Suspension culture of the great scallop Pecten maximus in Galicia, NW Spain--intermediate primary culture of hatchery produced spat.

ABSTRACT A technique for growing small P. maximus spat in suspension culture from rafts is described. Hatchery produced spat of initial size 3.0- and 4.3-mm shell height were transferred to a sea-based primary nursery system in May. Scallop spat (3.0 mm) grew to 16.8 [+ or -] 3.0 mm during 85 days. Survival was 70.0 [+ or -] 8.1%, but decreased due to presence of predators in 33.4% of the sampling units, to between 0 and 37.4%. Important predators were the crabs Atelecyclus undecimlineatus, Liocarcinus arcuatus, and Necora puber and the starfish Asterias rubens. A significant negative correlation was found between Asterias rubens arm length and survival of scallops. Initial shell height affected growth and survival significantly whereas stocking density (50-400 spat [quarter.sup.-1] and initial coverage of 1% to 17%) had significant effect on growth. Spat of 4.3 mm initial size were significantly bigger than the spat of 3.0 mm after 34 and 57 days, whereas survival was 96.4 [+ or -] 4.4% versus 71.4 [+ or -] 12.8% after 37 days. No mortality occurred during the second sampling period, but growth was negatively correlated to increased density of spat. The results give important information regarding development of economic production of scallops and showed promising opportunities for optimizing stocking density in intermediate primary culture. The choice of production methods in the next production step (secondary nursery stage) can be based on the results obtained.

KEY WORDS: Pecten maximus, great scallop, spat, intermediate primary culture, stocking density, mesh size, predation, Galicia

INTRODUCTION

The great scallop Pecten maximus is a highly appreciated and sought after species in Europe. The species has been heavily fished and the natural resources are generally depleted (Dao et al. 1999), which has brought about great interest in developing scallop aquaculture. The availability of spat is a key factor to successful scallop farming worldwide. Obtaining P. maximus spat from wild has been shown unreliable in Europe with the exception of a few sites in Ireland and Scotland (Dao et al. 1999). Commercial hatchery production of P. maximus was developed in France in the 1980s and later established in United Kingdom, Ireland, and Norway (Dao et al. 1999). Also in Galicia, Spain, the landings have decreased dramatically during the last years, and attempts to catch wild spat on collectors have not been satisfactory (Roman et al. 1987, Ramonell et al. 1990). Thus, to develop a commercially great scallop culture in Galicia, spat must be obtained from hatchery production or by import from another area.

Culture technique and handling practice will change according to production step and scallop size. The spat production techniques will likewise differ depending on the seed source, either from hatchery or natural collection. An experimental hatchery system has been developed at the Centro Oceanografico de A Coruna (COAC) in Galicia, Spain for producing scallop spat. Adult broodstock were brought into the hatchery for conditioning and spawning and the larvae reared in larval tanks before transfer to a settlement system for growth to a size of approximately 2 mm (Roman 1991). Further spat growth takes place in a nursery or intermediate culture system. According to Bourne & Hodgson (1991) the nursery stage are divided in 2, primary and secondary nursery. The primary nursery stage in Galicia has two growth phases. The first takes place in the hatchery and lasts from metamorphosis until a size of 2-4 mm, when the spat are transferred to the sea. The second phase is in suspension culture in the sea until roughly 20 mm shell height. The secondary nursery stage comprises juvenile growth from 20-60 mm shell height and final culture is to the commercial size of 100-mm SL.

The use of a primary nursery system is to ensure maximum growth and survival, but high mortalities of spat transferred from the hatchery to the sea are experienced (Bourne & Hodgson 1991, Grecian et al. 2000, Christophersen & Magnesen 2001). Both environmental conditions and husbandry practice affect growth and survival of scallops in suspension culture in the sea. Performance during early scallop spat stages are clearly affected by temperature, salinity, and diet (Paul 1980a, Paul 1980b, O. Foighil et al. 1990, Martinez et al. 1992, Heasman et al. 1994, Martinez et al. 1995, Heasman et al. 1996, Lu & Blake 1996, Laing & Psimopoulous 1998, O'Connor & Heasman 1998, Parrish et al. 1999, Robert & Nicolas 2000, Laing 2000, Christophersen & Magnesen 2001, Nicolas & Robert 2001, Laing 2002, Christophersen & Lie 2003, Christophersen & Strand 2003, Milke et al. 2004, Rupp & Parsons 2004). The effect of initial spat size, stocking density, type of equipment, fouling, and predation of spat in primary nursery are less documented. Parsons & Dadswell (1994) evaluated different gear types for intermediate culture of Placopecten magellanicus and also found an inverse relationship between growth and stocking density of 10-15 mm spat (Parsons & Dadswell 1992). Grecian et al. (2000) on the other hand did not find significant effects of density of smaller sized spat (1.5-3 mm) of the same species but an effect of gear type on growth. Initial spat size of P. magellanicus and P. maximus at deployment is important, as is the timing of deployment (Grecian et al. 2000, Christophersen & Magnesen 2001, Grecian et al. 2003). Although kept in a protected environment during suspension culture, the spat are exposed to predators and fouling organisms, because planktonic larvae can settle and colonize inside the equipment. Due to the small size of the newly transferred spat a wide range of predator species may feed on them (Dao et al. 1994), and, in particular, the presence of starfish may cause heavy losses (Levy et al. 1998, Nadeau & Cliche 2003).

