Experimental collection of postlarvae of Pecten maximus (L.) and other benthic macrofaunal species in the Bay of Saint-Brieuc, France. II. Reproduction patterns and postlarval growth of five mollusc species

Experimental collection of postlarvae of Pecten maximus (L.) and other benthic macrofaunal species in the Bay of Saint-Brieuc, France. II. Reproduction patterns and postlarval growth of five mollusc species

J. Exp. Mar. Biol. Ecol., 148 (1991) 181-200 © 1991 Elsevier Science Publishers B.V. 0022-0981/91/$03.50 181 JEMBE 01584 Experimental collection of...

2MB Sizes 0 Downloads 0 Views

J. Exp. Mar. Biol. Ecol., 148 (1991) 181-200 © 1991 Elsevier Science Publishers B.V. 0022-0981/91/$03.50

181

JEMBE 01584

Experimental collection of postlarvae of Pecten maximus (L.) and other benthic macrofaunal species in the Bay of Saint-Brieuc, France. II. Reproduction patterns and postlarval growth of five mollusc species G6rard Thouzeau Laboratoire d'Ocdanographie Biologiq~e, Unirersitd de Bretagne Occidentaie, Brest. France (Received 26 July 1990; revision received 21 December 1990; accepted 12 January 1991) Abstract: Spawning seasonality and postlarval (spat) growth of five mollusc species taken in spat collectors in the Bay of St-Brieuc in 1985 were analysed from size-frequency distributions. There is evidence for a short spawning period and a major spawning for all species. The respective abundance peaks of the three pectinid species in the collectors were shifted with time; the settlement ofPecten maximus occurred 15-30 clays before those of Chlamys sp. A spatial intracohort growth variability was found in P. maximus, depending on the collector height from the seabed. This variability appeared as early as the first benthic stages (mean height < 500 #m), with growth being lower by 25-30~o in the collectors located nearest to the bottom. Postlarval growth of P. maximus, A nomia ephippium and Crepidulafornicata also varied seasonally between cohorts, with growth rates decreasing during the reproductive season. Temperature and trophic resources were the main factors determining these variations, in P. maximus, another source of growth variation, the gamete quality, is suggested. These results, added to those obtained in a study of biotic interactions within the collectors (Thouzeau, 1991), help define ways to improve collection of pectinid spat in the Bay of St-Brieuc. Key words: Growth; Reproduction pattern; Spat collector

INTRODUCTION

There is an abundant literature on the postlarval (spat) growth of pectinids within artificial collectors, both in European (Buestel & Laurec, 1975; Minchin, 1976; Buestel et al., 1977, 1979; Pickett, 1978; Brand et al., 1980; Paul et al., 1981; Wallace, 1982; Richardson et al., 1982; Gillespie, 1983; Fegan, 1983~ Arzel, 1984; Roman & Cano, 1987) and American scallop beds (Naidu & Scaplen, 1979; Ruzzante & Zaixso, 1985; Dadswell et al., 1987). However, the factors influencing postlarval growth within the collectors are not always clearly determined, so that conditions for optimal growth in suspended culture remain unclear. On the other hand, a high growth variability through the water column has been observed for several pectinid species in suspended cultures (Leighton, 1979; Wallace & Reinsnes, 1984, 1985). In particular, Wallace & Reinsnes Correspondence address: G. Thouzeau, Laboratoire d'Oc6anographie Biologique, Universit6 de Bretagne Occidentale, 6 Avenue V. le Gorgeu, 29287 Brest C6dex, France.

182

G. THOUZEAU

(1984) indicate that the dry-weight increase of the adductor muscle of Chlamys islandica (MOiler) can vary by a factor of 7 as a function of the immersion depth of the culture baskets. Identifying the factors that determine those growth variations is thus a prime necessity for the aquaculture of those species. The present study on reproduction patterns and spat growth of five mollusc species (all epibenthic filter-feeders) in the Bay of St-Brieuc complements results obtained on settlement patterns, collector efficiency and biotic interactions within the suspended substrata (Thouzeau, 1991). Thouzeau & Lehay (1988) have shown that survival of scallop Pecten maximus (L.)juveniles on the seabed in this bay is a function ofindividual size during their first winter. The main purpose of this article is thus to define conditions allowing optimal growth of P. maximus spat within the collectors. The main spatio-trophic competitors of the scallop will also be considered in order to define the respective growth characteristics of the various species within the collectors. Fundamental ecological processes underlie these patterns such as changes in the gamete quality with time plus the effects of environmental factors on postlarval growth. MATERIALS AND M E T H O D S

The Bay of St-Brieuc (Fig. 1, in Thouzeau, 1991) is located in the western Channel (French coast) in inshore shallow waters (_<30 m). MEASUREMENT OF PHYSICAL AND CHEMICAL PARAMETERS

The bottom-water temperature was measured by means of a recording thermograph immersed in the eastern part of the bay (15 m depth) at Comtesses (see Fig. 1 in Thouzeau, 1991), from 10 July to mid-September of 1985. Salinity was not measured since it is known to be stable (34.9 + 0.1%o) in the bay in summer (R6seau National d'Observation, 1981). Nutrients (NO3, NO2, SiO3, PO4), Chl a and diatom concentrations were measured from April to September 1985; mean values were calculated for those parameters over the entire bay (on the basis of unpubl, data by Sournia). EXPERIMENTAL SYSTEM, COLLECTION SITE AND SAMPLING DESIGN

Characteristics of the spat collection system in 1985 are described elsewhere" the study was conducted with data from station Caffa (see Figs. 1 and 2 in Thouzeau, 1991). A line of four ropes, each one with four collectors, was placed at a depth of 15 m at Caffa on 14 July, above a coralline algae bottom, soon after a strong decrease of the gonad index of the sexually mature adult scallops had been observed (Paulet and Halary, pers. comm.) between 9 and 11 July. The immersion period for the four ropes varied from 18 to 59 days; the ropes were removed on 1, 16 and 28 August and 11 September, respectively. The sampling design allowed an estimate of the growth of each postlarval cohort between two successive recoveries of the collectors. Average daily growth was calculated for each time interval.

