation. It would be important for clam growers

to deploy mats over an extended period of time to sustain a highcollection rate. Furthermore, it would important for them toretrieve mats over an extended period of time while maintaining

a high collection rate. Young clams should reach a larger sizewith time. However, there are greater risks of losing spat fromthe mats because of greater turbulence in the fall. For instance,

currents up to 20 cm/sec were measured at the clam lease in theHavre-aux-Maisons lagoon during storms in early October,leading to turbulence up to 20 J/m3 (Redjah et al. 2010). Inparallel, it was observed that 10–20-mm clams buried in

medium sand were eroded and dispersed in an experimental

*Corresponding author. E-mail: bruno.myrand@merinov.ca

†Current address: Merinov, 107-125 chemin du Parc, Cap-aux-Meules,

Quebec, Canada, G4T 1B3

DOI: 10.2983/035.031.0105

Journal of Shellfish Research, Vol. 31, No. 1, 39–48, 2012.

39

flume when turbulence was # 10.1 J/m3 (Redjah et al. 2010).Indeed, small clams (<5 mm) are routinely redistributed by tidal

currents (Hunt &Mullineaux 2002). In addition, a large numberof mats would require a large area on a lease culture. Chandleret al. (2001) reported greater collection success at the mid in-tertidal level in the Bay of Fundy, which suggests that de-

ployment of mats could be restricted to this portion of theintertidal area. In contrast to the Bay of Fundy, the intertidalzone in Iles-de-la-Madeleine is narrow; the maximum tidal

amplitude is only 0.5 m (Guyondet & Koutitonsky 2008). Fur-thermore, the tidal conditions characterizing both sites proba-bly influence beach kinetics as well as clam predation and

desiccation when considering the different emersion times,whichmay influence the recruitment success of clams differentlyat both sites. Another way to minimize the area needed forcollection is to deploy groups of mats placed side-by-side rather

than place individual mats.The main objective of the current study was to determine

whether mats deployed in the sheltered lagoons of Iles-de-la-

Madeleine (Chevarie & Myrand 2006a, Richard et al. 2007)could be a reliable source of seed supply. The interannualvariability of spat collection was tested for 8 y (2002 to 2009)

to assess for the reliability of this approach. Several param-eters of importance for a commercial-scale operation were alsoexamined: (1) period of mat deployment, (2) period of mat

retrieval, (3) localization of mats in the intertidal zone, (4)organization of mats for large-scale collection, (5) spatial varia-tion of spat collection with mats, and (6) clam recovery andsorting.

These experiments are part of a larger research and devel-opment program aimed at developing a profitable softshell clamculture in Iles-de-la-Madeleine, southern Gulf of St. Lawrence

(Chevarie & Myrand 2006a, Chevarie & Myrand 2006b,Chevarie & Myrand 2007, Chevarie et al. 2008).

MATERIALS AND METHODS

Study Area

All experiments took place on a 4.7-ha experimental site inthe Havre-aux-Maisons lagoon (Iles-de-la-Madeleine, south-

ern Gulf of St. Lawrence; Fig. 1), hereafter referred to as theaquaculture site. Another site, the Pointe-a-Frank site, wascompared with the aquaculture site in one experiment. The

Pointe-a-Frank site is not used for commercial productionand was used only as a site for spatial comparison. Both sitesare located in the intertidal zone and are characterized by a

medium-sand substratum (mean grain size, 0.30 mm) (Redjahet al. 2010).

Interannual Variability

Experimental Astro-Turf mats (45 3 61 cm) covered with

16-mm-long bristles were used (Fig. 2). The mat size was chosento ease handling. Mats were fixed to the substratum by metalhooks at each corner at the mid intertidal level. The individualmats were separated from each other by ;1 m. From 2002 to

2009, a row of 10 mats was deployed in parallel to the shorelineon the same area in mid June and then retrieved in midSeptember. According to the year, 5–10 benthic cores of sub-

stratum (depth, ;10 cm; diameter, 0.02 m2) were taken atretrieval time as references. They were scattered about 3.5 m allaround the row of mats. At retrieval, mats and reference

samples were placed in individual plastic bags and broughtback to the laboratory. The mats were cleaned with pressurizedtap water, and the dislodged material was retained on a 2.5-mm

sieve and then on a 1-mm sieve. Themain analyses were done onthe clams retained on the first sieve because clams larger than2.5 mm are of interest for clam culture. Smaller clams are toodifficult to handle. However, clams retained on both sieves were

counted. Clams smaller than 15 mm were considered yearlyrecruits and were thus counted and measured. Larger clamswere discarded from the analyses based on previous observations

Figure 1. The experimental sites in Iles-de-la-Madeleine.