In spite of the economic importance of P. maximus in Europe and the fact that all the spat available in France and Norway are hatchery produced (Robert & Gerard 1999, Bergh & Strand 2001), a scarce amount of information exists regarding the primary nursery growth stage at a commercial scale. Growing spat in indoor hatchery-nursery facilities is a high cost procedure, and usually slower growth rates are obtained when comparing with spat growth in the sea (Roman, pers. obs.). Consequently the use of a sea-based nursery system is advantageous. This study deals with small sized (3-4 mm) hatchery produced scallop, Pecten maximus, spat cultured in a primary nursery growth system suspended from a raft in Ria de Arousa, Galicia, NW Spain. The objective was to study the effect of initial spat size, stocking density, and presence of predators on growth and survival to improve the husbandry practice of spat culture. The study was part of an EU CRAFT project (SCALQUAL) that intended to determine the effects of culture techniques, from spat to commercial size, on the viability of P. maximus during all production stages. Subsequent papers will be concerned with the secondary intermediate and final grow-out stages.

MATERIALS AND METHODS

The scallop spat originated from wild P. maximus that were spawned in the experimental hatchery of Centro Oceanografico de A Coruna (COAC) in Galicia, Spain during the winter 2001. The larvae were allowed to settle on artificial seaweed and maintained in the COAC hatchery system until May, when the spat were detached, size graded, and transferred to a sea-based nursery in O Grove, Ria de Arousa, NW Spain. The experiments took place between May 8 and August 1, 2001, and spat of the 2 size categories small (3.0 mm) and big (4.3 mm) were used.

The growth equipment was made up of rigid circular plastic cages (diameter 41.5 cm, height 8 cm, mesh size 10 mm) divided into 4 compartments. In each compartment a meshed (1.5, 3.5, or 4.5 mm) plastic box with a lid (quarter) was fitted, making the cages suitable for the growth of small spat in the primary nursery growth stage (Ill. in Roman et al. 2001). In May the spat were placed inside quarters of either 1.5 or 3.5 mm mesh and by June 18 relayed to quarters of 4.5 mm mesh size. Stacks of cages were suspended from a raft in the sea, and the spat were kept at a depth of 6 m during the experiments. Temperature, salinity, and fluorescence (Fig. 1) were recorded weekly by the Centro Control Calidade do Medio Marino with a CTD.

[FIGURE 1 OMITTED]

Experiment 1

This experiment lasted from May 8 to August 1 and was designed to study the possible effect of predators, which could settle inside the quarters, on the survival of the spat. The initial spat size was 3.0 [+ or -] 0.6 (SD) mm shell-height and the amount of spat added to each quarter was the volume of a spoonful, giving a stocking density of 127 [+ or -] 15 per quarter (n = 10). Small mesh (1.5 mm) quarters were used, and 54 quarters in total were deployed in 2 stacks of cages. Spat were handled on June 18 and transferred to quarters of 4.5 mm. Predators were not removed. At the end of the experiment all the scallops in each quarter (n = 54) were counted, shell height measured, and the present predators identified, counted, and measured (i.e., shell width of crabs and arm length of starfish). The number was related to the number of living scallops.

Experiment 2

This experiment lasted from May 15 to July 11 and was designed to compare the effect of initial spat size and stocking density on growth and survival. Initial spat sizes (mean [+ or -] SD) were 3.0 [+ or -] 0.6 and 4.3 [+ or -] 0.5 mm shell height, and the spat were placed in quarters of small (1.5 mm) and medium (3.5 mm) mesh size respectively. Spat of each size category was individually counted and stocked at densities of 50, 100, 200, 300, and 400 in four replicate quarters (i.e., 40 quarters). Two stacks, each of 5 cages were deployed. The quarters were randomly placed according to stocking density, but two quarters of each mesh size was placed in each cage. Initial coverage of the quarters ranged from 1.1% (small spat, 50 [quarter.sup.-l]) to 17.3% (big spat, 400 [quarter.sup.-1]) (see Fig. 4 later). At sampling June 18 and at the end of the experiment all the animals were counted, and 25 scallops from each quarter were measured. The predators were removed and identified, and the occurrence was related to the mesh size of the quarters.

[FIGURE 4 OMITTED]

Statistical Analyses

The analyses were carried out by using the statistical package Statgraphics Plus 5. Shell-height and calculated survival (%) at each sampling date were analyzed using multifactor ANOVA. All survival data were arcsine [x.sup.0.5] transformed and Kolmogorov-Smirnov test for normality and Bartlett test for homogeneity of variances performed prior to analysis. Student-Newman-Keuls test was used for posthoc comparisons.

RESULTS

Environmental Conditions

The temperature increased from 13.4[degrees]C at deployment date in May to a maximum of 16.3[degrees]C July 2, before decreasing to 14.8[degrees]C at the end of the experimental period August 1 (Fig. 1). Salinity ranged between 34.7 and 35.6 ppt, whereas chlorophyll a ranged between 0.5-1.2 [micro]g [1.sup.-1] during the period of study (Fig. 1).