REPRODUCTION PATTERNS AND POSTLARVAL GROWTH OF MOLLUSCS

183

SPECIES IDENTIFICATION AND BIOMETRIC PARAMETERS

While it was relatively easy to separate bivalve families (Mytilidae, Pectinidae, Anomiidae, Veneridae, Mactridae, etc.) from 200-250 l'.m upwards, species identification was sometimes difficult when individuals were small; this could be resolved after the individuals had grown larger. The three pectinid species were easily identified under the microscope from a height of 220 #m upwards, thanks to their characteristic ornamentation of the left valve of the dissoconch (Fig. 1), also described by Le Pennec (1978) and Le Pennec & Diss-Mengus (1987). This study of postlarval growth was undertaken on P. maximus and the four main species of epibenthic filter-feeding molluscs collected (see Thouzeau, 1991). The biometric parameter measured was shell height (P. maximus, Chlamys varia L., Chlamys opercularis L. and Anomia ephippium L.) or shell length (Crepidulafornicata L.). Postlarvae were measured to the nearest 12.5/am with an ocular micrometer mounted on a dissecting microscope. DATA ANALYSIS

Modal analyses of the size-frequency distributions of spat collected were performed according to the technique used by Gros & Cochard (1978). The method assumes that individual size at a given age is normally distributed. In this study, the size-frequency histograms were decomposed into a number of normal distributions, each of which represented a single cohort within the 1985 age class. The number of plausible Gaussian components and rough estimates of their means and standard deviations, were determined through the logarithmic difference method of Bhattacharya (1967) applied to a smoothed curve. These data were adjusted by the maximum likelihood optimization method of Hasselblad (1966), through the use of the NORMSEP software adapted to the IBM-PC. The optimization method allows standard errors to be defined for each component, as required by Hasselblad (1966), Grant et al. (1987) and Grant (1989). Settlement taate:, of P. maximus were estimated from the mean growth values found for each cohort and from the spatio-temporal monitoring of larval cohorts undertaken by Boucher (1987). Duration of the larval phase was obtained from the spawning dates determined by Paulet et al. (1988). RESULTS PHYSICAL AND CHEMICAL PARAMETERS IN THE ENVIRONMENT

Temperature Bottom temperatures varied from 15 to 16.7 °C between 10 July and 16 September 1985 (Fig. 2), with a maximum in late July. Daily variations were maximal in July (AT = 1.7 °C) while the magnitude was < 1 °C in August. Calculation of monthly and

"(u~.ul~S "IAI

~q sqd~Ji]O.l:~!m adoosoJ:mu uo.ll:~oio ) aeAZUllsod piui~:~od u! tOuooossip jo O^IUA1ja I jo ,~i~oloqdzoIAl "i 'itid mrl

001

'

mrl

>~,

f

¢,4/

001 ,~. P~.~.~" f--, ~,.

~ i

] ili ~

:!

.~

- .

-

2 ~

r~!ava :

samvlqD

!

m~

........

s!avlnaaodo

00I

¢.. C

s£mvlrl3

snm!xvm

uos:~Od

]V~IZFIOH~L "O

VS I

REPRODUCTION PATTERNS AND POSTLARVAL GROWTH OF MOLLUSCS

185

16.9 165 ~) l~

161 5.7 153

III

14.9

lO

t5

2'0

July 1985

is

I

;

lo

I

1~ August

~o

is

I I

~

1"o

,,+'

September TIME (days)

Fig. 2. Bottom-water temperature recorded at Comtesses between 10 July and 16 September 1985.

seasonal mean surface temperature indicated that the summer of 1985 was 0.42 °C cooler than the seasonal average calculated over 25 yr from 1962 to 1986 (Agoumi, unpubl, data). Phytoplankton biomass

Changes during the study period in the concentrations ofnutrients, Chl a and diatoms in the surface-water layer are shown in Fig. 3. Most of the phytoplankton consisted of fast-growing small-sized diatoms (Sournia, pers. comm.); their summer abundance (60-120 x 103 cells" 1- ~) and chlorophyll concentration in surface waters (0.2-1.0/ag" 1- ~) were both low, according to Sournia, when compared to the Bay of Mont St-Michel. The spring chlorophyll minimum of May-June was followed by a summer peak from mid-July to mid-August. NUMBER OF SPATFALLS FOR EACH SPECIES

P. maximus

Five spatfalls of P. maximus were deduced in the collectors at Caffa between 14 July and 11 September 1985 (Fig. 4). The postlarvae which had a mean height of 1705/am on 1 August settled around 21 July and could have originated from the 3 July local spawning. A decrease of four units of the gonad index at Caffa at that time (Halary, pers. comm.) and the retention of larvae in this area in July (Thouzeau & Lehay, 1988; Lehay, 1989) support this assumption. The second and third spatfalls settled within the collectors at the end of July (on 1 August, H2 = 750/am and H3 = 430/am), i.e., 18 days after the main spawning of9-11 July (Paulet et al., 1988). The corresponding larval cohort, part of which was located off Dahouet on 25 July, had indeed a pelagic phase of ~ 18 days (Boucher, 1987). The fourth settlement within the collectors, around 10 August (H4 = 890/am on 16 August), was unexpected from the monitoring of local gonad index variations; the corresponding spawning date is estimated as around 23 July. Settlement date of the last cohort collected was estimated as 30 August from spawning about 12 August; a corresponding August spawning was observed by Paulet

186

G. THOUZEAU

1.0

NITRATES

'


=L

o.o

i|

]



i

i

A

li

M

|

I

J

i

J

i

A

;



S NITRITES

0.10 ¢I =L

0.05

0.0

i

n

n

A

i

M

~

J

.

i

J

ii

u

A

l

i n

i

S SILICATES

~mb

=I. I

n

rx

r

i

im

I

A

M

i

i

J



J

I

A

s

u

!

|

PHOSPHATES

0.10

¢1 =I.



S

0,05

0.0

i

F

II

|

i

i

i

i

i

i

i

~11

CHLOROPHYLL a

1.0' =L

.500

!

'

'

'

"

!

A

!

!

J

J

!

A

!

!

!

S

120-

DIATOMS

d 100"

80-

60-

,n • !

J

!

F

!

M

!

A

I

M

I

J

l

J

I

A

ii I

S

!

O

I

N

D

Fig. 3. Mean values over entire bay of nutrients (NO3, NO2, SiO3, PO4),Chl a and diatom concentrations from April to September 1985 (Sournia, unpubl, data).

R E P R O D U C T I O N PATTERNS A N D POSTLARVAL GROWTH OF M O L L U S C S " ""

N = 81] 22

01" 0 8 " 1 9 8 5

zo.

N3 : 72 H3=429.8

N2:16 H2=749.9

s2 = 91.6

3

s2 = 8 3 . 9

%

N= !17

28-08-1985

~ 16" ¢1" 14, P 12, [;, 10'

4

14, 12

~

I

2

187

,o 8

°~1 t

.

.

.

.

65"

~

I SS:i

~

4 m

' 45 '

13s:i 2sl

96

N=

NI:

Si' bJ

N=

I

q

~

HI= 1704.6

1

15 II

2,

Height (ttm)

sl:

~

H -

18-08

92

~

~

~

~

o Hmm

r~4= 52

~

~

302.2

-1985

N3= 47

N2= 12

NI= 6

lltl= 2.67

H3= 4.64

I12= 6.10

HI= 7.49

stl = .9

S3= .61

s2= .44

sl=

N= 147

,51

11-09-1985

3

4

18 16'

3

,12. ,

10

2

4.

