Changes to Habitat
Between 1990 and 2020, the population along
the coast of Sussex County, Delaware, is expected to increase
in response to rapid population growth can lead to deterioration
of water quality and habitat in Delaware Bay. The examples
below illustrate some of the ways people can inadvertently
destroy critical horseshoe
How to "Lose" a Habitat
Method #1: Jetties and bulkheads, constructed to prevent shoreline erosion, obstruct horseshoe crab access to prime nesting grounds along Delaware Bay.
Method #2: Beach nourishment (replacing sand lost due to erosion) and dredging can alter spawning habitat characteristics. Changes in sediment size or drainage may impact embryo survival because eggs need a combination of oxygen and moisture to fully develop.
Method #3: Horseshoe crabs tolerate marine pollutants better than some marine arthropods. However, high concentrations of oil or heavy metals can affect juvenile survival.
Method #4: Delaware Bay
provides access to the largest oil-shipping port on
the East Coast. An oil spill during the spawning season could
be detrimental to adult horseshoe crabs and horseshoe crab
Although these are a few examples of the problems that face horseshoe crabs in and around the Delaware Bay region, similar and additional threats due to human population growth and development are present throughout the ranges of all horseshoe populations. Take the Japanese horseshoe crab, for example.
A World-Wide Problem
|Tatara Beach, Fukuoka, Japan: a horseshoe crab spawning beach.
Described in Japanese literature as early as the 1600s, the
Japanese horseshoe crab, Tachypleus tridentatus, was
once a common seaside visitor. Over the past century, however,
the number of horseshoe crabs inhabiting Japan's coast has
drastically declined. Coastal development and pollution have
taken their toll on the population. For example, land reclamation
projects in Kasaoka Bay have altered the quality of natural
breeding habitat. Flow patterns in the bay have changed, washing
sediment out to sea. Formerly a mix of sand and mud, regions
of Kasaoka Bay are now characterized by a hard bottom substrate.
This substrate restricts oxygen exchange between the sediment
and the water and is more difficult to burrow into. Currently,
the population of horseshoe crabs in Kasaoka Bay is estimated
to be between 40 to 50 pairs.
Hope for the Future
Despite the negative impact of development along much of Japan's
coast, the population of horseshoe crabs native to northern
Kyushu appears to be recovering. In the Bay of Hakata, Fukuoka,
horseshoe crab conservation groups continuously monitor the
status of the population. Almost 10,000 adult horseshoe crabs
are now reported to be living in this bay. Local government
and volunteer organizations have played central roles in the
protection of this species throughout Japan.
Recognizing the coastline bordering Kasaoka City as a key area for horseshoe crab breeding, the municipal government dedicated the coast as a national monument for horseshoe crabs in 1928. A protected area was also established on the God Island Aqueduct in 1971.
T. tridentatus embryos between the second and fourth embryonic molts.
The Kasaoka Municipal Horseshoe Crab Museum was built in 1990
to promote appreciation and interest in the conservation of Tachypleus
tridentatus. In addition to educational exhibits, the
museum rears horseshoe crab larvae for release into the wild,
and conducts research to learn more about the biology of the
horseshoe crab. Museum scientists are using radio telemetry
to track migration patterns, identify overwintering grounds,
and locate potential spawning beaches. Click
here to visit the museum's Web site.
Volunteer groups and conservation organizations participate in annual coastal cleanups to maintain the health of local horseshoe crab spawning beaches.
Harvesting For Bait An Even Greater Threat
Abrupt and gradual changes to habitat may prove harmful to horseshoe
crab populations over time. But a more serious and immediate
threat may be the recent, dramatic increases in horseshoe crab
harvests for bait in the eel and whelk fisheries.
Recently, a Fisheries Management Plan has been set in action
to prevent overharvesting of horseshoe crabs. Click
here to learn about horseshoe crab management policies designed
to protect horseshoe crabs.