Suspension culture of the great scallop Pecten maximus in Galicia, NW Spain: intermediate secondary culture from juveniles to young adults.
KEY WORDS: scallop, Pecten maximus, intermediate suspension culture, stocking density, handling, fouling
The great scallop, Pecten maximus, is a commercially important species in Europe, and aquaculture has been based on growth of spat coming either from hatchery production or collection from the wild (Dao et al. 1999). In areas like western Norway and Atlantic Spain scallop culture depends on hatchery produced spat (Bergh & Strand 2001, Louro et al. 2005). The scallops are commonly grown intermediately in suspension culture before final grow-out by ear-hanging or in bottom culture to commercial size >100 mm. Intermediate culture comprises a primary stage from [approximately equal to] 3-20-mm shell height (Louro et al. 2005) and a secondary stage from [approximately equal to] 20-60 mm. From the secondary stage onwards the same techniques can be applied for spat obtained from hatchery production and from natural settlement on collectors (Roman et al. 2003).
Suspension culture of scallops may increase growth because of more favorable temperature and food conditions than on the seabed (Wallace & Reinsnes 1985, Dadswell & Parsons 1992, Thorarinsdottir 1994, Lodeiros et al. 1998). Other factors related to culture technique and handling practice (i.e., stocking density, type of equipment, cleaning, grading, fouling, and predation) may have negative impact on scallop growth in suspension culture. Stocking density is one of the factors most often considered and many studies have shown an inverse relationship between growth and density (Widman & Rhodes 1991, Parsons & Dadswell 1992, Cote et al. 1993, Roman et al. 1999, Maguire & Burnell 2001, Roman et al. 2003). Although some authors have found that fouling does not affect scallop growth (Wallace & Reinsnes 1985, Widman & Rhodes 1991) and others suggest beneficial effects (Ross et al. 2002), fouling organisms are usually considered to limit growth, either by reducing water flow and food availability or by competing with the scallops for food (Paul & Davis 1986, Wildish et al. 1988, Cote et al. 1993, Claereboudt et al. 1994, Lodeiros & Himmelman 2000).
Removal of fouling organisms or changing the enclosures adds to operational costs in bivalve aquaculture (Young-Lai & Aiken 1986, Taylor et al. 1997). Cleaning procedures are labor intensive and may stress the scallops. The problem with fouling on the equipment is typically met by cleaning with high-pressure water or regular changes of nets or cages (Hardy 1991, Laing & Spencer 1997, Ross et al. 2002), whereas fouling on the shells may need to be removed by hand (Taylor et al. 1997). Frequent handling can affect growth and survival of P. maximus (Laing et al. 1999) and 23% mortality has been associated with repeated handling of Placopecten magellanicus during intermediate culture (Parsons & Dadswell 1992).
Previous suspension growth trials with P. maximus in O Grove (Ria Arousa, Galicia, northwest Spain) showed the area suitable for suspension culture (Roman et al. 2003). In the study of Roman et al. (2003) scallops originated from natural settlement, and growth, which varied with season (i.e., food availability) was affected by stocking density, but not by depth. The growth of juveniles to [approximately equal to] 60-mm shell height (young adults) took place from April to September, but subsequent growth between February and May was slow (Roman et al. 2003). For this reason it was considered a possibility to maintain higher stocking densities during winter, which from a commercial point of view would be an economical advantage.
Cost-effective commercial production is a compromise between operational costs and husbandry intensity. Therefore, knowledge of the interactions between stocking density, fouling, and cleaning frequency is important to optimize site-specific culture technique. Objectives of this paper were to study (1) the effect of stocking density on growth and survival of juveniles ([approximately equal to] 15-60 mm) from hatchery produced spat during the period from August to July; (2) the effect of fouling on cages and shells; and (3) the underlying effect of handling frequency. The work will contribute to a protocol describing the best husbandry practice during the intermediate secondary culture for great scallop in Galicia, Spain.
