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)