Delaware Bay Horseshoe Crab
Benjie Lynn Swan - Limuli Laboratories, Cape May, New
William R. Hall Jr. - Sea Grant College Program, University of Delaware
Carl N. Shuster Jr. - Virginia Institute of Marine Science, College
of William and Mary
The 11th annual Delaware Bay Spawning Survey was conducted during
the 2000 spawning season. Horseshoe crabs were enumerated on twelve
dates surrounding the moon phases in May and June. Eleven beaches in
both New Jersey and Delaware were surveyed.
Three peaks were found in New Jersey and Delaware on May 6th (2days
after the new moon), May 18th (full moon) and June 4th (2 days after
new moon) with the maximum peak estimate occurring on May 18th. Peak
numbers were similar to the two peak1999 estimates both in New Jersey
and Delaware. The maximum peak estimate was 272,770 which included surveys
of 22 beaches and is much lower than previous years' estimates. The
low estimate may be due in part to the beaches surveyed. When a comparison
of the estimates during the 12 survey dates from the same 6 "fixed"
beaches in Delaware and New Jersey was made, the 2000 total spawner
estimate was comparable to 1999.
The male to female ratio during the 12 survey dates was 3.67 during
the 2000 spawning season. The ratio throughout the years (1990-1999)
has been variable but the overrall trend in the male to female ratio
The Atlantic States Marine Fisheries Commission (ASMFC) Horseshoe
Crab Management Plan adopted in October 1998 included a yearly spawning
survey to be conducted along the shores of Delaware Bay. Since a Delaware
Bay volunteer spawning survey has existed since 1990, the ASMFC opted
for continuing this volunteer based survey with additional statistical
guidance from the United States Geological Service (USGS). The survey
was then modified to incorporate the statistical needs for the ASMFC
horseshoe crab monitoring program. The modified survey was successfully
implemented in 1999. The 2000 survey was conducted in the same manner
as the 1999 survey with only a few minor changes.
Horseshoe crabs were counted along the Delaware and New Jersey beaches
of Delaware Bay. Twelve beaches (6 beaches on each side of the Bay )
are "fixed" beaches, meaning they will be surveyed every year.
The 12 beaches are North Cape May, Cape Shore Lab (Green Creek), Highs,
Reeds, Gandys and Seabreeze in New Jersey and Primehook, Fowler, Big
Stone, North Bowers, Kitts Hummock and Woodland in Delaware. Eight additional
beaches were randomly selected for the 2000 survey. These additional
beaches were Higbees, Townbank, Pierces Point and Kimbles for New Jersey
and Broadkill, Bennetts Pier, South Bowers and Pickering in Delaware.
East Point, NJ and Slaughter Beach, DE were also surveyed, for a total
of 11 beaches surveyed on the New Jersey side and the Delaware side.
The survey was conducted on 12 dates during the horseshoe crab's spawning
season, which occurs in May and June along the Delaware Bay. These dates
were in groups of three: two days before the new or full moon, the day
of the moon and two days after the new or full moon. No additional dates
were surveyed during the 2000 survey. In 1999, additional dates or "off"
dates were scheduled, but after reviewing the data, the USGS determined
that surveying around the moons provided the statistical validity needed
for the survey.
Horseshoe crabs were enumerated by using a 1 meter quadrat. The number
of male and female horseshoe crabs within each quadrat were counted.
The quadrat was randomly placed 100 times along the water's edge, parallel
to the beach. Each time the survey was conducted a coin was flipped.
Depending if the coin was heads or tails, the count would begin on the
north or south end of the counting area.
The survey was performed by approximately 281 trained volunteers.
Many of the volunteers have participated in the survey since its inception.
Training workshops were given in Delaware and New Jersey prior to the
survey. The survey's printed materials and the quadrats were distributed
at the workshop.
