Oyster Growth Research Articles

Influence of Environmental Factors on Growth, Survival, and Heavy Metal Accumulation in Oyster, Crassostrea madrasensis (Preston, 1916) Cultivated in Negombo Estuary, Sri Lanka

The growth of marine bivalves is affected by the interactions of several environmental variables, particularly water salinity, temperature, and food supply. Influences of environmental parameters on daily weight gain (DWG), survival and heavy metal accumulation in 225 numbers of oysters, Crassostrea madrasensis (Preston, 1916), placed at five locations in the Negombo estuary, Sri Lanka, were investigated. One-way ANOVA indicated significantly higher (P < 0.05) DWGs (0.22 ± 0.01 and 0.16 ± 0.01 g.day-1) in Pitipana and Munnakaraya, where highest mean salinity (20.9 ± 0.34 ppt) and chlorophyll-a (5.41 ± 1.49 µg.L-1) were observed. Significantly lower growth rate (0.04 ± 0.02 g.day-1) recorded in Thaladuwa, where lowest salinity (13.29 ± 1.13 ppt), highest turbidity (19.26 ± 0.99 NTU) and ammoniacal nitrogen (0.368 ± 0.078 mg.L-1) were recorded. DWG showed a significant second-order polynomial relationships with chlorophyll-a (R² = 0.44, P < 0.05) and salinity (R² = 0.28, P < 0.05). Negative exponential relationships of DWG were evident with higher level of ammoniacal nitrogen (R² = 0.24, P < 0.05) and phosphate (R² = 0.25, P < 0.05). The high concentration of lead (1.883 mg.kg-1) exceeded the EU permissible limit of 1.5 mg.kg-1 (wet weight) in oysters’ tissue where urban wastewater is released to the lagoon. There appeared to be health concerns due to heavy metal accumulation in oyster tissues in polluted areas of the estuary. The findings of this study are useful for understanding the potential impacts of environmental changes on oyster resources and the long-term sustainability of oyster fisheries and aquaculture.

Stocking density effect on the growth and mortality of juvenile European flat oyster (Ostrea edulis Linnaeus, 1758)

AbstractThe effect of stocking density on the growth and mortality of the European flat oyster (Ostrea edulis Linnaeus, 1758) reared in lantern nets was evaluated, starting from spat up to the end of juvenile phase. Oysters were deployed in a raft on the Basque coast (SE Bay of Biscay) for 18 months. Oysters were divided into four densities ranging from 25% to 100% of surface coverage. At the end of the experiment, oysters from an initial mean height and wet weight of 22.79 ± 3.70 mm and 1.00 ± 0.30 g reached mean values of 55.86 ± 8.21 mm and 19.08 ± 9.66 g, respectively. Although significant differences were detected in the growth of oysters reared at different stocking densities, these differences were not density dependent. On the other hand, mortality increased with time showing relatively high values (27%–38%), but especially at the end of the study when oysters at the highest density showed values up to 50%. In conclusion, stocking density had no effect on the growth, but had a negative effect on the survival at the highest density. A stocking density of 75% is recommended as the optimum for rearing O. edulis in the Basque coast.

Climate warming promotes Pacific oyster (Magallana gigas) production in a subarctic lagoon and bay, Japan: Projection of future trends using a three dimensional physical-ecosystem coupled model

Bivalve aquaculture is a major industry supporting food production in coastal areas. Although sea water temperature increase due to climate change is expected to affect marine ecosystems in a variety of ways, it is important to determine whether changes in water temperature and associated changes in the trophic conditions will enhance or inhibit the production of bivalves. In this study, I conducted a field survey of environmental factors and growth rates of the Pacific oyster (Magallana gigas) in the subarctic estuarine areas (Akkeshi-ko Lagoon and Akkeshi Bay) and evaluated the effects of climate warming using numerical simulations. In situ cage experiments with two different year-classes were conducted and oyster growth varied between the two stations in the 2nd year-class mainly in response to spatial variation in water temperature. A three dimensional physical-ecosystem coupled model including a growth model of oyster was applied and the model could reproduce the differences in present (the year 2014) growth patterns between stations. The climate warming scenarios showed that oyster production would increase in both lagoon and bay. However, the timing and location of weight loss due to spawning will differ, so caution will need to be exercised regarding the timing of the oyster harvest in the future.

