Ecosystem Targets and Supporting Indicators

Horseshoe Crab Abundance

This indicator shows the abundance of Horseshoe Crabs in Connecticut and New York.

Show/Hide Table Data

Horseshoe Crab Abundance CT
CT DEEP (kg/tow)Niantic Bay (Count/tow)
1976-0.77
1977-0.598
1978-0.124
1979-0.205
1980-0.132
1981-0.175
1982-0.244
1983-0.197
1984-0.141
1985-0.162
1986-0.141
1987-0.24
1988-0.343
1989-0.319
1990-0.393
1991-0.305
19920.650.468
19930.770.543
19940.570.697
19950.200.372
19960.850.402
19970.660.385
19980.690.296
19990.910.248
20000.950.35
20011.110.175
20021.110.038
20031.310.198
20041.310.15
20050.580.043
20060.460.124
20071.060.077
20080.870.073
20090.860.064
20100.520.017
20111.000.047
20120.590.043
20130.890.128
20140.600.156
20150.450.068
20160.460.060
20170.540.051
20181.180.090
20190.730.026
20200.013
Horseshoe Crab Abundance NY (Count per tow)
Peconic EastManhasset and Little Neck West
19870.491.22
19880.701.89
19892.685.09
19901.661.54
19913.524.39
19923.601.47
19932.994.93
19942.600.76
19952.162.94
19962.402.43
19972.017.79
29981.265.49
19991.633.90
20000.945.03
20011.163.88
20020.505.33
20030.686.71
20040.731.80
20050.751.64
20061.172.47
20070.632.70
20080.632.41
20090.521.03
20100.231.08
20110.362.58
20120.332.07
20130.303.53
20140.241.52
20150.230.92
20160.132.09
20170.131.35
20180.191.61
20190.201.27
20200.05*NA*

WHAT ARE HORSESHOE CRABS?

The horseshoe crab (Limulus polyphemus) is an arthropod more closely related to spiders than other crabs and has a body form that evolved more than 200 million years ago. They are found from Nova Scotia to Mexico and live year-round in Long Island Sound. They are harvested as bait for other fisheries, but are not typically eaten for meat. They come onshore to spawn in May and June, with spawning tending to peak at night around the new and full moons.

WHAT DOES THIS INDICATE?

Since horseshoe crabs rely on beaches and the shallow intertidal environment to produce their young, their abundance is an indicator of the health and productivity of this transitional environment. Horseshoe crab eggs are an essential food source for migrating shorebirds, and their larvae and juveniles are consumed by many fish. They are most valuable to humans for their blood proteins which are extremely sensitive to bacteria. The extracted compound, known as LAL, is used to screen injected drugs and implanted biomedical devices for contamination.

STATUS

CT DEEP’s Long Island Sound Trawl Survey samples throughout the Sound in both Connecticut and New York waters during the spring (April, May, and June) and Fall (September, November, October). The Millstone Environmental Laboratory dataset in Niantic Bay reflects the abundance in the eastern Sound near the Connecticut shoreline. The Manhasset and Little Neck Bays NYSDEC seine survey reflects conditions in western Long Island Sound along the New York shoreline, while the trawl survey in Peconic Bay, which borders Long Island Sound, is indicative of conditions in eastern Long Island Sound near the New York shoreline.

Both the CT and NY indices show a decreasing abundance in Long Island Sound from the early ‘00s. Although American horseshoe crab populations increased in the 1990s, recent years show a declining trend due to various stressors (e.g., loss of habitat and illegal harvesting). To stabilize and restore the horseshoe crab populations, regulations have been implemented such as limiting harvesting permits, closed areas, seasonal closures, commercial quotas, and restricting the timing (lunar closures) and daily number of crabs a harvester can possess.

DATA NOTEs

  • The CT DEEP data are from the spring and fall trawl survey.
  • The Manhasset and Little Neck dataset combines the count per tow of two stations in western Long Island Sound.
  • The survey uses geometric mean instead of arithmetic mean to find the most frequently observed  number of  fish collected per tow. In a natural environment such as Long Island Sound  fish have a “patchy” distribution, i.e. some areas will have a very high abundance of fish, and other areas very low abundance or no fish at all. An arithmetic mean can be easily biased by unusually high or low values so that it doesn’t reflect the true center of a data set.  The geometric mean minimizes the effects of very high or low values using a log transformation and is a better average for this type of biological data.
  • Due to the Covid-19 pandemic sampling was either reduced (Peconic) or infeasible (WLIS) and therefore, data is either limited or unavailable in NY for 2020.
  • In 2020, CT DEEP was unable to conduct its Long Island Sound Trawl Survey due to COVID-19 restrictions.

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