Tidal Powered Upwelling Nursery Systems for Clam Aquaculture in Georgia

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1 Tidal Powered Upwelling Nursery Systems for Clam Aquaculture in Georgia Alan Power, Thomas Shierling, Todd Recicar, Joe Lambrix, Nelson Eller & Randal Walker 1 UGA Marine Extension Service, Shellfish Research Lab 20 Ocean Science Circle, Savannah, GA Tel: (912) ; Fax: (912) ; Website: 2 UGA Marine Extension Service, Advisory Services, 715 Bay Street, Brunswick, GA Tel (912) ; Fax: (912) ; Website: MARINE EXTENSION BULLETIN NO., NOVEMBER

2 ACKNOWLEDGEMENTS Financial support for the construction of three upwelling systems was provided by the Georgia Department of Natural Resources. We wish to thank Mr. Robert Baldwin of McClellanville, South Carolina and Mr. Perry Hall of St. Helena Island, South Carolina for allowing us to visit and observe their tidal-powered upwellers. We would also like to acknowledge Mike Townsend, Revis Barrow, Tommy Brown and Alvin Floyd for their assistance in constructing and testing our systems. Special thanks are given to G. Davidson and C. Ingram of the Georgia Sea Grant College Program for their editing and graphics expertise. ABSTRACT The hard clam, Mercenaria mercenaria aquaculture industry is a small-scale operation in Georgia. There is no commercial hatchery in the state, and therefore growers must import seed from hatcheries in South Carolina and Florida. Imported seed must be certified as free of pathogens by the Georgia Department of Natural Resources. Prior to the planting season, seed shortage is often an issue for our growers. Typically, Georgia growers purchase seed at a size of 8mm or larger. Clams smaller than this require a nursery facility prior to field planting. Predators can be excluded from a nursery facility and seed clams have sufficient food to ensure rapid growth. This can be quite labor intensive, and often mortality rates are high, however, the benefit is that smaller seed are in ready supply and are inexpensive. We examined the use of tidal-powered upwelling culture systems for the nursery grow-out of hard clam seed in the tidal creeks of coastal Georgia. We detailed the construction, cost, advantages, and the operation and maintenance of these systems. We also conducted growth trials over the course of a one year period. It is hoped that the use of tidal upwellers will lead to an increase in clam production for the state.

3 TABLE OF CONTENTS Title Page 1 Acknowledgements 2 Abstract 2 Introduction 4 Upweller Construction 4 Advantages of the Tidal Powered Upwelling System 6 Site Selection 7 Upweller Operation & Maintenance 7 Experimental Seed Growth & Survival 7 References 25 Appendix TABLES Table 1. Sample seed prices (2001) taken by averaging prices at four commercial hatcheries. 6 FIGURES Fig. 1 Upweller design from Baldwin et al. (1995). 5 Fig. 2 Base structure of the bottom of the upweller. 7 Fig. 3 Base structure covered with 3/4 plywood. 8 Fig. 4 Base structure with details of the side construction. 9 Fig. 5 Details of side post construction. 10 Fig. 6 Design for construction of the top structure of the upweller. 11 Fig. 7 Design of the decking on the top of the upweller. 12 Fig. 8 Structural assembly of the base, sidewall, and top for the upweller. 13 Fig. 9 Top structure with swing gates in funnel. 14 Fig. 10 Final assemblage of the upweller. 15 Fig. 11 Temperature (ºC), dissolved oxygen (mg/l), salinity (ppt) and turbidity (secchi disk depth in cm) recorded for the study area between October 2001 and April Fig. 12 Mean wet weight of seed clams (N=200) between October 2001 and April In December the seed was graded and sorted into two size classes. 19 Fig. 13 Mean shell length (± standard error) of seed clams (N=60) between October 2001 and April In December the seed was graded and sorted into two size classes. 20 Fig. 14 Photos illustrating the growth of the clam seed from an initial 1-mm size (A), to 8+mm (B) over the winter months. Out of an original 250,000 clams an extraordinary 246,000 were estimated to have survived (C). 21 Fig. 15 Temperature (ºC), dissolved oxygen (mg/l), salinity (ppt) and turbidity (secchi disk depth in cm) recorded for the study area between April 2001 and July Fig. 16 Mean wet weight of seed clams (N=200) between April 2002 and July Fig. 17 Mean shell length (± standard error) of seed clams (N=60) between April 2002 and July

