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A multi-year horseshoe crab tagging study will begin this spring in the Delaware Bay

April 15, 2003

Tags Help Researchers Understand Horseshoe Crab Populations

A multi-year horseshoe crab tagging study will begin this spring in the Delaware Bay. During April, scientists from DNREC Delaware Coastal Programs, U.S. Geological Survey, U.S. Fish and Wildlife Service and Cornell University are tagging and releasing thousands of male and female horseshoe crabs throughout the Bay. By recovering tagged horseshoe crabs as they come ashore to spawn, researchers can estimate the total number of horseshoe crabs that spawn in a given year, estimate survival from year to year, and study the migratory patterns of horseshoe crabs in Delaware Bay.

Beginning in 2004, scientists will also track the movement of individual female horseshoe crabs using radio telemetry. Over a two-year period, hundreds of female horseshoe crabs will be outfitted with a tiny transmitter weighing about 30 grams that will emit a unique signal each time she comes to shore. Radio receivers, positioned strategically on both sides of the Bay, will pick up that signal and create a record of where and when she spawned. This method will allow researchers to track the movements of individual horseshoe crabs throughout the entire spawning season, a task that would be impossible without this technology.

The tagging and radio telemetry study, coupled with on-going research like horseshoe crab egg monitoring and the volunteer based spawning survey, will increase our understanding of horseshoe crab migration, survival, and abundance and allow for better management of these special creatures and their important habitats.

You can help! If you see a tagged crab on the beach, please record the location, time, date and all tag numbers and tag types (some crabs may have more than one tag). In addition, please report its sex and whether it was found alive or dead. This information can be reported by calling 1-888-LIMULUS or by contacting Dave Smith at

horseshoe crab illustration

Figure showing possible locations of tags and tag types. Note that an actual horseshoe crab will have only one or two of the tags shown. A) Radio tag; B) Glued disk tag; C) Round button tag; and D) Squarish button tag. Also, an example tag is shown on the right with the tag number identified.