Effects of flood-associated stressors on growth and survival of early life stage oysters (Crassostrea virginica)

Oyster reefs provide essential ecosystem services but are severely degraded worldwide. Extreme flooding events, which can be intensified by water management decisions, reduce water quality in estuaries and further threaten oyster populations. Restoration and conservation of oysters is dependent on the success of early oyster life stages. This study examined the effect of water quality stressors associated with flooding events on the growth and survival of larval and juvenile oysters (Crassostrea virginica). In 96-h assays, we exposed D-stage larvae to a range of dissolved oxygen, microcystin-LR, pH, and salinity concentrations. These conditions were selected based on water quality data from the Mississippi Sound during a 2019 freshwater flooding event caused by the Bonnet Carré Spillway opening. There was no negative effect of microcystin-LR or pH on early veligers at the concentrations tested, but low salinity significantly reduced shell growth, and hypoxia (< 2 mg L−1 O2) decreased both larval growth and survival. Post-metamorphosis juvenile oysters were exposed to the same water quality stressors for 24 days in the lab. Low DO, pH, and salinity treatments reduced juvenile change in wet weight and shell growth rates, but had no effects on survival. These laboratory-exposed juveniles were subsequently deployed into the field to assess the ability of juveniles to recover from short-term exposure to simulated flooding-associated stressors. After deployment to natural conditions in the Mississippi Sound, juvenile oysters were able to compensate for reduced growth during the lab exposure, even though survival was reduced for juveniles previously exposed to low pH during the first two weeks in the field. In general, early oyster life stages were relatively tolerant of the duration and stressor concentrations tested, but negative sublethal impacts of flood-associated stressors must be considered in the face of increasing frequency and duration of flooding events due to climate change.

Subchronic exposure to high-density polyethylene microplastics alone or in combination with chlortoluron significantly affected valve activity and daily growth of the Pacific oyster, Crassostrea gigas.

Nowadays, pesticides and microplastics (MPs) are commonly found in coastal waters worldwide. Due to their widespread use, their persistence and toxicity, they may induce adverse effects on physiology and behaviour of marine organisms such as the Pacific oyster (Crassostrea gigas). This study explored the growth and valve activity of juvenile oysters exposed for 24 days to two frequently detected pollutants in the Pertuis Charentais (South West, France): a herbicide (chlortoluron, 85 µg.L-1) and high-density polyethylene microparticles (HDPE 20-25 µm, 112 MP.mL-1) alone or in combination (cocktail condition; 97 µg.L-1 of chlortoluron+108 MP. mL-1). The valve activity of juvenile oysters recorded by using a High Frequency and Non-Invasive valvometer (HFNI) was characterized by three parameters: the number of valve micro-closures (VMC), the Valve Opening Amplitude (VOA), and the Valve Opening Duration (VOD). Additionally, daily shell growth and the oyster daily rhythm were assessed. The exposure to MPs of oysters led to a significant increase of VMC and a decrease of VOD and shell growth. The exposure to chlortoluron showed a significant increase of VOA and a decrease of VMC. In combination with MPs, chlortoluron still increased VOA and decreased VMC but also reduced the shell growth. Chronobiological analysis did not reveal any effects on the daily rhythm of both contaminants. This work highlighted significant effects of high environmental concentrations of MPs and Chlortoluron on the behaviour and growth of the Pacific oyster.

Growth and survival of the native oyster Crassostrea gasar cultured under different stocking densities in two grow-out systems in tropical climate

ABSTRACT Survival and growth of the native oyster Crassostrea gasar along the juvenile and adult phases were evaluated in three different stocking densities [low (D), medium (2D) and high (3D)] and in two grow-out systems (fixed and floating system). The fixed system consisted of a rack made with PVC, fixed from the bottom with wood sticks. The floating system consisted of floating bags suspended by a rack made with PVC and maintained submerged from the seawater surface by eight floats. Survival and shell height of oysters cultured after 30, 60 and 90 days were registered in each phase and in each grow-out system. Results showed that the grow-out system did not affect survival and growth of C. gasar in the juvenile and adult phases. The tested densities affected the survival of oysters cultured over time in both phases but did not affect oyster growth. At times analyzed, it was observed positive growth in juvenile oysters grow after 90 days of culture. However, in the adult phase, no growth was observed after 90 days of culture. Oyster yield was higher in the density 3D, in both juvenile and adult phases. These findings contributed to the development of the oyster C. gasar culture.

