Abundance is a difficult thing to judge. Changes to the abundance of a particular species are relative to our perception of its abundance when we first observe the species. Our first observations become our baseline against which we measure changes in our natural world. Is this appropriate? Consider horseshoe crabs.

The fossil record shows that horseshoe crabs have been around, virtually unchanged, for about 350 million years. People harvest them for two reasons: to be used as bait in the eel and conch (whelk) fisheries, and to use a chemical in their blood to test for toxins in medical devices, vaccines and other injectable drugs (some blood is removed and then the live crab is returned to the water).

I first paid close attention to horseshoe crabs in Mattakesett Bay in the 1990s, although I had observed them occasionally before that. They were quite abundant, especially as they came ashore to breed on the Norton Point tidal flats in May and June. Now they seem less abundant, but this past month, I have been impressed with the large numbers of small horseshoe crabs (less than four inches across) that I find when I wade in the shallows of Mattakesett Bay. When (if?) these young crabs mature into adulthood, they may become more abundant than my baseline of 20 years ago. Am I justified to conclude that horseshoe crab populations are healthy?

Most naturalists say that horseshoe crabs are much less abundant now than they were 50 years ago. And they have data supporting these claims. Wellfleet Bay, a Massachusetts Audubon Society sanctuary on Cape Cod, has monitored the crabs breeding in the spring and discovered that their populations are declining at most of the sites studied on the outer Cape. And the annual spring and fall trawls conducted by the Massachusetts Division of Marine Fisheries show that the crabs have been declining in state waters since at least 1980. These all provide consistent baseline information over longer time frames.

Similarly declining populations of horseshoe crabs are documented in Rhode Island and Long Island Sound, where long-term studies are being conducted because of concern about declining populations of this species. These studies provide a wider geographic baseline.

Numerous articles and videos document the incredible abundance of horseshoe crabs in Delaware Bay in the spring, where females still clamber over each other to find a patch of sand to lay their eggs in. Red knots, a type of sandpiper, utilize this tremendous abundance of horseshoe crab eggs to sustain their northward migration from South America to their Arctic nesting grounds. However, the knots have declined by more than 50 per cent since the 1980s, a decline largely attributed to the decline of horseshoe crabs in Delaware Bay. The apparent abundance of horseshoe crabs there is deceiving; they used to be even more abundant!

The first European explorers and settlers found them to be very abundant along the entire Atlantic Coast, including on Cape Cod and Martha’s Vineyard. Now they are scarce, perhaps one percent or less of their historic abundance 500 years ago. Unfortunately, such extreme scarcity is accurate for a wide variety of animals, including cod, herring, eels, salmon, whales, seals, mammalian predators, herons, egrets, waterfowl, shorebirds, terns and on and on.

Remember that we started out with my baseline observation that horseshoe crabs are less common than they were 20 years ago. But my observation about August’s abundance of young horseshoe crabs in Mattakesett Bay is not useful because we do not know how many of them will survive to breeding age, and there are no observations from other Augusts. But a variety of local and state naturalists all agree that they are less abundant now than they were 50 to 100 years ago.

As we gather more information about the current status of horseshoe crabs geographically and historically, we get widespread agreement that these crabs are still declining, perhaps due to overharvesting, global warming and other environmental changes within the ocean. The literature makes it easy to determine that historically they used to be very abundant.

It is hard to believe that the wildlife we observe now represents a healthy and pristine natural world.

Robert Culbert leads guided birding tours and is an ecological consultant living in Vineyard Haven.