MATERIALS AND METHODS
The scallops were produced in the experimental hatchery of Centro Oceanografico de A Coruna (COAC) in Spain, by spawning adult P. maximus collected from the wild. Juvenile scallops were obtained after intermediate primary culture in sea-based nursery (Louro et al. 2005). Growth was in stacked rigid circular plastic cages (diameter 41.5 cm, height 8 mm, mesh size 10 mm) divided into 4 compartments (quarters), which secured an even distribution of scallops (Ill. in Roman et al. 2001). Each stack consisted of 8 cages where six held scallops, the top cage functioned as cover and the bottom cage was ballasted. The experiments took place in O Grove, Ria de Arousa, NW Spain, and the scallops were kept at 6-m depth in suspension culture from a commercial raft. Temperature, salinity, and chlorophyll a were recorded weekly by the Centro de Control da Calidade do Medio Marino with a CTD (Fig. 1.).
[FIGURE 1 OMITTED]
The study was carried out between August 1, 2001 and July 9, 2002 and divided into two experiments because of increased scallop size. In both experiments the effect of stocking density and handling frequency were studied. Handling involved change of fouled cages with clean ones, monthly (H1), bimonthly (H2), and every three months (H3). At each handling time scallop shell-heights were measured with a caliper to the nearest mm and the number of live scallops counted. Growth rate (mm [day.sup.-1]) was calculated on scallops handled monthly by the equation GR = ([h.sub.1] - [h.sub.0])/([t.sub.1] - [t.sub.0]), where [h.sub.1] and [t.sub.1] were the shell height and date at sampling, and [h.sub.0] and [t.sub.0] were the size and date of the previous sampling. Fouling on Cages was estimated occasionally after removal of scallops by subtracting the mean weight of a clean cage (581 [+ or -] 35 (sd) g, n = 25) from the wet weight of cages with fouling organisms. Fouling on the scallop shells was removed by hand before wet weight measurement. The weight of settled organisms found on scallops in each quarter was pooled and expressed as g [quarter.sup.-1].
The experiment started August 1 with juveniles of mean shell height 16.8 [+ or -] 3.0 (sd) mm (n = 200) stocked at densities of 24, 36, and 48 scallops [quarter.sup.-1] (760-1500 scallops [m.sup.-2]), which covered 17% to 34% of the cage bottom area. Scallops were deployed in 6 stacks, each having two replicate cages regarding stocking density. Until October 3 the scallops were kept undisturbed to acclimate in suspension culture because of high air and water temperatures. The scallops were transferred to clean cages and subsequent handling occurred on October 30 (H1), November 28 (H1 and H2), and finally on January 3 (H1 and H3). Two stacks of cages were sampled for each handling treatment, and fouling on cages was estimated for the HI and H3 groups. The experiment finished when the juvenile scallops had reached a mean size of [approximately equal to] 35-mm shell height.
On January 3 juveniles of mean shell height 35.5 [+ or -] 5.1 (sd) mm (n = 100) were stocked at densities of 6, 12, 18, and 24 scallops [quarter.sup.-1] (190-760 scallops [m.sup.-2]), which was equal to 18% to 73% coverage of the cage bottom area. Scallops were deployed in 6 stacks, each having two replicate cages with 6 and 12, one cage with 18 and one cage with 24 scallops [quarter.sup.-1]. The cages were randomly placed within stacks. Two stacks of cages were sampled at each handling date February 3 (H1), March 12 (H1 and H2), April 16 (H1 and H3), May 14 (H1 and H2), June 10 (H1), and July 9 (H1, H2 and H3). Fouling on cages was estimated at all sampling dates, whereas fouling settled on the scallop shells was weighed in April and July. The experiment ended when the scallops had reached a mean size of [approximately equal to] 60-mm shell height and were considered young adults.
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 one-way (monthly handling) and multifactor ANOVA (remaining cases), factors being stocking density and handling frequency. Comparison between fouling weight on the cages was performed employing one-way ANOVA for each handling treatment. 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 (SNK) test was used for posthoc comparisons. When necessary, natural log transformations were used. For comparison of fouling on the shells a multifactor ANOVA was designed, factors being handling frequency and stocking density. In most of the cases values did not follow the requirements of either normality or homogeneity of variances, and non-parametric Kruskal-Wallis tests were performed.
