Scallop Growth Research Articles

RESTORATION OF BAY SCALLOP (ARGOPECTEN IRRADIANS (LAMARCK)) POPULATIONS IN FLORIDA COASTAL WATERS: PLANTING TECHNIQUES AND THE GROWTH, MORTALITY AND REPRODUCTIVE DEVELOPMENT OF PLANTED SCALLOPS

Abstract Bay scallops (Argopecten irradians [Lamarck]) are a culturally and economically important component of Florida's nearshore marine community. However, many of the local populations that compose the bay scallop metapopulation in Florida have virtually disappeared since the 1950s. This study reports the results of a 3-year effort to restore bay scallop populations at 4 sites along the west central coast of the state (Tampa Bay, Anclote, Homosassa and Crystal River). During late summer of 1997, 1998 and 1999, wild adult scallops were retrieved from each of those four target sites and induced to spawn in the laboratory. The resultant offspring were grown to at least 20-mm shell height in a nursery setting and then transplanted to cages deployed at the site where their parents were originally harvested. The growth, survival and reproductive development of the planted scallops were recorded on an approximate 6-week schedule. Results suggest that caged scallops generally grew more slowly than their wild ...

Effect of varying dietary levels of ω6 polyunsaturated fatty acids during the early ontogeny of the sea scallop, Placopecten magellanicus

This study investigated the effects of selected microalgal diets on growth, survival and fatty acid composition of sea scallops, Placopecten magellanicus, from egg hatching to postlarvae. Scallops were offered Pavlova lutheri (MONO) and Pavlova sp. (strain CCMP 459, P459) in combination with Chaetoceros muelleri (CHGRA) and P459 alone. These diets exhibited varying proportions of ω6 docosapentaenoic acid (22:5ω6, DPA) which represented 8.9% of total fatty acids in P459 compared to 5.4% in P459–CHGRA and only 0.5% in MONO–CHGRA. For experiment I, larvae were fed three diets (MONO–CHGRA, P459–CHGRA or P459 alone) for 35 days after fertilization. For experiment II, larvae were fed P459–CHGRA for 21 days and split among the three diets until 39 days post-fertilization. For experiment III, 4-day-old larvae were exposed to P459–CHGRA until they become postlarvae. MONO–CHGRA failed to produce pediveligers when fed to early veligers but yielded competent larvae when fed to late veligers. This may be related to the low ω6 DPA content of this diet. High accumulation of DPA by early veligers suggests a nutritional requirement for this fatty acid. Unialgal P459 supported lower pediveliger production and lower growth rate than other diets containing CHGRA. This may be the effect of higher levels of energetic substrates in CHGRA-containing diets. Finally, P459–CHGRA yielded the best survival and growth rate during the whole larval period. All three experiments showed marked arachidonic acid (20:4ω6, AA) accumulation in early veligers (4 to 21 days) and in pediveligers (32 to 43 days). Accumulation of AA during metamorphosis might reflect stimulation of immune functions due to pathogen infection and degradation of larval organs or the role of AA in cell signalling processes associated with tissue reorganization. Early veligers markedly accumulate DPA likely in response to low docosahexaenoic acid (22:6ω3, DHA) concentrations in the three tested diets. The potential role and effect of DPA as a substitute for DHA is discussed.

Lion's paw scallop (Nodipecten subnodosus, Sowerby 1835) aquaculture in Bahia Magdalena, Mexico: effects of population density and season on juvenile growth and mortality

This study examined the effects of density and growing season on growth and survival of juvenile lion's paw scallops (Nodipecten subnodosus) in Estero San Buto, a mangrove channel in Bahía Magdalena, Mexico. Scallops were kept in plastic mesh bags in oyster trays at three population densities (500, 1000 and 2000 organisms per tray, or low medium and high density respectively) over a period of 2 months from October to December 2001 and from February to April 2002. Growth (shell height increment) was measured every 7–12 days and mortality was evaluated at the end of the experiment (dead shell count). Overall growth was fast (0.24–0.38 mm day−1) in comparison with other pectinids. Significant differences were found for both, density and season, with faster growth occurring at lower densities and during the fall season when the water temperature was higher. Mortality was low (0.5–3.0% 60 day−1) except for the high-density treatment in the fall (44% 60 day−1). Crowding together with high water temperatures and increased metabolic oxygen demand of the scallops and possibly competition for food were the probable reasons for this high mortality. Overall, mortality was lower in the spring, when temperatures were lower, O2 values were higher and food was more abundant.

