Estimation

Objectives

A recent fossil find in Manitoba places the horseshoe crab's origins at least as far back as 450 million years ago! ¹ It is a unique creature that inhabits the Delaware Bay, amongst other places, and most of my students have seen them at the beach. Our school is located within a half hour's drive to some of the most popular horseshoe crab spawning beaches on the planet. Each year, I assist one of our 8 ^th grade teachers, Garth Stubbolo, on his annual overnight "Green Eggs and Sand" fieldtrip to the beaches to count the spawning horseshoe crabs. Green Eggs and Sand is a multi-state coordinated environmental education program designed to teach students about the horseshoe crab/shorebird connection in the Delaware Bay. ² On the fieldtrip, it is always incredible to see the excitement and focus on a student's face as he/she holds a horseshoe crab for the first time and they can actually tell you about the different body parts and behaviors it exhibits. Its importance to the migratory shorebirds, commercial fishermen, the biomedical field, the major controversy surrounding its use and management, plus the fact that it lives right in our "backyard," - all these things combine to make the horseshoe crab a promising subject for creating an entertaining and educational math unit on estimation.

In 1990, the first annual horseshoe crab census was organized by the Delaware Sea Grant to estimate the number of spawning horseshoe crabs in the Delaware Bay. ³ This annual census continues to this day. However, with the implementation of the Atlantic States Marine Fisheries Commision's (ASMFC) fisheries management plan for the horseshoe crab in 2001, a more statistically rigorous scientific spawning survey was developed and has been in use since. ⁴

Why the need to estimate the number of horseshoe crabs in the Delaware Bay? Many people are directly affected by the laws pertaining to the harvesting of the horseshoe crab, especially the eel and conch fishermen. They use the horseshoe crab to attract conch and eel to their traps. The male horseshoe crab does not attract the eel the way the female horseshoe crab does, therefore, the eel fishermen are mainly interested in harvesting the females. Since the females are the ones who lay the eggs, many environmentalists worry that the harvesting of the females could be, if it has not been already, detrimental to the future of this species. To ensure future generations of this ancient mariner, many environmentalists want a complete moratorium on horseshoe crab harvesting. This occurred in Delaware, though the decision was reversed in 2001, mainly due to the fact that the livelihood of the fishermen would be greatly affected, and the fact that the horseshoe crab estimates do not warrant a moratorium at this time. ⁵

Another group of people who are interested in the laws affecting the harvest of the horseshoe crab are those who are concerned about the plight of the different species of shorebirds. There are many species of shorebirds that briefly stop over in the Delaware Bay in May to gorge themselves on the eggs of the horseshoe crabs, which are laid in the sand of the beaches. The horseshoe crab nests are usually just a few inches deep and are often disturbed by other nesting horseshoe crabs, thereby scattering many eggs near or on the surface making it easy for the shorebirds to eat them. One female horseshoe crab can lay up to 100,000 eggs in one season, so there are billions of eggs on the beaches during the peak mating season. ⁶ Although this is a huge number of eggs, with 1,000,000 or so shorebirds arriving in the Delaware Bay to feed on the eggs, some people believe that these numbers need to be sustained for the survival of the birds. The shorebirds are on an annual migration from the southern tip of South America to their nesting grounds in the Arctic, a trip that covers 10,000 miles for many of them. Some people feel there is a direct correlation between the number of horseshoe crabs and the number of shorebirds. This would mean that a decline in the number of horseshoe crabs will result in a decline in the number of shorebirds. (One species, a subspecies of the red knot, the most long-distance migrant to visit the Delaware Bay, has declined dramatically in population over the last decade and is now being considered for addition to the federal government's Endangered Species List. ⁷) The shorebirds also bring in an estimated annual income between $30 to $70 million combined, for Delaware and New Jersey, due to eco-tourism. ⁸ There are many people who come to the Delaware Bay each May to see the shorebirds and their visits generate money for the local economies. This money would certainly be missed if the shorebirds numbers decline and they no longer offer the current "Wow" appeal based on their sheer numbers.

The third major group that is interested in the laws governing the horseshoe crab is the biomedical industry. The coastwide biomedical industry currently has a harvest of approximately 500,000 horseshoe crabs, which are bled, then returned to the wild. It is estimated that 10% to 15% of these crabs do not survive the bleeding process. ⁹ Why is the biomedical field interested in harvesting horseshoe crabs? Actually we are all benefited by the horseshoe crab's contributions to the medical field and to our own lives. Horseshoe crab blood is used to test that medicines (those that are injected) are free of contamination from bacterial endotoxins. ¹⁰ This is the main current biomedical field use of horseshoe crabs. However new research is suggesting promise of even more benefits to human health, including discoveries in anti-viral and anti-cancer activities in the proteins of the horseshoe crabs' blood. ¹¹