Figure 2. Astro-Turf mats. (A) Mat details. (B) Deployment in the intertidal zone.

MYRAND ET AL.40

of the length distribution of clam spat found on suspendedcollectors (unpubl. data). Very few large clams were discarded.

These larger clams were probably moved by currents andturbulence onto the mats and in the areas sampled forreference. All clams were counted. The mean shell length(largest dimension) of clams larger than 2.5 mm found on

a given mat or in a given reference sample was measured withan electronic caliper (precision, ±0.1 mm). The mean shelllength was usually obtained from subsamples containing 30–50

clams. The clams from a given mat or a reference sample werewell mixed, and a portion of about 30 individuals was isolated.All clams from this subsample were measured. If there were

fewer than 30 clams, an additional portion of individuals wasisolated and measured. All clams were measured when therewere fewer than 30 individuals in the sample (usually referencesamples).

Deployment Date

In 2003, 3 deployment dates were tested (early, mid, andlate June) using 10 mats per date. The mats were installedrandomly on a single row at the mid intertidal level and were

retrieved at the same time in mid September. Ten referencesamples were also taken at 3.5 m all around the mats on theretrieval day. The experiment was repeated in 2004 and 2005,

but with 4 deployment dates throughout June and early July.The mats and the reference samples were treated as describedpreviously.

Retrieval Date

In 2003, 30 mats were all deployed the same day in mid June

and were retrieved at 3 different dates throughout September(early, mid, and late September). The mats were installedrandomly at the mid intertidal level in a single row. Ten mats

were retrieved at each date, along with 10 reference samples allaround the mats. The experiment was repeated in 2004 and2005, but with 4 retrieval dates (40 mats) throughout Septemberand early October. The mats and the reference samples were

treated as described previously.

Tidal Level

In 2005 and 2006, 10 rows of 10 mats each were deployedfrom the upper to the lower intertidal levels. During both years,

the rows were placed in the same area, separated by;12–18 m.Mats from a given row were separated from each other by ;1m. Row 1 was the most upper intertidal, whereas row 10 was

almost subtidal. The area covered by rows 1–3 was exposed toair at all low tides, whereas that covered by rows 8–10 was veryrarely exposed. The area covered by rows 4–7 corresponded tothe usual area for spat collection (see previous experiments),

and was sometimes unexposed at low tide. The mats weredeployed in mid June and retrieved in mid September. Atretrieval in 2005, 5 mats were sampled randomly from every

second row and treated as described previously to get individualresults about clam abundance and size. This was done becauseof logistical constraints. Data from the other mats were pooled

to provide an overall mean collection rate for each row. In 2006,all mats from all rows were examined individually, but only forthe 2.5–15-mm clams.

Groups of Mats Versus Individual Mats

It is important for clam growers to maximize spat collection

at a given site by concentrating mats over a limited area withoutcompromising collection success. In 2004, 3 matrices of 35(5 lines37 columns) mats (45361 cm) were placed side-by-sideand compared with 3 groups of 6 individual mats each

separated by the equivalent area of a single mat. Replicatesfrom both arrangements were laid alternately at the midintertidal level. Individual mats were deployed according to

the position of the following mats in the matrix: columns 2, 4,and 6 for both lines 2 and 4. All mats were deployed in mid Juneat the mid intertidal level and were retrieved in mid September.

At retrieval, all the individual mats and the corresponding6mats from eachmatrix were treated as discussed previously, butonly clams larger than 2.5mmwere examined for abundance and

size. Reference samples from sediments were not taken.

Intersite Variability

Spat collection success was measured between 2004 and 2007

at a second site also located on the northern part of theHavre-aux-Maisons lagoon: Pointe-a-Frank. At both sites, mats were in-stalled at the mid intertidal level in a single row parallel to the

shoreline.No reference samples were taken at the Pointe-a-Franksite because the aim of this experiment was to compare collectionsuccess on mats at both sites. Sites were about 3 km distant from

each other (Fig. 1). Results were compared with the aquaculturesite to characterize intersite variability for collection success ofclams larger than 2.5 mm.

Clam Recovery and Sorting

Thirty-sevenmats were cleaned with pressurized tap water in

2009. The material retained on the 2.5-mm sieve was placed ina small basin filled with saltwater, in which the algae fragmentswere retrieved easily because they were floating at the surface.The remaining material was sieved through immersed Vexar

plastic netting (7.4-mm mesh), which was moved gently up anddown to retain the large shell fragments while smaller itemspassed through the mesh. Time needed for this operation was

recorded.

