Hard Clam Densities of 2011 Survey Using Spline Interpolation with a 25M X 25M Grid
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Identify areas of high clam densities to focus efforts of reclassification of growing areas if possible Look for changes in clam densities and distributions from previous surveys Collect the necessary data for any potential aquaculture efforts Unfunded Federal mandate which strictly controls the harvest of bi-valve molluscan shellfish( clams, oysters, mussels, etc.) through State Shellfish Programs State Shellfish Programs establish harvest areas following strict regulations State Shellfish Programs are audited by U.S. Food and Drug Administration annually 1968 Grab Survey1 1976 Venturi Suction Dredge2 2011 Duplicated Sample technique of 1976 survey Dredge utilizes water pressure from 4” water pump to “vacuum” all clams and sediment to a minimum depth of 12 inches Assume 100% efficiency based on pre-sampling tests Collects clams that are less than 1 centimeter in length Restricted Harvest Seasonally Restricted # clams/m2 Figure 1. Rehoboth Bay site specific clam densities and harvest classifications Figure 2. Indian River Bay site specific clam densities and harvest classifications 1975-76 2010 0 1 2 Nautical Miles Figure 3. Density distribution of Mercenaria mercenaria Rehoboth Bay area 3 1975-76 2010 Figure 4. Density distribution of Mercenaria mercenaria Indian River Bay area3 Figure 5. Hard clam densities of 2011 survey using spline interpolation with a 25m x 25m grid. Table 1. Comparison of clam densities between years and between bays (Wilcoxon two-sample test). Statistical Test Result P-value Rehoboth Bay clam density: 1976 versus 2011 Not Significant P=0.45 Indian River Bay clam density: 1976 versus 2011 Not Significant P=0.36 Rehoboth Bay versus Indian River Bay clam density: 1976 Significant P<0.001 Rehoboth Bay versus Indian River Bay clam density: 2011 Significant P=0.049 Table 2. Comparison of the distributions of clam densities between years and between bays (Kolmogorov-Smirnov two-sample test). Statistical Test Result P-value Rehoboth Bay clam density: 1976 versus 2011 Not Significant P=0.31 Indian River Bay clam density: 1976 versus 2011 Not Significant P=0.34 Rehoboth Bay versus Indian River Bay clam density: 1976 Significant P<0.0001 Rehoboth Bay versus Indian River Bay clam density: 2011 Not Significant P=0. 296 0.40 Rehoboth Bay 1976 0.40 Indian River Bay 1976 0.35 0.35 0.30 0.30 0.25 0.25 0.20 0.20 0.15 0.15 0.10 0.10 Proportion of All Observations 0.05 Proportion of All Observations 0.05 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 Clams/m2 Clams/m2 0.40 Rehoboth Bay 2010 0.40 Indian River Bay 2011 0.35 0.35 0.30 0.30 0.25 0.25 0.20 0.20 0.15 0.15 0.10 0.10 Proportion of All Observations 0.05 Proportion of All Observations 0.05 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 Clams/m2 Clams/m2 Figure 6. Distribution of clam catches in the 1976 and 2011 surveys. General survey conducted at 10 stations using a bull rake and hand rake to determine if any bi-valve shellfish were present Only 9 clams were found at all 10 sites Sites were selected to represent all areas of the bay Majority of clams were found near Southern portion of the bay Use available data to make educated management decisions Identify survey method which provides statistically similar data but is less costly and intensive 1) Humphries, E. and F.C. Daiber. 1967. Shellfish Survey of Indian River Bay and Rehoboth Bay, Delaware. Technical Report. Northeast Marine Health Sciences Laboratory. Public Health Service. Narragansett, Rhode Island. 2) Cole, R.W. and L.W. Spence. 1976. Shellfish Survey of Rehoboth Bay and Indian River Bay. Technical Report. Delaware Department of Natural Resources and Environmental Control. Division of Fish and Wildlife. 3) Lynch, Rebecca. 2012. Inland Bay Clam Density Comparison Maps. Delaware Department of Natural Resources and Environmental Control. Division of Fish and Wildlife The Center for the Inland Bays 2 year project totaling $20,000 EPA pass-through funds Richard Wong, Statistical Analysis, DNREC Fisheries Bart Wilson-GIS Support, The Center for the Inland Bays Field Crew-Gene Shaner and Zac Moffitt Bryan Bloch-GIS Environmental Scientist II DNREC .