4 INTRODUCTION Marine clam farming techniques developed in other parts of the United States generally do not work well in Georgia. This is because Georgia has the greatest tidal amplitude along the eastern United States and Gulf of Mexico coastlines with the exception of upper coastal Maine. Georgia s average tidal range of 6-7 feet results in strong tidal currents which preclude local clam farmers from using common clam growing techniques. In Florida, which has an average tidal range of only 1-2 feet, small 2-mm seed clams are placed in wooden boxes filled with sand (Vaughan & Creswell, 1989). Boxes are covered with a mesh top which allows water to flow through the boxes and excludes predators. These boxes are then placed on lagoon bottoms, where clams are allowed to grow to market size. When growers attempted to use this technology in Georgia, the sand and clam seed were sucked out of the box by strong currents, and the boxes floated to the surface. Other clam farming techniques developed for different areas of the United States and the world have met with a similar fate when subjected to Georgia's tides and currents. The nursery phase (i.e., between hatchery and field grow-out) of clam culture is typically the most difficult part to accomplish in an economically feasible manner. At this size it is critical to protect the vulnerable seed while providing sufficient food and oxygen for growth and survival. Recent research has proven that upwellers are the optimal way to culture small shellfish seed through the nursery phase (Appleyard & Dealteris, 2002). This technique involves forcing plankton-rich seawater up through a partially fluidized bed of shellfish seed. Many upwelling designs have been developed. Some involve moving water through the system with compressed air and electrical devices. Many of these systems are expensive to purchase and operate. Here in Georgia tidal power presents an attractive alternative. Instead of being an impediment, Georgia s strong tidal currents can be beneficial by providing the energy required to run an upweller system. Currently, Georgia clam farmers are required to plant a larger seed size than farmers in other states. Experimental planting of clams less than 6 mm in size utilizing a variety of grow- out techniques has resulted in 100% mortality (Walker & Hurley, 1995). Typically seed are purchased at a size of 8-10 mm and are raised in mesh bags placed on the river bottom. Once they reach a size of 25 mm, they are planted out in bottom plots where they grow until they reach a harvest size of 45 mm. A crop requires approximately 18 months in the field before they reach harvestable size. Acquiring sufficient 25-mm seed from a commercial hatchery is difficult because they are expensive and usually in short supply. Tidalpowered clam nurseries offer a solution to this problem. UPWELLER CONSTRUCTION With funds (State Shellfish Lease Revenue) from the Coastal Resources Division of the Georgia Department of Natural Resources (GADNR), the University of Georgia's Marine Extension Service adapted tidal-powered clam nursery technology for Georgia clam growers. Originally developed in Maine, the model was subsequently used in South Carolina (Baldwin et al., 1995) before being modified for Georgia. The model s basic design consists of a floating tank structure with a wide scoop at the end, which directs incoming tidal water up into suspended bins that hold the seed mass secured on a screen (Fig. 1). The water moves up through the seed mass, passes out into a collecting trough above, and exits at the rear of the unit. These systems are anchored in the river and turn with the tide, so the scoop always faces the tidal flow. They also require ample area to rotate with the turning of the tide. Because of this, they must be moored in open areas so as not to hinder boat traffic.