Statistical Analyses

Abundance data were subjected to square root transforma-tion tomeet the assumptions for ANOVAwhen needed (Sokal &Rohlf 1981). Treatments were usually compared with 2- or 3-way

ANOVAs. Post hoc comparisons were done with series of t-testson LS means. A sequential Bonferroni correction was thenapplied to keep an overall a level ¼ 0.05 (Rice 1989). t-Tests

were used when only 2 treatments were compared.

RESULTS

Interannual Variability

Spat have been collected successfully with mats since 2002.There were significant treatment3 year interactions for clamslarger than 2.5 mm (F(7, 121)¼ 6.48,P < 0.0001) and for the totalnumber of clams (F(7, 121) ¼ 3.32, P ¼ 0.008), but not for the

small (1–2.5 mm) ones (F(5, 108) ¼ 1.05, P ¼ 0.39). Planned post

BENTHIC SPAT COLLECTION OF CLAMS 41

hoc comparisons were performed for each of the size classes tocompare (1) the abundance of clams onmats versus reference sites

for each year and (2) the annual abundance of clams on matsamong years. For each size class, the number of clams on matswas always significantly greater (all P # 0.0041) than in thenearby sediments at the reference sites (Table 1).On average, over

the years, there was a 7-fold increase for clams of interest foraquaculture (>2.5 mm) on mats compared with the sediments(1,599 clams per m2 vs. 230 clams/m2). Furthermore, the very

small clams (1–2.5 mm) and the total number of clams were 3.5and 2.4 times more abundant onmats than in the sediments, with2,092 clams per m2 vs. 873 clams/m2 and 3,694 clams per m2 vs.

1,067 clams/m2, respectively. Regardless of size class, the numberof clams found on themats showed significant differences (allP#0.0019) among years with, for example, values ranging from 883–2,422 clams/m2 for clams of interest for aquaculture (Table 1).

There was a significant year 3 treatment interaction(F(7112)¼ 3.64,P¼ 0.0014) for the mean shell length (the longestdimension) of clams of interest for aquaculture (>2.5 mm). Post

hoc comparisons were done as noted previously. There were nosignificant differences among years according to the mean shelllength of clams found on the mats following a sequential

Bonferroni correction (all P $ 0.015). However, the mean shelllength of clams was significantly greater in the sediments than onmats for some years (Table 1). On average over the years, the

mean shell length tended to be smaller and less variable on themats (range, 7.3–9.9 mm; mean, 8.2 mm) than in the sediments(range, 7.1–12.3 mm; mean, 9.4 mm).

Deployment Date

Therewere significant differences (F(3,33)# 12.73,P# 0.0001)

among treatments for all size categories in 2003: very small clams(1–2.5 mm), 2.5–15-mm clams, and 1–15-mm clams (Fig. 3). Forall sizes, the number of clams at the reference sites was

significantly fewer than on the mats. However, there were nosignificant differences among deployment dates. An overall

mean of 1,498 clams larger than 2.5 mm/m2 was found on themats deployed between early and late June. This is 6.7 timesgreater than the number of clams larger than 2.5mm found in thesediments. Mean size of the larger than 2.5-mm clams was not

significantly different among deployment dates (F(2,24) ¼ 0.81,P ¼ 0.46), with values between 7.8 ± 0.2 mm and 8.3 ± 0.3 mm,and an overall mean of 8.0 mm.

The experiment was repeated in 2004 and 2005, but with anadditional deployment date in early July. The overall patternwas similar to 2003. There were no significant year3 treatment

interactions for all size categories (F(4,88) # 1.49, P $ 0.21).Therefore, both factors were examined independently. For allsize classes, there were significant differences among treatments(F(4,88) $ 15.02, P < 0.0001). Clams at reference sites were

always significantly less abundant than those on mats (Fig. 3).However, there were no significant differences among thedeployment dates of the mats. For example, the mean abun-

dance of clams larger than 2.5 mm in 2004 to 2005 was 150clams/m2 at the reference sites compared with 1,566 clams/m2

on mats (1,286–1,650 clams on mats deployed between early

June and early July). This is 10.4 times greater than in thesediments. Spat collection was significantly greater in 2005 than2004 for all size categories (Fig. 3). For instance, the mean

abundance of clams larger than 2.5 mm (including those fromthe sediments) was 1,542 clams per m2 versus 955 clams/m2 in2005 and 2004, respectively.