The counts were conducted during the night time high tide. Counts
began when the tide was just starting to recede. The times of the counts
this year were similar to last year, falling in the evening hours, 7:00-10:00PM
at the reference station, Breakwater Harbor, DE. The time of arrival,
the start time and end time of the survey was recorded. Data collected
also included weather conditions, wave height and the relative amount
Table 1-A and table 1-B (see attached) and Figure 1 summarize the
survey data during the year 2000. The table lists the beaches surveyed,
the density of horseshoe crabs and the estimated number of horseshoe
crabs on each of the twelve dates. Three peaks were identified in each
state during the survey period. The peaks on each side were on the same
dates, May 6th, May 18th and June 4th. The maximum peak occurred during
the full moon date on May 18th.
Weather plays a critical role, greatly affecting the spawning activity
of the horseshoe crabs. Weather conditions during the 2000 spawning
season were favorable for spawning with the exception of June 2nd (the
new moon date). A severe thunderstorm occurred during the night high
tide on June 2nd adversely affecting both volunteers and horseshoe crabs.
Many of the volunteers could not conduct the count due to the dangerous
lightening and high amplitude waves kept the horseshoe crabs from spawning
in great numbers.
The South Cape Shore lab beach supported the majority of the spawners
from all the New Jersey beaches, 37%. The highest density of horseshoe
crabs was recorded on Highs Beach on May 6th (2 days after the new moon)
with a density of 15.91. In Delaware, 25% of the spawners were found
on Slaughter Beach. The highest density of crabs in Delaware was 27.21
on Pickering Beach on May 18th (full moon).
The maximum peak estimate for 2000 was 272,770 during the full moon
date, May 18th. This estimate is the total of the estimates on 11 beaches
both in New Jersey and Delaware. In Delaware, 202,477 spawners were
estimated during the May 18th, 2000 peak count and the total estimate
for New Jersey was 70,293 spawners. This year's estimate is much lower
than previous years' estimates Figure 2; (Table 2 see attached). It
was equivalent, however, to the numbers reported for the 1977 peak spawning
(= 222,000 males and 51,000 females) by Shuster and Botton (1985).
The 2000 estimate from the 6 "fixed" beaches was compared to
last year's estimate both in New Jersey and Delaware (Figure 3. A and
B). The estimates from the twelve dates for each beach in 1999 and 2000
were added and the number compared between the two years. In New Jersey,
fewer crabs (8%) were found during the 2000 spawning season than during
the 1999 season on the "fixed" beaches. During the 1999 counts,
80% of the total number of spawners surrounding the moon phase were found
on the 6 New Jersey "fixed" beaches. During the 2000 survey,
65% of the total spawners were found on these beaches.
The estimates from the 6 "fixed" beaches in Delaware were
compared between the two years. The numbers remained relatively the
same (447,756 in 1999 and 450,114 in 2000). In 1999, 77% of the total
number of horseshoe crabs were found on the "fixed" beaches
whereas in 2000 only 49% were found on these beaches.
Male to female ratios were calculated based on the actual number of
males and females counted during all the survey dates. The overall male
to female ratio this year was 3.67, in Delaware the ratio was 3.87 and
in New Jersey, 3.38. This ratio is similar to last year's ratio of 3.72.
The ratios from the previous years 1990 -1998 were 2.19, 2.64, 3.13,
2.49, 2.46, 2.74, 2.45, 3.48, and 4.66 respectively.
The peak numbers were similar to the 1999 counts, with the peaks in
New Jersey totaling 75,033, 70,293 and 67,092 on May 6th, May 18th and
June 4th respectively and in Delaware, 151,376, 202,477 and 128,583.
Last year (1999) only two peaks were identified. This year, the moon
phase in May was early, May 4th, whereas last year (1999), the first
moon related count started on the 13th of May. This may in part explain
the three peaks determined during the 2000 survey. During these three
peak days, 53% of the total number of spawners were estimated in New
Jersey and 52% were estimated in Delaware.
