Commercial Oyster Farms Research Articles

Efficacy and effects of three desiccation intervals on biofouling and Eastern oysters (Crassostrea virginica) at three commercial oyster farms in the Chesapeake Bay, MD

Oyster aquaculture is a growing industry both globally and nationally. Productive coastal waters support high-density containerized culture of oysters, but also sustain many other macrofaunal species which can settle on oysters and cage materials (biofouling), causing problems for oyster farms. Desiccation, or periodic air drying, of farm infrastructure and oysters has been used to control colonization and growth of biofouling organisms on oyster farms worldwide, but the minimum necessary interval of desiccation has yet to be thoroughly assessed. Given the known stresses to oysters associated with periods of emersion, an understanding of the effects of different weekly desiccation intervals is needed. This study investigated the use of desiccation in controlling biofouling and the associated impacts to oyster growth and mortality at three oyster farms within Maryland's portion of the Chesapeake Bay, including one low salinity site in an upper Chesapeake Bay tributary and two sites in the middle portion of the Chesapeake Bay. Three desiccation treatments were applied at each site: control (no desiccation), 8 consecutive hours of desiccation per week, and 24 consecutive hours of desiccation per week. Oysters were deployed between July–December 2018 and the percent valve coverage by individual biofouling species was monitored, along with worm abundance, oyster growth, and oyster mortality. Total biofouling was significantly greater on non-desiccated oysters but minimal differences in biofouling coverage were observed among oysters desiccated for 8 or 24 h weekly. Seven biofouling species were observed among the sites, and all except for a single macroalgal species (Ulva intestinalis) were controlled by the 8- and 24-h desiccation treatments. No clear treatment-derived differences in oyster growth or mortality were observed. Overall, results suggest desiccating weekly for either 8 or 24 consecutive hours may be a suitable biofouling management strategy for gross biofouling reduction on oyster farms.

Pathology associated with summer oyster mortality in North Carolina

Expansion of oyster aquaculture industries throughout the southeastern United States has been met with recurring, often catastrophic mortality events, limiting capacity for industry growth. Numerous etiologies have been proposed, including regional extensions of enigmatic mortality episodes that have been observed in triploid oysters in Chesapeake Bay as well as the mid-Atlantic and Gulf of Mexico coasts. To better understand associated pathology and begin to address etiology, we conducted a longitudinal histological survey at a commercial oyster farm in North Carolina that experiences recurring mortality. Our survey was timed with dramatic mortality experienced across North Carolina oyster farms, with farmer-reported mortalities exceeding 90% of near market-sized oysters. Surveyed oysters presented with persistent digestive diverticula pathology in the weeks leading up to and through the mortality event and suggest that mortality was ultimately a result of nutritional deprivation. These observations are inconsistent with pathology associated with recent observations of triploid associated mortality in eastern oysters, however similar diverticula pathology has been observed with summer mortality in Pacific oysters. Expanded research into comparative etiology throughout impacted regions and across oyster species will help address oyster mortality events worldwide. Further efforts for eastern oysters are now being concentrated toward understanding variation in resistance to recurring mortality events across commercially available oyster lines.

Seasonal and spatial patterns of mudblister worm Polydora websteri infestation of farmed oysters in the northern Gulf of Mexico

Mudblister worms Polydora websteri bore holes into oyster shells, and oysters respond by secreting extra layers of shell, creating a mudblister. When shucked, mudblisters can burst and release anoxic mud. Thus, infestation devalues oysters, particularly on the half-shell market. This study quantified oyster condition index and worm abundances over 2 full growing seasons at commercial oyster farms on the US Gulf of Mexico coast, and our results indicate that oyster growth rate, manipulated through ploidy and stocking densities, had little effect on worm infestation. Larval spionid worms were found year-round. Larval abundances were slightly higher within than away from farms, and larval size distributions were skewed toward smaller larvae within the farms, suggesting that farms may be a source of larvae. Triploid oysters had higher or comparable condition index values to diploids, but during summer months, when worm infestation was high, worm infestation was not correlated with condition index. Previously infested shells deployed at farms became more infested than uninfested shells at moderate infestation levels, but re-infestation was influenced more by farm location than by previous infestation condition. Higher infestation at a farm with more variable salinity as well as higher infestation in 2017 when salinity was lower suggest that salinity may be a potential driver of mudblister worm infestation. Results indicate that oyster farmers on this coast should use desiccation to treat oysters for mudblister worms frequently during the summer, but that manipulating stocking density or ploidy is unlikely to be effective in preventing mudblister worm infestation.