Shell length was significantly larger in 2005 than in 2004

(F(1,71) ¼ 21.07, P < 0.0001) for clams found on the mats, withmean values of 8.0 mm versus 7.2 mm, respectively. However,there was no significant difference in the mean size of clams

larger than 2.5 mm among deployment dates (F(3,71)¼ 0.23,P¼0.88), with mean values ranging from 7.5–7.6 mm, and anoverall mean of 7.5 mm for all deployment dates.

TABLE 1.

Spat collection (mean % SE) between 2002 and 2009 obtained with mats deployed in mid June and retrieved in mid September.

Year

Clams, 1–2.5 mm

Clams, 2.5–15 mm

Clams, 1–15 mm

Mats (No./m2) Controls (No./m2)

Mats Controls

Mats (No./m2) Controls (No./m2)No./m2 mm No./m2 mm

2002 318 ± 39e 15 ± 4 2,422 ± 190a 9.1 ± 0.3B 19 ± 4 12.3 ± 0.7A 2,741 ± 210de 34 ± 5

2003 1,331 ± 16d 256 ± 53 1,488 ± 145abcd 8.0 ± 0.3B 283 ± 113 10.9 ± 1.1A 2,818 ± 270cde 539 ± 137

2004 1,676 ± 264cd 545 ± 98 1,384 ± 301bcd 7.3 ± 0.3 101 ± 43 7.1 ± 0.6 3,059 ± 531cd 646 ± 129

2005 2,807 ± 415b 885 ± 167 1,916 ± 384abc 7.7 ± 0.3 200 ± 51 9.4 ± 0.9 4,722 ± 733abc 1,085 ± 195

2006 648 ± 50e N/A 951 ± 88cd 9.9 ± 0.1 267 ± 59 11.4 ± 1.1 1,599 ± 122e N/A

2007 2,828 ± 121b N/A 1,899 ± 121ab 7.9 ± 0.2 412 ± 106 7.9 ± 0.9 4,726 ± 203ab N/A

2008 2,228 ± 200bc 776 ± 117 883 ± 66d 7.6 ± 0.2B 218 ± 53 11.1 ± 1.0A 3,111 ± 190bcd 994 ± 127

2009 4,195 ± 363a 2,763 ± 554 1,850 ± 206ab 7.9 ± 0.2 339 ± 114 7.7 ± 0.7 5,713 ± 439a 3,103 ± 636

Mean 2,004 (2,092*) 873 1,599 8.2 230 9.4 3,561 (3,694*) 1,067

* Mean excluding 2006 and 2007.

Clams were separated into size classes. The 2.5–15-mm clams are of interest for clam culture, and their mean shell length is also provided. Ten mats

were used each year, and 5–10 benthic cores of substratumwere taken at retrieval time as references (¼ controls) according to the year. A sequential

Bonferroni correction was applied to keep an overall a level¼ 0.05, following the comparisons of LS means. Values from a given column followed

by different letters were significantly different. Whatever their size, the number of clams found on mats was always significantly higher than in the

controls (¼ reference sites) for all years. Clam shell lengths greater than 2.5 mm followed by different capital letters were significantly different

among mats and controls.

MYRAND ET AL.42

Retrieval Date

In 2003, mats were all deployed the same day in mid June, butwere retrieved at 3 different dates throughout September. For allsize classes, there were no significant retrieval date3 treatment

interactions (F(2,52) # 0.65, P $ 0.53) and no significant dif-ferences (F(2,52) # 2.52, P $ 0.09) among retrieval dates (Fig. 4).For example, the overall mean abundance of clams larger than

2.5 mm on the mats was 1,441 individuals/m2 regardless of theretrieval date (1,288–1,633 clams/m2). In contrast, there weresignificantly more clams on themats than at the reference sites for

all size classes (F(1,52) $ 76.67, P < 0.0001). For instance, clamslarger than 2.5mmwere, on average, 3.6 times more abundant on

themats than in the sediments (1,441 clams/m2 vs. 403 clams/m2).Therewas no significant difference (F(2,47)¼ 0.10,P¼ 0.91) in the

mean shell length for clams larger than 2.5 mm among retrievaldates, with an overall mean of 8.1 mm.