The 2000 estimate was 272,770 for the 11 beaches surveyed in Delaware
and New Jersey. This estimate is much lower than the 1999 estimate of
422,775, which was comprised of estimates from 13 beaches in New Jersey
and 8 Delaware beaches. New Jersey's low estimate of 70,293 spawners
could be explained in part due to sampling fewer beaches and sampling
beaches with less spawning activity. In 1999, three beaches which historically
exhibit good spawning activity, Sunray, Fortescue and Moores beaches,
were sampled, contributing 40,176 individuals to the total estimate.
In the 2000 survey, these three beaches were not surveyed and Higbees
beach, a beach with low spawning activity was surveyed.
The Delaware count of May 30th, on the full moon date in 1999 was
281,056 for 9 beaches. Most of the Delaware beaches surveyed in both
1999 and 2000 had lower estimates in 2000. The two additional beaches
surveyed in Delaware for the 2000 survey were South Bowers and Bennetts
Pier. Big Stone beach, due to its long expanse (7.6 km), was divided
for the 2000 spawning survey to ensure that an accurate sample was taken.
Therefore, Big Stone beach was split to include a 5.6 km stretch on
the south end and a 2.6 km stretch, Bennetts Pier, on the north end.
Comparing the total estimates from the two years poses some problems
since not all of the same beaches surveyed, therefore estimates from
the 6 "fixed" beaches were compared. The NJ beach estimate
was slightly lower, 8%, in 2000 than in 1999. On the Delaware side,
the numbers were comparable in 1999 and 2000.
The selection of beaches for surveying is critical for obtaining the
peak overall estimate. Horseshoe crabs do not utilize the whole Delaware
bay shore similarly. Horseshoe crab spawning activity is greatest along
the middle beaches of the bay. The abundance of horseshoe crabs, if
plotted along the axis of the Delaware Bay, creates a bell shaped curve.
Historically this is the case and during the 11 years of the volunteer
spawning survey, this generalization has still proven to be correct.
In past years, surveying was scheduled first and foremost on the middle
beaches. If additional help was available, the outlying beaches were
surveyed. The selection of beaches under the direction of the USGS is
random, although beaches are somewhat categorized based on geographic
location and historical spawning activity. This random process may not
be the best methodology to use for the selection of beaches. Slaughter
Beach was not one of the beaches randomly selected for surveying this
year, however it produced the the greatest number of spawners.
The beaches were covered more extensively this year with fewer survey
dates missed. New Jersey missed 16.7% of the dates in 1999 and 9.7%
dates in 2000. In Delaware, 29% of the dates were missed in 1999 and
18% in 2000. The June 2nd date was missed on almost all of the beaches
due to a severe thunderstorm. Additional dates were missed because of
shortages in volunteer help or last minute emergencies.
Male to female ratios can be instructive in assessing a population.
The ratio of males to females during the spawning season usually results
in more male spawners than females, which is a behavioral aspect of
the horseshoe crab's spawning activity. This ratio is also affected
by weather conditions, calm weather allows more males to congregate
around a female and rough weather allows just the attached male to hang
on. Since 1997, the ratio of males to females has climbed considerably.
One factor which negatively affects this ratio is the collection of
female horseshoe crabs as the preferred bait for eel and conch (whelk)
The Delaware Bay Spawning Survey continues to be a proven methodology
to be an extremely useful tool in estimating the spawning population
of horseshoe crabs. The project is supported by many dedicated non-governmental
organizations (NGOs) and volunteers who brave the dark, the bugs, the
water, the weather and sleepless nights to assist in the count. As always,
thank you, thank you, thank you to all the volunteers!!!
Shuster, C.N. Jr. and M. L. Botton. 1985. ÒA contribution to
the population biology of horseshoe crabs, Limulus polyphemus (L.),
in Delaware Bay.Ó Estuaries 8 (4): 363-372.