Mapping and Assessing Critical Horseshoe Crab Spawning Habitats in Delaware Bay: Update to 2007/10 and Post-Sandy

horseshoe crab and shore photos

Investigators: Richard G. Lathrop1, Larry Niles2, Daniel Merchant1, Timothy Farrell1, Christopher Licitra1

1 Grant F. Walton Center for Remote Sensing and Spatial Analysis
2 Conserve Wildlife


The horseshoe crab update 2013 was part of the Regional Conservation & Management Plan Development and Habitat Restoration & Optimization for the Horseshoe Crab: Delaware Bayshore project funded by the Clear Into The Future program. The Rutgers University Center for Remote Sensing and Spatial Analysis component of the project was funded through a grant provided by the Conserve Wildlife Foundation of New Jersey. The post-Sandy aerial reconnaissance mapping was undertaken by Larry Niles, Amanda Dey, and Joanna Burger.

Acknowledgements for previous 2005-06 work:

Conducted in cooperation with the American Littoral Society with funding provided by the Regina Frankenburg Fund and the Mushett Family Foundation. We gratefully acknowledge the role of Tim Dillingham, Executive Director of the American Littoral Society, in initiating this project and securing the necessary funding. For previous project work and findings, please refer to the 2006 published report linked below on this webpage.

Delaware Bay, on the United States Middle Atlantic seaboard, provides essential spawning habitat for horseshoe crabs, Limulus Polyphemus (Atlantic States Marine Fisheries Commission, 1998). Delaware Bay also serves as critical stopover habitat for migrating shorebirds, especially during the spring migration when it supports some of the highest numbers recorded in the lower 48 states (Clark et al., 1993). Many of these migrants, including the red knot (Calidris canutus), rely heavily on the eggs of horseshoe crabs (Myers, 1986; Tsipoura and Burger, 1999; Niles et al., 2008). Because a significant proportion of the Western Hemisphere’s population of red knots, a species in decline and in consideration for protection as a US federally listed Threatened species, moves through Delaware Bay during the spring migration, this area is of international conservation concern.


In 2005, the Rutgers Center for Remote Sensing & Spatial Analysis (CRSSA), in collaboration with the American Littoral Society, initiated the Delaware Bayshore Horshoe Crab Spawning Habitat Mapping Project. As part of this project, CRSSA undertook a systematic survey of Delaware Bayshore beaches to assess their suitability as horseshoe crab spawning habitat (Lathrop et al., 2006). Visual interpretation of high spatial resolution (1 meter or better) color/color infrared digital aerial photography acquired in 2002 was used to map five categories of horseshoe crab spawning habitat suitability: optimal, suitable, less suitable, avoided and disturbed.  Given ongoing shoreline erosion in the Delaware Bay region, there was growing concern about the continued availability of prime horseshoe crab spawning habitat. Accordingly, an update with more recent aerial photography (i.e. from 2010-2011) and, in the immediate aftermath of the Superstorm Sandy, aerial reconnaissance and sketch mapping was undertaken.

The resulting updated mapping did not reveal major changes in the horseshoe crab spawning suitability of the Delaware Bay shoreline of Delaware and New Jersey between 2002 and 2010. In both cases, approximately 70% of the shoreline did not show a change in habitat suitability. Of the 30% that exhibited a change, some areas showed a decline in mapped habitat suitability, while other areas showed increases in suitability. Overall, there was a slight decrease in mapped habitat suitability with a greater percentage of shoreline experiencing a decline in habitat suitability vs. an increase.  Approximately 13% of the Delaware shoreline showing improvement and 18% showing degradation while approximately 11% of the New Jersey shoreline showing improvement and 18% showing degradation.

The post-SuperStorm Sandy mapping suggests that this “extreme” event had a greater negative impact on horseshoe crab spawning habitat than the prior 8 years  of “normal” shoreline dynamics. Approximately 30% of the New Jersey Delaware Bay shoreline showed a decline in habitat suitability pre vs. post-Sandy as compared to 18% between 2002 and 2010.  Portions of Cumberland and Cape May Countys’ shoreline appear to have been the most heavily impacted by the storm.


Rick Lathrop
Director, Center for Remote Sensing and Spatial Analysis (CRSSA)

Grant F. Walton Center for Remote Sensing and Spatial Analysis (CRSSA)
School of Environmental and Biological Sciences
Rutgers, The State University of New Jersey
14 College Farm Road, Cook Campus
New Brunswick, NJ USA 08901-8551
Tel: 848-932-1582
Fax: 732-932-2587

Web site composed and updated by the Grant F. Walton Center for Remote Sensing and Spatial Analysis (CRSSA), Rutgers University, © 2006; 2015.