Growth of postlarval sea scallops, Placopecten magellanicus, on microalgal diets, with emphasis on the nutritional role of lipids and fatty acids

Culture of the sea scallop, Placopecten magellanicus, is constrained by a reliable supply of high-quality postlarvae, yet little is known about the diets and essential nutrients required to maximize growth and survival during these vulnerable stages. Therefore, post-settlement sea scallops were exposed to binary microalgal diets consisting of a flagellate: Pavlova lutheri, Pavlova sp. (Pav, CCMP 459) or Tetraselmis striata (Plat-P) and a diatom: Chaetoceros muelleri, Thalassiosira weissflogii or Fragilaria familica for 28–30 days. The combination Pav 459/C. muelleri provided a superior diet for sea scallop postlarvae, yielding a growth rate of up to 28 μm day−1. However, when these algae were offered singly, a 32% (Pav 459) and 64% (C. muelleri) decrease in growth rates was observed, indicating that both species made a significant contribution to the success of the mixed diet. The two species are characterized by unique signatures of n−6 polyunsaturated fatty acids (PUFAs): C. muelleri has high levels of arachidonic acid (AA), and Pav 459 (in contrast to P. lutheri) has high levels of n−6 docosapentaenoic acid (DPA). DPA was selectively incorporated into tissues of scallops fed all binary diets except the one rich in DPA (Pav 459/C. muelleri). These results, coupled with a marked increase in n−6/n−3 ratios between the diet and tissues, provide evidence that n−6 PUFAs may play an important and previously underestimated role in scallop nutrition. The diet of T. striata and C. muelleri, which yielded the lowest growth rate (8.1 μm day−1) contained dietary docosahexaenoic acid (DHA; 22:6n−3) concentrations 4.5–7× lower than any other binary treatment. Low DHA levels may thus explain the poor performance of this diet. Tissue protein and triacylglycerol concentrations reflected diet performance, with higher values generally associated with more successful diets. This work suggests that DHA, as well as the n−6 PUFAs AA and DPA, may be essential for optimizing growth of sea scallop postlarvae.

Growth and survival of the scallop Lyropecten (=Nodipecten ) nodosus (L. 1758) in suspended culture in the Cariaco Gulf (Venezuela) during a non-upwelling period

Growth and survival of the scallop Lyropecten nodosus were studied in 1997 at two sites (inner and outer Turpialito Bay) during a non-upwelling period normally occurring between August and November. Individuals had an initial shell height of 4.86 cm (SD=1.64 cm). Both experimental groups were held in suspended plastic baskets at the same depth (4 m). Measurements of shell height and dry weights of shell, gonad, digestive gland, remaining tissues and shell biofouling were taken at monthly intervals. Environmental parameters, including temperature, phytoplanktonic biomass, total particulate material (TPM) and associated organic (POM) and inorganic (PIM) fractions, were recorded simultaneously. At the end of the study, significant differences in growth and survival of scallops were observed between the two experimental sites. Scallops maintained inside the bay showed a 22% greater increase in shell height (7.41 ± 0.27 cm) than those placed outside the bay (6.37 ± 0.41 cm). Survival of scallops inside the bay was 31% higher compared with scallops outside. The greater availability of food of phytoplanktonic origin during the first two experimental months (July and August) together with greater POM throughout the whole experimental period except September, at the inner bay site, probably explained survival and growth differences observed between the two locations. Results suggest that, during the non-upwelling period (characterized by low primary productivity and high water temperatures), POM of sedimentary origin may play an important role as an energy source required for metabolic and reproductive activities of L. nodosus.

Nursery growth, survival and chemical composition of great scallop Pecten maximus (L.) spat from different larval settlement groups

Scallop Pecten maximus spat (1.3–2.1 mm shell height) from different settlement groups were transferred from hatchery to land-based nursery at different ages and sizes. Chemical content, growth and survival were compared at transfer time and after 1 and 8 weeks of nursery growth. Growth was lowest and mortality highest in the first week after transfer. Mean shell height growth was 21.5–71.4 μm day−1 and ash-free dry weight (AFDW) growth −2.7 to 10.3 μg day−1. Spat from the first settlement group attained a larger size and weight than spat from larvae settled 3 days later, but had a lower daily growth rate (%). Keeping the late-settled spat a longer time in the hatchery to reach a bigger size before transfer seemed not to improve subsequent nursery growth. Survival showed a large variation with mean survival ranging from 32% to 74%. A substantial reduction in lipid content was found after transfer to the nursery. Sterol content at transfer was the only lipid class correlating with survival in the nursery. Based on the results, it is justified that spat groups of different settlement age are included in production of 15-mm great scallop spat if they are transferred from the hatchery at the same age.