Experiment 1

After 85 days (May 8 to August 1) growth in the sea of the mean shell height was 16.8 [+ or -] 3.0 mm and the overall survival 57.0% [+ or -] 22.3%. The survival in quarters without predators was 70.0 [+ or -] 8.1%. In the quarters with potential predators the mean survival ranged between 0% and 37.4% (Table 1). Potential predators occurred in 33.4% of the 54 quarters, and the species identified were the crabs Atelecyclus undecimlineatus, Liocarcinus arcuatus, and Necora puber and the starfish Asterias rubens (Table 1). The most active predator was A. undecimlineatus. The relationship between starfish arm length and living scallops was described by the equation: survival (%) = 119.525 - 4.088 x Asterias rubens arm length (mm). A significant (P < 0.01) negative correlation was found and the correlation coefficient of -0.766 indicated a moderately strong relationship between the variables.

Experiment 2

Growth and Survival

Initial shell height affected growth and survival significantly while stocking density had significant effect on growth. Shell height growth and survival were assessed at start, after 34 (June 18) and 57 (July 11) days in the sea (Fig. 2, Fig. 3). Spat of 4.3 mm initial size were significantly bigger than the spat of 3.0 mm in June and July, showing mean shell-heights (all stocking densities) of 12.9 mm versus 10.7 mm and 17.8 mm versus 16.7 mm respectively. By July a negative correlation between stocking density (both sizes categories) and final shell height was found: 50 spat [quarter.sup.-1] (21.4 mm), >100 spat [quarter.sup.-1] (19.5 mm), >200 spat [quarter.sup.-1] (16.8 mm), and >300 spat [quarter.sup.-1] (14.7 mm) = 400 spat [quarter.sup.-1] (13.7 mm) (Table 2, Table 3, Table 4). During the first month only minor differences in growth was related to the initial coverage (1% to 17%) of the quarter area (Fig. 4, Table 3). However, for the larger spat (>9.0-12.2 mm and >11.6-14.6 mm) there was a clear relationship between initial spat density and percent coverage shown as decreased growth rates (Fig. 4, Table 3). The small spat suffered 25% to 35% mortality between deployment date and sampling in June, whereas neither the small nor the big sized spat underwent additional mortality of importance between June and July (Fig. 3). In June, mean survival was 96.4 [+ or -] 4.4% versus 71.4 [+ or -] 12.8% and in July 97.0 [+ or -] 5.5% versus 70.0 [+ or -] 11.5%, for the big and small spat respectively. Within each size category there was no significant difference in survival between June and July.

[FIGURES 2-3 OMITTED]

Presence of Predators

At sampling, June18 predators were found only in quarters of 1.5 mm mesh with low stocking density of spat and not inside quarters of 3.5 mm mesh (Table 5). The predators were different crab species (five A. undecimlineatus of 10-13 mm and one L. arcuatus of 9 mm) and were found in 75% of the quarters holding 50 spat and in 25% of the quarters with 100 spat. At final sampling July 11 the starfish Asterias rubens (arm length 7-9 mm) was found only in quarters of the stocking densities 200, 300, and 400 spat per quarter (Table 5). Mean survival [+ or -] SD (all stocking densities pooled) in June was 68.0 [+ or -] 20.8% in quarters of 1.5 mm mesh with predators inside and 72.2 [+ or -] 10.8% in quarters without predators. In July the survival was 70.1 [+ or -] 13.3% in quarters with predators versus 70.0 [+ or -] 11.7% in quarters without. No significant difference in survival (1-way ANOVA, P > 0.05) was found between quarters with or without presence of potential predators at any date.

DISCUSSION

According to Bourne & Bunting (2001) paramount in production of seed supply is an efficient nursery system that permits maximum growth with minimum mortalities of juveniles. This study showed that by using the right culture procedures in the nursery growth system developed for P. maximus in Galicia, spat of 10-15 mm shell-height can be obtained after 1 mo growth in the sea and >20 mm spat after 2 mo. Deployment of hatchery produced spat of 3-mm and 4.3-mm size in late spring showed that growth and survival was affected by initial spat size, stocking density, and presence of potential predators.

Effect of Spat Size and Deployment Time

The success of scallop spat depends on the conditions of the ambient water and of the development stage or size of the scallop (Paul 1980a, Paul 1980b, Heasman et al. 1996, O'Connor & Heasman 1998, Rupp & Parsons 2004). In this study the spat of the largest initial size (4.3 mm) showed better survival than the smallest (3.0 mm) after transfer from the hatchery to sea-based nursery in late spring. By transfer of spat >4 mm, survival up to 99% could be achieved. P. maximus spat >4 mm have been shown to tolerate transfer to cold water in early spring better than spat <3 mm (Christophersen & Magnesen 2001). Likewise, Grecian et al. (2000) found higher growth rates and recovery of Placopecten magellanicus spat >3mm than of spat from 1.4-2.9 mm deployed in October and retrieved in July. Based on the former (Grecian et al. (2000, Christophersen & Magnesen 2001) and present results, 4 mm seems to be a critical spat size to withstand a change of environment, which is in accordance with the stage when the gill function is almost fully developed (Beninger et al. 1994, Veniot et al. 2003).

