 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
Nautical Miles
Figure 3. Density distribution of Mercenaria mercenaria Rehoboth Bay area 3
2
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
0.35
Rehoboth Bay 1976
0.30
0.25
0.20
0.15
0.10
0.05
0.40
Proportion of All Observations
Proportion of All Observations
0.40
0.30
0.25
0.20
0.15
0.10
0.05
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
Rehoboth Bay 2010
0.30
0.25
0.20
0.15
0.10
0.05
0.40
Proportion of All Observations
Proportion of All Observations
0.35
Indian River Bay 1976
0.00
0.00
0.40
0.35
0.35
Indian River Bay 2011
0.30
0.25
0.20
0.15
0.10
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