~L2

1

6 4 2

,

~~t~!~

H mill N4= 44

N3:23

NP= IB

H4:.B9

H3:1.68

H2:2.42

HI: ] . t ,

s4:

s3:

s2:

SI= .2~

.24

.22

.26

Hmm

NI= 7 H5= l.R2

H4~ 4.82

H3- 7.P,6

H?= Q.RI

HI= 12.i7

~;5= .45

S4= .86

53= .77

$2= .6q

sl=

.42

Fig. 4. Modal analyses of size-frequency distributions of P. maximus spat collected at Caffa in 1985. Graphical method of Bhattacharya (1967) optimized by maximum likelihood iterations of Hasselblad ( i 966). N, number of individuals; H, mean height (/a m or mm); s, s E. Postlarval cohorts are numbered from l to5.

et al. (1988) after the rematuration of the individuals which had spawned during the first half of July. Some settlement has been observed in the Bay of St-Brieuc after 11 September 1985 (Halary, pers. comm.). However, no prerecruitment corresponding to this late spawning was sampled on the seabed after the first winter in March 1986 (Thouzeau & Lehay, 1988). Comparison of relative abundances of newly collected spat at each collector removal shows that the 3 July spawning represented 5.9~o of the postlarvae settling in the collectors, the 9-11 July spawning 57.9~, the late July spawning 28.9~o, and the 12 August spawning 7.2~. These values remained stable in time since modal analysis of the height histograms for 11 September gives respective percentages of 6.1, 59.9, 26.5 and 7.5 ~ for the four spawnings. This would indicate that spat mortality rate has been similar for each cohort within the collectors.

188

G. THOUZEAU

C. varia

Four cohorts of black scallop were collected in 1985 (Fig. 5). The main spatfall was recorded during the first half of August, corresponding to a July spawning. The spawning date cannot be determined precisely since the duration of the larval phase varies considerably in this species. Le Pennec (1978) observed metamorphosis after 25-30 days in water thermoregulated at 18 + 1 °C (experimental rearing), while the breeding programmes conducted by the industrial hatchery of the "Soci6t6 Atlantique de Mariculture" (SATMAR) in Barfleur produce pediveliger larvae (mean length: 210/am) in 14 days (in Le Pennec & Diss-Mengus, 1987) in water at 17 + 1 °C.

N= 53

01-08-1985

%

N= 389

28-08-1985

1

40¸

2

3s 30

•..::: ::.':':: ::: ~

25

3

iii!l!i!ii

i

Hmm

Height (l~m)

NI= 53

N3:43

i11:3'J3.4

sl=

103.6

N= 309

15-08

- 1985

N2= 303

N I : 42

I13:

.6~

H2= 1.59

H;: Z . l l

~3:

.16

s2:

sl:

.37

.26

11-09

N= 357

-1985

2I 1

/jli '

m. . . . . . . .

o

~,

o

1

_

H mm

Hmm

~?= 19R It;?:

841 . l

sT~ 155

'il-

110

114- 56 .~]l

II1- 1 2 4 4 . 6

114:

~I= 1 3 6 . 5

S4= .26

N3= BO

N2= 126

NI= 91

li3= 2.21

112= 3.26

III= 4.3

s3~ .47

~2= .37

sl=

.67

Fig. 5. Modal analyses of size-frequency distributions ofC. varia spat collected at Caffa in 1985. Postlarval cohorts are numbered from 1 to 4.

C. opercularis

Two spatfalls of the queen scallop were recorded during the second halves of July and August (Fig. 6); the August spat collection presented abundances twice as high as those from the first spawning. The two spawnings probably occurred in the second

R E P R O D U C T I O N PATTERNS A N D POSTLARVAL GROWTH OF MOLLUSCS

N= 32

01-08-1985

i

N= 128

1

28-08

189

-1985

2

40 35

t

30 25 20

15 :'::::::. :::::::: ::::.

hi= 3~

,

o

o

8

Height

(izm)

240.1

N = 27

16-08-1985

..d

I

8

H1 = 5 5 6 . 3 sl=

7l

.".

::::::::

.

.

.

.

.

.

.

.

{,)

¢d

¢d

NI= 41

t.12=

.fi9

HI:

2.7

r, 2=

.18

¢,1:

.f, 7

N= 109

io 16 14 12

2

I~l

H mm

85

N2=

24 22

4

i

10 5

11-09-1985

2

,-"--"r 1

---

.':':: i:::!:i:: ::::::::: ::'.-.

o

8 Height

rll=

27

It1= I ? I B . 5 sl: 415.5

H

([J.m) N2=

73

NI~

HZ-

i .T,;

ii~

.',.7~

~Z~

.60

sl=

1.22

mm

35

Fig. 6. Modal analyses of size-frequency distributions of C. opercularis spat collected at Caffa in 1985. Postlarval cohorts are numbered from 1 to 2.

halves of June and July, if we allow a mean larval phase duration from 25 to 35 days (Beaumont in Le Pennec, 1978; Brand et al., 1980).

A. ephippium The spat were collected between mid-July and late August (Fig. 7); the spatfalls corresponding to a main spawning (i.e., cohorts 1, 2 and 3, making up altogether 76 % of the total numbers) appeared in the collectors from 1 August on, and showed maximum abundances on 16 August. Given the postlarval growths observed for the first three settlings (50-80 #m. day- :) and since mean height at metamorphosis is 190 #m (Le Pennec, 1978), the postlarvae probably settled between 25 and 30 July; this corresponds to spawning in late June and early July (larval duration ~ 30 days; Camarena Luhrs, unpubl, data). The fourth settlement, recorded on 16 August, accounted for 19~ of the newly collected spat of this species, with a further 5.5 % of all postlarvae being collected in late August. The respective postlarval growth rates of these two cohorts

190

G. THOUZEAU

01 - 0 8 - 1 9 8 5

N= 382

28-08

N= 307

-1985

3

2 4

o

o

o

Height (pro)

%

Hmm NS= 42

N3= 77

NZ= 235

NI= 6B

H3= 278.5

H2:490.5

It1= 6 4 5 . 6

s3= 49.4

s2= 5 6 . 3

sl=

H5= .70 s5=

119.2

16-08

N= 740

-1985

.14

N4~ 81

N3= 129

H4= 1 . 2 4

1t3= 1 . 6 7

H2= Z.14

HI-

2.79

s4=

s3=

s2=

sl=

.18

.ll

.17

N1= 41

NI= 13

.II

N = 81

11-09-1985

14

12 8 6

4

4 2

H mm ~;4

1,I:,

I;]-

Z~'~'

~;2

3.r,O I .47

Htl= . 7 9

1t3= 1 . 0 3

II?-

s4= .2

s3= .12

s2= .15

rll:

H

21

HI= 1 .RI

145= 1 .OR

1t4= 1 . 7 9

t11= 2 . 4 3

t17:2.91

I11- ?4.64

sl= .IB

st,= .23

s4=

s3=

.~P= .IR

sl=

.30

.IB

.14

I

I I

Fig. 7. Modal analyses of size-frequency distributions of A. ephippium spat collected at Caffa in 1985. Postlarval cohorts are numbered from 1 to 5.

