Delaware Bay Horseshoe Crabs

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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

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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

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Ke

lle

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on

Is

W es

Site

Po

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Site

ave trawl

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Ki tts

Ea

Ea

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Eg

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1

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Density/100 sq m +1

W es

Site

ave dredge ave trawl

B we rs

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ave trawl

Ki tts

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Bo

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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

Thank You (Patent Pending)