Regional biodiversity monitoring reveals severe population decline of the Atlantic horseshoe crab (Limulus polyphemus) in Long Island Sound, USA

Horseshoe crabs are an ancient species facing modern threats. While their importance to the conservation of endangered birds has garnered them increasing protections in recent years, disparate and insufficient management strategies across regions have precluded population recovery. Six datasets spanning 22-46 years for Atlantic horseshoe crabs (Limulus polyphemus) in the Long Island Sound, USA region were analyzed from regional monitoring programs conducted for general biodiversity assessments that had not been previously aggregated for the evaluation of this species. In these datasets, a decline of 2-9% per year in L. polyphemus count per sample was observed, with significant individual declines in 5 of the 6 datasets (all datasets from bays and harbors). The central Long Island Sound dataset exhibited a significantly slower rate of decline, driven by higher counts per sample in recent years, which should be monitored to determine if an improving trend will be sustained or if the recent higher counts were anomalies. An increasing proportion of females, mean prosomal width, and mean weight of L. polyphemus were also observed. With management activities at the local, regional, and national level impacting the potential for population recovery in this species, it is critical for policymakers at all levels to be aware of the severe decline in observations of this species across data sources, pointing to a need for urgent action to prevent local extinction.