(40 and 30 #m. day- l) would indicate that spawnings occurred in the first week of July (4) and in mid-July (5).

C. fornicata This species, which first appeared in the Bay of St-Brieuc in 1974 (Dupouy & Latrouite, 1979), was collected from mid-July to mid-September (Fig. 8) with maximum abundances on 16 and 28 August. Duration of the planktonic phase is ~ 1 month (Chipperfield, 1951; Coum, 1979), following 2-4 wk incubation inside capsules laid on the substratum (Chipperfield, 1951; Dupouy & Latrouite, 1979). By subtracting 1.5-2 months from the observed settling dates, the main spawning period ran from early June to mid-July in 1985, with a partial spawning during the second half of July. The relative abundances of the various spatfalls could not be estimated quantitatively since a high mortality rate (45 and 4 0 ~ on 16 and 28 August) was observed in the collectors, in contrast to the other species collected.

REPRODUCTION PATTERNS A N D POSTLARVAL GROWTH OF MOLLUSCS

:'"

N= 85

01.08.1985

%

N= 390

191

28.08.1985

26

I.= 18

4

24

3

22 20 18 16

,,o,

I~,,2

14 12 10 8' 6

2

. . . . . . . . . . . •

m

o

,

.

.

.

m

o

.

,

m

o

,

m

.

~

o

~

,

.

,

,

~

u,

~

~

.

,

.

,~

,

w

~

~

,



L e n g t h

(p°m)

,

o

.

-

,

,

i

g

,

i

i

° o

t

$

D

~

,

l

1

,

N4:31

N3= 40

t12= 8

"I: 5

N6= 142

N5= 105

N4= 98

N3= 31

N2- q

i.4= 554.1

L3= 1102.2

L2= 1491.9

LI= 1890.1

L6= 583.0

L5= 1164.8

1.4= 1530.3

L3" 2015.4

L2- 7504.r~

s4= 105.2

s3= 138.6

s2= 81.6

sl= 73.8

~6= 1 5 0 . 3

s5= 1 9 5 . 2

s4= 1 7 1 . 6

s3= 1 6 7 . 2

s?= ?r,2.6

16.08.1985

N = 311

4

"]",o

,

$

,1 o,

18

5

14

1:

Fi

5

3

i L m ~

~

c~

~

~

~

~

~

~

~ L ,n

0

0

0

0

,~

m 0

m 0

~ 0

(,n 0

m 0

N5: 79

N4= 157

N3= 56

L b : bb~.Z

L~ = I U I ~ . ~

L3= ; 4 4 ' , . t ,

Le:

N2= !R A~J.~

s5= 100.0

s4= 125.4

s3= 13S.R

s2= 202.4

±

L lain

N7-

74

llft=

NS= 66

N4- 15

N ] : '~

LI=

5R,I.tl

L6= l l l t a . ~

LS= 1R08.7

L4- ? 3 8 4 . 4

L~= 3'~5f,.0

s6 = 237.3

sS- 251.3

~4= 1 7 1 . 6

~t-

s7= 1 4 5 , 3

87

173.4

Fig. 8. Modal analyses of size-frequency distributions of C.fornicata spat collected at Caffa in 1985. Postlarval cohorts are numbered from 1 to 7.

POSTLARVAL GROWTH WITHIN THE COLLECTORS Mean postlarval size increases for each time interval are listed in Table I and illustrated in Fig. 9 for bivalve species. P. maximus

Postlarvae originating from the first two spawnings (early summer) presented the highest mean daily growth rates (comparisons made at a given size). These decreased during the reproductive season and the growth rate was 25 ~/o lower in the fourth than in the second settlement. Growth variations also occurred between Spatfalls 2 and 3 (originating form the same spawning), with a faster growth of Cohort 2 (mean shell height > 15~o after 60 days). Significant differences (95~o c.l.) of postlarval growth according to the collector height from the seabed were found; the mean heights observed for Cohort 3 on 1 August were 479 + 77/am in Collector A, 445 + 71/am in B, 368 + 84/am in C and 409 + 83/am in D. Values obtained for this cohort on 28 August

192

G. T H O U Z E A U TABLE I

Mean daily growth (#m) of mollusc postlarvae collected at Caffa in 1985. Collectors were immersed on 14 July 1985. Shell height (bivalves) or shell length (gastropod) increases were determined between two successive recoveries of collectors. Mean daily growth ( # m . d a y - ~) of shell From To

01-08-1985 16-08-1985

16-08-1985 28-08-1985

28-08-1985 11-09-1985

Mean from: 01-08-85 to 11-09-85

1 2 3 4

126 108 82 -

325 311 248 150

335 277 239 156

256 225 184 154

1 2 3

57 -

123 62 -

i13 119 113

95 93 113

1 2

44 -

124 -

221 63

128 63

1 2 3 4 5

78 65 50 -

81 56 53 37 -

61 55 54 39 27

73 59 53 38 27

--

--

P. maximus Cohort

C. varia Cohort

C. opercularis Cohort

A. ephippium Cohort

C. fornicata Cohort

l

2 3 4 5 6

__

31 23 31 -

--

88 48 43 50 -

74 61 46 38

48 45 48 38

in B, C and D were, respectively, 5352 + 1018, 4573 + 638 and 3978 + 367/~m. On 28 August, postlarvae from Cohort 4 had a shell height of 2277 + 462/~m (B), 2239 + 554/~m (C) and 1583 + 665/~m (D). Growth rates were slower near the bottom than higher in the water column. Growth variations ranged from 25 to 30% between A and D; they were observed from the first benthic stages on (differences significant at 99~o c.l. on 1 August). Postlarval growth #7 other species The mean daily growth of C. varia (shell height) was 94 #m for the first two cohorts collected. In contrast to P. maximus, the spatfall collected in late summer (Cohort 3) presented a higher growth rate (113 ~um ' d a y - t ) . Of the three pectinid species, C. opercularis showed the highest growth variations according to time (five-fold increase of growth rate in 1 month).