The highest temperatures were recorded during summer and autumn 2001, with values ranging between 14.0[degrees]C and 18.6[degrees]C. A sharp decrease was observed from November 5 down to a minimum of 9.0[degrees]C reached December 26. From January to July 2002 the temperature increase was slow but constant from 12.0[degrees]C to 15.7[degrees]C (Fig. 1). Salinity generally ranged between 35.0 and 35.5 psu except for occasional drops to 33.3 psu (Fig. 1). Chlorophyll a concentrations ranged from 0.5-1.0 [micro]g [L.sup.-1] from July to October 2001, whereas lower values (<0.5 [micro]g [L.sup.-1]) were recorded between November 2001 and March 2002. From spring to summer 2002 the concentration oscillated and showed several peaks between from 1.2-2.7 [micro]g [l.sup.-1] (Fig. 1).
Survival and Growth
Survival after two months (August 1 to October 3) in suspension culture was between 98.0% and 99.1%. No significant differences in survival were recorded throughout the experimental period, neither between scallops initially stocked at different densities or between groups handled at different frequencies. The pooled survival was 98.4 [+ or -] 2.6% at first sampling and 98.2 [+ or -] 3.6% at the end of the experiment (January 3).
Handling did not significantly affect shell-height growth in this experiment (Table 1), but significant differences related to stocking density (P = 0.017) appeared as soon as October 30 (Fig. 2). At this date scallops stocked at the lowest density, 24 [quarter.sup.-1], were significantly larger (31.4 [+ or -] 4.6 mm, n = 196) than scallops grown at 48 quarter 1 (29.7 [+ or -] 5.1 mm, n = 370). At November 28 the scallops stocked at 24 [quarter.sup.-1] were larger (36.2 mm [+ or -] 4.9, n = 191) than scallops stocked at both 36 (34.5 [+ or -] 4.9 mm, n = 192) and 48 (34.0 [+ or -] 4.8 mm, n = 192) [quarter.sup.-1] (Table 1). The growth rates were 0.163, 0.148, and 0.142 mm [day.sup.-1]. Even if slower growth rates (0.039, 0.025, and 0.017 mm [day.sup.-1]) were observed for all groups from November 28 to January 3 (Fig. 2), the final shell height differed significantly between all stocking densities, showing 37.6 [+ or -] 5.1 mm (n = 378), 35.2 [+ or -] 4.5 mm (n = 384) and 34.3 [+ or -] 4.9 mm (n = 384) for scallops held at 24, 36, and 48 [quarter.sup.-1], respectively (Fig. 2, Table 1).
[FIGURE 2 OMITTED]
Coverage of the cage bottom area had increased from 17% to 34% to 43% to 78% after 2 mo growth, and was 79% to 131% by January when the experiment ended. Growth rates varied with season and showed decreasing trends (fitted to power functions) with increased coverage percentage (Fig. 3a).
[FIGURE 3 OMITTED]
Fouling on Cages and the Shells
The weight of fouling organisms on cages was determined from October 30 for the H1 and H3 treatments. Monthly handling resulted in low accumulation (mean wet weight) of fouling organisms on the cages, with the highest build-up in October (94 [+ or -] 24.59 g) (Fig. 4). In November fouling on cages ranged from 18-41 g and in January from 29-101 g. Fouling was about five times higher (513.0 [+ or -] 89.5 g) on cages that had been in the sea for three months before handling (Fig. 4). Scallop shells were sparsely fouled with small specimens of the sponges Sycon sp and Leucosolenia sp.
[FIGURE 4 OMITTED]
Survival and Growth
No significant differences in survival were found neither related to stocking density nor to handling frequency during the six months of experimental time. Final survival ranged from 93.2% to 96.9% and final mean shell-height from 54.8 mm to 67.2 mm. Shell-growth was mainly affected by stocking density and less affected by handling frequency (Table 2). Scallops handled monthly (H1) did not differ significantly in size after one month (P = 0.05), but after 5 mo significant mean shell heights (P = 0.000) were shown between all stocking densities, showing 62.8, 59.3, 55.5, and 53.3 mm for scallops held at 6, 12, 18, and 24 [quarter.sup.-1], respectively (Fig. 5). The multifactor ANOVA showed similar growth for the two highest densities until May 14, whereas in July all densities showed significant different sizes (Table 2).
