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Abstracts
Workgroup: Aquaculture and Hatchery Issues
Optimum Size for Planting Hatchery Produced Oyster Seed
Principal Investigator(s):
Richard K. Wallace, Auburn University Marine Extension and Research Center, rwallace@acesag.auburn.edu
Co-Investigator(s):
David B. Rouse: Auburn University, Department of Fisheries and Allied Aquacultures
Funding Period: 1998-2001
Hatchery produced seed has not gained acceptance in the Gulf of Mexico Region primarily because survival of seed is poorly understood, particularly in relation to cost. Four size classes ( 5 mm, 6-10 mm, 11-15 mm and 16-20 mm) of two types of oyster seed ( set on microcultch and set on whole shells) were planted at two sites (high and low salinity) in replicated plots during 1999. Three size classes (<15 mm, 15 - 25 mm and > 25 mm) of both types were planted in 2000. Oyster seed set on microcultch did not survive regardless of size, location or year, while survival of seed set on whole shell ranged from 42 to 62% after 32 weeks at the low salinity site in 1999 but did not survive in 2000. Drought conditions in 2000 resulted in the low salinity site being higher in salinity than expected and oyster drills were prevalent.
In laboratory predation experiments, oyster drills ate more large (> 25mm) single oyster seed than the other two size classes (< 15 mm and 15-25 mm) and than seed set on whole shell. When oyster drills were provided oyster seed set on whole shell only, they ate more of the largest size class. A similar study using blue crabs resulted in more medium and small oysters being consumed than the largest size class and more single oysters were consumed than those set on whole shell.
Survivorship of the three size classes of single oyster seed when exposed to hypoxic conditions (dissolved oxygen 0.01- 0.23 ppm) in a series of laboratory tests ranged from 100% after 72 hrs to 0% after 144 hrs. The mean estimated LT-50's for the three size classes of oyster seed (<20mm, 22-34mm, >35mm) were 99.37 h, 93.62 h, 93.03 h respectively, with no apparent relation between size and survival.
Analysis of survival for oyster seed set on whole shell from year one of this study and the published price of oyster seed of various sizes indicated there was no advantage in paying more for larger seed. Even though the smallest seed had the lowest nominal survival, the cost per oyster after 32 weeks was still lowest for the smallest seed size (<5 mm). The cost per oyster of the largest seed (16-20 mm) after 32 weeks was about 5 times higher than the smallest seed. For the conditions encountered during this study and the range of seed oyster sizes used, there is no economic advantage in paying more for larger seed.
IMPACTS and/or BENEFITS: The Gulf of Mexico region is the largest producer of oysters in the US. Continued high production is dependent on many factors but some experts believe that more intensive oyster farming can play an important role in maintaining or increasing production. Hatchery produced seed is an important element of oyster culture and the information from this study can give guidance to oyster farmers who are considering buying seed oysters. Significant savings can be obtained by purchasing spat set on whole shell at a smaller, less expensive size and by carefully selecting sites for growout.
PROJECT PUBLICATIONS:
Wallace, R. K. , D.B. Rouse, F.S. Rikard, J.C. Howe, B.A. Page, D.B. Gruber and J.K. Dunne. 2001. Experiments in determining optimum size for planting hatchery produced oyster (Crassostrea virginica) seed. World Aquaculture Society Book of Abstracts. Aquaculture 2001. p.675.
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