REPRODUCTION PATTERNS AND POSTLARVAL GROWTH OF MOLLUSCS ,~. 14-

193

A5-

Pecten maximus

E

1

Chlamys v a r i a

g

~

1

F- 4

(.~ 10n

I.I.I

..j ..i IJJ 2: u~

8'

3

6'

ul 2:3 j .J

2

2: "~2

3

4 4'

1

2' 0 10

4

5 i

20

|

30

i

40

i

50

!

O

60

!

10

20

|

i

!

!

30

40

50

60

D A Y S OF I M M E R S I O N

~8. E 7

DAYS OF IMMERSION

Chlamys opercularis

A

4

Anornia ephippium ~

E E v

1

I-

6

2

m

..I ..I uJ 4 2: ¢n 3

,J ,_1 4 ffl 5

y 0

10

!

20

|

30

|

40

!

50

|

60

D A Y S OF I M M E R S I O N

0

10

.

20

.

30

.

.

40

50

6'0

DAYS OF IMMERSION

Fig. 9. Mean postlarval growth of the various bivalve cohorts collected at Caffa in 1985 (95% c.l. represented). Cohorts are numbered in Arabic numerals.

Growth rates of 38-73 # m . d a y - ' after 4-6 wk of settled life were observed for A. ephippium. This species presented the same characteristics as P. maximus regarding the relative growth of the various spatfalls collected; maximum growth was observed for the early summer spawnings and growth rate decreased by 48 % from the first to the fourth settlement. Growth of the first two cohorts of C. fimticata could not be calculated over the entire monitoring period since the postlarvae dropped off the collectors before mid-August (Cohort 1)or in early September (Cohort 2). A mean daily length increase ranging from 38 to 4 8 # m ' d a y -~ was observed for Cohorts 2-6. Maximum growth was 9 0 / a m . d a y - ~ after 1 month in the collectors.

194

G. THOUZEAU DISCUSSION

RELIABILITY OF THE OPTIMIZED GRAPHICAL METHOD

Grant et al. (1987) and Grant (1989)recently pointed out the limitations of graphical methods to separate modal structure, particularly when sample size is small. In this study, however, the clearly well-defined peaks in the postlarval shell-size frequencies as well as the low SE values (7 values usually separated by > 2.5 SD units), give confidence in estimates of modal structure and mean sizes. Furthermore, the time series available on the 2-month survey allowed comparisons between consecutive times of collection, for each species, which increased the reliability of inferences drawn from such small samples. The five modes separated in the analysis, for P. maximus, agree with the number of spawnings determined by Paulet (1990) and with the size structure of Age 1 juveniles sampled on the seabed in March 1986 (Thouzeau & Lehay, 1988). Size-frequency histograms are based on the contents of the four collectors, immersed at different depths. Growth variations within the water column did not affect the separation of modes and were included in the modes variance; between-modes differences in scallop shell height were higher than intracohort variations reported earlier (Fig. 4). RELATIONSHIP OF SETTLEMENTS TO SPAWNING ACTIVITY

Thouzeau & Lehay (1988) showed that, for P. maximus, spat collection data are reliable indicators of spawnings that will result in juveniles recruiting to the seabed. The size-frequency structure of spat collected on 11 September 1985 was similar tO that of the Age 1 juveniles sampled in March 1986 (same number of modes, same principal mode, and few differences in the relative frequencies of the five cohorts). Spat collection data were thus representative of the reproductive cycle in 1985. The spawning-settlement and collector-bottom correlations suggest that the bay was an enclosed/semienclosed system, without major larval dispersion outside the bay in 1985. Hydrographic processes support this result since gyre-clockwise residual currents at Caffa during the summer of 1985 (Thouzeau & Lehay, 1988; Lehay, 1989) allowed larvae to be retained within the sovthwestern area of the bay. Modelling of the horizontal movement of suspended particles also showed that particles were contained within the bay, for an 18-day drift (duration of the larval phases), in most areas except for the northwest. In the British an Irish beds, as in the Bay of Brest, the spawning season ofP. maximus runs from April to September-October. However, the season appears to be much shorter in the Bay of St-Brieuc, from late June to late August (Paulet et al., 1988). There is no spring spawning in the bay, in contrast to the other European scallop beds (Gibson, 1956; Mason, 1958; Comely, 1974; Fraser & Mason, 1987; Wilson, 1987; Paulet et al., 1988). The main spawning period of P. maximus in 1985 was in July, providing 92.8% of the total number of scallop spat. The existence of such a main spawning that resulted in a major spatfall in the collectors agrees with Buestel et al. (1979). The spat collected

REPRODUCTION PATTERNS AND POSTLARVAL GROWTH OF MOLLUSCS

195

after mid-September (Halary, pers. comm.) could not be related later to any juveniles on the seabed (Thouzeau & Lehay, 1988). In C. varia, the four 1985 settlements corresponded to local spawnings occurring between late June and the third week of August. The lack of collection data after mid-September prevented the detection of an autumn spawning, as for C. opercularis and C. fornicata. The bimodal (April-June/September-November) character of reproduction in some black scallop populations of the Atlantic littoral (Lubet, 1959; Reddiath, 1962; Lucas, 1965; Conan & Shafee, 1978; Shafee & Lucas, 1980) remains under question in the Bay of St-Brieuc. The two spawnings of C. opercularis in 1985 occurred between mid-June and late July in the bay, which agree with Brand et al. (1980) and Fraser & Mason (1987). The occurrence of a second spawning phase in September-October, as reported elsewhere (Soemodihardjo, 1974; Brand et al., 1980), could not be determined from this study. The three pectinid species displayed a temporal shift of their abundance peaks in the collectors: the main settlement of P. maximus was observed on 1 August, compared with mid-August for C. varia, and late August for C. opercularis. Such variations are also reported between P. maximus and C. opercularis, in a number of studies (Ventilla, 1977; Pickett, 1978; Slater, 1979; Brand et a!., 1980; Fegan, 1983; Gillespie, 1983; Roman & Cano, 1987). The deduced spawning period of A. ephippium in 1985 was confined apparently to a few weeks from late June to mid-July. However, Camarena Luhrs (unpubl. data) reported spawnings from May to September in the Bay of St-Brieuc, with larval abundance peaks observed in late June and late July, corresponding to main spawnings occurring in those same months. Results of this study agree with those of Cahour (1968, unpubl, data) for the Bay of Brest. The collection period did not cover the spawning period of C. fornicata which runs from February-March to November in Breton waters (Marteil, 1963; Lubet & Le Gall, 1972; Coum, 1979; Quiniou & Blanchard, 1987).' In the Norman-Breton Gulf, maximum spawning runs from June to August (Quiniou & Blanchard, 1987); spatfaUs collected in 1985 were likely to represent the main spawning period, though autumn spawnings are reported in a number of studies (Chipperfield, 1951; Polk, 1962; Lubet & Le Gall, 1972; Coum, 1979; Quiniou & Blanchard, 1987). COMPARATIVE ANALYSIS OF POSTLARVAL GROWTH