Skewed sex ratio and gametogenesis gene expression in eastern oysters (Crassostrea virginica) exposed to plastic pollution

Plastic pollution is ubiquitous in the oceans, and numerous studies have documented the dangers from entanglement, ingestion, or exposure to individual xenoestrogenic plastic additives. Plastic degradation in seawater poses a complex threat through sorption of harmful organic pollutants, release of complex chemical ingredients that compose plastics, and fragmentation of plastic pieces. Therefore, a more holistic approach is needed to understand the impact of plastic exposure on marine organisms. The eastern oyster (Crassostrea virginica) is a keystone species that serves as an ecosystem engineer in western Atlantic estuaries. Individuals undergo annual gametogenesis and are able to change sex from one year to the next, typically from male to female. A 1:1 sex ratio is maintained at the population level by younger/smaller oysters that are mostly male and older/larger oysters that are mostly female. As sessile bivalves, oysters are susceptible to damaging effects from plastic that settle on the seafloor. The objective of this study was to quantify changes in survival, growth, sex ratio, and gene expression of eastern oysters in response to polyethylene terephthalate (PET) plastic exposure. Hatchery-produced larvae settled onto either oyster shell or PET plastic and metamorphosed into juveniles. One set of juvenile oysters was reared in an aquaculture facility for three months, and a second set was placed on natural reefs in the St. Mary's River of the Chesapeake Bay for ten months. Juvenile oysters grown on PET showed significantly higher mortality rates and reduced growth than those grown on shell after three months. After ten months of growth and maturation in natural waters, oysters grown on shell were 82% male while oysters grown on plastic had a significantly lower proportion of males (56% male). We measured differential gene expression of eight candidate genes involved in gametogenesis. In general, expression patterns followed expectations based on putative roles in either spermatogenesis or oogenesis. Exposure to plastic did not have a significant impact on gene expression. Small sample sizes limited statistical power to detect small effect sizes. Although not significant, females on plastic had higher median and mean expression than those on shell for all genes, except for the mean expression level of vitellogenin-6 (vit-6). Our findings suggest that plastic exposure alters the sex ratio of first-year oysters, resulting in more females than expected. Additional research into the expression of underlying genes that impact gamete differentiation and therefore sex determination is warranted.

Evidence of deleterious effects of microplastics from aquaculture materials on pediveliger larva settlement and oyster spat growth of Pacific oyster, Crassostrea gigas

Plastic is currently used in aquaculture as a material for settlement and magnification of oyster spats. Plastic weathering and fragmentation under natural conditions can lead to the production of micro and nanoparticles and additive leakage, with potential toxic effects on marine life. This study investigates the effects of the exposure to microplastic (MPs) cocktail derived from aged aquaculture material on oyster pediveliger larvae (Crassostrea gigas). The cocktail was made of high-density polyethylene (HDPE), polypropylene (PP) and polyvinyl chloride (PVC). The concentrations tested were 0, 0.1, and 10 mg MP·L−1. During the 7-day fixation phase, pediveliger larvae (17 days) were exposed to the MP cocktail in laboratory-controlled conditions. After exposure, the success of settlement was significantly lower for larvae exposed to 10 mg MP·L−1 (49 ± 0.9%) compared to control ones (61.8 ± 1.6%). No malformations or metamorphosis abnormalities were observed. Growth of pediveliger and spat stages was monitored up to 11 months. During the first twenty-eight days of development, spat growth was significantly lower for the two MPs exposure conditions (0.1 and 10 mg MP·L−1; respectively −51.8% and −44.4%) compared to control group. Subsequently, the previously exposed oysters grew faster than the control condition, resulting in a significantly greater growth (0.1 and 10 mg MP·L−1: +18.3% and +19.7%) than the control group at the end of follow-up. The nearly one-year follow-up highlighted the potential effects of MPs from aquaculture on larvae and spat of C. gigas.

Seagrass-driven changes in carbonate chemistry enhance oyster shell growth.

