Identifying the Habitat and Distribution of Juvenile Horseshoe Crabs in Delaware Bay William Burton Fred Kelley
US Army Corps of Engineers Philadelphia District
Versar, Inc. 9200 Rumsey Road Columbia, MD 21045
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 Problem Sampling 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
Schematic of Suction Dredge Sampling Methods Boat
100-foot Trash pump hose
Bottom
Suction Head
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 suctiondredge.
• 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
Dredge versus Trawl
(standardized to #s / 100 sq meters; 11 sq m dredge 350 sq m trawl) Dredge vs Trawl - September
Dredge vs Trawl - July
1000
100
10
ave trawl ave dredge
Density/100 sq m +1
100
10
ave trawl ave dredge
1
1
gg tE
rt M ah Po
Ki tts
Is
on
Is y lle
Ea
Ke
st
Hu m m
Po
oc k
in t
Is g Eg Ea
Site
Dredge vs Trawl - October
Dredge vs Trawl - August
1000
100
10
ave trawl ave dredge
Density/100 sq m +1
1000
100
10
ave trawl ave dredge
1 ave dredge ave trawl
k
Is gg tE
rt M ah
Ke
lle
y
on
Is
W es
Site
Po
Ki tts
Hu m m
oc
Po st Ea
st
Eg
g
in t
Is
B Bo
tE
gg
Is
on Po
rt M ah
y lle Ke
we rs
oc Hu m m
Is
W es
Site
ave trawl
k
in t Po st
Ki tts
Ea
Ea
st
Eg
g
we rs
Is
B
ave dredge
Ea
1
Bo
Density/100 sq m +1
W es
Site
ave dredge ave trawl
B we rs
tE
gg
Is
on W es
Po
rt M ah
y lle Ke
Bo
Is
oc k
nt
Hu m m
st P
ave trawl
Ki tts
Ea
Ea
Bo
st E
oi
gg
we rs
Is
B
ave dredge
st
Density/100 sq m +1
1000
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 Juvenile HSC Dredge - July Kelly Island
Juvenile HSC Dredge - August Kelly Island 50
40 30 20 10
Offshore Mid Nearshore
0 KI01
KI02
KI03
Catch per Tow
Catch per Tow
50
30 20 10
Offshore
0 KI01
KI04
Juvenile HSC Dredge - September Kelly Island
KI02
Nearshore KI03
KI04
Juvenile HSC Dredge - October Kelly Island 50
40 30 20 10
Offshore
0
Catch per Tow
50
Catch per Tow
40
40 30 20 10
Offshore Mid Nearshore
0
KI01
KI02
Nearshore KI03
KI04
KI01
KI02
KI03
KI04
Juvenile Horseshoe Crab Densities at Egg Island Juvenile HSC Dredge - August East Egg Island
Juvenile HSC Dredge - July East Egg Island
50
40 30 20 10
Offshore
Catch per Tow
Catch per Tow
50
40 30 20 10
EEI01
EEI02
Nearshore EEI03
EEI01
EEI02
EEI04
EEI03
EEI04
Juvenile HSC Dredge - October East Egg Island
Juvenile HSC Dredge - September East Egg Island
50
40 30 20 10
Offshore Mid Nearshore
0 EEI01
EEI02
EEI03
EEI04
Catch per Tow
50
Catch per Tow
Offshore Mid Nearshore
0
0
40 30 20 10
Offshore Mid Nearshore
0 EEI01
EEI02
EEI03
EEI04
Sediment percent slit/clay did not correlate with juvenile crab catches Juvenile/sediment - Aug
350 300
Horseshoe crab density
Horseshoe crab density
Juvenile/sediment - July
R2 = 0.012
250 200 150 100 50 0 0
20
40
60
80
200 2
150
R = 0.0103
100 50 0
100
0
20
35 2
R = 0.0201
20 15 10 5 0 0
20
40
60
Percent silt/clay
80
100
Juvenile/sediment - Oct Horseshoe crab density
Horseshoe crab density
Juvenile/sediment - Sept
25
60
Percent silt/clay
Percent silt/clay
30
40
80
100
60 50
R2 = 0.0428
40 30 20 10 0 0
20
40
60
Percent silt/clay
80
100
Sediment Total Organic Carbon did not correlate with juvenile crab catches Juvenile/sediment - Aug
350 300
Horseshoe crab density
Horseshoe crab density
Juvenile/sediment - July
R2 = 2E-06
250 200 150 100 50 0 0
2
4
6
8
10
200 R2 = 0.0061
150 100 50 0
12
0
2
4
TOC
Horseshoe crab density
Horseshoe crab density
30
2
R = 0.0172
25 20 15 10 5 0 4
6 TOC
10
12
Juvenile/sediment - Oct
35
2
8
TOC
Juvenile/sediment - Sept
0
6
8
10
12
60 50
R2 = 0.042
40 30 20 10 0 0
2
4
6 TOC
8
10
12
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