[FIGURE 5 OMITTED]
At the end of the experiment, shell height also showed an inverse relationship with stocking density within all handling treatments (Fig. 5). Scallops handled monthly (H1) and bimonthly (H2) did not differ in size at any of the sampling dates, but they were always significantly larger than scallops handled every three months (H3) (Table 2). The slowest growth occurred during January and February, whereas the highest growth rates were recorded from March to June. Reduced growth rates were again shown between June and July mainly at the higher stocking densities (Table 3).
Coverage of the cage bottom area increased from 18% to 73% to 22% to 165% during the experimental period (Fig. 3b). Growth rates showed a steeper decrease with increased coverage percentage than in experiment 1. The trend lines fitted to power functions, except for in June where it fitted a second order polynomial function (Fig. 3b).
Fouling on Cages
The mean wet weight of fouling organisms on cages, handled monthly (H1), ranged between 19.8 [+ or -] 13.4 and 97.6 [+ or -] 43.5 g (Fig. 6). A slight increase was observed in the summer months. A significant higher amount of fouling organisms accumulated on cages kept for two months in the sea (H2), with mean weights ranging from 161.4 [+ or -] 33.7 387.7 [+ or -] 72.5 g (Fig. 6). A significant increase in weight was observed from winter to summer (March < May < July, P = 0.000). The most fouling was on cages changed every 3 mo (H3), where the mean value in April, 469.9 [+ or -] 137.2 g, was significantly lower than the weight 654.6 [+ or -] 202.3 g recorded in July (Fig. 6).
[FIGURE 6 OMITTED]
Fouling on the Shells
The overall mean weight of fouling organisms found on scallop shells was 170.7 [+ or -] 155.4 g [quarter.sup.-1] (n = 110), but in April (322.7 [+ or -] 114.5 g [quarter.sup.1], n = 46) the weight was significantly higher (Kruskal-Wallis test, P = 0.0000) than in July (61.4 [+ or -] 58.5 g [quarter.sup.-1], n = 64) (Fig. 6). Handling frequency did not affect the settlement of fouling organisms on the shells in April (Kruskal-Wallis test, P = 0.4486), whereas in July significant lower amounts (one-way ANOVA, P = 0.0014) were found on shells handled every 3 mo (H3) than on shells handled more frequently (H1 and H2). The weights were 87.9 [+ or -] 82.8, 71.4 [+ or -] 54.2, and 33.8 [+ or -] 23.7 g [quarter.sup.-1] for the H1 (n = 16), H2 (n = 24), and H3 (n = 24) groups, respectively.
Stocking density affected fouling on the shells significantly in April (Kruskal-Wallis test, P = 0.0000) and in July (Kruskal-Wallis test, P = 0.0003) (Fig. 7). In April fouling was higher at 6 [quarter.sup.-1] (442.4 [+ or -] 81.6 g, n = 14) than at 12 [quarter.sup.-1] (330.6 [+ or -] 78.2 g, n = 16), which in turn was higher than 18 [quarter.sup.1] (222.0 [+ or -] 11.8 g, n = 8), whereas 18 and 24 [quarter.sup.-1] (198.2 [+ or -] 21.6 g, n = 8) not differed significantly. In July fouling at 6 [quarter.sup.1] (107.6 [+ or -] 80.9 g, n = 20) was higher than the other densities, 12 [quarter.sup.-1] (51.7 [+ or -] 32.4 g, n = 20), 18 [quarter.sup.-1] (37.3 [+ or -] 15.2 g, n = 12), and 24 [quarter.sup.1] (24.8 [+ or -] 8.4 g, n = 12). Frequent handling affected fouling at the two lowest stocking densities more than the two higher densities. Within each handling group the fouling showed a decreasing trend with increased density, and the lowest density always had more fouling than the others (Fig. 7). In April H1 scallops kept at 6 and 12 [quarter.sup.-1] were affected by handling and had significant more fouling than scallops at the higher densities. In July scallops kept at 6 [quarter.sup.-1] showed a significant relationship with handling frequency (H1 > H2 > H3), whereas handling had no significant effect on fouling at the other stocking densities.
[FIGURE 7 OMITTED]
This study showed that juvenile P. maximus had high survival and continuous growth during intermediate secondary culture in Galician waters from August 2001 to July 2002. Deployment of scallops of [approximately equal to] 16 and [approximately equal to] 35-mm shell height showed survival >98% and >93%, respectively, which were similar to results obtained for small (>4 mm, Louro et al. 2005) and larger (>26 mm, Roman et al. 2003) spat grown at the same site. The high survival indicated suitable environmental conditions, low predator impact, and acceptable husbandry practice; though stocking density and handling frequency influenced on growth.