5 Fig.. Upweller design from Baldwin et al. ( ).

6 We modified this design, so that it would better suit conditions in Georgia. Our upweller has two scoops, one on each end of the system. This allows the nursery system to be anchored in small tidal creeks so that it remains stationary. It can also be attached alongside exiting pilings or docks, such as those found in marinas. Figures 2-10 illustrate the construction of our upweller design. Further drawings and a detailed material list and cost estimate are provided as an appendix. The estimated cost per unit in 2003 is $2,998. It requires approximately 160 man-hours to construct the system. ADVANTAGES OF THE TIDAL POWERED UPWELLING SYSTEM There are many advantages to using this type of upwelling system in Georgia. First of all, the system is relatively inexpensive to construct and operate. Since it is powered by tides, it has no fuel or electricity costs, and it is also environmentally friendly. It can be anchored in a stationary position in small tidal creeks, or it can be attached to exiting pilings or docks, such as those found in marinas. It does not require a special permit or lights. The upweller can also hold a large number of seed in a small space where they can be inspected easily. The floating dock provides extra workspace for the operation and maintenance of the system. By attaching lockable doors over the central tank area, the seed can be protected from theft, vandalism and predation by otters and minks. The system is also mobile and can be moved to another location if conditions become unfavorable. Because it is capable of raising seed from a size of 1 mm, the problem of limited seed availability is reduced. Finally, 1 mm-clam seed is less expensive than 8-mm seed (Table 1), and therefore offers growers the potential to increase their production levels. Table. Sample seed prices ( ) taken by averaging prices at four commercial hatcheries. Mesh Size Clam size # Clams/liter $ per,.. -.,.. -.,.. -.,.. -.,.. -.,.. -.,.. -.,

7 Figure. Base structure of the bottom of the upweller.

8 Figure. Base structure covered with / plywood.

9 Figure. Base structure with details of the side construction.

10 Figure. Details of side post construction.

11 Figure. Design for construction of the top structure of the upweller.

12 Figure. Design of the decking on the top of the upweller.

13 Figure. Structural assembly of the base, sidewall, and top for the upweller.

14 Figure. Top structure with swing gates in tunnel.

15 Figure. Final assemblage of the upweller.

16 SITE SELECTION Selecting a suitable site to deploy the upweller is critical. In terms of water quality, the maximum growth will occur where the salinity ranges from 25-35, the temperature from 20-28ºC /68 82ºF, and where dissolved oxygen levels are greater than 4mg/L (Ansell, 1968; Eversole, 1987; Chesapeake Bay Program, 1987; Lorio & Malone, 1995). Any deviation from the optimal salinity range can reduce the clams tolerance for high temperatures. Conversely, optimal temperatures enhance the clams tolerance to unfavorable salinities. The upwelling system requires a water current of at least 0.5 knots to open the doors inside the scoop ends. However, a faster current speed is desirable. The current should not be so fast that causes the seed to bounce on the bin screens, which reduces their ability to filter plankton, and thus inhibits growth rates. In Georgia, soft muddy substrates predominate. In order to reduce the amount of siltation in the upweller system, a depth of feet at low water is advisable. Finally, it is wise to choose a site away from intense wave action, boat wakes, agricultural runoff and frequent boat traffic. UPWELLER OPERATION & MAINTENANCE Seed should be purchased and placed into the upweller bins early in the growing season. Once in place, the seed should be inspected visually for the presence of predators (crabs) as regularly as possible. Fouling organisms (e.g., sea grapes) should be removed regularly from the system (upweller bottom and sides, bin mesh, and weed screen), because they compete for food and also will restrict the flow of water to the seed. The best cleaning technique for the delicate bin mesh bottoms is to brush them gently, immerse them in freshwater, and allow to air dry. It is advisable to have replacement bins to use while the other bins are going through a cleaning cycle. In the summer months, when fouling organisms are abundant, cleaning is required once per week. In winter, once per month should suffice. Once a year the upweller should be removed from the water entirely for a comprehensive cleaning. The best time for this is during the warmest summer months just after the clams have been planted in the field. All shellfish do not grow at the same rate. To ensure the highest survival and growth rates, it helps to periodically sort and grade the seed into similar size classes. Slow-growing seed will do much better if separated from the more competitive larger clams (Baldwin et al., 1995). Additionally, the larger seed should be placed into a bin with a larger mesh screen bottom, which allows greater water flow to the seed. The number of bins should be increased whenever the seed size doubles. Growers should pay close attention to this before initially stocking the upweller (e.g., if two bins are initially filled, by the time they have reached planting size a total of eight bins will be filled). EXPERIMENTAL SEED GROWTH & SURVIVAL In Georgia, clam grow fastest in spring and fall and slowest during the winter months. Summer growth rates fall somewhere in between. The upweller was placed in a tidal creek in McIntosh County, Georgia at a depth of about 20 ft. The water was sampled regularly for turbidity with a secchi disk, dissolved oxygen with a probe, temperature with a thermometer and salinity with a refractometer (Fig. 11). The upweller was stocked with 250, mm seed. Every two weeks, three sets of 200 clams were randomly