In 2004 and 2005, an additional retrieval date was added inearly October, and there were no significant year3retrieval date3treatment interactions (F(6142) # 1.46, P $ 0.20) and no year3retrieval date interactions (F(3142) # 1.93, P $ 0.13) for all sizecategories (Fig. 4). There were no significant year3 treatment

interactions (F(1142) # 2.76, P $ 0.10) for the number of smallclams (1–2.5 mm) and the total number of clams (1–15 mm). For

Figure 3. Mean number (%SE) of spat collected from 2003 to 2005

according to deployment dates across each size category: 1–2.5 mm, 2.5–

15 mm, and 1–15 mm. Reference sites sampled 3.5 m away from the mats.

Figure 4. Mean number (%SE) of spat collected from 2003 to 2005

according to retrieval dates across each size category: 1–2.5 mm, 2.5–

15 mm, and 1–15 mm. Black bars represent clam abundance in the nearby

sediments (reference sites sampled 3.5 m away from the mats) at retrieval

time.

BENTHIC SPAT COLLECTION OF CLAMS 43

these 2 size classes, all main factors were examined indepen-dently, and there were significant differences for each one

(years: F(1, 142) $ 10.34, P# 0.0016; retrieval dates: F(3, 142) $20.40, P < 0.0001; and treatments: F(1, 142) $ 328.73, P <0.0001). Indeed, there were significantly more clams in 2005than 2004, and also significantly more clams on the mats than

at the reference sites (Fig. 4). For both size classes, clams weresignificantly less abundant on mats retrieved in early Septem-ber than later in the season, whereas the greatest abundance

was found on mats retrieved in late September and earlyOctober (Table 2). In contrast, there was a significant year3treatment interaction (F(1, 142) ¼ 4.57, P ¼ 0.03) for clams of

interest for aquaculture (>2.5 mm). The lowest abundance ofclams was found at the reference sites in 2004 (117 clams/m2)and 2005 (185 clams/m2), and there was no significant differ-ence among these treatments. The greatest abundance was

found on the mats in 2005 (2,453 clams/m2), followed by matsin 2004 (1,622 clams/m2). There was also a significant differ-ence among retrieval dates (F(3, 142) ¼ 10.47, P < 0.0001), with

more clams found on mats retrieved in late September to earlyOctober (Table 2). There were significantly fewer clams onmats retrieved in early September. An average of 2,462 clams

larger than 2.5 mm/m2 was found on the mats in lateSeptember to early October 2004 to 2005. This is about 11.8times the number of clams larger than 2.5 mm found in the

sediments for the same retrieval dates (209 clams/m2). Theproportion of clams larger than 2.5 mm relative to the totalnumber of clams (1–15 mm) was comparable over the retrievaldates (Table 2), although their proportion in late September to

early October tended to be lower than in early September:48.3–52.0% versus 55.6%, respectively.

There was no significant year3retrieval date interaction for

the mean size of clams larger than 2.5 mm (F(3,71) ¼ 0.11, P ¼0.96). Furthermore, the mean shell length was similar for bothyears (F(1,71)¼ 3.87,P¼ 0.053) and for all retrieval dates (F(3,71)¼1.30, P ¼ 0.28). The overall mean shell length of clams largerthan 2.5 mm was 7.4 mm.

Tidal Level

In 2005, there were significant differences for all sizecategories relative to the mat position in the intertidal zone(F(4,17)$ 3.80,P# 0.022). Clams larger than 2.5 mmwere more

abundant at the mid intertidal level (rows 5–7), whereas thelowest abundance was observed at the lower intertidal level(rows 9 and 10; Table 3). There was also a significant differenceamong the tidal levels for the mean shell length of clams larger

than 2.5 mm (F(4,17) ¼ 11.11, P < 0.0001). The clams weresmaller at the lower intertidal level (Table 3).

The experiment was repeated in 2006, but only clams

larger than 2.5 mm were examined. There was sand accumu-lation (;30 cm) on the mats at the lower intertidal level (rows8–10) so that very few clams were found. As a result, the

abundance on mats was significantly different among tidallevels (F(9,90) ¼ 90.37, P < 0.0001). The abundance was similar

TABLE 2.

Mean number of young clams observed on the mats in 2004 and2005 according to the retrieval date.

Retrieval

Clams,

1–2.5 mm

(No./m2)

Clams,

2.5–15 mm

(No./m2)

Clams,

1–15 mm

(No./m2)

Proportion

of 2.5–15-mm

Clams (%)

Early September 1,245c 1,557b 2,802c 55.6

Mid September 2,241b 1,650b 3,891b 42.4

Late September 2,649a 2,475a 5,124a 48.3

Early October 2,259ab 2,449a 4,709ab 52.0

Clams were separated according to different size classes. Ten mats were

retrieved at each date. The 2.5–15-mm clams are of interest for clam

culture, and their proportion relative to the total number of spat is also

presented. Results are based on post hoc comparisons of LS means fol-

lowed by a sequential Bonferroni correction to keep an overall a level¼0.05 for a given set of comparisons. Values from a given column

followed by different letters showed significant differences.

