Horseshoe Crab Science

In addition to various tagging efforts, every year a number of state, federal, and non-governmental organizations coordinate to conduct a standardized spawning survey of horseshoe crabs at select Delaware Bay beaches in May and June. These surveys help with species management by:
  • Providing a reliable index to compare variations in seasonal and geographical spawning activity
  • Increasing our understanding of the relationships between spawning factors and environmental conditions
  • Promoting public awareness of the role horseshoe crabs play in shorebird population dynamics, Atlantic coast fisheries, and human health and the biomedical industry

Baywide Spawning Activity by Lunar Cycle from 1999-2017
Horseshoe crabs begin gathering nearshore to spawn in early May. This pie chart shows that peak spawning on average falls within the second lunar cycle, which typically occurs in the end of May. The next highest average is the third lunar cycle, typically the beginning of June. By the end of June, most horseshoe crabs have left the beaches. Spawning activity is based on the mean number of spawning female horseshoe crabs per square meter of available beach. View the data.
Sex Ratio
The ratio of male to female horseshoe crabs has ranged from 3.1:1 to 5.2:1 over the 19 years of the survey. Tracking this data is important to horseshoe crab harvest management strategies in the Delaware Bay region. The current all-male harvest strategy has given rise to questions regarding impacts to spawning and egg fertilization; however, trends in male spawning in both Delaware and New Jersey have been slightly positive, indicating no cause for concern at this time.
If you see a tagged horseshoe crab, please fill out the U.S. Fish and Wildlife Service Horseshoe Crab Resighting Form. This data can be used to show where tagged horseshoe crabs have spawned historically and increase our understanding of their needs and preferences in habitat.

Page reviewed 1/7/19