Aquaculture Center Switches to Vibratory Screening of Oysters
CAPE MAY, NJ — Seed oyster production at The New Jersey Aquaculture Innovation Center at Rutgers University (AIC) is a large-scale operation by any measure. The 22,000 sq ft (2046 sq m) facility next to the Cape May Canal in Cape May, NJ, produces disease-resistant, fast-growing seed oysters for commercial grow-out, research, and restoration of native oyster beds. Annual production is more than 10 million seed oysters to meet customer specifications, in particular those of East Coast commercial oyster producers.
Sorting by size for counting and redistribution in the "nursery" is a frequent and essential operation in the seed oyster production cycle. Originally, sorting with a 2 X 2 ft (0.6 X 0.6 m) hand-held screener was a daunting task, according to Matt Neum.
Ian, lead researcher and technician for hatchery production at the AIC. "It took all day for a three- or four-man crew to complete each round of sorting," he explains. "And the added weight of seawater mixed with the product took a huge toll on everyone's shoulders."
The search for a better way led to replacing manual screening with a dramatically more productive, mechanical 30 in. (762 mm) diameter VIBROSCREEN circular vibratory screener manufactured by Kason Corporation of Millburn, NJ. With one operator now completing each day's sorting in less than two hours, the equipment paid for itself in the first season.
Aquaculture innovation at Rutgers
As the leading research and education hub for New Jersey's aquaculture community, the AIC studies and teaches methods of commercially raising seafood, benefiting a wide range of stakeholders — from aquaculture entrepreneurs and restoration specialists, to governmental resource managers and non-governmental organizations, to the fishing industry.
Among AIC’s projects is mass culturing of micro-algae using algal photo-bioreactors to increase yields of biochemical compounds with high potential value for food and pharmaceutical applications, as well as for shellfish (i.e., oysters) cultured for human consumption.
Seed oyster production — mimicking and accelerating nature
The Eastern Oyster (Crassostrea virginica) is among the shellfish species successfully cultured at the AIC using a micro-algae diet. The facility has been producing seed oysters from this strain since 2008. Seed oysters are small oysters, about 2–25 mm (0.8–1 in.) long, provided primarily to oyster growers for the half-shell market. They are also used to restore natural oyster populations or natural ecosystems, and for research.
While natural oyster populations in the Delaware Bay spawn in late June or early July, at the AIC broodstock (adult oysters which produce baby oysters) are moved into temperature-controlled tanks in January, given plenty of microalgae food, and are ready to reproduce by late February. Fertilized eggs become microscopic oyster larvae, which are raised in filtered, sterilized seawater in 1,500 gal (5,678 l) culture tanks. After two to three weeks, the larvae metamorphose into juvenile oysters called "spat." The growth cycle from spat to seed oysters takes several weeks to several months, depending on their intended size. Where oysters naturally clump together, AIC's methodology yields individual oyster "singles" suitable for the half-shell market.
The first sorting operation in this growth cycle occurs when juvenile oysters are still under a millimeter in size, after spending about two weeks in filtered seawater in large "downweller" tanks, with regular feedings of cultured algae. Manual sorting in the downweller tank with a 1 X 2 ft (0.3 X 0.6 m) hand-held screen works efficiently for distributing the oysters at this tiny size to a series of 18 in. (457 mm) diameter "upweller" silos fed by raw seawater and whatever food it naturally contains.
As they grow, juvenile oysters are regularly sorted by size, counted, and redistributed in the upwellers to optimize growth and survival. The number of seed oysters placed in individual silos is based on volume determined by sub-samples. At full production late in the growth cycle, the AIC has 130 upwellers in use.
The AIC supplies some growers with seed oysters as small as 2 mm for cultivation in their own upweller systems. The remaining oysters at AIC grow at about a millimeter per week. Sorting to prevent overcrowding and to separate oysters by size becomes more frequent as they grow, and — with the increasing weight of the oysters — more challenging to sort with hand-held screens.
Looking for a better way
"More frequent" means dozens of sorting operations as seed oysters grow to 8 mm size and larger, and daily sortings during the last weeks of the growing cycle. Faced with staff diversions for sorting manually — as well as aching shoulders — the AIC's Neuman took the lead in researching options. "We looked at every supplier, and learned that pretty much every clammer in New Jersey uses Kason screening equipment," he says.
He credits independent representative Chris Dugan for streamlining his evaluation and purchase. While visiting Kason's headquarters and laboratory, Neuman and an AIC staff engineer observed the circular vibratory screener during test runs using a batch of seed oysters in different sizes. Neuman decided on a 30 in. (762 mm) diameter model for optimizing sorting yields and processing time.
Problem solved with circular vibratory screener
The K30-1-SS VIBROSCREEN circular vibratory screener now in use at the AIC is a single-deck model with four interchangeable screens sized at 8mm, 6mm, 4mm, and 2mm mesh. Typical sorting operations involve several screen changes. Neuman also looked at Kason's multi-deck models for sorting multiple sizes simultaneously but determined "they were more than we needed for the scale of our operation."
The screener's corrosion-resistant stainless steel construction is essential for the AIC's salt water environment. Because salt water is also corrosive to tools, Neuman specified a clamshell lid, quick change option to allow easy screen changeover without tools.
The deck of the circular vibratory screener is suspended on springs that allow screens to vibrate freely. An imbalanced-weight gyratory motor creates multi-plane inertial vibration that controls the flow path of material on the screen surface and maximizes the rate at which undersize seed oysters pass through the screen while oversize ones migrate in controlled pathways to the periphery and through the discharge spout into a small tub. Undersize oysters exit from the lower discharge into a separate container.
Flow patterns can be adjusted by repositioning a bottom eccentric weight relative to the top eccentric weight. The AIC unit was delivered with factory settings of 45 degrees, which Neuman fine-tuned to 60 degrees, maximizing efficiency for his application. "Adjusting the angle on a bottom weight involves adjustment of only one bolt," he says.
Oysters are hand-fed slowly from a large funnel into the center of the screener at a rate of about a liter (0.264 gal) over 30 seconds. Neuman's team is developing a new feed method using a corkscrew design to speed up this process while controlling the flow rate for a steady feed.
The AIC set up its vibratory screener on a mobile cart and plugs it in where needed. The screener allowed Neuman and his team to discard their old upweller hand-held screens, making life easier on the seed oyster production line. He says, "The new screener has freed seasonal staffing for other tasks, and it allows us to get bigger seed oysters out earlier, which our growers like."
NJ Aquaculture Innovation Center at Rutgers University
The operator scoops seed oysters into the circular vibratory screener. Oysters larger than the mesh openings discharge into the container in foreground. Smaller oysters pass through the screen and discharge from the lower deck into a separate container.
These 2 cm size oysters are ready for commercial farmers to place in the Delaware Bay to finish growing out.
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