Effect of Stocking Density on Growth
Stocking density was the main factor affecting growth as previously reported for juvenile P. maximus in suspension culture (Cano et al. 2000, Maguire & Burnell 2001, Roman et al. 2003, Louro et al. 2005). Consequently, juveniles kept at the lowest densities, 760 [m.sup.-2] (16-mm shell height) and 190 [m.sup.-2] (35-mm shell height), obtained the highest growth rates and final size. Seasonal impact on growth is shown for tropical (Lodeiros & Himmelman 2000) and temperate (Maguire & Burnell 2001) scallops, and was also the case in our study. Stocking density influenced growth during all seasons. The scallops grew continuously but slower during the winter months, suggesting growth at higher densities may be feasible at colder temperatures (<15[degrees]C). This assumption was supported by our results where scallops kept at the two highest densities had similar shell-height before December (1,150 and 1,500 [m.sup.-2]) and after restocking the following year, between January and May (570 and 760 [m.sup.-2]). Growth rates <0.1-mm per day were only observed from December to February, but reducing stocking density could double the growth rate. The effect of stocking density was most pronounced in spring and summer, when conditions for growth were more favorable because of increased temperature and chlorophyll a. In spring coverage above 100% still gave good growth, but the highest growth rates were observed at coverage below 50%. This agrees with earlier observations of low coverage ([less than or equal to] 30%) facilitating better growth in P. maximus cultures (Maguire & Burnell 2001, Roman et al. 2003, Louro et al. 2005).
Fouling on Cages and Scallop Shells
Culture method, site, placement in the water column, current speed, and season are among factors that affect fouling (Claereboudt et al. 1994, Ross et al. 2002). Fouling on the cages increased with rising sea temperature, whereas high stocking density significantly reduced fouling on cages. We observed some species-specific colonizing patterns, which could have affected the environmental conditions inside the cages. The solitary tunicate Ciona intestinalis, a common nuisance in bivalve cultures (Uribe et al. 2001, Carver et al. 2003, Ross et al. 2004), settled mainly underneath the cages. Colonial tunicates (i.e., Didemnum candidum, Diplosoma listerianum, Botryllus schlosseri, Botrilloides leachi, Dendrodoa grossularia, and Polyclinidae), which often covered the cage mesh completely may have caused major reduction in water flow and food availability. Other solitary tunicates, Phallusia mamillata, Ascidia spp, Ascidiella spp, Molgula spp, and the introduced species Styela clava settled inside and outside the cages. They also settled on scallop shells, mainly on the left, flat shell.
In our study the colonization of colonial and solitary tunicates and Porifera on the scallop shells might have prevented settlement of mussels and barnacles, which is a great problem in ear hanging cultures in southern Spain (Cano et al. 2000). Culture on the seabed produces P. maximus shells with little fouling (Minchin 2003) compared with long-term suspension culture in the upper level of the water column, which enhance fouling (Claereboudt et al. 1994). P. maximus in Galicia are often associated with more fouling organisms than the scallop Aequipecten opercularis when cultured in the same location (Roman, pers. comm.). A similar difference in resistance to fouling, related to natural habitat, has also been suggested between the species Euvola ziczac (recessed) and Lyropecten nodosus (epifaunal) (Lodeiros et al. 1998).
Effect of Foaling and Handling Frequency on Growth
Less handling increased fouling on the cages, whereas fouling on shells was highest for scallops handled frequently. Similar, scallop growth was negatively affected in heavy fouled cages handled less frequently. Most likely the fouling on cages reduced the flow and food availability inside the cages and inhibited growth of the scallops and fouling organisms on the shells. To counteract the effect of flow reduction the use of biological control organisms may be a solution to remove fouling on equipment and scallop shells (Ross et al. 2004). Fouling on the shells was, however, considered of minor importance to growth compared with the effect of stocking density in our study.