Growth of Nodipecten subnodosus (Bivalvia: Pectinidae) in La Paz Bay, Mexico

Nodipecten (Lyropecten) subnodosus (Sowerby 1835), the largest pectinid in tropical waters, has been continuously exploited. In support of sustainable development for this species, we started basic growth studies to establish aquacultural practices. N. subnodosus was cultured using triangular plastic cages in La Paz Bay (1995–1996) to describe growth and survival; we determined relationships between quantity of available food and growth and behaviour of body component indices. This culture system allows an isometric shell growth of scallops over 13 months; 56.8±2.1 mm height, 54.5±2.1 mm length and 23.0±1.1 mm width. Allometric growth was found between weight and height, length and width (survival was 44.3%). Body component indices and growth increased progressively until the summer of 1996 when all indices decreased negatively, influenced by high water temperatures. Gonad tissue developed during the first year. Gametogenic activity was evident, presenting the first spawn in summer. Particulate organic and inorganic matter and chlorophyll concentrations were relatively constant during the study, and there was no evidence relating these values to growth and body component indices. Culture of N. subnodosus is feasible in La Paz Bay. However, high water temperatures in summer reduced growth and survival rates.

Biological control of fouling in suspended scallop cultivation

Fouling of scallop shells and cultivation nets by living organisms is costly to remove and can reduce scallop growth. Here we investigate biological control of fouling in suspended scallop ( Pecten maximus) cultivation. In preliminary trials in the Irish Sea, off the Isle of Man, sea urchins ( Echinus esculentus and Psammechinus miliaris) and hermit crabs ( Pagurus spp.) removed fouling from nets more efficiently than did a range of other invertebrates. Sea urchins and hermit crabs were subsequently deployed from August 2000 to January 2001 at various densities (hermit crabs and E. esculentus at 1, 2, or 3 per net and P. miliaris at 3, 5 or 7 per net) in pearl nets containing scallops. After 6 months, the survival of biological control organisms and their effect on scallop growth and mortality, and fouling of nets and scallop shells, were assessed. Only P. miliaris was associated with increased scallop shell growth, but no biological control organism reduced scallop growth or survival. All three biological control organisms significantly reduced the weight of fouling on nets (often by as much as 50%) and fouling of scallop shells; results were largely independent of biological control organism density. Sea urchins were most effective, removing hydroids and solitary tunicates efficiently; they could thus be commercially exploited alongside scallops in a form of polyculture. These results suggest that biological control could be an efficient and environmentally sound method of addressing the problem of fouling in scallop cultivation.

Modelling potential effects of petroleum exploration drilling on northeastern Georges Bank scallop stocks

A set of numerical models was used along with laboratory and field observations to evaluate the potential risk of exploratory oil and gas drilling on northeastern Georges Bank to sea scallop ( Placopecten magellanicus) stocks. The models were used to predict the drilling waste zone of influence and the impact of chronic exposure on scallop growth and reproduction. Growth and reproduction are generally considered to be the most important sublethal effects of chronic contaminant exposure. The highest near-bottom concentrations of drilling waste (water-based mud) from a hypothetical 92-day exploration well was predicted to occur along the side of the bank (>100 m depth). Laboratory information on drilling mud toxicity threshold concentrations indicated a potential for 0–48 days of growth inhibition depending upon the site, settling velocity of the mud, and area over which results are averaged. Scallop stocks on the side of the bank are relatively sparse, but dense aggregations are found in some areas and it is possible that changes in reproductive output could have detectable effects at the population level. Growth inhibition in the tidal front region, which has the densest scallop stocks, was predicted to be more localised and confined to a range of 0–15 days. Growth loss in the central mixed region (<65 m) was predicted to be negligible (<2 days). These results illustrate the importance of site location and waste settling velocity on potential effects. The magnitude of effects predicted at each site was closely related to bottom stress ( u ∗) as this determines how rapidly drilling mud reaching the seabed are redistributed and diluted both horizontally and vertically.

