 |
|
|
|
|
Summary In 1991, N.C. Sea Grant College Program launched an intensive two-year effort to develop a method for culturing oysters using new materials and techniques. The project was a joint effort between Sea Grant's Marine Advisory Service and Tipper Tie Inc., a private corporation. The project was continued for a third year by Sea Grant to refine some of the procedures. Help from the N.C. Division of Marine Fisheries and 30 private oyster growers in North Carolina enhanced the project's success. This report describes the evolution of the project from first ideas to final product, called the chub ladder. The chub ladder is an innovative oyster growing system that resembles a ladder floating on the water. Each rung of the ladder is a tubular mesh bag (chub) containing oysters and a float. The ends of the chubs are closed with aluminum clips that fasten them to two parallel stabilizer ropes or the legs of the ladder. The entire apparatus is attached to two parallel longlines that position it over a lease. The system has a number of advantages over traditional bottom culture and is suitable for commercial-scale culture of oysters. The chub ladder is different from other oyster growing methods. It lifts oysters off the estuarine bottom and floats them near the water's surface where supplies of oxygen and food are plentiful. The system utilizes a single stocking of seed sealed in the growing containers until harvest. It is adaptable to onshore assembly-line construction and to a labor-saving routine of air drying for fouling control. It is mobile and easily retrievable for harvest. The chub ladder can be constructed with either disposable or reusable materials depending on oyster farming strategy and economic sensitivity analyses. Oysters in chub ladders appear to grow to market size in about half the time it takes oysters in the wild. And the faster growing chub oysters tend to be healthier and less likely to suffer from diseases such as Dermo. Results of the development effort at 31 sites in North Carolina showed an average of 86 percent of oysters (Crassostrea virginica) grew from 22 millimeters to market size (> 75 mm) in 18 months. The best sites produced 100 percent market size oysters for the same time. Mortality was negligible the first four months and averaged 82 percent during the last 14 months. 1 Current address: 1199 Prides Run, Lake In the Hills IL 60102 INTRODUCTION The chub ladder shellfish culture method is the final product of an informal two-year cooperative effort between Tipper Tie Inc. and N.C. Sea Grant. Tipper Tie is a private manufacturer of packaging materials and equipment. Its principal products are aluminum clip closures for food packages (turkeys, hams, etc.) and associated equipment. N.C. Sea Grant Marine Advisory Service is a university-based marine technology extension program. It performs applied research and transfers information to appropriate users. The project was successful because the participants were interested and committed and there was no formal written plan and contract to constrain the research. The flexibility of an informal work plan was critical to the project when there was need to change materials and fabrication methods in response to early results. Had the tests been tied to a single design, the project would have run longer and may not have been as successful.Tipper Tie's manager of product development contacted N.C. Sea Grant's specialist in shellfish culture to discuss the potential for Tipper Tie products in the aquaculture industry. Product samples were shipped and mutual site visits followed. Further discussions led to several ideas for combining modern packaging technology with current shellfish culture methods. Tipper Tie provided engineering expertise and purchased all materials and equipment for the project, including shellfish seed stock. N.C. Sea Grant provided aquaculture expertise, labor and a research sanctuary in Bogue Sound, N.C., for the tests.Several goals were developed early and maintained throughout the project.
