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Research & Monitoring

Status and Trends: LISS Environmental Indicators

Horseshoe Crab Abundance

CTDEEP, NYSDEC, and Millstone Environmental Laboratory

Horseshoe Crab Abundance CT
  CTDEEP West (kg/tow) Niantic Bay East (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
1992 0.35 0.468
1993 0.45 0.543
1994 0.6 0.697
1995 0.13 0.372
1996 0.61 0.402
1997 0.33 0.385
1998 0.55 0.296
1999 0.8 0.248
2000 0.74 0.35
2001 0.94 0.175
2002 0.76 0.038
2003 1.33 0.198
2004 0.96 0.15
2005 0.39 0.043
2006 0.25 0.124
2007 0.86 0.074
2008 0.62 0.073
2009 0.65 0.064
2010 0.52 0.017
2011 0.81 0.047
2012 0.55 0.043
2013 0.70 0.098
2014 0.66 0.123
2015 0.48 0.068
2016 0.52 0.060
Horseshoe Crab Abundance NY (Count per tow)
  Peconic East Manhasset and LIttle Neck West
1987 0.49 1.22
1988 0.70 1.89
1989 2.68 5.09
1990 1.66 1.54
1991 3.52 4.39
1992 3.60 1.47
1993 2.99 4.93
1994 2.60 0.76
1995 2.16 2.94
1996 2.40 2.43
1997 2.01 7.79
1998 1.26 5.49
1999 1.63 3.90
2000 0.94 5.03
2001 1.16 3.88
2002 0.50 5.33
2003 0.68 6.71
2004 0.73 1.80
2005 0.75 1.64
2006 1.17 2.47
2007 0.63 2.70
2008 0.63 2.41
2009 0.52 1.03
2010 0.23 1.08
2011 0.36 2.58
2012 0.33 2.07
2013 0.30 3.53
2014 0.24 1.52
2015 0.23 0.92
2016 0.13 2.09

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 on shore 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 for 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

Both the CT and NY indices show a stable or increasing abundance in western Long Island Sound through the 1990s, with high abundance in 2003, 2007, and 2011.  Although abundance was equally strong in the eastern Sound and Peconic Bay during the early 1990s,  it declined from 1995 through the 2000’s. While values remain low, they do seem to have stabilized or improved slightly over the last couple of years.

CTDEEP’s trawl survey samples mainly in the western and central Sound, while the Millstone Environmental Laboratory dataset in Niantic Bay reflects 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.

DATA NOTE

  • The CTDEEP data are from the spring 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.
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