Deployment date is an important factor affecting spat survival in sea-based nursery because temperature and food conditions are season dependent in temperate waters (Grecian et al. 2000, Christophersen & Magnesen 2001, Grecian et al. 2003). The COAC's hatchery does only produce spat at the end of the winter resulting in spat deployment always in late spring, suggesting that the seasonal effects of the sea environment is of minor importance. In this study the spat were deployed in May, and temperature during the growth period was between 13[degrees]C and 16[degrees]C, which should be well within the range (10[degrees]C to 17[degrees]C) shown to support good growth of P. maximus spat (Laing 2000).

Survival during the first month was related to initial spat size, but could also have been a consequence of the mesh size of the quarters. The smallest spat that were placed in the quarters of smallest mesh size (1.5 mm) had the highest mortality. No subsequent mortality could be related to initial size when the spat were placed in quarters of larger and equal mesh size (4.5 mm). Although we cannot separate the effects mesh size and spat size to elucidate the causes of mortality, the silting and clogging of the small mesh may have resulted in a deterioration of the environment inside the quarters. Clogging or fouling affects the water flow rate, which in turn may affect scallop growth and survival (Wildish et al. 1987, Lodeiros & Himmelman 1996, Ross et al. 2002).

Effect of Stocking Density

Stocking density was shown to be a major factor affecting the final shell height during 2 mo growth in the sea. Growth was negatively affected by stocking density even at the lowest densities (50 and 100 per quarter) of the smallest sized spat. Likewise the percent coverage of the quarter bottom area related to shell-height pictured the effect of stocking density. Our results showed an effect of stocking densities on growth even at the lowest area coverages (<25%), whereas ~30% coverage is shown feasible for larger P. maximus in suspended culture (Maguire & Burnell 2001, Roman et al. 2003) and other cold water pectinids (Ventilla 1982, Parsons & Dadswell 1992, Roman et al. 1999). The increased negative effect of coverage rate with larger spat size may be due to an increase in biomass competing for space and the ability of the small spat to detach and resettle elsewhere (Beninger & Le Pennec 1991, Minchin 1992) than at the bottom of the quarter. The spat were likely to have settled under the lid on the walls of the quarter thereby increasing the available area. Culture of scallop postlarvae and small spat at higher densities has shown small growth differences (Parsons & Dadswell 1992, Heasman et al. 2002, Rupp et al. 2004), which yet make it cost effective to grow scallop spat at high densities during the primary nursery stage.

Along with spat growth, the environment inside the equipment is changed due to higher competition for space and food between the scallops and fouling organisms. At >10 mm size most of the spat were detached resulting in a more crowded environment at the bottom of the quarters. The probability of shell biting increases at high densities, affecting normal growth of the scallops or even causes mortality (Hardy 1991). In spite of the poorer growth rate related to the highest stocking densities (>175%) we did not find either increased mortality, or deformed scallops, which could be expected based on the results from Parsons & Dadswell (1992). According to Ross et al. (2002) the effects of fouling on pearl nets used for scallop culture were not necessarily negative. Grecian et al. (2000) found no relationship between fouling and growth of small P. magellanicus spat grown in sea-based nursery from October to July. Thus, due to the short experimental period (57-85 days) in our study, the exposure to fouling organisms (mainly recently settled sponges Leucosolenia sp and Sycon sp and very young tunicates of different species attached to the walls) was not considered a source of mortality.

Effect of Predators

Exposure to predators is another factor that must be considered during the primary nursery growth period because smaller scallops are more vulnerable to predation than larger ones. In the experimental hatchery of COAC spat can only be produced in the winter months. As a result, spat were deployed in the spring coinciding with the settlement of potential predators as crabs and starfish. Despite the fact that deployment of spat during the settlement time of predators is recommended not to take place (Grecian et al. 2003, Nadeau & Cliche 2003), the environmental conditions in spring support rapid growth and favor scallop spat culture.

Survival of spat of initial size 3 mm was shown to be affected when predators were allowed to stay inside the quarters during the 2-3 mo nursery growth. In experiment 2, where the potential predators were removed after 1 mo of growth, survival was higher. Predation was the most likely cause of the reduced mean survival (0-37% vs. 70%) in experiment 1, because no significant difference in survival was found between the smallest spat in quarters without predators in the two experiments. In general few numbers of predators were found, and in the quarters holding the larger spat predators hardly occurred. This might be due to the bigger size of the spat making them unavailable as prey or to lower settlement because of the large mesh size of the quarters. The settlement of predator larvae may be associated with the amount of fouling, which is shown to vary with equipment type (i.e., mesh type and size) (Parsons & Dadswell 1994, Maguire & Burnell 2001, Grecian et al. 2003).

The harm on the scallop spat is subjected to the predator size and species. Starfish increases its predatory efficiency with size (Nadeau & Cliche 2003) as shown in experiment 1. The same trend related to predator size was shown for the crabs in this study. In experiment 2, the survival was less affected by the presence of predatory species. During the first month only crabs were identified that were believed to not prey on scallops (except one voracious A. undecimlineatus), whereas starfish occurred during the second month. The starfish may have started to settle in May, but were not able to prey on the scallop spat before they had reached a larger size in June to July. Grecian et al. (2003) confirmed that starfish settlement was one factor determining the window of opportunity of deployment to a sea-based nursery. Thus, it is important to avoid peak settlement of starfish, remove predators during the growth period, or deploy spat larger than the potential predators.