Quantifying the strength of non-trophic interactions exerted by foundation species is critical to understanding how natural communities respond to environmental stress. In the case of ocean acidification (OA), submerged marine macrophytes, such as seagrasses, may create local areas of elevated pH due to their capacity to sequester dissolved inorganic carbon through photosynthesis. However, although seagrasses may increase seawater pH during the day, they can also decrease pH at night due to respiration. Therefore, it remains unclear how consequences of such diel fluctuations may unfold for organisms vulnerable to OA. We established mesocosms containing different levels of seagrass biomass (Zostera marina) to create a gradient of carbonate chemistry conditions and explored consequences for growth of juvenile and adult oysters (Crassostrea gigas), a non-native species widely used in aquaculture that can co-occur, and is often grown, in proximity to seagrass beds. In particular, we investigated whether increased diel fluctuations in pH due to seagrass metabolism affected oyster growth. Seagrasses increased daytime pH up to 0.4 units but had little effect on nighttime pH (reductions less than 0.02 units). Thus, both the average pH and the amplitude of diel pH fluctuations increased with greater seagrass biomass. The highest seagrass biomass increased oyster shell growth rate (mmday-1) up to 40%. Oyster somatic tissue weight and oyster condition index exhibited a different pattern, peaking at intermediate levels of seagrass biomass. This work demonstrates the ability of seagrasses to facilitate oyster calcification and illustrates how non-trophic metabolic interactions can modulate effects of environmental change.

Estimating the Influence of Oyster Reef Chains on Freshwater Detention at the Estuary Scale Using Landsat-8 Imagery

Oyster reef chains grow in response to local hydrodynamics and can redirect flows, particularly when reef chains grow perpendicular to freshwater flow paths. Singularly, oyster reef chains can act as porous dams that may facilitate nearshore accumulation of fresh or low-salinity water, in turn creating intermediate salinities that support oyster growth and estuarine conditions. However, oyster-driven freshwater detention has only been confirmed by limited, point-scale observational data, and simplified models. Oyster reef-driven freshwater detention in real ecosystems at the estuary scale remains largely unexplored. In this study, we analyzed the visible bands in 30-m resolution remote sensing (RS) images recorded by the Operational Land Imager aboard Landsat-8 to characterize the freshwater detention effect of oyster reef chains across a set of hydrologic conditions. Our results support prior findings indicating that 30-m resolution RS images recorded by the Operational Land Imager aboard Landsat-8 are useful for analyzing coastal dynamics after atmospheric correction, despite having been originally designed for terrestrial studies. Statistical models of water-leaving reflectance revealed that freshwater detention by oyster reefs was evident across the estuary, with the greatest effect occurring in the region closest to shore. Additionally, statistical modeling results and spatial patterns apparent in the satellite images suggested that reef-driven freshwater detention occurred under high riverine discharge conditions, but was less evident when flow was low. Beyond offering insight on the potential role of oyster reefs as mediators of estuarine hydrology, this study presents a transferable methodological framework for exploring estuarine biophysical feedbacks in blackwater river estuaries using satellite remote sensing.

Insulin-Like Peptide Receptor-Mediated Signaling Pathways Orchestrate Regulation of Growth in the Pacific Oyster (Crassostrea gigas), as Revealed by Gene Expression Profiles.

The involvement of insulin/insulin-like growth factor signaling (IIS) pathways in the growth regulation of marine invertebrates remains largely unexplored. In this study, we used a fast-growing Pacific oyster (Crassostrea gigas) variety “Haida No.1” as the material with which to unravel the role of IIS systems in growth regulation in oysters. Systematic bioinformatics analyses allowed us to identify major components of the IIS signaling pathway and insulin-like peptide receptor (ILPR)-mediated signaling pathways, including PI3K-AKT, RAS-MAPK, and TOR, in C. gigas. The expression levels of the major genes in IIS and its downstream signaling pathways were significantly higher in “Haida No.1” than in wild oysters, suggesting their involvement in the growth regulation of C. gigas. The expression profiles of IIS and its downstream signaling pathway genes were significantly altered by nutrient abundance and culture temperature. These results suggest that the IIS signaling pathway coupled with the ILPR-mediated signaling pathways orchestrate the regulation of energy metabolism to control growth in Pacific oysters.