Modelling the effect of food depletion on scallop growth in Sungo Bay (China)

Sungo Bay (China) has a mean depth of 10 m, a total area of 140 km 2 and is occupied by several types of aquaculture, whilst opening to the ocean. The production of scallops ( Chlamys farreri) cultured on long lines is estimated to exceed 50 000 tonnes (total weight) per year. Selection of sites for scallop growth and determination of suitable rearing densities have become important issues. In this study, we focused on the local scale (e.g. 1000 m) where rearing density, food concentration and hydrodynamics interact. We have developed a depletion model coupling a detailed model of C. farreri feeding and growth and a one-dimensional horizontal transport equation. The model was applied to assess the effect of some environmental parameters (e.g. food availability, temperature, hydrodynamism) and spatial variability on growth, and to assess the effect of density according to a wide range of hydrodynamical and environmental conditions. In the simulations, food concentrations always enabled a substantial weight increase with a final weight above 1.5 g dry weight. Compared to a reference situation without depletion, a density of 50 ind m –3 decreased growth between 0% and 100%, depending on current velocity when maximum current velocity was below 20 cm s –1. The mean ratio between food available inside and outside the cultivated area (depletion factor) varied with the percentage of variation in scallop growth that was due to density. Our model suggests that scallop growth was correlated with maximum current velocity for a given density and current velocity below 20 cm s –1. The model was integrated within a Geographical Information System (GIS) to assist in making decisions related to appropriate scallop densities suitable for aquaculture at different locations throughout the bay. Concepts (depletion), methods (coupling hydrodynamics and growth models), and the underlying framework (GIS) are all generic, and can be applied to different sites and ecosystems where local interactions must be taken into account.

Growth advantages of ear hanging compared to cage culture for sea scallops, Placopecten magellanicus

A study of field growth in sea scallops, Placopecten magellanicus, was conducted to establish possible advantages of ear hanging over cage rearing for suspended culture. Several hundred juvenile scallops (mean shell height of 44.3 mm) were allocated to both Nortene culture cages and ear-hanging arrays distributed at several depths in a shallow (7 m) study site in Lunenburg Bay, Nova Scotia. To account for growth variation, water quality data (e.g. temperature, salinity, current speed and chlorophyll- a) were collected hourly by in situ instruments moored at 1 m above the bottom, and direct water sampling was conducted biweekly. Growth rates of shell and soft tissue were calculated over several periods during the 5-month-long study. Culture method and height above bottom were both important to tissue growth, but their influence was neither consistent with time nor tissue type. Whenever culture technique was a significant effect, however, we found that ear-hung scallops grew faster than those enclosed in 6-mm-mesh plastic cages. In particular, adductor muscles of caged scallops grew by 45% over the study, whereas ear-hung scallops grew by 94%, although faster growth of ear-hung scallops was not apparent at all times. Growth differentials in soft tissues as a result of culture method were most apparent over the winter, when caged scallops had minimal growth (muscle tissue) or lost biomass (remaining soft tissue) at a greater rate than ear-hung scallops. Mortality of scallops in cages and ear-hung arrays was negligible. Although factors such as fouling may be problematic in ear-hung scallops, the growth advantages of this method coupled with its favourable economic stance in terms of gear and labour costs suggest it as an advantageous culture option.

Fertilization, larval rearing and post-larval growth of the Tehuelche scallop Aequipecten tehuelchus D'Orb., 1846