Husbandry Strategy

The 2-phased primary culture of scallop spat, with growth from metamorphosis to 3-4 mm in the hatchery and further to a size of 20-30 mm in a sea-based nursery, is to be recommended if the seawater conditions ensure good growth. The strategy of producing P. maximus spat is similar in France and Norway (Fleury et al. 1995, Christophersen & Magnesen 2001) except for a smaller size at deployment ([less than or equal to] 2 mm) and different types of equipment. Because survival was related with the spat size at deployment it would be advantageous to grow the spat indoors in the hatchery until a size of 4 mm. On the other hand, the intensive growth is more expensive than suspension culture in the sea. The spat of smaller size (3 mm) must necessarily be placed in equipment of smaller mesh size (1.5 mm) than the larger spat (3.5 mm). In this study, the mortality (ca. 30%) of the smallest spat during the first month could have been caused by the negative effects of the small quarter mesh size. Thus, there must be a trade off between what spat size and equipment mesh size to deploy.

The mesh size of the equipment is an important factor in suspension culture because smaller mesh openings reduce the flow rate (Brake & Parsons 1998). Clogging of the equipment walls and overstocking are other reasons that further reduce the water flow through the quarters. Optimization of stocking density depends on factors like initial area coverage of the equipment, spat growth rates, and labor costs during handling (i.e., predator removal and relaying). The results in this study confirm recommendations given by others that low coverage (<30%) ensure maximum growth of scallops (Ventilla 1982, Maguire & Burnell 2001, Roman et al. 2003) whereas others argue optimum stocking density to be site and species specific (Parsons & Dadswell 1992). The smaller spat may be stocked to a higher coverage because growth was less affected and survival was not.

Because the sea conditions during the spring and summer season also enhance growth of other species than the scallop spat, the presence of predators inside the quarters must be controlled. Based on the results and knowledge of the settlement time of potential predators like starfish (A. rubens) at the site in Galicia, it is suggested to handle the spat of initial size 3 mm during the 2- to 3-month growth in primary nursery. The spat should be detached after 1 mo and relayed into quarters of larger mesh size after removal of the predators to improve survival. The survival was not further affected until reaching 20 mm, therefore, the use of small spat and the smaller meshed equipment may counteract the investment of keeping spat in land-based nurseries to a larger (>4 mm) size.

Bourne and Bunting (2001) state that any improvements that can be made in technology to increase growth and survival of juveniles and hence reduce costs are important. The husbandry technique during the first phase of the primary nursery stage affects the number of available spat for deployment (Heasman et al. 2002). The methods used in this study resulted in survival during the second primary nursery phase of 70% and >95% depending on culture strategy. Compared with results from Norway and Canada showing up to 60% and 80% survival respectively (Grecian et al. 2000, Christophersen & Magnesen 2001), this study has shown that it possible to achieve increased survival during the spat nursery stage. It has also shown that spat of >20 mm for intermediate secondary culture could be available after 2 mo in the sea. By growing P. maximus spat in Galicia a high spat yield can be expected, thereby increasing the availability of spat on the European market.

TABLE 1.
Species of predators, number inside each quarter (n = 54), percent of
occurrence and arm length (starfish) or shell width (crabs), number of
living spat per quarter and mean percent survival [+ or -] sd.
Initial number of scallops 127 [+ or -] 15. (Experiment 1)

    Species           [N.sup.er]/    % Occurrence     Size Range
                      Quarter                        of Predators

A. rubens                  1             20.4           8-22 mm
A. rubens                  2              3.7          11-22 mm
N. puber                   1              3.7          15-21 mm
A. umdecimlineatus         1              3.7             27 mm
L. arcuatus                1              1.9             18 mm

    Species              No               Survival (%)
                      Scallops

A. rubens               3-82          37.4 [+ or -] 15.9
A. rubens              19-43          24.4 [+ or -] 13.4
N. puber               33-60          36.6 [+ or -] 15.0
A. umdecimlineatus        0                   0
L. arcuatus              27                  21.3

TABLE 2.
Initial shell heights, stocking densities and final shell heights
of spat ([+ or -] sd) after two consecutive samplings.

 Initial      Density    Mean Size           Mean Size
Size (mm)    (# spat)     June 18      Sd     June 11      sd

   3.0          50         12.2       1.8      19.9       3.0
   3.0         100         11.4       1.8      18.8       3.2
   3.0         200         10.8       1.5      16.6       3.2
   3.0         300          9.9       1.7      14.0       3.0
   3.0         400          9.0       1.7      13.3       2.5
   4.3          50         14.6       1.6      22.5       2.8
   4.3         100         13.6       1.6      19.8       2.8
   4.3         200         12.8       1.7      16.7       2.8
   4.3         300         11.6       1.6      15.1       2.7
   4.3         400         11.7       1.7      13.8       3.1

Table 3.
Student-Newman-Keuls test comparing the effects of spat initial size
(3.0 and 4.3 mm height) and stocking density (50, 100, 200, 300 and
400 spat [quarter.sup.-1]) on growth.