Nutritional contribution of seaweed Ulva lactuca single-cell detritus and microalgae Chaetoceros calcitrans to the growth of the Pacific oyster Crassostrea gigas

The Pacific oyster Crassostrea gigas (Thunberg, 1793) is the most cultivated bivalve in the world. Nonetheless, the massive production of microalgae as feed represents a substantial cost during laboratory production stages. The use of single cell detritus (SCD) from seaweed Ulva lactuca, cultivated in fish farm effluents, has been proposed as an alternative to microalgae Chaetoceros calcitrans in oyster culture, with the aim of reducing microalgae production costs. Seaweed meal was subjected to an acid and enzymatic digestion process to obtain SCD particles smaller than 20 μm to feed the oysters. Five levels of SCD (w:w) replacing microalgae were evaluated: 0, 25, 50, 75, and 100% in a feeding assay of 5 weeks. At the end of the experiment, growth parameters, condition index, enzymatic activity in the digestive gland (amylase, protease, lipase, and aminopeptidase) were analysed. In addition, during the course of the experiment, the stable isotope ratio of nitrogen (δ15N) was analysed at natural abundance levels in both feed sources and in the mantle tissue of oysters reared on different feeding regimes. Contribution to growth was estimated using an isotope mixing model. Better growth (74 to 94% dry weight gain) and condition indexes (81–75) were observed in oysters fed on experimental regimes having from 0 to 50% substitution of microalgae with SCD, showing no significant differences among them. Oysters under the latter treatments also showed similar enzymatic activity for amylase, alkaline-protease, and lipase. At higher substitution levels of microalgae (75–100%), oysters presented lower growth (13 to 34% dry weight gain) and poor condition indexes (<60); 100% of SCD also elicited higher amylase, alkaline-protease, and lipase activities, whereas (Leu) aminopeptidase N activity was lower. The digestive capacity of lipase was improved in oysters fed with 50 and 75% levels of SCD. Isotopic equilibrium for nitrogen (δ15N) was reached by day 14 in the 50% substitution treatment. Metabolic turnover rates of nitrogen decreased (0.15 to 0.08 day−1) whereas elemental half times in tissue increased (4.1 to 8.2 days) with higher microalgae substitution. Oysters under treatment with 50% microalgae substituted by SCD incorporated similar amounts of dietary nitrogen and dry matter from the microalgal biomass as from SCD to meet the nitrogen requirement for oyster growth. In conclusion, results suggest that SCD from U. lactuca can substitute up to 50% of microalgae C. calcitrans without modifying C. gigas productivity.

The Impacts of Boring Sponges on Oyster Health across Multiple Sites and Tidal Heights

Clionaid boring sponges (Cliona sp.) may affect individual oyster growth and condition while destroying oyster habitats, which can impact restoration efforts and aquaculture for eastern oysters (Crassostrea virginica). Since both oysters and boring sponges are affected by similar abiotic variables (i.e., salinity, tidal exposure), this study explored the potential trade-offs between these abiotic variables and sponge presence on growth, condition index, and survival of oysters using a fully-factorial in situ experiment across 3 sites in coastal GA, USA. Across all sites and tide heights, oysters with sponge grew 20% slower and had 10% lower condition index than oysters without sponge. Savannah State, the site with the lowest salinity (mean ± SD = 24.5 ± 2.1, range 20–28), exhibited growth rates that were 1.4–1.7 times faster than the other two sites, with even sponge-infested oysters exhibiting higher growth rates than uninfested oysters at the other two sites. In addition, growth rates of sponge-infested oysters were reduced by 26% relative to uninfested individuals in the low intertidal tier, but only by 12% in the upper intertidal zone, suggesting the negative impact also varies with tidal location. Therefore, oysters exposed to air for prolonged periods and placed in areas with polyhaline salinity exhibit fast growth and high relative condition index, although salinity may be confounded by other variables (food availability, fouling organisms). This study highlights the importance of considering site and tidal location for oyster restoration and culture.

Pelagic stocks and carbon and nitrogen uptake in a pearl farming atoll (Ahe, French Polynesia)

This study reports the first measurements of nitrogen uptake and new data on carbon fixation (15N/13C incorporation) for two size-fractionated phytoplankton (<2 μm and >2 μm), on organic matter, and phytoplankton stocks in Ahe lagoon. Data were collected between November and December 2017, during the hot season with prevailing trade winds. Ammonium and nitrate uptake data (7.58 to 39.81 and 1.80 to 21.43 μmol N m−3 h−1, respectively) suggest a rapid turn-over of N-nutrients in the water column and show that primary production was largely sustained by recycled nitrogen providing 68% of the pelagic N demand. These results highlight the spatial heterogeneity of the measured processes linked to the local hydrodynamics, exhibiting higher regenerated production in the more exploited southwestern part of the lagoon and a higher proportion of new production in the north. Intense nutrient recycling appears to promote nanophytoplankton production which is critical for pearl oyster growth.