It may take some time before accumulated fouling on equipment affect scallop growth. A major difference in settlement of fouling organisms was demonstrated between pearl-nets that had been deployed for 2 and 16 wk in the Irish Sea (Ross et al. 2002). Grecian et al. (2000) showed that fouling influencing growth of Placopecten magellanicus varied by season and gear type. We did not see any effect of fouling on the cages after 2 mo, when mean wet-weight of the fouling ranged between 0.06 and 0.15 g [cm.sup.-2]. After 3 mo, however, when fouling ranged from 0.19-0.26 g [cm.sup.-2], scallop growth was reduced. Moya (1998) found that the growth rate of Argopecten purpuratus, cultured in pearl-nets in Chile, was affected already after 2 mo, when fouling on the pearl-nets was 0.12 g [cm.sup.-2]. Likewise, fouling on pearl nets strongly affected E. ziczac growth (Lodeiros & Himmelman 1996) and muscle and soft tissue yield of P. magellanicus (Claereboudt et al. 1994). The lower final shell height observed in experiment 2 for scallops kept undisturbed in the sea for three months could be explained by heavier fouling on cages, but because no growth differences were found in experiment 1, despite a considerable difference in fouling on cages, we considered fouling on cages of little importance during the winter.
Scallops handled monthly and bimonthly grew faster during spring and summer than scallops handled every three months. Despite the fact that scallops handled every three months were less disturbed, growth was most probably reduced because of heavier fouling on cages. Our growth results agree with Pit & Southgate (2003) who found that less frequent disturbance (every 16 wk) affected shell growth of Pinctada margaritifera negatively. They found the lowest survival in trays cleaned every 4 wk, whereas we did not find any differences in survival regarding our handling regimen. In accordance with the work of Taylor et al. (1997) on Pinctada maxima, we found that fouling did not affect survival of 1-y-old P. maximus. Handling did not affect growth during autumn and winter months when the chlorophyll a content in the sea was the lowest. This contrast the results of Laing et al. (1999) who found that growth was reduced by frequent handling during winter-spring months with little food naturally available. They suggested increased level of stress in scallops disturbed monthly compared with bimonthly and every three months.
This study intended to produce information on the longest interval between cleaning that would provide best growth at the lowest labor costs. Scallops in cage culture will routinely be exposed to handling, and the present study considered handling stress associated with changing cages. In long-term suspension culture thinning of animals will be necessary, dirty cages must be replaced periodically by clean ones, inspection of cages for removal of predators can be advisable, and growth and survival should be surveyed at regular intervals. In our study no growth differences were found between scallops handled monthly and bimonthly. Thus, we suggest replacing fouled cages with clean ones every second month in spring-summer and every three months in autumn-winter when fouling is a lesser problem.
Experimental sampling as measurement, counting, and removal of fouling from the scallop shell was carried out simultaneously as relaying onto clean trays. Thus, possible stress related to disturbance and air exposure was limited and comparable to industrial husbandry procedures. Scallops are known to be sensitive animals, but juvenile P. maximus has been shown to withstand short-term emersion (Christophersen 2000, Maguire et al. 1999), and therefore should not be seriously affected by the air exposure during sampling or transfer to clean cages.
When comparing with culturing smaller spat in intermediate primary stage (Louro et al. 2005), less management is required in the secondary stage. The larger cage mesh size used during secondary culture delays the time before silting, and clogging affects water flow and thus growth. The presence of predators must, however, be controlled, but this was considered manageable during secondary culture. Frequent handling involved removing small predators from cages, which prevented them from growing to a size potentially harmful to the scallops. Settlement of larvae from the most potential predators was low by end of summer when the secondary culture started, but peaked in spring and early summer. At that time the scallops had reached at least 50-mm shell height, a size too large to be predated on (Grefsrud 2006).
Removing fouling organisms and changing cages are time consuming and adds to operational costs. The present study revealed that no advantage regarding growth and survival was obtained by monthly compared with bimonthly handling, even during summer months. Thus, effective production during intermediate secondary culture in Galicia can be obtained by handling maximum every second month. A further saving can be achieved by operating with higher densities during periods of slower growth (i.e., winter conditions). In the period September to February coverage above 100% had little influence on scallop growth, but the rest of the year growth was maximized at low stocking densities. We therefore propose a strategy where spat are initially stocked at high density (48 [quarter.sup.-1]) in August, transferred to clean cages in October and restocked to 12 [quarter.sup.-1] in clean cages in January. With a procedure of cleaning or changing cages every second month after that, scallops are expected to reach suitable size for final culture (>60 mm shell height) in July of the same year.
