Horseshoe crabs (Limulus polyphemus) are a marine arthropod found along the Atlantic coast from northern Maine to the Yucatan Peninsula and the Gulf of Mexico. The Delaware Bay supports the largest spawning population in the world. Adults either remain in estuaries or migrate to the continental shelf during the winter months. Migrations resume in the spring when the horseshoe crabs move to beach areas to spawn. Juveniles hatch from the beach environment and spend the first two years in nearshore areas.
Spawning usually coincides with the high tide during the full and new moon. Breeding activity is consistently higher during the full moon than the new moon and is also greater during the night. Adults prefer sandy beach areas within bays and coves that are protected from surf. Eggs are laid in clusters or nest sites along the beach with females laying approximately 90,000 eggs per year in different egg clusters.
The eggs play an important ecological role in the food web for migrating shorebirds. The Delaware Bay Estuary is the largest staging area for shorebirds in the Atlantic Flyway. An estimated 425,000 to one million migratory shorebirds converge on the Delaware Bay to feed and rebuild energy reserves prior to completing their northward migration.
Horseshoe crabs provide the backdrop for one of the most interesting marine resource management issues along the Atlantic coast. In addition to their role as a food source for birds, horseshoe crabs provide bait for commercial American eel and conch fisheries along the coast. Their unique blood is also used by the biomedical industry to produce Limulus Amoebocyte Lysate (LAL). The challenge of fisheries managers is to ensure that horseshoe crabs are managed to meet all these diverse needs, while conserving the resource for future generations.
From the 1850s to the 1920s, between 1.5 and two million horseshoe crabs were harvested annually for fertilizer and livestock feed. Harvest dropped throughout the 1950s and ceased in the 1960s. Between 1970 and 1990, reported commercial harvest ranged from less than 20,000 pounds to greater than two million pounds annually. Since the mid- to late 1990s, commercial harvest has been sold primarily as bait for the American eel and whelk pot fisheries. Increased need for bait in the whelk fishery likely caused an increase in horseshoe crab harvest in the 1990s, with a peak of nearly six million pounds in 1997. Preliminary coastwide commercial landings for bait in 2011 were approximately 650,000 horseshoe crabs.
Commercial fishermen have adopted new gear such as bait bags and cups allowing them to effectively catch eel and conch while using as little as a tenth of the bait. The majority of horseshoe crab harvest comes from the Delaware Bay Region, followed by the New York, New England, and Southeast Regions. Trawls, hand harvests, and dredges make up the bulk of commercial horseshoe crab bait landings. Discard mortality occurs in various dredge fisheries and may vary seasonally with temperature, impacting both mature and immature horseshoe crabs; however, the actual rate of discard mortality is unknown.
Horseshoe crabs are also collected by the biomedical industry to support the production of LAL (short for Limulus amoebocyte lysate), a clotting agent that aids in the detection of human pathogens in patients, drugs, and intravenous devices. No other procedure has the same accuracy as the LAL test. Blood from the horseshoe crab is obtained by collecting adults, extracting a portion of their blood, and releasing them alive. Following bleeding, most crabs are returned of the waters where they were captured. However, since 2004, states have the ability to enter bled crabs into the bait market and count those crabs against the bait quota. In recent years, the total estimate of horseshoe crabs caught for medical usage is around 500,000 per year on the Atlantic coast. Estimated mortality on biomedical crabs not counted against state bait quotas has increased from about 45,000 crabs in 2004 to 80,800 crabs in 2011.
A close-up view of spawning horseshoe crabs. Photo credit: Dr. Rob Robinson, British Trust for Ornithology.
Little is known about the status of the horseshoe crab population. Limited time-series of horseshoe crab population data make it difficult to assess its status. However, the 2009 stock assessment and peer review concluded increasing trends in abundance in the Southeast and Delaware Bay regions, and decreasing abundance in the New York and New England regions.
Horseshoe crabs are managed under the Interstate Fishery Management Plan for Horseshoe Crab (December 1998). In 2000 under Addendum I, the Commission established state-by-state quotas in all Atlantic states for crabs harvested for bait. In 2006, in response to decreasing migratory shorebird populations Addendum IV was approved. This enabled the Commission to reduce quotas in New Jersey and Delaware and added additional protection in Maryland and Virginia to increase horseshoe crab and egg abundance in and around Delaware Bay. These measures were in place from 2006 - 2012.
In 2012, using its Adaptive Resource Management (ARM) Framework, the Commission's Horseshoe Crab Management Board approved set a harvest limit of 500,000 Delaware Bay male horseshoe crabs and zero female horseshoe crabs for the 2013 season. The ARM Framework, established through Addendum VII, incorporates both shorebird and horseshoe crab abundance levels to set optimized harvest levels for horseshoe crabs of Delaware Bay origin. It was developed in recognition of the relationship between horseshoe crab eggs and shorebirds in the Delaware Bay Region. The optimized harvest level will be reevaluated annually, allowing for management to adapt to the changes in the population levels of horseshoe crabs and shorebirds as a result of the regulations.