The Effect of Adding Kepok Banana Peels (Musa paradisiaca) to Powder Media on the Growth of White Oyster Mushrooms (Pleurotus ostreatus)

This study aims to see the effect of Kepok banana peel on the growth of white oyster mushroom (Pleurotus ostreatus). The research was conducted in the white oyster mushroom kumbung Sawangan, Depok between February and July 2018. The research method used was an experimental method with a research design using a completely randomized design (CRD). The study was conducted by giving 6 treatments and five replications where there were 3 parameters used, namely fresh weight, dry weight and number of oyster mushroom fruit bodies. The treatments referred to were treatment A (control), B (1 gram of Kepok banana peels), C (3 grams of Kepok banana peels), D (5 grams of Kepok banana peels), E (7 grams of Kepok banana peels), and F (9 grams of banana peel kepok). The results showed that the growth of white oyster mushrooms with D treatment had the most effect on mushroom weight and dry weight with an average of 62.38 grams and 6.35 grams. Whereas the number of fruiting bodies that had the most influence on treatment F was 16.9 grams. The results of the analysis of variance of one white oyster mushroom growth factor on the fresh weight parameter shows that Fhitung (9.46) > Ftable (0,01) (3.90). This means that oyster growth was very significant and the dry weight parameter indicates that Fhitung ( 2.79) > Ftable (0,05) (2.62), which indicates that the growth of oyster mushrooms was significant. The parameter of the number of fruiting bodies showed that Fhitung (5.06) > Ftable (0,01) (3.90), which indicates that the oyster mushroom growth was significant. This proves that offering Kepok banana peels affects the growth of white oyster mushrooms (Pleurotus ostreatus).

Context‐dependent carryover effects of hypoxia and warming in a coastal ecosystem engineer

Organisms are increasingly likely to be exposed to multiple stressors repeatedly across ontogeny as climate change and other anthropogenic stressors intensify. Early life stages can be particularly sensitive to environmental stress, such that experiences early in life can “carry over” to have long‐term effects on organism fitness. Despite the potential importance of these within‐generation carryover effects, we have little understanding of how they vary across ecological contexts, particularly when organisms are re‐exposed to the same stressors later in life. In coastal marine systems, anthropogenic nutrients and warming water temperatures are reducing average dissolved oxygen (DO) concentrations while also increasing the severity of naturally occurring daily fluctuations in DO. Combined effects of warming and diel‐cycling DO can strongly affect the fitness and survival of coastal organisms, including the eastern oyster (Crassostrea virginica), a critical ecosystem engineer and fishery species. However, whether early life exposure to hypoxia and warming affects oysters' subsequent response to these stressors is unknown. Using a multiphase laboratory experiment, we explored how early life exposure to diel‐cycling hypoxia and warming affected oyster growth when oysters were exposed to these same stressors 8 weeks later. We found strong, interactive effects of early life exposure to diel‐cycling hypoxia and warming on oyster tissue : shell growth, and these effects were context‐dependent, only manifesting when oysters were exposed to these stressors again two months later. This change in energy allocation based on early life stress exposure may have important impacts on oyster fitness. Exposure to hypoxia and warming also influenced oyster tissue and shell growth, but only later in life. Our results show that organisms' responses to current stress can be strongly shaped by their previous stress exposure, and that context‐dependent carryover effects may influence the fitness, production, and restoration of species of management concern, particularly for sessile species such as oysters.

Mitigation of Eutrophication and Hypoxia through Oyster Aquaculture: An Ecosystem Model Evaluation off the Pearl River Estuary.

Shellfish aquaculture has been proposed to abate eutrophication because it can remove nutrients via shellfish filter-feeding. Using a three-dimensional physical-biogeochemical model, we investigate how effective oyster aquaculture can alleviate eutrophication-driven hypoxia off the Pearl River Estuary. Results show that oysters reduce sediment oxygen consumption and thus hypoxia, by reducing both particulate organic matter directly and regenerated nutrients that support new production of organic matter. The hypoxia reduction is largest when oysters are farmed within the upper water of the low-oxygen zone, and the reduction increases with increasing oyster density although oyster growth becomes slower due to food limitation. When oysters are farmed upstream of the hypoxic zone, the farming-induced hypoxia reduction is small and it declines with increasing oyster density because the nutrients released from the farm can increase downstream organic matter production. An oyster farming area of 10 to 200 km2 yields a hypoxic volume reduction of 10% to 78%, equaling the impact of reducing 10% to 60% of river nutrient input. Our results demonstrate that oyster aquaculture can mitigate eutrophication and hypoxia, but its effectiveness depends on the farming location, areal size, and oyster density, and optimal designs must take into account the circulation and biogeochemical characteristics of the specific ecosystem.