17 selected and weighed to determine mean wet weight (Fig. 12). On each occasion, 60 clams were also randomly selected and their shell length measured (longest possible measurement, i.e., anterior-posterior) under a dissecting scope. Six months later in April 2002, the survival rate was 98% with 66% (estimated by volumetric displacement) of the clams averaging 8.43 mm (± 0.16 S.E.) and 33% averaging 7.55 mm (± 0.10) (Fig. 13). Figure 14 illustrates the growth and survival from the experiment s initiation to its termination. Although growth was slow over these winter months, the experiment showed that clams could be grown successfully in the upweller at this time of the year with high survival rates. upweller during slack water. Mortalities may have been reduced somewhat by stocking the upweller bins less densely. Based on these experiments it is not recommended that clams remain in the upweller systems in the warmest summer months. If this upweller system can be used to successfully raise two batches of seed per year in coastal Georgia, then we recommend purchasing seed and stocking the upweller in early-mid September, and again in mid-late March. The experiment was started again in April 2002, when another batch of 250,000 1-mm seed were placed into the upweller. Water quality (Fig. 15), mean wet weight (Fig. 16), and shell length (Fig. 17) were again monitored as before. Growth was much more rapid at this time of year with clams reaching a planting size in half the time it took over the winter months. However, the clams survival rate was very low with approximately 67,000 (26.4%) remaining at the end of July In May approximately 230,000 clams were alive, but this number was dramatically reduced to 83,000 in June. Several stressor factors may have contributed to the high mortalities observed during June including higher water temperature, which climbed to 28ºC in June. Water temperature peaked at 30ºC in July Dissolved oxygen levels were consistently low throughout this experimental period. They fell below the recommended level of 5mg/L, dropping from 3.72 to 2.4 mg/l between May and June In a 48-hour period prior to the June sampling date, approximately five inches of rain fell. This resulted in a much reduced salinity, down from 32 ppt in April and May to 20ppt. In addition, predatory crabs, fouling organisms, and bacteria were much more abundant in the growing bins at this time of year. A combination of these effects is likely responsible for the high mortality rate. Effects probably were intensified in the

18 Temperature (C) DO (mg/l) Salinity (ppt) Turbidity (inches) Oct-01 4-Nov-01 4-Dec-01 4-Jan-02 4-Feb-02 4-Mar-02 4-Apr-02 Fig.. Temperature (ºC), dissolved oxygen (mg/l), salinity (ppt) and turbidity (secchi disk depth in cm) recorded for the study area between October and April.

19 35 30 Wet Weight (g) 200 Clams Oct Oct Nov Nov Nov Dec Dec Jan Jan Feb Feb Mar Mar Apr Apr-02 Fig.. Mean wet weight of seed clams (N= ) between October and April. In December the seed was graded and sorted into two size classes.