TABLE 3.

Clam abundance and size by mat position in the intertidal zone in 2005.

Position

1–2.5 mm2.5–15 mm

1–15 mm

Individual Data Pooled DataIndividual Data

Pooled Data Individual Data Pooled Data

No./m2 No./m2 No./m2 (mm) No./m2 No./m2 No./m2

1 (upper intertidal) 1,019 1,540 2,559

2 1,703 ± 364b 2,029 1,260 ± 109bc 9.2 ± 0.5a 1,853 2,963 ± 469ab 3,882

3 1,949 1,924 3,873

4 1,597 ± 211ab 1,764 1,700 ± 332ab 8.8 ± 0.4ab 1,607 3,298 ± 511ab 3,381

5 1,660 2,138 3,798

6 1,773 ± 333ab 2,116 2,622 ± 299a 9.0 ± 0.2ab 2,823 4,395 ± 587a 4,939

7 1,733 2,192 3,926

8 3,246 ± 498a 3,828 1,551 ± 498ab 7.5 ± 0.3bc 1,493 4,797 ± 461a 5,321

9 5,248 1,156 6,404

10 (lower intertidal) 1,285 ± 216b 1,257 586 ± 186c 5.9 ± 0.2c 508 1,870 ± 395b 1,765

Tenmats were deployed at each level. Individual data (mean ± SE) are provided for positions 2, 4, 6, 8, and 10. Overall values are provided for all 10

positions. Clams were separated by size. Because the 2.5–15-mm clams are of interest, their shell size at positions 2, 4, 6, 8, and 10 is also provided.

Results are based on post hoc comparisons of LS means followed by a sequential Bonferroni correction to keep an overall a level¼ 0.05 for a given

set of comparisons. Values from a given column followed by different letters showed significant differences.

MYRAND ET AL.44

(894–1,261 clams/m2) for rows 1–7, and much lower (5–127clams/m2) for those at the lower intertidal level (rows 8–10;

Table 4). There was also a significant difference in shelllength among tidal levels (F(9,79) ¼ 4.22, P ¼ 0.0002). Themean size of clams tended to be smaller at the lower intertidallevel (Table 4).

Groups of Mats Versus Individual Mats

There were no significant differences (t-tests for all size

categories, df ¼ 4, P$ 0.64) for clam abundance relative to thearrangement of mats: single mats or groups of 35 mats placedside-by-side (Fig. 5). In this experiment performed in 2004,

mean abundance of clams larger than 2.5 mm for both groupswas similar, with 721 ± 48 clams/m2 versus 738 ± 152 clams/m2.The mean shell length of clams larger than 2.5 mm was alsosimilar (t-test, df¼ 7, P¼ 0.38) for both types of arrangements,

with an overall mean of 7.2 mm.

Intersite Variability

The aquaculture site was compared with another site in theHavre-aux-Maisons lagoon, Pointe-a-Frank site, over a 4-yperiod (2004 to 2007). Only clams larger than 2.5 mm found on

mats were examined. There was a significant site 3 yearinteraction (F(3,67) ¼ 8.76, P < 0.0001). Although the overallmean abundance was similar at both sites over the 4 y

(1,538 clams m2 vs. 1,503 clams/m2), annual spat collectionwas more variable at the Pointe-a-Frank site than at theAquaculture site (Table 5). Indeed, the abundance of spat atthe aquaculture site varied between 951 clams/m2 and 1,916

clams/m2 compared with values between 552 clams/m2 and2,931 clams/m2 at the Pointe-a-Frank site, so that the maxi-mum density-to-minimum density ratio was 2.0 and 5.3,

respectively.

There was also a significant site3year interaction (F(3,55) ¼8.46, P ¼ 0.0001) relative to shell length. The mean size of spatat the aquaculture site varied between 7.3 mm and 9.9 mm, withan overall mean of 8.2 mm, compared with values between 6.0

mm and 10.6 mm, with an overall mean of 7.8 mm at the Pointe-a-Frank site (Table 5).