Cathodically protected steel as an alternative to plastic for oyster restoration mats

Oyster populations in many coastal areas have decreased as a result of overharvesting and habitat degradation. In order to help restore oyster populations and natural water filtration, many restoration efforts utilize plastic mesh to reseed oyster reefs. However, plastics do not break down or mineralize in seawater, instead they break down into smaller and smaller pieces eventually becoming what is termed microplastics. One alternative may be the use of a cathodically protected steel which develops a mineral accretion layer and enhances calcareous marine growth. In order to test the efficacy of this material for oyster restoration and its ability to promote oyster growth, a field experiment was designed to compare it to traditional plastic mats at three locations. At all sites, the steel mats were able to promote the recruitment of oysters at a rate equal to or greater than the plastic mats. The amount of mineral accretion and the total number of oysters present on the mats were dependent on the environmental and ecological conditions of the test site. The steel mats were easy to work with, provided sufficient oyster settlement, and gain in weight over time. This makes them an attractive alternative to plastic oyster restoration mats, with implications for creating artificial reefs or living shorelines.

Growth and survival of Pinctada martensii (Dunker) postlarvae under concurrent variation in temperature, algal ration and stocking density

The growth and survival of pearl oyster postlarvae (Pinctada martensii) were studied under laboratory conditions. A face-centered central composite experimental design was used with 3 factors, temperature (18―34 °C), algal ration (5―100 cells μL―1) and stocking density (0.2―3.0 in.. mL―1). Pearl oyster postlarvae were fed with a 1: 1 mixture of Isochrysis zhanjiangensis and Pavlova viridis during the experiment. Temperature, algal ration and stocking density exerted significant effects on postlarvae growth and survival. Temperature was the most dominant factor, followed by algal ration. Significant interactions among the 3 factors were inhibitory (negative) to growth and survival. Within the ranges of 3 factors tested, growth and survival increased with 3 factors up to a pinnacle, beyond which they began to decrease. The conditions required for the optimal growth could also ensure the optimal survival. Through simultaneously maximizing the reliable models of postlarvae growth and survival, the optimal trifactor culture conditions of 27.86 °C/50.85 cells μL―1/1.60 in.. mL―1 were obtained. These optimal culture conditions resulted in the greatest specific growth of 8.95 % d―1 (95 % confidence interval, 8.73―9.18 % d―1) and survival of 21.5 % (95 % confidence interval, 21.06―21.94 %), with desirability as high as 99.6 %. Due to practical feasibility, the optimal trifactor culture conditions were recommended to apply to practice to maximize growth and survival of the pearl oyster P. martensii postlarvae martensii. This study provides valuable new insights into refining the production efficiency of the pearl oyster postlarvae.

Spatio-Temporal Variation in Growth Performance and Condition of the Winged Pearl Oyster Pteria penguin

Environmental conditions can strongly influence the growth performance of pearl oysters and affect pearl farm production schedules. Growth and condition index (CI) of two age cohorts of Pteria penguin were measured for 13 months to investigate differences in growth performance between four culture sites within the northern (Vava’u) and southern (Tongatapu) island groups of the Kingdom of Tonga. Environmental conditions were also measured at culture sites and used to explore potential effects on oyster growth and condition. Between island groups, growth performance of P. penguin was superior at northern sites and was most strongly related to higher water temperatures at these sites. Within the southern island group, growth performance varied significantly between sites and may be driven by differences in wave energy. Monthly growth rates (GM) of P. penguin also showed significant temporal variation related to age and environmental conditions. This study demonstrated significant variation in the growth performance of P. penguin at latitudinal and local scales and suggests that in oligotrophic marine environments with minimal terrestrial inputs, such as Tonga, water temperature and wave exposure may be the primary environmental conditions influencing the growth performance of P. penguin. This study therefore recommends that optimal culture sites for P. penguin in Tonga are characterized primarily by warmer water temperatures (25–30°C) and low wave exposure (&amp;lt;15 joules m2 day–1). Culture of P. penguin at sites with more suitable environmental conditions enables pearl production to begin up to 34.2 % (6.5 months) earlier than at less-suitable sites and this may greatly influence mabé pearl farm profitability and feasibility.