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Identifying the Habitat and Distribution of Juvenile
Horseshoe Crabs in Delaware Bay
William BurtonFred Kelley
Versar, Inc.9200 Rumsey Road
Columbia, MD 21045US Army Corpsof Engineers
Philadelphia District
The Need
The USACE proposes to use dredged material to reconstruct two sections of the Delaware Bay shoreline that are eroding.
Delaware Bay shoreline currently host horseshoe crab spawning.
The eggs are an important food resource for migrating shorebirds in the spring.
This study was undertaken to evaluate juvenile crab use of near shore habitats prior to restoration.
Proposed Island Restoration Sites
Kelly Island, DE
Severe marsh erosion
Egg Island, NJ
Severe marsh erosion
No marsh erosion
The ProblemSampling newly hatched horse crabs in
shallow water adjacent to spawning beaches posed a challenge as few quantitative sampling
methods exist that can target 5 – 30 mm juvenile horseshoe crabs.
Necessity being the mother of invention……
The
Business
End
8 Hp
Trash
Pump
Discharge
Hose
Configuration
Collection box bottom fitted with 5-mm hardware cloth
Boat
Suction Head
100-foot Trash pump hose
Bottom
Schematic of Suction Dredge Sampling Methods
Clearing the system between stations
Ready to sample -15 meter tows at each station
- measured with a tag line
Collection basin wash down
Examples of Juvenile Horseshoe Crab Collected by Suction Dredge
Survey Methods• Four stations along three inshore to offshore
transects were sampled at three study beaches and four reference areas.
• Surveys were conducted monthly from July to October.
• Juvenile horseshoe crabs collected using suction-dredge.
• 5 meter trawl sampling with 7-mm mesh cod end liner was conducted at all stations and events
• Trawl catches of 5-30 mm horseshoe crabs compared to suction dredge data at each station
Suction Dredge Sampling Stations at Kelly Island and Port Mahon
Suction Dredge Sampling Stations at Egg Island
Suction Dredge Sampling Stations at East Point Reference Areas
Suction Dredge Sampling Stations at Kitts Hummock and Bowers Beach Reference Areas
Bowers
B
East E
gg Is
East P
oint
Kitts H
ummoc
k
Kelley
Is
Port M
ahon
Wes
t Egg
Is
ave trawlave dredge
1
10
100
1000
Den
sity
/100
sq
m +
1
Site
Dredge vs Trawl - July
ave trawlave dredge
Bowers
B
East E
gg Is
East P
oint
Kitts H
ummoc
k
Kelley
Is
Port M
ahon
Wes
t Egg
Is
ave trawlave dredge
1
10
100
1000
Den
sity
/100
sq
m +
1
Site
Dredge vs Trawl - August
ave trawlave dredge
Bowers
B
East E
gg Is
East P
oint
Kitts H
ummoc
k
Kelley
Is
Port M
ahon
Wes
t Egg
Is
ave trawlave dredge
1
10
100
1000
Den
sity
/100
sq
m +
1
Site
Dredge vs Trawl - September
ave trawlave dredge
Bowers
B
East E
gg Is
East P
oint
Kitts H
ummoc
k
Kelley
Is
Port M
ahon
Wes
t Egg
Is
ave trawlave dredge
1
10
100
1000
Den
sity
/100
sq
m +
1
Site
Dredge vs Trawl - October
ave trawlave dredge
Dredge versus Trawl (standardized to #s / 100 sq meters; 11 sq m dredge 350 sq m trawl)
Juvenile Horseshoe Crab Sampling Gear Results
• Order of magnitude more juvenile horseshoe crabs collected with suction-dredge relative to trawls
– Dredge towed for 15 meters– Trawl tows averaged 109 meters in length
• Suction dredge an effective gear to monitor newly hatched horseshoe crabs
Juvenile Horseshoe Crab Densities at Kelly Island
KI01 KI02 KI03 KI04
NearshoreMid
Offshore0
10
20
30
40
50
Cat
ch p
er T
ow
Juvenile HSC Dredge - JulyKelly Island
KI01 KI02 KI03 KI04
Nearshore
Offshore0
10
20
30
40
50
Cat
ch p
er T
ow
Juvenile HSC Dredge - AugustKelly Island
KI01 KI02 KI03 KI04
Nearshore
Offshore0
10
20
30
40
50
Cat
ch p
er T
ow
Juvenile HSC Dredge - SeptemberKelly Island
KI01 KI02 KI03 KI04
NearshoreMid
Offshore0
10
20
30
40
50
Cat
ch p
er T
ow
Juvenile HSC Dredge - OctoberKelly Island
EEI01 EEI02 EEI03 EEI04
Nearshore
Offshore0
10
20
30
40
50
Cat
ch p
er T
ow
Juvenile HSC Dredge - JulyEast Egg Island
EEI01 EEI02 EEI03 EEI04
NearshoreMid
Offshore0
10
20
30
40
50
Cat
ch p
er T
ow
Juvenile HSC Dredge - AugustEast Egg Island
EEI01 EEI02 EEI03 EEI04
NearshoreMid
Offshore0
10
20
30
40
50
Cat
ch p
er T
ow
Juvenile HSC Dredge - SeptemberEast Egg Island
EEI01 EEI02 EEI03 EEI04
NearshoreMid
Offshore0
10
20
30
40
50
Cat
ch p
er T
ow
Juvenile HSC Dredge - OctoberEast Egg Island
Juvenile Horseshoe Crab Densities at Egg Island
Juvenile/sediment - Aug
R2 = 0.0103
0
50
100
150
200
0 20 40 60 80 100
Percent silt/clay
Hor
sesh
oe c
rab
dens
ity
Juvenile/sediment - July
R2 = 0.012
050
100150200250300350
0 20 40 60 80 100
Percent silt/clay
Hor
sesh
oe c
rab
dens
ity
Juvenile/sediment - Sept
R2 = 0.0201
05
101520253035
0 20 40 60 80 100
Percent silt/clay
Hor
sesh
oe c
rab
dens
ity
Juvenile/sediment - Oct
R2 = 0.0428
0
10
20
30
40
50
60
0 20 40 60 80 100
Percent silt/clay
Hor
sesh
oe c
rab
dens
ity
Sediment percent slit/clay did not correlate with juvenile crab catches
Juvenile/sediment - July
R2 = 2E-06
050
100150200250300350
0 2 4 6 8 10 12TOC
Hor
sesh
oe c
rab
dens
ity
Juvenile/sediment - Aug
R2 = 0.0061
0
50
100
150
200
0 2 4 6 8 10 12TOC
Hor
sesh
oe c
rab
dens
ity
Juvenile/sediment - Sept
R2 = 0.0172
05
1015
20253035
0 2 4 6 8 10 12TOC
Hor
sesh
oe c
rab
dens
ity
Juvenile/sediment - Oct
R2 = 0.042
0
10
20
30
40
50
60
0 2 4 6 8 10 12TOC
Hor
sesh
oe c
rab
dens
ity
Sediment Total Organic Carbon did not correlate with juvenile crab catches
Juvenile Horseshoe Crab Survey Results
• Crabs less abundant off of Kelly Island as compared to reference beaches; crab abundance lower off of most New Jersey beaches.
• Juvenile horseshoe crab use of near shore habitats for about 2-3 months after hatching before moving to deeper habitats.
• Densities of juveniles not correlated to sediment type suggesting little preference to mud or sand
Thank You(Patent Pending)
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