WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2017 67 (CONTINUED ON PAGE 68) light to heavy. By September, mean overset was lowest on oysters from the Low Pro cages and 2×2 sub-tidal cages (Fig. 4), while 2×2 and semi-buoyant intertidal cages had moderate to heavy overset (Fig. 5). This study demonstrates that single oyster cultivation is possible in Georgia and that a marketable oyster of 50 mm or greater (Fig. 6) can be grown within 16 months and that oyster stocks can be held over summer with survival greater than 60 percent. Oyster survival over the summer is critical for growers to manage stocks that are not legal size by the time that the wild harvest season closes at the end of May. High survival can be achieved by holding oysters sub-tidally in large mesh bags or in semi-buoyant cages. Although mean growth was lower in sub-tidal cages, the effort to maintain sub-tidal cages was much less than for the intertidal cages. Semi-buoyant cages performed well, but fouling of the cage was noticeably greater than on other cages. This study indicates that gear, gear location, and mesh size can greatly affect summer survival of oysters, and the success of semibuoyant cages indicates that floating cages and/or bags needs to be examined. The effect of overset needs to be further evaluated along with the time and effort to help determine best gear and bag combination to hold oysters in the summer because Georgia has very high natural set rates of oysters (10,000/m2). Georgia is still in the early stages of oyster aquaculture, but under current conditions, it can be successfully done. There are plans to evaluate the use of floating gear and triploid oysters in the coming years to develop a robust oyster industry using diploid and triploid oysters. Notes Thomas H. Bliss, Justin Manley, and Rob Hein, University of Georgia Marine Extension and Georgia Sea Grant: Shellfish Research Laboratory, 20 Ocean Science Circle, Savannah, GA 31411, tbliss@uga.edu were removed and censored from survival calculations. From 1 May to 5 October, cages and bags were cleaned twice monthly using a freshwater power washer to remove fouling (Fig. 2) and once monthly bags were retrieved and the height, length and width of 28 oysters were measured to the nearest 0.1 mm. To compare growth by treatment, mean height (mm) of oysters from each bag was calculated and the mean initial size in May was subtracted. A two factor (gear type × mesh size) analysis of variance was used to evaluate growth differences among treatments. Survival was determined by counting and removing dead oysters from each bag monthly and Kaplan-Meier analysis was used to determine overall survival. To estimate hard fouling a visual ranking of light to heavy was used to rank overset. A ranking of light fouling was scored <3, moderate fouling >3 and <7, and heavy fouling >7. Survival was greatest in Low Pro cages with 4-mm mesh and semi-buoyant cages with 6-mm mesh (75 percent). Survival of oysters held sub-tidally in 9-mm mesh bags was 73 percent and in 12-mm mesh bags was 66 percent. For all mesh size combined, overall survival was greatest in semi-buoyant cages (67 percent) followed by sub-tidal (49 percent), Low Pro (48 percent), and 2×2 intertidal cages (26 percent). Growth, as measured by increase in oyster height, was similar for oysters in all treatments. Oyster growth from May through September was 16.2 mm in semi-buoyant cages, 15.9 mm in Low Pro cages, 13.2 mm in 2×2 intertidal cages, and 12.8 mm in 2×2 sub-tidal cages. Overall, the mean length of oysters was 59.5 mm which is greater than the 50 mm market size for single oysters. Statistically there was no difference in growth among gear types, bag mesh sizes, or between gear type × mesh size. Overset was found on oysters in all gear types and mesh sizes, ranging from FIGURE 3. Low Pro cage after power washing (front) and before power washing (back). (Photo: UGA Marine Extension Georgia Sea Grant) FIGURE 4. Oyster with low overset. (Photo: UGA Marine Extension Georgia Sea Grant)
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