First Year The initial designs were configured to resemble other currently used oyster culture cages. Mesh materials were knitted in a tubular form with a width of 21-24 inches. (These were later abandoned in favor of smaller width mesh sizes.) Initial mesh openings tested were 3/16, 3/8 and 5/8 inch. Testing began with bottom culture because of the perceived difficulty associated with obtaining water column leases and the $500-per-acre annual fee to use the water column.Tipper Tie used two primary fibers in its mesh knitting operations: 5-mil polyethylene (HDPE) flat yarn and 640 denier polyester (PET) multifilament yarn. These materials were knitted to our specifications for initial tests. Oval-shaped rings of rigid aluminum wire (a Tipper Tie product) were made and inserted into the tubular mesh to act as a frame for the chubs and to hold meshes open for water exchange. Chub is an industry term for a package closed at the ends. A manually operated Tipper Tie clipper was used to close the chubs with aluminum clips. Usually chubs are arranged and produced in a linear fashion (for engineering efficiency) similar to sausage links. The first chubs were made linearly, loaded with oyster seed and placed on the bottom of the research sanctuary. The clipper was used to close the end of each chub and to begin the next chub. Some of the mesh was too bulky for the manual clipper. Cable ties were used for these materials until a larger pneumatic clipper could be obtained. Twenty-five thousand oyster seed (20-30 mm) were purchased from Aquacultural Research Corp. in February 1991. The seed were held in flowing water raceways 2 feet wide x 12 feet long x 2 inches deep and supplied with seawater by a 1 1/2 hp centrifugal pump until used in tests. It soon became obvious that the original design and materials would not be sufficient as a bottom oyster grow-out method. Wave action over the research sanctuary, especially at low tides, washed sediment over oysters in chubs and bent the aluminum wire frames. This early result led to the development of off-bottom testing. Chubs were made linearly and attached to parallel 1/4-inch diameter polypropylene stabilizer ropes. Flotation was provided by stringing net floats onto the stabilizer ropes, which were tied to screw-type anchors at both ends. Wave action and low-strength mesh materials caused this second configuration to fail. The lightweight food-grade plastic mesh was chafed and broken by the wire and wave action. Also, blue crabs appeared to easily pick holes through the mesh enabling them to preyon seed oysters. Crabs eventually became the primary concern in choosing mesh materials for chubs.Oysters were observed to group into a pouch instead of remaining evenly distributed over the bottom of larger chubs. Chubs were downsized to a smaller 12-inch lay-flat width and stocked with fewer oysters. These were fitted with internal flotation as opposed to floats on ropes. Since Tipper Tie had no strong fibers that could be knitted into new material, mesh was coated with paints and resins in an attempt to strengthen it. The polyethylene yarn resisted coatings, but the multifilament polyester was easily stiffened. Of those tried, polyester resin was the most successful at preventing crab predation and was used through the remainder of year-one tests. Wave induced chafing later wore holes into the coated chubs. Soft chubs (without wire ring frames) were made both linearly and attached perpendicularly to the stabilizer ropes. These were observed to chafe less than the chubs with rings, although crab predation was severe. Tension on the linear string of chubs caused mesh openings to close, eliminating some of the water flow to those oysters.The next configuration of chubs was a ladder, the final arrangement, (see Figure 1). Tubular mesh of 12-14-inch lay-flat width (8-10 inch diameter) was cut into 36-inch lengths and loaded with oysters and an internal float. The ends of the chub were then clipped to two parallel stabilizer ropes to ease tension on the mesh. The internal floats were long and narrow rather than round and flat or spherical. This type of float held the mesh open for water exchange and provided a relatively flat bottom to eliminate pouching. A substantial portion of surviving oysters reached market size (> 75 mm) after the first year, encouraging a second year of testing. Because the project emphasized the development of oyster culture methodology using Tipper Tie products, oysters had not been measured for growth. Second Year One hundred thousand oysters averaging 22 mm in length were purchased from Harbor Branch Oceanographic Institution in May 1992. The seed was held in flowing water raceways until used in second-year tests. To avoid the extra step of coating mesh to avert crab predation, an intensive effort was made to source stronger fibers that Tipper Tie could knit into tubular mesh. Letters asking for product samples were mailed to over 100 knitting and fiber manufacturers. Twelve fibers had potential for knitting. Of those, only three could be packaged for use by Tipper Tie knitting machines andfabricated into chub ladders with oyster seed. Two fabrics immediately failed. The third fiber, Spectra by Allied Fibers Co., resisted crabs. However, it was a flexible fabric, not stiffened. Crabs could not pick holes in the mesh, but they were able to crush the thin shells of small seed oysters through the flexible mesh and prey on them.Tipper Tie initiated discussions with a plastic extruding firm to produce a specific product. This material was extruded into a medium-density polyethylene tubular mesh with a 12-inch lay-flat width (8-10 inches diameter). The stiffer extruded plastic mesh adequately prevented crab predation. Remaining seed was stocked into chub ladders of this material during the next month. Two 100-foot parallel longlines of 3/16-inch stainless steel cable were anchored 30 feet apart as a field on the research sanctuary. Chub ladders were made 30 feet long and attached perpendicularly between the two longlines. Chubs were spaced at 12-inch intervals along the stabilizer ropes. Fouling by marine organisms was severe at times. Fouling during the first summer of growth was most successfully controlled