     Source of                 F        P
     Variation         Df    Ratio    Value     Student-Newman-Keuls
                                                        Test
June 18, 2001
A spat initial size    1     59.71    0.0000    Initial size 3.0 mm <
                                                  initial size 4.3 mm
B stocking density     4     15.59    0.0000    400 = 300 <200;200 =
                                                  100; 100 = 50
A x B interaction      4              0.8166
July 11, 2001
A spat initial size    1     12.12    0.0016    Initial size 3.0 mm <
                                                  initial size 4.3 mm
B stocking density     4     85.68    0.0000    400 = 300 < 200 < 100
A x B interaction      4     1.91     0.1338      < 50

Table 4.
Student-Newman-Keuls test comparing the effects of spat initial size
(3.0 and 4.3 mm height) and stocking density (50, 100, 200, 300 and
400 spat [quarter.sup.-1]) on survival.

     Source of         Df       F         P       Student-Newman-Keuls
     Variation                Ratio     Value             Test

June 18, 2001
A spat initial size    1      82.42    0.0000    Initial size 3.0 mm <
                                                   initial size 4.3 mm
B stocking density     4       0.43    0.7837    400 = 300 = 200 =
                                                   100 = 50
A x B interaction      4       1.04    0.4000
July 11, 2001
A spat initial size    1     121.23    0.0000    Initial size 3.0 mm <
                                                   initial size 4.3 mm
B stocking density     4     1.12      0.3640    400 = 300 = 200 =
                                                   100 = 50
A x B interaction      4     2.20      0.0932

Table 5.
Number of predators inside quarter and percent survival of spat
according to stocking density and quarter mesh size (Experiment 2).

                            June 18
Stocking   Quarter Mesh    % Survival   Quarter 3.5     % Survival
 Density      1.5 mm                       mm Mesh

   50          5 (1)       77.3 vs.           0            94.5
                            66.0 (2)
  100      1 A. undecim-   40.0 vs.           0            99.3
             lineatus       73.3 (2)
  200            0            72.1            0            95.4
  300            0            76.8            0            98.2
  400            0            68.4            0            94.9

           Initial Mesh     July 11     Initial Mesh
            Size 1.5 mm                  Size 3.5 mm

Stocking   Quarter Mesh    % Survival   Quarter Mesh    % Survival
 Density      4.5 mm                       4.5 mm
   50            0            75.5            0            91.0
  100            0            62.5            0            98.5
  200       1 A. rubens    79.5 vs.           0            96.3
                            62.8 (2)
  300       1 A. rubens    81.6 vs.           0            99.2
                            60.7 (2)
  400            0            68.9       1 A. rubens    98.8 vs.
                                                         97.6 (2)

(1) 1 quarter with 1 A. undecimlineatus; 1 quarter with 2 A.
undecimlineatus, 1 quarter with 1 L. arcuatus + 1 A. undecimlineatus.

(2) Quarters with predators vs. quarters without predators.

ACKNOWLEDGMENTS

The authors thank Recursos Marinos Grovenses (REMAGRO) for the participation and use of facilities, the crew of the ship REMAGRO II, and Juan Fernandez-Feijoo and Carmen Vazquez-Vazquez for their assistance. This study was financed by EU, project SCALQUAL contract no Q5CR 2000-70310. The CTD data for O Grove were provided by the Centro de Control da Calidade do Medio Marino.

LITERATURE CITED

Beninger, P. G., S. A. P. Dwiono & M. Le Pennec. 1994. Early development of the gill and implications for feeding in Pecten maximus (Bivalvia: Pectinidae). Mar. Biol. 119:405-412.

Beninger, P. G. & M. Le Pennec. 1991. Functional anatomy of scallops. In: S.E. Shumway, editor. Scallops: biology, ecology and aquaculture. Amsterdam: Elsevier. pp. 133-223.

Bergh, O. & O. Strand. 2001. Great scallop, Pecten maximus, research and culture strategies in Norway: a review. Aquacult. Int. 9:305-318.

Bourne, N. & B. L. Bunting. 2001. Nursery systems: are they an integral part of scallop culture? 13th International Pectinid Workshop, Coquimbo, Chile, April 18-24, 2001. Book of abstracts. Ed J. E. Illanes & E. von Brand. Universidad Catolica del Norte, Coquimbo, Chile. pp. 2-3.

Bourne, N. & C. A. Hodgson. 1991. Development of a viable nursery system for scallop culture. In: S. E. Shumway & P. Sandifer, editors. An international compendium of scallop biology and culture. World Aquaculture Workshops no 1. Baton Rouge, LA: World Aquaculture Society. pp. 272-280.

Brake, J. & G. J. Parsons. 1998. Flow rate reduction in scallop grow-out trays. Bull. Aquacul. Assoc. Canada 98-2:62-64.

Christophersen, G. & T. Magnesen. 2001. Effects of deployment time and acclimation on survival and growth of hatchery-reared scallop (Pecten maximus) spat transferred to the sea. J. Shellfish Res. 20:1043-1050.

Christophersen, G. & O. Lie. 2003. Nursery growth, survival and chemical composition of great scallop Pecten maximus (L.) spat from different larval settlement groups. Aquacult. Res. 34:641-651.