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 valuable assistance. This study was financed by EU, project SCALQUAL contract no Q5CR 2000-70310. The CTD data were provided by the Centro de Control da Calidade do Medio Marino da Conselleria de Pesca da Xunta de Galicia.
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TABLE 1. Result of multifactor ANOVA and Student-Newman-Keuls (SNK) test showing effects of stocking density (24, 36, and 48 scallops per quarter), and handling frequency (H1, H2, and H3) on shell-height growth of juvenile great scallops in Experiment 1. Source of Variation Df F Ratio P Value November 28, 2000 Stocking density (SD) 2 10.08 0.0003 Handling (H) 1 1.02 0.3180 Interaction SD x H 2 0.03 0.9712 January 3, 2001 Stocking density (SD) 2 40.08 0.0000 Handling (H) 1 0.27 0.6040 Interaction SD x H 2 0.35 0.7040 Source of Variation Results of SNK Test November 28, 2000 Stocking density (SD) 48/q = 36/q < 24/q Handling (H) Interaction SD x H January 3, 2001 Stocking density (SD) 48/q < 36/q < 24/q Handling (H) Interaction SD x H TABLE 2. Result of multifactor ANOVA and Student-Newman-Keuls (SNK) test showing effects of stocking density and handling frequency (H1, H2, H3) on shell-height growth of juvenile great scallops in 2002. Mean shell-height (mm) at each stocking density, expressed as number of scallops per quarter (#/q), are shown in brackets. Multifactor ANOVA Date Main Effects SS DF MS F-Ratio March 12 A: handling 1.8408 1 1.8408 2.76 B: density 51.0021 3 17.0007 25.51 Interact AB 0.0788 3 0.0263 0.04 April 16 A: handling 30.0833 1 30.0833 40.43 B: density 94.6408 3 31.5469 42.4 Interact AB 0.0775 3 0.0258 0.03 May 14 A: handling 0.1633 1 0.1633 0.07 B: density 175.703 3 58.5675 24.87 Interact AB 0.3375 3 0.1125 0.05 July 09 A: handling 42.9356 2 21.4679 21.23 B: density 544.997 3 181.66 180.52 Interact AB 4.8228 6 0.8038 0.8 Multifactor ANOVA Date Main Effects p-Value Results of SNK Test March 12 A: handling 0.1160 H1 = H2 B: density 0.0000 6/q (44.4) > 12/q (42.4) > 18/q Interact AB 0.9892 (41.1) = 24/q (40.6) April 16 A: handling 0.0000 H1 (49.1) > H3 (46.7) B: density 0.0000 6/q (50.9) > 12/q (48.5) > 18/q Interact AB 0.9910 (46.6) = 24/q (45.6) May 14 A: handling 0.7957 H1 = H2 B: density 0.0000 6/q (57.2) > 12/q (53.5) > 18/q Interact AB 0.9857 (51.2) = 24/q (50.2) July 09 A: handling 0.0000 H1 (61.2) = H2 (60.5) > H3 B: density 0.0000 (58.4) Interact AB 0.5804 6/q (65.8) > 12/q (61.5) > 18/q (57.2) > 24/q (55.6) TABLE 3. Shell-height growth rates (mm day') of juvenile great scallops in 2002 grown at different stocking densities, expressed as number of scallops per quarter (#/q), and handled monthly (H1), bimonthly (H2) and every three months (H3). Handling Frequency Date Density 6/q 12/q 18/q 24/q H1 February 13 0.098 0.085 0.063 0.059 March 12 0.194 0.137 0.121 0.108 April 16 0.209 0.199 0.182 0.173 May 14 0.190 0.144 0.120 0.114 June 10 0.199 0.208 0.163 0.118 July 09 0.152 0.127 0.094 0.105 H2 March 12 0.125 0.096 0.079 0.070 May 14 0.206 0.180 0.163 0.159 July 09 0.169 0.159 0.116 0.095 H3 April 16 0.138 0.116 0.097 0.086 July 09 0.168 0.142 0.123 0.125
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) Department of Biology. University of Bergen, Norway
* Corresponding author. E-mail: firstname.lastname@example.org