Experiments on fertilization (self-fertilization and cross-mating), embryonic density, larval feeding, survival and larval settlement on natural substrata were carried out in order to identify some of the basic requirements of the early life history of Aequipecten tehuelchus reared under hatchery conditions. In self-fertilization, the main developmental stages were represented by the initial stages, mainly nonfertilized oocytes and embryos going through early divisions. In cross-matings, there was a displacement towards the stages characteristic of the culmination of the incubation phase, namely trocophores and D-larvae. The number of larvae finally obtained from self-fertilized crossings was lower than that reached in cross-matings ( P<0.05). The relation 10 sperms: 1 oocyte, commonly used in hatchery manipulation, yielded low fertilization rates (10%). Fertilization success obtained from adding 30 and 50 spermatozoids/oocyte was significantly higher (36.01% and 32.64%, respectively). Cross-fertilized eggs were incubated at three different densities (354, 708 and 1062 oocytes mm −2). Nonsignificant differences were observed in the number of D-larvae produced (SNK: P>0.05). One-week-old larvae of A. tehuelchus were cultured to the pediveliger stage in order to assess larval performance under three different dietary regimes. Growth and survival were assessed on larvae fed on: (1) Isochrysis galbana (T-Iso)+ Chaetoceros calcitrans, (2) Pavlova lutheri+ C. calcitrans, and (3) T-Iso+ P. lutheri+ C. calcitrans. Maximum growth rates were detected in larvae fed diets that included P. lutheri. Larval mortality rate varied along the culture period, being maximum during the first week of culture (0.24–0.43), and furtherly decreasing towards the end of the planktonic life (0.15 and 0.29). Pediveliger settlement was performed at the hatchery using the preferential substrata used by this species in the natural grounds: scallop shells (San Matı́as Gulf), and Ulva lactuca thalli (San José Gulf). Larvae massively settled both on shells (0.613 individuals cm −2) and thalli (0.624 individuals cm −2). Spat density on experimental collectors was directly related to the surface offered ( P<0.05; r=0.74), but not to the substrate type ( P>0.05). Scallop growth rate was similar on both substrates in the experimental period (59 days) even when growth decreased on U. lactuca at the end probably due to habitat deterioration on algal thalli (increase of organic matter and associated microfauna).

Growth and gametogenesis in the lion-paw scallop Nodipecten (Lyropecten) subnodosus

The lion-paw scallop ( Nodipecten subnodosus) represents an important fishery resource along the Baja California Pacific coast in Mexico and has high potential for aquaculture. Although some previous studies have analyzed the growth rate of this species, knowledge of its reproduction is very limited, and no studies exist on the pattern of energy storage and utilization in relation to its reproductive cycle. Spat obtained in the laboratory from wild adults were cultured at Bahı́a Magdalena (Baja California Sur, Mexico). Growth and gametogenesis were analyzed over 18 months and biochemical composition of several tissues was analyzed during the last 6 months of culture. Growth rate, in terms of increase in shell height, averaged 0.22 mm/day. The weight of adductor muscle was 20 g after 12 months, and 55 g after 18 months of culture, respectively. Gonad maturity was observed mainly from July to September during the first year (1999) and started again in April in the second year (2000). Lipid and carbohydrate concentrations in female gonad increased with gonad development, but protein concentration remained unchanged. Adductor muscle index and carbohydrate concentration increased in parallel to gonad index during the second year (2000). However, a slight but significant decrease in adductor muscle proteins was observed as gonad developed. A continuous increase in digestive gland index concomitant to gonad development might indicate an increased availability of nutrients. From an aquaculture point of view, these results indicate that Bahı́a Magdalena is an adequate site for scallop growth and reproduction. From a physiological point of view, these results indicate that natural food availability could sustain the full cost of gametogenesis in this species in optimal environmental conditions, without or with minimal energy transfer from storage tissues.

A functional model of responsive suspension-feeding and growth in bivalve shellfish, configured and validated for the scallop Chlamys farreri during culture in China

A dynamic growth model is presented for the suspension-feeding scallop Chlamys farreri. The model is configured and validated for C. farreri cultured in Sungo Bay, China, using functional relations to simulate rapid and sensitive adjustments in feeding and metabolism as observed in response to the highly changeable environment there. Notable novel elements include resolving significant adjustments in the relative processing of living chlorophyll-rich phytoplankton organics, non-phytoplankton organics and the remaining inorganic matter during both differential retention on the gill and selective pre-ingestive rejection within pseudofaeces. We also include a facility to predict the energy content of non-phytoplankton organics. This is significant, for living phytoplankton contributed less than 20% towards suspended particulate organic matter within Sungo Bay. Further, the energy content of non-phytoplankton organics was very much more variable than for phytoplankton organics. Whether using that facility or assuming an average value for the energy content of non-phytoplankton organics, resolution of the relative processing of different particle types allows simulation of how the rates, organic compositions and energy contents of filtered, ingested and deposited matter change in response to differences in seawater temperature, seston availability and seston composition. Dependent relations predict rates of energy absorption, energy expenditure and excretion. By these means, our model replicates dynamic adjustments in feeding and metabolism across full ranges of relevant natural variability, and successfully simulates scallop growth from larvae or seed to harvestable size under different temporal and spatial scenarios of culture. This is an important advance compared with simpler models that do simulate responsive adjustments. Only by modelling the complex set of feedbacks, both positive and negative, whereby suspension feeding shellfish interact with ecosystem processes, can one realistically hope to assess environmental capacities for culture.