by drying chub ladders. Sheets of foam insulation (1-2 inches thick, 2 feet wide and 8 feet long) were pushed underneath chub ladders on longlines. These foam sheets floated up underneath the chubs, lifting them above the water surface and exposing them to the air. The foam was left in place for five to 12 hours until most of the fouling organisms were killed. The foam was removed, returning the chub ladders to the water and allowing oysters to resume growth. Dead fouling organisms disintegrated and drifted away with the water currents. Barnacles were not usually controlled by drying. It was observed, however, that waves (especially during storms) caused the oysters to rub together inside chubs, effectively grinding most barnacles from their shells or simply preventing barnacles from attaching. Effort in the second year also focused on methodology, although attempts were made to measure growth. After four months, up to 45 percent (one batch) of 22 mm seed oysters, the first stocked in extruded mesh chubs, had grown to market size on the research sanctuary. The average was 30 percent. Oysters were measured in centimeter groups with the division between groups occurring at the 5-mm marks. Oysters measuring 65-75 mm were in the 7-cm group, those measuring 75-85 mm were in the 8-cm group. Measurements falling exactly on fives were rounded to the closest even number. Oysters in 8-cm and larger groups were market size, those in 7-cm and smaller groups were sub-market size. It became apparent that the extruded polyethylene plastic chubs were breaking down from photodegradation. Conversations with the manufacturer confirmed that the material had been made with no color or ultraviolet (u/v) stabilizers. Black u/v-stabilized material of two strand weights were specified to continue the tests. A similar product made with polypropylene was obtained from another manufacturer. Three lightweight extruded tubular materials were also obtained for testing. The lightweight products were more flexible than the heavier material but less expensive (see Figure 2). The longline field on the research sanctuary held about 600-700 chubs made from the failing extruded material. There was not enough labor to rechub all tests. Four one-day workshops were held to educate individuals about the chub ladder and to gather enough labor to complete the task quickly. The workshops (more appropriately, work days) were very successful. Thirty-five people helped rebag all failed chubs into new chubs of different materials. Oysters from failing chubs were collected and stocked into new chub ladders each workshop day. Several length-frequency measurements were taken of workshop oysters. Mortality was not recorded but was negligible, and dead oysters were discarded. The workshop participants took the chub ladders stocked with oysters to 30 additional sites. The N.C. Division of Marine Fisheries allowed Sea Grant to continue developing the chub ladder by designating a 540-square-foot area at each site as a research sanctuary. The participants' responsibilities were to record observations about the chub ladder system and make suggestions for improving it. They agreed to split the surviving oysters at harvest with Sea Grant. Third Year Follow-up visits were made to all sites except two. Observations were made of oyster growth and survival, mesh performance, fouling and chub ladder design. Final growth measurements were taken at 24 sites. At one site, the chub ladders were split between an open-water location and a protected boat basin location. The small boat basin received periodic rainwater runoff, which affected the oysters there but not those oysters in the open-water location.At another site, the participant posted signs claiming to "close" the entire 10-acre shellfish lease associated with the research sanctuary in hopes of deterring potential theft of other oysters on the lease. At a fourth site, the participant placed the chub ladders immediately adjacent to a duck hunting blind owned by someone else to intentionally render the blind useless. At a fifth site, the ladders were split between two one-half acre saltwater ponds. The ponds were being used for minnow production and received freshly pumped saltwater on a regular basis. At some sites, the ladders were tethered or tied between posts and left without follow-up input. Most of the participants made some effort to secure the chub ladders properly and followed up with input as needed to maintain the ladders. A few of the participants made significantly more effort to manage the chub ladders, eliminate fouling and take a more active role in developing the method. Some developed their own ideas and made significant inputs of time and expenditure.Oysters at most sites were measured two months, five months and 14 months after the workshop. At the best sites, up to 68, 76 and 100 percent of the workshop oysters sampled were market size after two, five and 14-months respectively. The worst sites produced 13, 7 and 60 percent market size oysters for the same time periods. The average of all sites for the two, five and 14-month growth periods were 42, 53 and 86 percent market size respectively (see Table 1). Oyster growth varied considerably among sites. The best sites appeared to be in the mid- to high- salinity ranges. However, growth was probably more closely related to a combination of energy level and consistency and quality of food supply than to salinity. Oyster weight and volume were not measured but would be a good indicator of oyster growth as a comparison among sites.Some first-year oysters were opened and meat weights were recorded at a follow-up workshop in February 1993. Wild oysters were obtained from local markets, opened and weighed for comparison. At final harvest, second-year oysters from a high-energy and a low-energy site were opened and weighed. Results showed greater meat-to-shell ratios for cultured oysters than for wild oysters. Meat weights of cultured oysters exceeded or compared favorably to wild oysters. Cultured oysters of small shell length often contained disproportionately large meats (see Table 2). The heavier mesh materials performed best for the duration of the tests. Lightweight materials performed