People are taking too many crabs to be sold and used as conch and eel bait. Another big problem is horseshoe crabs sometimes get stranded on the beach. They get flipped over by big waves, and they can't get back up. If they stay on their backs too long, the sun will dry out their gills, and the crabs will weaken and die. Then gulls, raccoons, or other beach predators come along and eat them.
Pollution also is a possible threat to horseshoe crabs.
People are using more pesticides on their lawns and farmland, and New Jersey and Delaware are spraying more for mosquito control. As a result, you get more chemicals washing into the water from the urban and agricultural runoff. Junk in the water causes problems, too. Sewage, factory waste, rainwater carrying pollutants from parking lots, yards and streams can kill clams and other animals that crabs use for food. Water pollution may weaken or kill horseshoe eggs and larvae.