Christophersen, G. & O. Strand. 2003. Effect of reduced salinity on the great scallop (Pecten maximus) spat at two rearing temperatures. Aquaculture 215:79-92.

Dao, J.-C., P.-G. Fleury & J. Barret. 1999. Scallop seabed culture in Europe. In: B. R. Howell, E. Moksness & T. Svasand, editors. Stock enhancement and sea ranching. Fishing News Books. Oxford, UK: Blackwell Science. pp. 423-436.

Dao, J.-C., P.-G. Fleury, M. Norman, J.-P. Mikolajunas & O. Strand. 1994. Concerted Action: Scallop seabed cultivation in Europe. Intermediate report no 2 (work 1994). AIR-CT93-1647.

Fleury, P.-G., C. Halary & J.-C. Dao. 1995. The intermediate culture of Pecten maximus in Brittany, France. 8th International Pectinid Workshop, Cherbourg (France). May 22-29:1999, No 17. pp. 95-100.

Grecian, L. A., G. J. Parsons, P. Dabinett & C. Couturier. 2000. Influence of season, initial size, depth, gear type and stocking density on the growth rates and recovery of sea scallop, Placopecten magellanicus, on a farm-based nursery. Aquacult. Int. 8:183-206.

Grecian, L. A., G. J. Parsons, P. Dabinett & C. Couturier. 2003. Effect of deployment date and environmental conditions on the growth rate and retrieval of hatchery-reared sea scallops, Placopecten magellanicus (Gmelin, 1791), at a sea-based nursery. J. Shellfish Res. 22:101-109.

Hardy, D. 1991. Scallop Farming. Oxford, England: Fishing News Books. 237 pp.

Heasman, M. P., W. A. O'Connor & A. W. Frazer. 1994. Detachment of commercial scallop Pecten fumatus, spat from settlement substrates. Aquaculture 123:401-407.

Heasman, M. P., W. A. O'Connor & A. W. J. Frazer. 1996. Ontogenetic changes in optimal rearing temperatures for the commercial scallop, Pecten fumatus Reeve. J. Shellfish Res. 15:627-634.

Heasman, M. P., W. A. O'Connor, A. W. Frazer, Y. Languet & S. J. O'Connor. 2002. Alternative means of nursery culture for commercial scallop (Pecten fumatus Reeve) spat. Aquaculture 213:323-338.

Laing, I. 2000. Effect of temperature and ration on growth and condition of king scallop (Pecten maximus) spat. Aquaculture 183:325-334.

Laing, I. 2002. Effect of salinity on growth and survival of king scallop spat (Pecten maximus). Aquaculture 205:171-181.

Laing, I. & A. Psimopoulous. 1998. Hatchery cultivation of king scallop (Pecten maximus) spat with cultured and bloomed algal diets. Aquaculture 169:55-68.

Levy, L., G. J. Parsons & P. Dabinett. 1998. Effect of deployment date and environmental conditions at a farm-based nursery on growth and recovery of hatchery-reared sea scallops (Placopecten magellanicus). Bull. Aquacul. Assoc. Canada 98-2:73-75.

Lodeiros, C. J. M. & J. H. Himmelman. 1996. Influence of fouling on the growth and survival of the tropical scallop, Euvola (Pecten) ziczac (L. 1758) in suspended culture. Aquacult. Res. 27:749-756.

Lu, Y. T. & N. J. Blake. 1996. Optimum concentrations of Isochrysis galbana for growth of larval and juvenile bay scallops, Argopecten irradians concentricus (Say). J. Shellfish Res. 15:635-643.

Maguire, J. A. & G. M. Burnell. 2001. The effect of stocking density in suspended culture on growth and carbohydrate content of the adductor muscle in two populations of the scallop (Pecten maximus L.) in Bantry Bay, Ireland. Aquaculture 198:95-108.

Martinez, G., L. A. Caceres, E. Uribe & M. A. Diaz. 1995. Effects of different feeding regimens on larval growth and the energy budget of juvenile Chilean scallops, Argopecten purpuratus Lamarck. Aquaculture 132:313-323.

Martinez, G., M. Torres, E. Uribe, M. A. Diaz & H. Perez. 1992. Biochemical composition of broodstock and early juvenile Chilean scallops, Argopecten purpuratus Lamarck, held in two different environments. J. Shellfish Res. 11:307-313.

Milke, L. M., V. M. Bricelj & C. C. Parrish. 2004. Growth of postlarval sea scallops, Placopecten magellanicus, on microalgal diets, with emphasis on the nutritional role of lipids and fatty acids. Aquaculture 234:293-317.

Minchin, D. 1992. Biological observations on young scallops Pecten maximus. J. Mar. Biol. Assoc. UK. 72:807-819.

Nadeau, M. & G. Cliche. 2003. Investigation of starfish (Asteria vulgaris) predatory impact on scallop collector bags. 14th International Pectinid Workshop, St. Petersburg, Florida. Book of abstracts, pp. 18-19.

Nicolas, L. & R. Robert. 2001. The effect of food supply on metamorphosis and post-larval development in hatchery-reared Pecten maximus. Aquaculture 192:347-359.