Between-sites or interannual comparisons of postlarval growth within the collectors were made at similar sizes and at the same months of year, in order to study spatio-temporal variability. In the Bay of St-Brieuc, Buestel et al. (1979) found a mean daily growth of 250 #m for P. maximus in August 1977 (mean height increasing from I to 9 mm), similar to that recorded for Settlement 1 in 1985. Buestel et al. (1977) found a mean daily growth of 87.5/~m after 20 days within the collectors. The present values are higher for Cohorts 1

196

G. THOUZEAU

and 2 (126 and 108 #m. day-~), and similar for the third settlement (82 #m. d a y - ' ) . Maximum spat growth rates of 300-350 #m. day-~ have been recorded in the bay (Buestel & Laurec, 1975; Buestel et al., 1977; Arzel, 1984), which agree with this study. Growth of P. maximus spat in the collectors is slower further to the north off Cornwall (170 #m. day-~ for an early summer spawning; Pickett, 1978) while a higher growth rate (360 #m. day-~) was found on the southeast coast of Spain (Roman & Cano, 1987). Postlarval growth of C. varia in 1985 was similar to that recorded by Roman & Cano (1987) for the Iberian bed (mean daily growth of 98 #m between 1 and 5 mm height). These authors found a mean daily height increase of 210 #m. day- ~ for C. opercularis, 1 month after settlement, similar to that observed in this study (221 #m.day-~). Richardson et al. (1982) found a mean summer growth of 120 #m. day- ~ in the Firth of Clyde, close to that of 1985 (128/~m. day- ~). No reference was found in the literature on postlarval growth of A. ephippium in the sea. Le Pennec (1978) recorded a daily growth of 30-40 #m (shell height) from 190 to 400/zm in experimental cultures. Growth within the collectors in 1985 was probably lower than on the seabed since collectors are not suitable for spat growth. Unlike the Pectinidae, the daily growth rate ofA. ephippium did not increase with time. Postlarvae dropped off the collectors from a mean shell height of 1500 #m; no spat > 3800 #m was found in the bags. The high variability ofjuveniles and adults growth (Seed, 1980) was also found for the postlarvae: separation ofthe modes in the size-frequency analysis was more difficult than for the Pectinidae. Postlarval growth of C. fornicata is highly variable according to sites and environmental conditions; growth rate ranges from 50-55 to 152 #m. day- ~ (shell length) in the literature (Polk, 1962; Marteil, 1963; Coum, 1979; Dupouy & Latrouite, 1979). The lower growth rates in this study are due to smaller individuals. C. fornicata also dropped off the collectors from 1500/~m on; the bags contained few postlarvae > 2400 #m. Coum (1979) showed that collectors were not suitable for postlarval survival and growth of this species. The high mortalities observed during the second half of August 1985 may be partly explained by higher vulnerability to predators, compared to bivalves, because of a more fragile postlarval shell (Pilkington & Fretter, 1970). FACTORS REGULATING POSTI.ARVAL GROWTH

The definition of growth-regulating factors is of much wider interest than improving spat production techniques in suspended cultures. Thouzeau & Lehay (1988) have shown that survival of P. maximus juveniles on the seabed was a function of individual size during their first winter. Considering the high winter mortality observed in 1985-86 (up to 60%), growth-regulating factors might strongly influence the juveniles (and recruitment) abundance. This study points to four sources of growth variation within and between post-larval cohorts, i.e. the collector height from the seabed, water temperature, trophic resources, and possible differences in the gamete quality. Minchin (1976) also found a lower growth

REPRODUCTION PATTERNS AND POSTLARVAL GROWTH OF MOLLUSCS

197

of spat settled in the collectors located near the bottom, while variations were not significant in GiUespie (1983). Fegan (1983)for the Oban bed found higher postlarval growth in the collectors located at 5-7 m from the bottom, compared to those placed at 9 m. On the Canadian coasts, P. magellanicus postlarvae grew faster in collectors located 1 m above the seabed than at 4-7 m (Dadswell et al., 1987). High variations in the growth of juvenile Pectinidae in suspended cultures were also reported by Gillespie (1983) and Wallace & Reinsnes (1984, 1985), these being interpreted as a function of the immersion depth of the structures. Inversely, Leighton (1979) and Richardson et al. (1982) did not find significant variations in the 30-40 upper m of the water column. In the Bay of St-Brieuc, growth variations introduced by the collector height did not derive from any thermal differences, since temperature was homogeneous in the bottom 3 m at Caffa (Lehay, 1989). Trophic quality of the environment could be the determinant factor; the contamination of the bottom-water layer by particulate inorganic matter (PIM) accumulated at the water-sediment interface, induces an increase of the PIM" POM ratio near the seabed (Sournia, pers. comm.). Vahl (1980) and Wallace & Reinsnes (1985) have shown that such an increase inhibits nutrition and growth in C. islandica above a PIM" POM ratio value of 3.5-4. Although no value of the ratio was calculated at St-Brieuc, one might expect that food resources regulated growth in the water column. Temperature and trophic factors may explain the temporal growth variations observed between the various cohorts of P. me:,irnus, A. ephippium and C. fornicata in 1985. Mean bottom-water temperature decreased from 16-16.2 °C before 6 August to 15.8-15.9 °C before 2-3 September and to 15.6 °C after 3 September (Fig. 2). Chla concentration also decreased (two-fold reduction) from 23 August on (Fig. 3). Thus, the sharp decreases of postlarval growths in September coincided with high temperature and food variations. The preponderant role of trophic factors in regulating spatio-temporal growth variations was emphasized by Vahl (1980) and Wallace & Reinsnes (1984, 1985) for C. islandica and by Leighton (1979) for Hinnites multirugosus Gale. Growth variations between the two cohorts of P. maximus originating from the main spawning might be related to physiological processes, i.e., differences in the gamete quality during a spawning phase (Paulet, 1990). Such variations can be maintained at the juvenile stage; thus, the mean shell height at the first annual ring was 17~o lower in Cohort 3 (19.9 + 1.7 mm) than in Cohert 2 (23.9 + 1.1 mm; Thouzeau & Lehay, 1988). CONCLUSION

Modal analysis of the size-frequency distributions of the molluscs collected in the Bay of St-Brieuc enabled settlement peaks to be related to spawning peaks, and postlarval growth to be studied in 1985. These results, added to those obtained during the study of biotic interactions within the collectors (Thouzeau, 1991), should help define the