A field test investigating the influence of brood stock origin and ploidy on the susceptibility of Crassostrea virginica to “triploid mortality” in the Chesapeake Bay

Mass mortalities of cultured triploid Crassostrea virginica in late spring, or “triploid mortality,” have been reported on farms in the Chesapeake Bay since 2012. Typical causes, such as disease or poor husbandry, were not responsible, and mortalities occurred without clear signs of biological or physical stressors. Previous comparisons of the effects of genetic origin on triploid mortality have been uncontrolled, initiating this investigation of the effect of brood stock source and ploidy on triploid mortality. Four triploid and four diploid crosses, produced by crossing different combinations of brood stock of Virginia, Louisiana, and Maine origin in early 2015, were tested at four commercial oyster farms in Virginia throughout 2016. From February to November, oysters from all crosses and sites were regularly sampled, and growth (shell height), condition (meat weight), and pathology were monitored, as were environmental conditions. Compared to diploids made from Virginia brood stock, diploids with Maine genetic origin had high mortality, and diploid Virginia-Maine hybrids exhibited mid-parent heterosis. A triploid mortality event occurred in late spring at only one site and only affected the triploid crosses. Evidence for substantial disease pressure from Haplosporidium nelsoni or Perkinsus marinus or of especially stressful environmental conditions based on temperature, salinity, pH, and dissolved oxygen was absent during the triploid mortality event. At 18 months, shell height was similar in the diploids and triploids with the most similar genetic origin. Triploids maintained meat weight through the summer, while meat weight in diploids dropped sharply. Triploids may be especially susceptible to late spring mortality events in the Chesapeake Bay, which justifies their classification as “triploid mortality” and warrants further investigation on traits in triploids that may affect susceptibility.

Elevated mortalities of triploid eastern oysters cultured off-bottom in northern Gulf of Mexico

After unusually high mortalities of eastern oysters (Crassostrea virginica) were reported at commercial farms in Alabama during the summer of 2016, this study was conducted to determine whether the mortalities were reproducible, site-specific, and linked to ploidy. Seed of triploid and diploid oysters were deployed in November 2016 at four sites adjacent to commercial oyster farms in Alabama: Mobile Bay, Navy Cove, Grand Bay, and Dauphin Island. Mortality, growth, condition index, gametogenesis, and Perkinsus marinus infection intensity were monitored until October 2017. Mortalities were significantly greater in triploids by the end of the study, but the magnitude and timing of mortalities differed among sites. Triploids and diploids at Mobile Bay experienced severe mortalities in summer (100% in about two months), and the start of this mortality event was delayed in diploids. Elevated triploid mortalities were observed at Navy Cove (77%) and Dauphin Island (60%) in late spring through fall, while oysters at Grand Bay suffered the lowest triploid mortalities (34%). Possible causes of the high mortalities included environmental conditions, such as prolonged periods of low salinity and high temperature in summer at Mobile Bay, as well as, heavy P. marinus infection intensities in fall at Grand Bay. Despite elevated triploid mortalities, both triploids and diploids showed faster shell height growth at Navy Cove and Dauphin Island. Conversely, the site with minimal triploid mortality had the slowest shell height growth overall, but the greatest difference in shell height between triploids and diploids. Analysis of gametogenic stages and gonad-to-body ratios over the summer revealed diploids primarily in the spawning stage and triploids showing delayed and diminished gametogenesis, with lower gonad-to-body ratios than diploids.

Two epizootic Perkinsus spp. events in commercial oyster farms at Santa Catarina, Brazil.

Perkinsus spp. have been detected in various bivalve species from north-east Brazil. Santa Catarina is a South Brasil state with the highest national oyster production. Considering the pathogenicity of some Perkinsus spp., a study was carried out to survey perkinsosis in two oyster species cultured in this State, the mangrove oyster Crassostrea gasar and the Pacific oyster Crassostrea gigas. Sampling involved eight sites along the state coast, and oyster sampling was collected during the period between January 2013 and December 2014. For the detection of Perkinsus, Ray's fluid thioglycollate medium (RFTM) and histology were used, and for the identification of the species, PCR and DNA sequencing were used. Perkinsus spp. was found by RFTM in C.gigas and C.gasar from São Francisco do Sul. This pathology was also detected in C.gasar from Balneário Barra do Sul both, by RFTM and histology. Perkinsus marinus was identified in C.gigas and C.gasar from São Francisco do Sul and Perkinsus beihaiensis in C.gasar from Balneário Barra do Sul. This is the first report of P.marinus in C.gigas from South America. Results of this preliminary study suggest that both oyster species tolerate the species of Perkinsus identified, without suffering heavy lesions.