O'Connor, W. A. & M. P. Heasman. 1998. Ontogenetic changes in salinity and temperature tolerance in the doughboy scallop, Mimachlamys asperrima. J. Shellfish Res. 17:89-95.

O Foighil, D., B. Kingzett, G. O. Foighil & N. Bourne. 1990. Growth and survival of juvenile Japanese scallops, Patinopecten yessoensis, in nursery culture. J. Shellfish Res. 9:135-144.

Parrish, C. C., J. S. Wells, Z. Yang & P. Dabinett. 1999. Growth and lipid composition of scallop juveniles, Placopecten magellanicus, fed the flagellate Isochrysis galbana with varying lipid composition and the diatom Chaetoceros muelleri. Mar. Biol. 133:461-471.

Parsons, G. J. & M. J. Dadswell. 1992. Effect of stocking density on growth, production, and survival of the giant scallop, Placopecten magellanicus, held in intermediate suspension culture in Passamaquoddy Bay, New Brunswick. Aquaculture 103:291-309.

Parsons, G. J. & M. J. Dadswell. 1994. Evaluation of intermediate culture techniques, growth, and survival of the giant scallop, Placopecten magellanicus, in Passamaquoddy Bay, New Brunswick. Canadian technical report of fisheries and aquatic sciences No. 2012. pp. 29.

Paul, J. D. 1980a. Upper temperature tolerance and the effects of temperature on byssus attachment in the Queen scallop, Chlamys opercularis (L.). J. Exp. Mar. Biol. Ecol. 46:41-50.

Paul, J. D. 1980b. Salinity-temperature relationships in the Queen scallop Chlamys opercularis. Mar. Biol. 56:295-300.

Ramonell, R., G. Roman, C. P. Acosta & M. Malvar. 1990. Captacion de semilla de pectinidos en colectores. Resultados de la campana de prospeccion en Bueu (Ria de Pontevedra, Galicia) en 1988. Actas III Congreso Nac. Acuicult. pp. 439-444.

Robert, R. & A. Gerard. 1999. Bivalve hatchery technology: The current situation for the pacific oyster Crassostrea gigas and the scallop Pecten maximus in France. Aquat. Living Resour. 12:121-130.

Robert, R. & L. Nicolas. 2000. The effect of seawater flow and temperature on metamorphosis and postlarval development in great scallop. Aquacult. Int. 8:513-530.

Roman, G. 1991. Spain. In: S. E. Shumway, editor. Scallops: biology, ecology and aquaculture. Amsterdam: Elsevier. pp. 753-762.

Roman, G., M. J. Campos, C. P. Acosta & J. Cano. 1999. Growth of the queen scallop (Aequipecten opercularis) in suspended culture: influence of density and depth. Aquaculture 178:43-62.

Roman, G. C. & J. P. Cano. M. J. L. Campos & J. I. L. Lopez. 2001. Biologia y cultivo de la vieira en Malaga. Junta de Andalucia, Spain: Recursos pesqueros, Pesca y acuicultura, Consejeria de Agricultura y Pesca. 75 pp.

Roman, G., F. Fernandez Cortes, C. P. Acosta & E. Rodriguez Moscoso. 1987. Primeras experiencias con colectores de pectinidos en las Rias de Arosa y Aldan. Cuad. Mariq. Publ. Tec 12:375-380.

Roman, G., A. Louro & J. P. De la Roche. 2003. Intermediate culture of king scallop (Pecten maximus) in suspension in cages: effect of stocking density and depth. J. Shellfish Res. 22:647-654.

Ross, K. A., J. P. Thorpe, T. A. Norton & A. R. Brand. 2002. Fouling in scallop cultivation: help or hindrance? J. Shellfish Res. 21:539-547.

Rupp, G. S. & G. J. Parsons. 2004. Effects of salinity and temperature on the survival and byssal attachment of the lion's paw scallop Nodipecten nodosus at its southern distribution limit. J. Exp. Mar. Biol. Ecol. 309:173-198.

Rupp, G. S., G. J. Parsons, R. J. Thompson & M. M. De Bern. 2004. Effect of depth and stocking density on growth and retrieval of the postlarval lion's paw scallop, Nodipecten nodosus (Linnaeus, 1758). J. Shellfish Res. 23:473-482.

Veniot, A., V. M. Bricelj & P. G. Beninger. 2003. Ontogenetic changes in gill morphology and potential significance for food acquisition in the scallop Placopecten magellanicus. Mar. Biol. 142:123-131 and Erratum 827-832.

Ventilla, R. F. 1982. The scallop industry in Japan. Adv. Mar. Biol. 20: 310-382.

Wildish, D. J., D. D. Kristmanson, R. L. Hoar, A. M. DeCoste, S. D. McCormick & A. W. White. 1987. Giant scallop feeding and growth responses to flow. J. Exp. Mar. Biol. Ecol. 113:207-220.

ANGELES LOURO, (1) GYDA CHRISTOPHERSEN, (2) THOROLF MAGNESEN (2) AND GUILLERMO ROMAN (1) *

(1) Instituto Espanol de Oceanografia, Centro Oceanografico de A Coruna, Spain; (2) Centre for Studies of Environment and Resources, University of Bergen, Norway

* Corresponding author. E-mail: [e-mail]