Growth, condition, reproductive potential, and mortality of bay scallops, Argopecten irradians, in response to eutrophic-driven changes in food resources

Anthropogenic nutrient enrichment of coastal waters is changing the habitat and food resources of bay scallops, Argopecten irradians. As land-derived nitrogen (N) enters estuaries, phytoplankton abundance, particulate organic matter, and nitrogen content of seston may increase, providing a higher quantity and quality of food. Understanding these changes is important for monitoring declining populations or developing field aquaculture systems. To examine if changes in food resources due to nutrient enrichment will affect growth, condition, reproductive potential, and mortality, we conducted a field experiment in seven estuaries each having a different land-derived N load. We placed juvenile bay scallops within these estuaries for approximately 12 weeks while monitoring food quantity and quality. Stable isotopic signatures suggested that scallops assimilated food from the specific estuaries in which we placed them. Growth rates were relatively high and did not increase with higher phytoplankton concentrations, suggesting that at the densities we deployed, ambient phytoplankton concentrations were in excess of consumption ability. Growth rates decreased at sites with lower salinities, and where high densities of competitors (barnacles and slipper shells) fortuitously settled on the scallops. Condition index significantly increased with higher growth rates. Gonad index and mortality were not related to food resources, but mortality increased with lower salinities. Land-derived N load seems unlikely to directly alter condition, reproductive potential, or mortality. These results suggest that estuaries undergoing anthropogenic nutrient additions may provide food concentrations above the maximum ration assimilable, resulting in high bay scallop growth rates.

Effects of the dinoflagellate Alexandrium tamarense on early development of the scallop Argopecten irradians concentricus

The effects of Alexandrium tamarense (strain ATHK) on early development of the bay scallop Argopecten irradians concentricus were studied under laboratory conditions. The algal culture was verified by HPLC to produce paralytic shellfish poisoning (PSP) at a level of 37.48 fmol/cell. Survival of the scallop larvae was not affected when they were grown with A. tamarense at concentrations of 500–10,000 cells/ml for 48 h. However, the activity of D-shape larvae was inhibited after 48-h exposure to A. tamarense at the algal cell density of 10,000 cells/ml. Scallop growth was inhibited significantly by A. tamarense during a 14-day exposure starting at the eye-spot larval stage. The size of juvenile scallops in the group of 10,000 cells/ml was only about 32% of that of the controls, although no obvious effect of A. tamarense was found on the rate of larval metamorphosis. All juvenile scallops survived in algal concentrations of 600–2400 cells/ml, however, attachment rates were significantly lower than control values after a 5-h exposure to A. tamarense at concentrations >600 cells/ml, while they were not obviously reduced after only 1 h of exposure. At concentrations >600 cells/ml, the climbing ability of juveniles was clearly reduced by exposure to A. tamarense after only 1 h. The climbing rate and height were only 55% and 45%, respectively, of those of the controls, when exposed to A. tamarense at a concentration of 600 cells/ml. The results indicated that A. tamarense blooms may have detrimental impacts on shellfish at early life stages, therefore, special attention should be paid to the toxic algal blooms in shellfish breeding area.

Response on scallop culture to enhanced nutrient supply by experimental fertilisation of a landlocked bay

