The Horseshoe Crabs of Delaware Bay
There are several ongoing surveys around the Delaware Bay that can be used to estimate trends in the population of horseshoe crabs. All of these surveys indicate that the population of horseshoe crabs has dropped off significantly since 1990.
A census of the number of spawning crabs around the Delaware Bay found a two thirds decrease in the number of crabs since 1991. Most of the decrease has appeared as fewer crabs spawning on New Jersey's shores, the number of crabs spawning on Delaware's shores has not shown a similar decline. Trawl surveys done by Delaware show a similar decline in the overall numbers of crabs with a high correlation to the total numbers found by the spawning census.
Unequal distribution of horseshoe crabs around the bay is not unprecedented. There are reports from earlier this century and last century that indicate that crabs were found in unequal numbers on the opposite sides of the bay. These reports indicate that there was usually a shift, with a decrease on one side corresponding with an increase on the other.
Of greatest concern to shorebirds is the fact that the decline in spawning horseshoe crabs on the New Jersey shores has also resulted in the decline of surface eggs that are available to shorebirds for food. When sampling that was originally done in 1990 & 1991 was performed performed again in 1995 & 1996, it showed a 90% decrease in the numbers of eggs available to shorebirds. The effects of this on the shorebird numbers was most pronounced this year (see Shorebird Surveys).
One shortcoming of the following surveys is that most of them began in 1990 so that they do not indicate what the population trends were before that point. The surveys do indicate the importance of collecting a variety of data to better understand the trends in the horseshoe crab population.
This census (Swan, Hall, & Shuster, 1996) organized by Limuli Labs
and performed by volunteers estimates the numbers of spawning crabs
on 30 beaches around the Delaware Bay. Eighteen 5 meter segments are randomly
selected on each beach before spawning occurs and then the number of crabs
in each segment is counted a half hour after high tide. These samples are
then used to generate an estimate for the number of crabs spawning along
the entire beach and these estimates are added to get totals.
Since this work is performed by volunteers all surveys are done on
Saturdays.
Between 1990 and 1994, a single Saturday was chosen for the census
several months in advance. Since 1994, censuses are done on the
4 Saturdays closest to the full and new moons during May and June.
The peak count is shown in the accompanying graph.
The number of crabs spawning can vary based on weather
and tide height so these numbers need to be viewed
with some caution. Some other issues are included here.
#-1994 was a cool year and the preselected Saturday had
a very low count of crabs, a 2nd census was performed later that year
and those numbers are shown in the graph. *-1995, the peak
spawning event happened in early May and was not recorded by the
census.
The graph shows a rapid decrease in the total number of spawning crabs after
1991 but the total seems to be stable after that point. Looking at New Jersey's
numbers, the graph shows a steady decline from 1990 through 1994.
Delaware's numbers are variable but do not seem to show much of a
trend in either direction.
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This survey was originally performed in 1990 & 1991
(Botton, Loveland, & Jacobsen, 1994) to estimate the
egg densities of horseshoe crab eggs on
6 beaches on the New Jersey side of the Delaware Bay.
Samples were taken from the surface (0-5cm)
to estimate the density of eggs available for shorebirds.
Samples were also taken at a depth of 15-20cm, the depth at which
eggs are initially deposited, to estimate
the abundance of eggs.
These surveys were repeated in 1995 & 1996 after several organizations became concerned about the number of horseshoe crabs spawning on the New Jersy beaches. Comparison of the results showed that there had been a 90% reduction in the number of eggs available to shorebirds. |
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Trawl surveys that are used to estimate the populations of commercially harvested fishes are performed each year by New Jersey and Delaware. These trawls do not specifically target horseshoe crabs but crabs are caught in the process and the method is consistent. (Trawl nets are dragged along the bottom but are raised slightly to avoid getting caught on rocks. Since horseshoe crabs crawl and burrow in the mud along the bottom many of them are missed by a trawl net. Since the method is consistent though, any trends in the numbers caught should reflect trends in the overall population.)
Delaware's trawl survey shows
a strong decline in horseshoe crab numbers after 1991 and
has a high correlation with the spawning census.
New Jersey's trawl is done along the Atlantic coast and not in the
Delaware Bay. The trawl results do show a peak in 1990
that has not been matched since but overall it does not
show a decline. This raises questions about what the
other surveys would show if they had been performed
in years prior to 1990.
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Dredge surveys are performed by New Jersey to determine the population of surf clams along the Altantic coast south of Egg Harbor inlet. These surveys are not targeted at horseshoe crabs but crabs are caught and counted, and the method is consistent. Unlike trawl surveys, a dredge should catch a higher percentage of the crabs present since it scrapes the bottom. This survey shows a decline after 1992 and its one sample prior to 1990 shows a population that is more similar to 1990 & 1992 than subsequent years. This contrasts with the pre-1990 data from the New Jersey trawl survey |
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