1. Identifying the Habitat and Distribution of Juvenile Horseshoe Crabs in Delaware Bay
William Burton
Fred Kelley
Versar, Inc.
9200 Rumsey Road
Columbia, MD 21045
US Army Corps
of Engineers
Philadelphia District

2. 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.

3. Proposed Island Restoration Sites
Figure 1-1 from report

4. Kelly Island, DE
Severe marsh erosion

5. Egg Island, NJ
Severe marsh erosion
No marsh erosion

6. 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.

7. Necessity being the mother of invention……

8. The
Business
End

9. 8 Hp
Trash
Pump

10. Discharge
Hose
Configuration

11. Collection box bottom fitted with 5-mm hardware cloth

12. Schematic of Suction Dredge Sampling Methods
Boat
100-foot Trash pump hose
Suction
Head
Bottom

13. Clearing the system between stations

14. -15 meter tows at each station - measured with a tag line
Ready to sample
-15 meter tows at each station
- measured with a tag line

15. Collection basin wash down

16. Examples of Juvenile Horseshoe Crab Collected by Suction Dredge

17. 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.

18. 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

19. Suction Dredge Sampling Stations at Kelly Island and Port Mahon

20. Suction Dredge Sampling Stations at Egg Island

21. Suction Dredge Sampling Stations at East Point Reference Areas

22. Suction Dredge Sampling Stations at Kitts Hummock and Bowers Beach Reference Areas

23. Dredge versus Trawl (standardized to #s / 100 sq meters; 11 sq m dredge 350 sq m trawl)

24. 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

25. Juvenile Horseshoe Crab Densities at Kelly Island

26. Juvenile Horseshoe Crab Densities at Egg Island

27. Sediment percent slit/clay did not correlate with juvenile crab catches

28. Sediment Total Organic Carbon did not correlate with juvenile crab catches

29. 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

30. Thank You (Patent Pending)