Human Impact

Changes to Habitat

Between 1990 and 2020, the population along the coast of Sussex County, Delaware, is expected to increase by 75%. Coastal development 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 crab habitat.

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 eggs.

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.