The effect of an enhanced nutrient supply to coastal waters of a landlocked bay, Hopavagen in Central Norway, on the phytoplankton production and biomass, and on growth of scallops (Pecten maximus) was studied in 1997–1999. Nitrogen, silicon and phosphorous (N:Si:P = 16:8:1, atomic) were added daily between May and October in 1998 at a level of 0.4 mg P m−3 day−1. The concentration of nutrient addition was doubled in 1999 during the same period. High addition of nutrients (1999) resulted in a significantly higher phytoplankton biomass in the summer period, expressed as chlorophyll a content, than without nutrient (1997) and low nutrient (1998). The respective mean chlorophyll a levels were 2.4 in 1999, 1.6 in 1998 and 1.2 μg l−1 in 1997. The mean primary production during the summers generally increased with the addition of nutrients from an average level of 320 mg carbon m−2 day−1 in 1997 to 1200 mg carbon m−2 day−1 in 1999. Scallops placed at 10 m depth in Hopavagen showed an increase in growth rate of the outer scallop shell in the period July–September from 0.16% day−1 in 1997 to 0.53% day−1 in 1998. Scallops grown in an unfertilised control station in the fjord outside Hopavagen had a significantly lower growth rate than those grown in the fertilised water of Hopavagen. The results showed decreased growth rate with increasing shell sizes. However, for all size groups studied a higher growth rate of the scallops was observed when nutrients were added to the bay. The tissue dry weight content of scallops grown in Hopavagen was 2–4 times higher than in the control scallops.

Family differences in success of triploid induction and effects of triploidy on fecundity of catarina scallop ( Argopecten ventricosus)

Mass induction of triploidy in the catarina scallop ( Argopecten ventricosus) results in low success in the percentage of triploids produced. To understand whether this is a treatment effect affecting all eggs equally, families were individually induced to triploidy with cytochalasin-B (CB), comparing their survival from egg to D-larvae and spat, and the percent of triploidy within families. Differences in percent triploidy success were evident between families, obtaining some with no triploids, and some with high triploidy. Among the possible causes for these differences are quality of eggs, different developmental rates, and differences in susceptibility to the treatment (CB or DMSO) itself. Regardless of those differences, overall triploidy production was increased by inducing individually eggs of each scallop rather than in mixed egg batches. In the first experiment, it was improved by 17%, and in the second experiment by 42%, as indicated by the weighted mean of triploids among the families, and when compared with previous results with this same species, where triploidy success was 58%. In a second experiment with three different families, the growth and fecundity of triploid and diploid catarina scallops were evaluated. The growth superiority of triploids was confirmed. The results indicated that triploid catarina scallops had a significantly reduced fecundity when compared with diploid scallops. The reduced fecundity appears to be mostly of a random nature, possibly associated with a reduced capability to produce balanced gametes. Whereas the successful production of tetraploid catarina scallop from fertile triploid scallops is in principle possible, the low number of eggs shed by triploid catarina scallops could diminish that success rate, even more if single triploid females are required to optimize tetraploid induction.

Drilling position of the ear affects growth and mortality of scallop ( Patinopecten yessoensis, Jay) in ear-hanging culture

We evaluated the effect of where the hole is drilled in the shell when using the ear-hanging or Nimai-ake method in scallop aquaculture. We examined its influence on growth and mortality during 8-month growth trials on the scallop Patinopecten yessoensis at Shikabe, Japan. Our observations showed: (1) no differences in growth and mortality between drilling the hole in the posterior compared to the anterior ear; (2) that drilling at positions next to the ligament causes increased mortality; (3) that the lowest increase in shell height and total weight is obtained when the hole is drilled next to the ligament on the anterior ear; and (4) that growth is greatest when the hole is drilled near the corners of the ears (either on the anterior or posterior ears).

Triploid catarina scallop ( Argopecten ventricosus Sowerby II, 1842): growth, gametogenesis, and suppression of functional hermaphroditism

Triploidy was induced in catarina scallop, Argopecten ventricosus, using two concentrations of cytochalasin-B (CB). Growth of triploid scallops exceeded that of diploids in all evaluated traits. The largest percent difference between the diploid control group and the treated groups was seen after diploid scallops reached the peak of sexual maturation and began spawning. The gonad of triploid scallops was easily recognizable by visual inspection because of a brownish discoloration and lack or few egg masses present. That characteristic allowed for the separation of ‘putative triploids’ (PTs), which when compared with diploid scallops from within the same treatment (treated diploids), had a significantly larger muscle weight than the differences seen between treated and control groups. Gametogenesis and the condition of hermaphroditism in this normally functional hermaphrodite were greatly affected by the triploid condition. In the female portion of the gonad, few oocytes developed. The male portion of the gonad was arrested early during development, and the male acini were replaced by female acini, producing all female gonads in increasing percentages during the culture period, suppressing the normal condition of hermaphroditism. Oocytes of diploids were larger than those of triploids early in the culture, probably because of delayed maturation in triploids. Later during the culture, oocytes of triploids were larger than those of diploids only when compared with treated diploids.