Clam Recovery and Sorting

Broken shells, algae fragments, gastropods (Hydrobia spp.),and very few mussel spat were found along with clam spat on

the 2.5-mm sieve after cleaning the mats. This operation took13 min per mat on average. The elimination of broken shellsand other large items with the 7.4-mm mesh netting required,on average, an additional 1 min per mat. A mix of tiny shell

TABLE 4.

Clam abundance and size (mean% SE) by mat position in the

intertidal zone in 2006.

Position

2.5–15 mm

No./m2

mm

1 (upper intertidal) 1,171 ± 85a 10.2 ± 0.2abc

2 1,185 ± 93a 10.9 ± 0.1ab

3 894 ± 94a 10.9 ± 0.1ab

4 904 ± 92a 11.2 ± 0.3a

5 969 ± 41a 10.4 ± 0.3abc

6 1,136 ± 83a 9.8 ± 0.3abc

7 1,261 ± 128a 9.0 ± 0.2bc

8* 5 ± 5c 9.8†

9* 29 ± 7bc 8.4 ± 1.1c

10 (lower intertidal)* 127 ± 56b 9.4 ± 0.6abc

* A deposition of ;30 cm of sand was observed on mats from these

rows.

† Only 1 clam was measured.

Ten mats were deployed at each level. Results are based on post hoc

comparisons of LS means followed by a sequential Bonferroni correc-

tion to keep an overall a level ¼ 0.05 for a given set of comparisons.

Values from a given column followed by different letters showed

significant differences.

Figure 5. Mean number (%SE) of spat collected in 2004 relative to mat

organization: groups of 35 mats or single mats. Abundance is presented by

spat size categories: 1–2.5 mm, 2.5–15 mm, and 1–15 mm.

TABLE 5.

Abundance and size (mean% SE) of clams larger than 2.5 mmcollected onmats at 2 different sites in theHavre-aux-Maisons

lagoon between 2004 and 2007: aquaculture site (AQUA) and

Pointe-a-Frank site (PAF).

Year Site

Clams, 2.5–15 mm

No./m2

mm

2004 AQUA 1,384 ± 301bcd 7.3 ± 0.3bc

PAF 552 ± 37d 6.6 ± 0.3cd

2005 AQUA 1,916 ± 364abc 7.7 ± 0.3b

PAF 2,938 ± 394a 6.0 ± 0.3d

2006 AQUA 951 ± 88bcd 9.9 ± 0.1a

PAF 1,714 ± 714abc 10.6 ± 0.1a

2007 AQUA 1,899 ± 121ab 7.9 ± 0.2b

PAF 809 ± 72cd 7.8 ± 0.1b

Overall mean AQUA 1,538 8.2

PAF 1,503 7.8

Ten mats were deployed each year at both sites. Results are based on

post hoc comparisons of LS means followed by a sequential Bonferroni

correction to keep an overall a level ¼ 0.05 for a given set of com-

parisons. Values from a given column followed by different letters

showed significant differences.

BENTHIC SPAT COLLECTION OF CLAMS 45

fragments, clam spat, and small gastropods lay on the bottomof the basin at the end of this procedure. The rare remaining

mussels were easy to retrieve by hand.

DISCUSSION

During an 8-y period (2002 to 2009), clams larger than 2.5mm were, on average, 7 times more abundant on the mats thanin the nearby sediments in Iles-de-la-Madeleine (southern Gulf

of St. Lawrence). Furthermore, benthic collection of clam spatusing mats provided repeatable results, with an overall meanabundance of 1,599 clams larger than 2.5 mm/m2 and a co-

efficient of variation of 32.8%. During this period, mats weredeployed repeatedly in the same area at the mid intertidal levelin mid June, and were retrieved in mid September. The meanshell length of clams for aquaculture (>2.5 mm) was relatively

stable throughout the years, with values of 7.3–9.9 mm and anoverall mean of 8.2 mm. The bristles on the mats possiblyincrease larval settlement by decreasing water movement locally

and/or providing a barrier to reduce spat dispersal caused bybedload transport. They possibly also intercept spat movingaround via passive dispersal.