198

G. THOUZEAU

factors that could improve pectinid spat collection in the bay. In order to increase production yields, a valid compromise must be found between optimal depths for spat collection, survival and growth of postlarvae. The best-adapted solution would be an experimental protocol with spat collection between 1 and 2 m above the seabed, and spat growth higher in the water column, into the zone of maximum phytoplankton production. In 1985, all the species presented a main spawning, the product of which would directly determine the abundance of that year's pre-recruitment. A long-term generalization of this result requires monitoring these processes for several years. In this regard, the study of spat collection in 1986 (Thouzeau, 1989) is interesting because 1986 presented a strong negative thermal anomaly compared to 1985; this anomaly caused different patterns in the ontogenic cycles. ACKNOWLEDGEMENTS

We thank H. Lebris and A. Menesguen for their help in data analysis. The work was supported by an IFREMER Operating Grant to G. Thouzeau. REFERENCES Arzel, P., 1984. ~tude de l'incidence d'une centrale nucl6aire sur une population exploit6e: la coquille Saint-Jacques. Contrat Electr. Ft. CNEXO E!969, Rapp. 3, 125 pp. Bhattacharya, C.G., 1967. A simple method of resolution of a distribution into Gaussian components. Biometrics, Vol. 23, pp. 115-135. Boucher, J., 1987. D6terminisme du recrutement de la coquille Saint-Jacques. Programme et r6sultats actuels. Program. Natl. D~term. Recrut. Inf., Vol. 5, pp. 1-25. Brand, A. R., J. D. Paul & J.N. Hoogesteger, 1980. Spat settlement of scallops Chlamys opercularis (L.) and Pecten maximus (L.) on artificial collectors. J. Mar. Biol. Assoc. U.K., Vol. 60, pp. 379-389. Buestel, D. & A. Laurec, 1975. Croissance de la coquille Saint-Jacques (Pecten maximus) en rade de Brest et en baie de Saint-Brieuc. Haliotis, Vol. 5, pp. 173-177. Buestel, D., P. Arzel, P. Cornillet & J.C. Dao, 1977. La production de juv6niles de coquilles Saint-Jacques, Pecten maximus (L.). 3rd Count. Meet. Int. Counc. Explor. Sea Work. Group Maricuit. Brest Actes Colloq. CNEXO, Vol. 4, pp. 307-315. Buestel, D., J. C. Dao & G. Lemarie, 1979. Collecte de naissain de pectinid6s en Bretagne. Rapp. P.V. R~un. Cons. Int. Explor. Mer., Vol. 175, pp. 80-84. Chipperfield, P.N.J., 195 l. The breeding of Crepidulafornicata (L.) in the river Blackwater, Essex. J. Mar. Biol. Assoc. U.K., Vol. 30, pp. 49-70. Comely, C.A., 1974. Seasonal variations in the flesh weights and biochemical content of the scallop Pecten maximus L. in the Clyde sea area. J. Cons. Int. Explor. Met, Vol. 35, pp. 28 !-29~. Conan, G., & M. S. Shafee, 1978. Growth and biannual recruitment ofthe black scallop Chlamys varia (L.) in Lanveoc area, bay of Brest. J. Exp. Mar. BioL Ecol., Vol. 35, pp. 59-71. Coum, A., 1979. La population de cr6pidules Crepidulafornicata (L.) en rade de Brest; 6cologie et dynamique. Th~se 3~me Cycle Univ. Bretagne Occident. Brest, 133 pp. Dadswell, M.J., R.A. Chandler & G.J. Parsons, 1987. Spat settlement and early growth of giant scallop, Placopecten magellanicus in Passamaquoddy Bay and the Bay of Fundy, Canada. 6th Int. Pectinid Workshop Menai Bridge Wales, 31 pp. Dupouy, H. & D. Latrouite, 1979. Le d6veloppement de la cr6pidule sur le gisement de coquilles SaintJacques de la baie de Saint-Brieuc. Sci. Pdch. Bull. hist. P~ch. Mar., Vol. 292, pp. 13-19.