adequately unless they were in a high-energy site or until the thin edges of fast-growing oysters chafed holes in the material. Polypropylene plastic mesh was generally more durable than polyethylene material of the same weight. No difference in growth rates were noted between mesh types, material weights or manufacturer. Oysters grew into the meshes of material with larger openings more often than into meshes with smaller openings. Fouling reduced water flow through chubs, presumably slowing growth (chubs were not dried during the second year of testing). In locations with high wave energy, fouling sometimes protected oysters from shell abrasion. It was observed that unfouled oyster shells in high energy areas were thicker and more rounded than in areas of low wave energy. On final harvest, chubs with the most severe fouling also had many juvenile oysters attached to the stocked oysters. Fouling organisms such as boring sponges contributed to oyster mortality. Chub ladders from high energy sites usually had oysters with very clean, almost polished shells. However, at many of these sites the oysters had not been leveled and were tightly pouched in one end of chubs with severe fouling. This restricted growth of the oysters. Mortality was recorded for most sites at final harvest. Average survival was 82 percent for all sites sampled 14 months after the workshop. Survival was generally higher in low salinity sites. No difference in survival was noted among mesh types. Oysters were not tested for the parasite Dermo. However, oysters from another experiment on the original Sea Grant research sanctuary tested positive for Dermo during the first growing season. The chub-grown oysters were probably also infected with Dermo. Because of the greater oxygen levels and food supplies associated with the water surface, oysters in floating chub ladders were thought to be protected from Dermo by fast growth and good condition. The oysters were harvested by unhooking floating chub ladders from the longline field, towing them to shore and transporting them to a work shed. Chubs were cut lengthwise, floats were removed, and oysters were poured from chubs onto a table for sorting and final measurements. Small quantities of marketable oysters were sold by some of the participants to retail customers usually at 16 cents each for oysters sold in 100-count boxes. Medium-size oysters (average size 87 mm) were selected for sale in this package. A survey of 12 restaurants within a 150-mile area showed a wholesale price of 16 to 58 cents each for single oysters. The economic feasibility of the chub ladder is contingent on controlling costs of inputs and reducing mortality. Outside of labor, seed and mesh costs are the largest expenses of the system. Survival rate has a large effect on economic return. The projected economics of the system are shown in Figure 4. Future Development The chub ladder has been developed to the point that the concept is sound and proven. The method is centered on the concept of a single stocking of seed oysters into a series of small closed packages. However, many details need refinement (see Figure 3). New materials should be examined for cost-effectiveness and resistance to abrasion and crab predation. Modifications could be tested, experimenting with smaller and larger chubs, different amounts of flotation and vertical arrangements that use more water column. Stocking densities, seed sizes and planting seasons should also be examined. There is a range in oyster sizes at harvest that requires holding slow growers for another season, discarding them or finding markets for them. Markets may be found for small cup-shaped oysters with large meats if this feature can be promoted. Stocking larger seed oysters may provide a more uniform product at harvest. Additionally, because their shells are harder, large seed could be stocked into chubs made of less expensive flexible mesh without crab predation. This could improve the cost-effectiveness of the chub ladder. Reusing heavy weight chub materials might also improve profitability. Stocking regimens may allow development of out-of-season markets for oyster sales throughout the year. Sale of oysters is possible year-round from leases in North Carolina. Summer oysters are usually considered less desirable, although it was noted that many oysters opened during the summer months were in apparently good condition. This condition was probably assoicated with a greater food supply at the surface than at the bottom and the full-time submergence without low-tide exposure. Wild spat collection could be a less expensive source of single oyster seed. Portland cement-coated materials have shown great promise as oyster spat collectors. In another test, up to 50 spat were collected on 2-foot pieces of cement coated 1/8-inch diameter aluminum wire (a Tipper Tie product). Biodegradable materials such as twine, cord, rope and netting could also be cement-coated for spat collection. Birds, especially sea gulls and terns, often used chub ladders as roosts. Birds may cause fecal coliform contamination of oysters or possibly introduce other bacterial pathogens to the area. Methods should be developed to prevent birds from resting on chub ladders. Various methods should also be tested to facilitate chub ladder drying. For example, a hollow PVC pipe grid could be constructed as an airlift. It would be positioned beneath a row of chub ladders and pumped full of air to float chub ladders out of the water. Once dried, the grid could be filled with water and sank before moving to another series of chub ladders. Because different shellfish species feed on slightly different foods, double cropping should also be tested. Oysters could be grown in floating chub ladders while clams or scallops are grown in bottom beds. The chub ladder should be tested on other shellfish species to compare biological and economic feasibility with currently used methods. Chub ladders might be used to grow clams and facilitate harvesting, now one of the most difficult and labor-consuming tasks. Following is the best management recommendation for commercial use of the chub ladder, based on development testing (also see Figure 3, Oyster Chub Ladder Production Guide):
|