Effects of Waste Disposal: Temporo-Spatial Patterns of Bacterial Community Composition on Oyster Farms in Estuary Regions of Bahia, Brazil

Oyster farming in Brazil is limited by a number of problems, such as lack of sanitary control in production areas. The aim of the present study was to use the multiple correspondence analyses (MCA) to correlate the microbiological quality of oyster production in the state of Bahia, Brazil, by linking it to environment parameters. Samples were randomly collected from 15 different oyster farms, from two different cultivation techniques (lantern and pillow baskets), corresponding dry and rainy periods. Twenty eight samples were collected, stored at 5 °C and send to the laboratory. The analyzed environmental parameters were pH, temperature, dissolved oxygen, salinity, and polluting sources like sewage outflow. Coliforms were counted at 35 °C and 45 °C: Escherichia coli, Salmonella sp., Pseudomonas sp. and Aeromonas sp. Statistical analyzes were performed using the data sizing reduction procedure (MCA), through the SPPS Program, version 18.0. A positive relationship was found between the dry period and lantern net farming with poor sanitary conditions. MCA allowed the analyzed commercial oyster farms to form three distinct groups, helped define the intervention strategies of these commercial oyster farms and provided the basis for implementing health protection measures.

Effects of ploidy and gear on the performance of cultured oysters, Crassostrea virginica: Survival, growth, shape, condition index and Vibrio abundances

In 2011 at a commercial oyster farm in Grand Bay, Alabama, we conducted an experimental field test of ploidy (triploids and half-sibling diploids) and gear type: 1) LowPro bottom cages (Chesapeake Bay Oyster Company — CBOC), 2) adjustable long-line baskets (BST, Ltd.), 3) OysterGro floating cages (Ketcham Supply) and 4) floating bags (CBOC). Eastern oysters, Crassostrea virginica, were deployed in the four gear types from May 5 to October 11 (166days), with replicate bags of each ploidy assigned to each gear type (n≥3). Survival, growth (both shell dimensions and weight measurements, adjusted to account for differences between the ploidies at the onset of the experiment), and oyster shape (‘fan’ & ‘cup’ ratios) were quantified at the conclusion of the experiment. Condition indices were determined in both August and October, while abundances of the bacteria, Vibrio vulnificus and V. parahaemolyticus, were quantified in August and September. Survival was equivalent between ploidies, but differed significantly among gear types with poor survival in the bottom cages (affected by the oyster drill, Stramonita haemastoma). In terms of growth, triploids grew better than diploids for almost all metrics. Among gear types, growth was poorest in the bottom cages. For dry tissue weight, there was a significant ploidy by gear interaction; within floating bags, there was no difference between triploids and diploids, but triploids had higher dry tissue weight than the diploids in any other gear type. No differences were observed in fan ratios among treatments, but triploids had significantly higher cup ratios than diploids. For condition index, unexpectedly there was no clear pattern explained by ploidy in the August sample. In October, however, triploid condition index exceeded the diploid condition index. Finally, there was no significant effect of gear or ploidy on the abundances of the two Vibrio spp. assessed, but there was a tendency for these abundances to be lower in triploids than diploids. This study adds to the growing body of published evidence of the benefits of genetic triploidy to the Eastern oyster aquaculture industry. We conclude that oyster farmers could expect to benefit from raising triploid oysters, but that the magnitude of these benefits will depend on the type of gear selected.

An Atlantic infaunal engineer is established in the Northeast Pacific: Clymenella torquata (Polychaeta: Maldanidae) on the British Columbia and Washington Coasts

The Northwest Atlantic bamboo worm Clymenella torquata, believed to have been imported with commercial oyster culture, was last formally reported from the American Pacific coast more than 30 years ago from a single location. We report here that it is broadly distributed in British Columbia and is now established in Washington. In Samish Bay, Washington, this tubiculous infaunal worm creates a spongy, porous substrate that has proved detrimental to commercial oyster farms by causing the oysters to sink into the sediment and suffocate. Little is known about the ecological or economic impacts of this invasion in the Pacific Northwest.

Impact of varying cultivation depths on growth rate and survival of the European flat oyster Ostrea Edulis, L.

European oysters, Ostrea edulis, L. were kept in plastic mesh cages at three depths at two commercial oyster farm sites within Mali Ston Bay, Croatia, between April 2004 and August 2005. Oysters from each cultivation depth were sampled at both sites at approximately 3-month intervals for growth parameters, mortality rate control and condition index calculation. The growth parameters of experimental groups from both sites only revealed a considerable distinction for shell length growth (P=0.011). Total weight gain and soft tissue weight gain were the highest for groups cultivated in the middle of the water column as compared with the top and bottom positions for both studied sites. Survival rates were very similar in all experimental groups, situated at different depths of both experimental sites, and differences in the mortality peak during the summer months were not significant. Condition indices fluctuated throughout the trial period and were the highest in spring and summer, with a reduction during the winter months and improvement again in the spring.