20 9 Mean Shell Length (mm) ± Standard Error Oct Oct Nov Nov Nov Dec Dec Jan Jan Feb Feb Mar Mar Apr Apr Date Fig.. Mean shell length (± standard error) of seed clams (N= ) between October and April. In December the seed was graded and sorted into two size classes.

21 A C Fig.. Photos illustrating the growth of the clam seed from an initial -mm size (A), to +mm (B) over the winter months. Out of an original, clams an extraordinary, were estimated to have survived (C). B

22 Temperature (C) DO mg/l Salinity ppt Turbidity Apr-02 May-02 Jun-02 Jul-02 Fig.. Temperature (ºC), dissolved oxygen (mg/l), salinity (ppt) and turbidity (secchi disk depth in cm) recorded for the study area between April and July.

23 40 35 Wet Weight (g) 200 Clams Apr May Jun Jul-02 Fig.. Mean wet weight of seed clams (N= ) between April and July.

24 10 9 Mean Shell Length (mm) ± Standard Error Apr May Jun Jul-02 Fig.. Mean shell length (± standard error) of seed clams (N= ) between April and July.

25 REFERENCES Ansell, A. D., The rate of growth of the hard clam Mercenaria mercenaria (L.) through out the geographical range. Conseil Permanent International pour l'exploration de la Mer, Journal du Conseil 31(3): Appleyard, C.L. and J.T. Dealteris, Growth of the northern quahog, Mercenaria mercenaria, in an experimentalscale upweller. Journal of Shellfish Research, 21 (1): Baldwin, R.B., W. Mook, N.H. Hadley, R.J. Rhodes and M.R. DeVoe, Construction and operations manual for a tidal-powered upwelling nursery system. South Carolina Sea Grant Consortium, Charleston. 44pp. Vaughan, D.E. and R.L. Creswell, Field grow-out techniques and technology transfer for the hard clam, Mercenaria mercenaria. Aquaculture Report Series, Florida Department of Agriculture and Consumer Services, Tallahassee, Florida. 42pp. Walker, R.L. and D.H. Hurley, Biological feasibility of mesh bag culture of the northern quahog Mercenaria mercenaria (L.) in soft-bottom sediments in coastal waters of Georgia. The University of Georgia Marine Extension Bulletin No. 16, August Chesapeake Bay Program, "Habitat Requirements for Chesapeake Bay Living Resources." Chesapeake Bay Living Resources Task Force. Eversole, A.G., "Species Profiles: Life Histories and Environmental Requirements of Coastal Fishes and Invertebrates (mid-atlantic): Hard Clam." U.S. Fish and Wildlife Service Biological Report 82(11.75). U.S. Army Corps of Engineers TR EL Lorio, W.J and S. Malone, Biology and culture of the northern quahog clam (Mercenaria mercenaria). Southern Regional Aquaculture Center. SRAC publication Number 433.

26 APPENDIX TIDAL POWERED UPWELLING SYSTEM CONSTRUCTION DRAWING LIST DWG = Drawing DWG B-1: Base Structure DWG B-2: Item #2 From DWG B-1 DWG M-16: Final Assembly Detailed Material List and Cost Estimate DWG B-3: Items 1, 3, & 4 From DWG B-1 DWG B-4: DWG S-5: Base Structure Plywood Sidewall Structure DWG S-6: Detail B & Item 8 From DWG S-5 DWG S-7: Items 9 & 10 From DWG S-5 DWG S-8: Sidewall Panel Layout DWG S-9: Item 14 Details From DWG S-8 DWG S-10: Items 11, 12 & 13 From DWG S-8 DWG T-11: DWG T-12: DWG M-13: Top Deck Structure Top Structure Decking General Arrangement-Misc Items DWG M-14: Item 24 From DWG M-13 DWG M-15: Items 19, 20, 21, & 22 From DWG M-13

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