Female Horseshoe Crabs in Decline in Raritan Bay

A decline in female Horseshoe Crabs (Limulus polyphemus) is taking place in Raritan Bay and Sandy Hook Bay in New Jersey.  For about every 20 or 30 single males there are only about 1 or 2 single females, which is out of proportion to the average of 5 to 10 single males for every single female.

This information is according to the Bayshore Watershed Council, an all-volunteer environmental organization dedicated to protection and restoration of Raritan Bay and Sandy Hook Bay. For the past five years, since 2009, volunteers with the council have been monitoring Horseshoe Crab populations at five sites along the estuary: 1) Plum Island at Sandy Hook, 2) the mouth of Many Mind Creek in Atlantic Highlands, 3) Leonardo public beach in Middletown Township, 4) Conaskonck Point in Union Beach, and 5) Cliffwood Beach in Aberdeen Township.

After five years of monitoring and tagging adult Horseshoe Crabs the good news is that the overall Horseshoe Crab population has been increasing. The total crab population in 2009 was 1,174. In 2013, the total crab population was 2,916. In general the crab population approximately doubled within the study area over five years. Perhaps the rapid decline in abundance during  the 1990s has been halted and the crab population has stabilized or is increasing.

The most disturbing surprise, however, was finding that the female population has not remained stable over five years. Although there has been an increasing trend in male density, female density has decreased. When the study began in 2009, there were 330 single females and a population ratio of 72 single males for every 27 single females. In 2013, there were only 40 single females in the entire study area. This is not consistent with monitoring sites in Delaware Bay that report a ratio of 3:1 or 5:1, with males outnumbering females. This means there is a tremendous amount of stress on the female population to function.

While male Horseshoe Crabs normally outnumber female Horseshoe Crabs on spawning beaches in the northeast, which usually corresponds to greater genetic diversity, the sizable difference in the sex ratio in Raritan Bay and Sandy Hook Bay appears atypical. The increase in mating pairs and clusters over five years suggests that adult sexually active females are being fought over quickly by single males.

The wide difference between single male and female populations puts the overall population in Raritan Bay and Sandy Hook Bay on a razor thin edge. Any disturbance to the female population will automatically cause a crash in the local Horseshoe crab population. Of course, more research is needed to see if this trend continues.

What could be causing this wane in female Horseshoe Crabs, at least in the short-term? New research indicates it very well could be attributed in great part to excessive harvesting in neighboring New York State.

New York State is the second largest harvester of Horseshoe Crabs in the United States behind Maryland. In 2013, New York harvested 167,723 crabs, while Maryland harvested 169,087 crabs. That’s a lot of crabs! Why so many?

Horseshoe Crabs, especially females which are rich with eggs, are used as bait to catch eels and whelks, which in turn are sold for money to overseas markets in Europe and Asia. Horseshoe Crabs have also been known to be used as bait to catch Blue Crabs and certain species of fish, such as catfish.

The laws concerning harvesting Horseshoe Crabs in New York State can be generous. While a permit is needed for commercial harvest, there is no permit needed for recreational use. Anyone can catch five crabs per day, whether a resident or not, as long as the crabs are used for the harvester’s own personal use and not offered for sale. That has the potential to be a lot of crabs per day.

Although New Jersey has instituted a moratorium on harvesting Horseshoe Crabs in 2007, there is no such law in New York State. People are still able to harvest crabs. This action puts the crab population under severe threat in Lower New York Bay, including Raritan Bay and Sandy Hook Bay.

The over-harvesting of Horseshoe Crabs, especially females, combined with other factors including climate change, habitat loss, sea level rise and water temperature fluctuations, may well limit the distribution and breeding of these crabs, resulting in localized population declines. This could have several implications.

Horseshoe Crabs are an important part of the ecology of coastal communities around Lower New York Bay. During the nesting season in May and June, Horseshoe Crab eggs become the major food source for migrating birds. Over 50 percent of the diet of many shorebird species consists of Horseshoe Crab eggs. Many bird species, including the endangered Red Knot, and Ruddy Turnstone have been observed feeding on Horseshoe Crab eggs. In addition, many fish species rely on Horseshoe Crab eggs for food.

Horseshoe Crabs are also extremely important to the biomedical industry because of their unique, copper-based blue blood contains a substance called Limulus amebocyte lysate. The substance, which coagulates in the presence of small amounts of bacterial toxins, is used to test for sterility of medical equipment and virtually all intravenous drugs. Research on the compound eyes of Horseshoe Crabs has led to a better understanding of human vision.

All this activity will be adversely affected if Horseshoe Crabs decline.

The results of the study by the watershed council indicates there is more work to be done in managing Horseshoe Crab populations not only in Delaware Bay, but up north in Raritan Bay and Sandy Hook Bay. New York State needs to develop a new Horseshoe Crab harvest adaptive management plan that incorporates better protection for female crabs at the very least.

The Bayshore Watershed Council enlists the public to help monitor adult spawning Horseshoe Crab populations in May and June along Raritan Bay and Sandy Hook Bay. We meet on new moon and full moon evenings during high tide. If you would like to be a volunteer and help with the programs, please visit the watershed website at www.restoreourbay.org or send us an email to bayshorewatershed@comcast.net.

For more information, pictures and year-round sightings of wildlife in or near Sandy Hook Bay, Raritan Bay, and Lower New York Bay, please check out my blog entitled, Nature on the Edge of New York City at http://natureontheedgenyc.blogspot.com