Growth rates of the venus clam Ruditapes largillierti grown under experimental culture conditions in New Zealand

The growth rates and survivorship of juvenile and adult Ruditapes largillierti grown in mesh bags on two commercial oyster farm leases were assessed from March 1997 to January 1998. Clams in Okiwi Bay grew faster than those in the Mahurangi Harbour. Subtidal clams in Okiwi Bay grew faster than intertidal clams (1.6 and 1.2 mm month −1, respectively). However, in the Mahurangi Harbour, clams grew faster in the intertidal: growth rates ranged from 1 to 1.2 mm month −1. This was attributed to increased siltation occurring in the subtidal regions around the oyster farms in the Mahurangi Harbour. No growth was recorded for adult clams ongrown in the Mahurangi Harbour. Survivorship of both juvenile and adult clams was generally high; initial mortalities (∼10%) in the Mahurangi Harbour were attributed to the stress of transportation. However, intertidal clams in Okiwi Bay experienced 20% mortality in January 1998, probably resulting from desiccation due to the high temperatures that occurred during this period. The results presented here suggest that R. largillierti may be suitable for commercial culture.

Raw oyster consumption and outbreaks of viral gastroenteritis in New Zealand: evidence for risk to the public's health.

To investigate the relationship between consumption of raw Pacific half shell oysters and outbreaks of Norwalk-like virus (NLV) gastroenteritis in Auckland in the last third of 1999. Ten outbreaks were investigated as retrospective cohorts using standardised questionnaires relating to food and drink exposures. Trace back of oysters and site inspections of implicated commercial growing areas were performed. Virological analyses compared oysters linked to outbreaks and faecal samples from cases. Eighty-six cases were identified, of whom 32 (37.2%) were confirmed NLV positive on faecal analysis. The summary risk estimate for illness among oyster consumers for all outbreaks was RR 8.23 (95% CI 4.55-14.90; p< 0.001) and in five of seven outbreaks permitting statistical analysis, the risk for those consuming raw oysters was greater than five-fold that of non-consumers. NLVs were identified in two batches of oysters from different growing areas, implicated in four outbreaks. In both the strain (Genogroup II/3 'Mexico-like virus') from cases matched that in the oysters from the same harvest batch. The epidemiological and virological evidence implicates oysters as the source of a number of outbreaks of NLV gastroenteritis. This is the first time NLVs have been identified in commercially farmed Pacific oysters in New Zealand. Sewage effluent from recreational boats was the likely source of faecal contamination of growing waters in one site. Combined use of virological and epidemiological methods have proved invaluable in investigating NLV outbreaks. Further research is needed into enteric viral contamination of commercial oyster farms.

The evaluation of two treatments to reduce mud worm ( Polydora hoplura Claparède) infestation in commercially reared oysters ( Crassostrea gigas Thunberg)

The quality of commercially reared oysters is markedly lowered by infestation with the burrowing polychaete Polydora. Two methods to eradicate Polydora hoplura in the Pacific oyster Crassostrea gigas were tested in the laboratory and followed by field trials. Oysters infested with Polydora, obtained from a commercial oyster farm (Algoa Bay, South Africa), were (1) exposed to fresh water for 12 h and, (2) exposed to heated (70 °C) sea water for 40 s. After treatment the oysters were placed in plastic crates and returned to the sea for a recovery period of 2 months. The crates were suspended from long lines following the practice of the commercial oyster farm. Fresh water and heat treatments did not significantly increase oyster mortality. The Polydora infestation per oyster was significantly reduced by both treatments ( F 0.05(1),2,18 = 27.663, P < 0.05). Heat treatment yielded the lowest average infestation (1.13 polydorids per oyster) when compared with the average infestation of the fresh water treatment (1.59 polydorids per oyster) and the untreated control (3.05 polydorids per oyster). Treatment of commercially reared Crassostrea gigas with fresh water or heated sea water seems to be a practical means to reduce Polydora infestation.

Light and Electron Microscope Study of Urosporidium cannoni N. Sp., a Haplosporidian Parasite of the Polyclad Turbellarian Stylochus sp.

ABSTRACT. Urosporidium cannoni n. sp. was found in most tissues of a polyclad turbellarian associated with commercial oyster farms from eastern Moreton Bay, Australia. Mature spores had 11 to 13 episporal tails at irregular intervals around the spore wall. The spherule, prominent in development, dispersed as the spores matured. Spores were found within cysts that contained up to a few hundred spores at the same stage of development. Mature cysts occurred in groups and their masses of dark, golden spores resulted in black patches on the otherwise red‐brown turbellarian. The infection may have commercial importance.