REPRODUCTION PATTERNS AND POSTLARVAL GROWTH OF MOLLUSCS

199

Fegan, D.F., 1983. Scallop spat collection in the Oban area, 1982; 4th Int. Pectinid Workshop Aberdeen Scotland, 20 pp. Fraser, D.I. & J. Mason, 1987. Pectinid spat studies in selected areas of the west coast of Scotland 1982-1986. Count. Meet. Int. Counc. Expior. Sea CM-ICES/K, No. 4, 10 pp. Gibson, F.A., 1956. Escallop (Pecten maximus L.) in Irish waters. Sci. Proc. R. Dublin Soc., Vol. 27, pp. 258-270. Gillespie, M.J.S., 1983. Results of recent pectinid R and D carried out at MFU, Ardtoe. 4th Int. Pectinid Workshop Aberdeen Scotland, 9 pp. Grant, A., 1989. The use of graphical methods to estimate demographic parameters. J. Mar. BioL Assoc. U.K., Vol. 69, pp. 367-371. Grant, A., P.J. Morgan & P.J.W. Olive, 1987. Use made in marine ecology of methods for estimating demographic parameters from size-frequency data. Mar. BioL, Vol. 95, pp. 201-208. Gros, P. & J.C. Cochard, 1978. Biologie de Nyctiphanes couchii (Crustacea, Euphausiacea) dans le secteur Nord du Goife de Gascogne. Ann. Inst. Oceanogr. Paris, Vol. 54, pp. 25-46. Hasselblad, V., 1966. Estimation of parameters for a mixture of normal distributions. Technometrics, Vol. 8, pp. 431-441. Lehay, D., 1989. l~tude de l'hydrologie et de rhydrodynamique de la bale de Saint-Brieuc. Approche du r61e de rhydrodynamisme sur la coquille Saint-Jacques. Th~se Dr Univ. Bretagne Occident. Brest, 338 pp. Leighton. D. L, 1979. A growth profile for the rock scallop H#mites multirugosus, held at several depths off La Jolla, California. Mar. BioL, Vol. 51, pp. 229-232. Le Pennec, M., 1978. G6n6se de la coquille larvaire et post-larvaire chez divers bivalves marins. Th~se Dr. Univ. Bretagne Occident. Brest, 2 tomes. Le Pennec, M. & B. Diss-Mengus, 1987. Aquaculture de Chlamys varia (L.): donn6es sur la biologic de la larve et de la post-larve. Vie Mar., Vol. 8, pp. 37-42. Lubet, P., 1959. Recherches sur le cycle sexuel et r6mission des gametes chez les mytilid~s et les pectinid~s (Mollusques bivalves). Rev. Tray. Inst. Pdch. Mar., Vol. 23, pp. 387-548. Lubet, P. & P. Le Gall, 1972. Recherches pr61iminaires sur la structure des populations de Crepidulafornicata Philb., mollusque m~sogast~ropode. Bull. Soc. Zool. Ft., Vol. 97, pp. 211-222. Lucas, A., 1965. Recherches sur la sexualit~ des moilusques bivalves. Bull. Biol. Ft. Belg., Voi. 29, pp. i 15-217. Marteil, L., 1963. La cr6pidule (Crepidulafornicata) en France. Sci. P~ch., Vol. 121, pp. 51-56. Mason, J., 1958. The breeding of the scallop Pecten maximus (L.), in Manx waters. J. Mar. Biol. Assoc. U.K., Voi. 37, pp. 653-671. Minchin, D., 1976. Pectinid settlement. 1st Int. Pectinid Workshop Baltimore Ireland 11-16 May" 1976, 21 pp. Naidu, K.S. & R. Scaplen, 1979. Settlement and survival of giant scallop Placopecten magellanicus larvae, on enclosed polyethylene film collectors. In, Advances i'l aquaculture, edited by T. V. R. Pillay & W. A. Dill, Fishing News (Books), London, pp. 379-381. Paul, J.D., A.R. Brand & J.N. Hoogesteger, 1981. Experimental cultivation of the scallops Chlamys opercularis (L.) and Pecten maximus (L.) using naturally produced spat. Aquaculture, Vol. 24, pp. 31-44. Paulet, Y.M., 1990. R61e de la reproduction sur le d~terminisme du recrutement de Pecten maximus (L.) en baie de Saint-Brieuc. T.h~se Dr. Univ. Bretagne Occident. Brest, 68 pp. + annexes. Paulet, Y. M., A. Lucas & A. Gerard, 1988. Reproduction and larval development in two Pecten maximus (L.) populations from Brittany. J. Exp. Mar. Biol. Ecol., Vol. 119, pp. 145-156. Pickett, G. D., 1978. Spat collection on the English coast 1976-1978.2ndhu. Pectinid Workshop Brest France, 17 pp. Pilkington, M. C. & V. Fretter, 1970. Some factors affecting the growth ofprosobranch veligers. Helgol. Wiss. Meeresunters., Vol. 20, pp. 576-593. Polk, P., 1962. Waarnemingen aangaande het voorkomen, de voortplanting, de settling en de groei van Crepidula fornicata. Ann. Soc. R. Zool. Belg., Vol. I, pp. 47-80. Quiniou, F. & M. Blanchard, 1987. Etat de la prolif6ration de la Cr6pidule (Crepidulafornicata L.) dans le secteur de Granville (golfe Normano-Breton, 1985). Haliotis, Vol. 16, pp. 513-526. Reddiath, K., 1962. The sexuality and spawning of Manx pectinids. J. Mar. Biol. Assoc. U.K., Vol. 42, pp. 683-703. R6seau National d'Observation (RNO), 198 I. Synth6se des travaux de surveillance 1975-1979 du R6seau National d'Observation de la qualit6 du milieu marin. Publ. Minist. Environ. CNEXO Paris, 358 pp.

200

G. THOUZEAU

Richardson, C.A., A.C. Taylor & T.J. Venn, 1982. Growth of the queen scallop Chlamys opercularis, in suspended cages in the firth of Clyde. J. Mar. Biol. Assoc. U.K., Vol. 62, pp. 157-169. Roman, G. & J. Cano, 1987. Pectinid settlement on collectors in Malaga, S. E. Spain, in 1985.6th Int. Pectinid Workshop Menai Bridge Wales, 32 pp. Ruzzante, D.E. & H.E. Zaixso, 1985. Settlement of Chlamys tehuelchus (D'Orb) on artificial collectors. Seasonal changes in spat settlement. Mar. Ecol. Prog. Ser., Vol. 26, pp. 195-197. Seed, R., 1980. Reproduction and growth in Anomia ephippium L. (Bivalvia, Anomiidae) in Strangford Lough, Northern Ireland. J. Conchyl., Vol. 30, pp. 239-245. Shafee, M.S. & A. Lucas, 1980. Quantitative studies on the reproduction of black scallop Chlamys varia (L.) from Lanveoc area (bay of Brest). J. Exp. Mar. Biol. Ecol., Vol. 42, pp. 171-186. Slater, J., 1979. An investigation into the occurrence and development ofpectinid veligers at the Loch Ceann Traigh spat collection site. White Fish Author. Field Rep., No. 782. Soemodihardjo, S., 1974. Aspects of the biology ofChlamys opercularis L. (Bivalvia) with comparative notes on four allied species. Ph.D. thesis. University of Liverpool. Thouzeau, G., 199 I. Experimental collection ofpostlarvae ofPecten maximus and other benthic macrofaunal species in the bay of Saint-Brieuc, France. I. Settlement patterns and biotic interactions among the species collected. J. Exp. Mar. Biol. Ecol., Vol. 148, pp. 159-179. Thouzeau, G., 1989. D~terminisme du pr~-recrutement de Pecten maximus (L.) en baie de Saint-Brieuc. Th~se Dr. Univers. Bretagne Occident. Brest, 545 pp. Thouzeau, G. & D. Lehay, 1988. Variabilit~ spatio-temporelle de la distribution, de la croissance et de la survie des juveniles de Pecten maximus (L.) issus des pontes 1985, en baie de Saint-Brieuc. Oceanol. Acta, Vol. I 1, pp. 267-284. Vahl, O., 1980. Seasonal variations in seston and in the growth rate of the Iceland scallop, Chlamys islandica (O.F. Miiller) fi,~.n Balsfjord, 70°N. J. Exp. Mar. Biol. Ecol., Vol. 48, pp. 195-204. Ventilla, R.F., 1977. Further investigations into the collection ofnatural scallop spa: offthe Ardnamurchan coast. White Fish Author. Field Rep., No. 536, 18 pp. Wallace, J.C., 1982. The culture of the Iceland scallop Chlamys islandica (O. F. Miiller). I. Spat collection and growth during the first year. Aquaculture, Vol. 26, pp. 31 !-320. Wallace, J.C. & T.G. Reinsnes, 1984. Growth variation with age and water depth in the Iceland scallop (Chlamys islandica, Pectinidae). Aquaculture, Vol. 41, pp. 141-146. Wallace, J.C. & T.G. Reinsnes, 1985. The significance of various environmental parameters for growth of the Iceland scallop, Chlamys islandica (Pectinidae), in hanging culture. Aquaculture, Vol. 44, pp. 229-242. Wilson, J. H., 1987. Spawning ofPecten maximus (Pectinidae) and the artificial collection of juveniles in two bays in the west of Ireland. Aquaculture, Vol. 61, pp. 99-11 I.