It seems that clam growers operating in the Havre-aux-Maisons lagoon could have some flexibility in the managementof their spat collection using mats. Large numbers of mats

would be required on a commercial scale, and it would beimportant to deploy and retrieve them over extended periods oftimewhile sustaining a high collection rate. Young clams shouldreach a larger size with time. However, there are greater risks of

losing spat from the mats as a result of greater turbulence in thefall. In the current study, Astro-Turf mats could be deployedany time during a 5-wk period between early June and early July

with no impact on the collection success of clams larger than 2.5mm. In addition, mats could be retrieved anytime during a 5-wkperiod between early September and early October with no

negative effects on collection success of clams larger than 2.5mm. Indeed, the abundance of clams on mats was greater in lateSeptember and early October. However, there is no need to waitlater than late September for retrieval, because growth ceases at

this time, as shown by the similar mean shell lengthmeasured onclams retrieved throughout the 5-wk period. Furthermore, thereare greater risks of losses associated with wind storms in late

fall. In southern areas, juvenile clams stop growing later in theseason—for example, in early November inMaine (Beal 2007a).Furthermore, spawning period and duration of larval develop-

ment are also probably different in southern areas (Brousseau1978, Anonymous 1998). Therefore, deployment and retrievaldates for successful spat collection could be different according

to the site. It would be interesting to look at these parameters inother areas.

The success of spat collection was relatively consistent at alltidal levels, except at the lower intertidal level, where fewer and

smaller clams were found. In addition, a substantial amountof sand was deposited over the mats in the lower intertidal in2006. Clams found on mats at the lower intertidal level were

also smaller than those from upper levels, although theycould take advantage of longer feeding periods (¼ longerimmersion periods). This may be related to a preferential

predation against larger clams and/or a dispersal of clamsthrough bedload transport earlier in the season. In the lattercase, clams at the lower intertidal level were possibly recruited

later in the season, and thus reached a smaller size. Lowabundance of small clams has been reported frequently at

lower intertidal levels as a result of higher bedload transportand/or higher predation by benthic fish and large crustaceanstaking advantage of a longer foraging time (Beal 2006b). In theBay of Fundy, clam spat were also more abundant on mats

placed at the mid intertidal level (Chandler et al. 2001), but incontrast to the current study, greater numbers were also foundat the lower intertidal level and a low abundance was noted at

the upper intertidal level. An estimated 60-m strip located atthe mid to upper intertidal levels could be used for successfulspat collection at the culture site in Havre-aux-Maisons

lagoon. Mats could also be placed in groups to minimize thearea needed for spat collection.

Success of spat collection varied among years and amongsites, as shown by the results obtained from 2 sites relatively close

from each other in the Havre-aux-Maisons lagoon. This result isin agreement with other studies that reported that bivalve spatcollection can differ spatially within the same body of water

(Giguere et al. 1995, Bourque & Myrand 2008). Furthermore,mats used at other sites near clam flats in Quebec provided poorresults (Chevarie et al. 2007). Local beach kinetics and sedimen-

tary characteristics are probably very influential for successfulspat collection with mats. Collection success is thus very sitespecific and must be assessed accordingly.

The very low abundance of mussel spat on benthic mats is insharp contrast with pelagic collectors (Chevarie &Myrand 2006a,Chevarie et al. 2008, Vassiliev et al. 2010), and clam sorting ismuch easier. It took 13 min, on average, to clean a mat, and an

additional 1 min to get rid of the large items (mainly brokenshells). More work is still needed to develop a quick and easysorting procedure, but the remaining mix of tiny shell fragments,

clam spat, and small gastropods (Hydrobia spp.) could probablybe overwintered with no problem for small clams. The smallfragments of shells should continue to break and disintegrate with

time, and pass through the mesh of the overwintering cage.The unit cost of mats was CAN$4.36, and their life

expectancy is probably at least 10 y (Werstink et al. 2010). In2009, production costs for spat collection using mats was

estimated at about CAN$4/1,000 clams, with a mean shelllength of 8–9 mm (Werstink et al. 2010). In comparison, clamspat of similar size produced in U.S. hatcheries were sold for

more than CAN$14/1,000 clams in 2006 (Calderon et al. 2007).Thus, seed supply obtained through benthic collection ischeaper than the purchase of clam seed from hatcheries in the

Iles-de-la-Madeleine context.

ACKNOWLEDGMENTS

We thank the technical staff assigned to the R&D programMIM (French acronym for softshell clam culture in Iles-de-la-Madeleine), whose entire collaboration was essential to the

success of this project. This study was part of the R&D programMIM, which was supported by funds from Ministere del’Agriculture, des Pecheries et de l’Alimentation du Quebec

(MAPAQ); Ministere du Developpement Economique, Inno-vation et Exportation du Quebec (MDEIE); DeveloppementEconomique Canada; Societe de Developpement de l’Industrie

Maricole (SODIM); and Centre Local de Developpement desIles-de-la-Madeleine (CLD-Iles-de-la-Madeleine). Thanks alsoto the anonymous reviewers for their helpful comments.

MYRAND ET AL.46

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