Estimation of Short‐Term Tag‐Induced Mortality in Horseshoe Crabs Limulus polyphemus

Horseshoe crabs Limulus polyphemus range along the East Coast of the United States and over 150,000 of them have been marked with U.S. Fish and Wildlife Service disk tags. It has been assumed that the tags do not harm the animals and are similar to common epibionts often found on the shells of horseshoe crabs. We investigated whether newly tagged adult female horseshoe crabs would exhibit higher short‐term mortality rates than untagged adult females. All crabs were collected from a beach in Connecticut and then were transported to a laboratory for the experiment. Tagging involved drilling a small hole through the carapace in the lower back corner of the prosoma and then inserting the tag into the hole. Overall mortality of tagged and untagged females held in flow‐through raceways for 44 d was minimal (0% mortality among 53 tagged crabs; 4% mortality among 52 untagged crabs). None of the horseshoe crabs lost or shed their disk tags over the course of the experiment. In field mark–recapture studies of horseshoe crabs, typically between 11% and 20% of the initial numbers of tagged crabs are recaptured. Crabs that are recaptured but reported dead are unlikely to have died from the initial tagging process. Our results indicate that newly tagged adult horseshoe crabs have the potential to survive as well as untagged crabs through the Connecticut spawning season (∼30–45 d). Recapture data also suggest that these crabs can survive for as long as 8–10 years with the tags in place.