Scientists track state’s horseshoe crabs

BROOKSVILLE – A prehistoric survivor that has remained virtually unchanged for millions of years may be facing pressures from a relatively new kid on the evolutionary block.

Unregulated harvesting of horseshoe crabs for bait has resulted in a drastic reduction in the population along the mid-Atlantic coast, which in turn has increased harvesting pressures in Maine where, until two years ago, there were no limits on harvests.

Although the state now limits the taking of horseshoe crabs, scientists are trying to learn more about these “living fossils” and how they might have been affected by human activity.

Each year after the full moon in May and the new moon in June, horseshoe crabs migrate into Maine’s estuaries seeking spawning areas along the shores. The strange-looking marine creatures predate dinosaurs and are not really crabs at all, but are more closely related to scorpions and spiders.

For the past three years, about 70 volunteers from local environmental groups have searched the coast as part of a weeklong, state-sponsored survey of the horseshoe crabs, according to Department of Marine Resources scientist Pete Thayer, who oversees the annual count.

Volunteers measure the water temperature and salinity and then scour the shores for the horseshoe crabs. Early counts this year were low, probably because the water has remained colder than normal, Thayer said.

The last formal assessment of the population was done in the late 1970s. Although that study did not tabulate specific numbers, it pinpointed population concentrations along the coast which have been targeted for the annual count.

The counts, so far, have been inconclusive, Thayer said. Because it takes a horseshoe crab about 10 years to reach sexual maturity, the DMR scientist still is uncertain how much of the population is being measured.

“We’ve gone about 20 years without seeing what they’ve been up to,” Thayer said last week. “Now we’re trying to develop a solid baseline of information.”

Anecdotal stories from residents along some of Maine’s rivers, however, tell of past populations of horseshoe crabs so thick they had to be bulldozed off the beaches, Thayer said. They also tell of massive harvests of the creatures. Great Salt Bay on the Damariscotta River was one of the most heavily harvested areas along the coast, he said, and residents there reported as much as a truckload a day being taken from the bay.

“We don’t really know what effect that level of harvesting is having on the population,” the scientist said.

Horseshoe crabs inhabit the East Coast from the Gulf of Mexico to Taunton Bay, which is their northernmost documented spawning area, Thayer said. It’s likely that the colder water farther Down East limits their range.

The Taunton Bay horseshoe crabs also have shown some genetic differences from crabs in Southern areas, although scientists don’t know whether they are genetically isolated at this point.

Although they track the spawning crabs in the spring, scientists have little information on what happens to them during the rest of the year.

Wildlife biologist Sue Schaller, who has worked with Thayer on the crab count, has started tagging horseshoe crabs to determine whether they return to the same spawning area annually. This year, as part of an overall assessment of the effects of dragging operations in Taunton Bay, Slade Moore, a DMR scientist, hopes to use sound waves to monitor the movement of horseshoe crabs.

“We suspect that crabs in the New England area behave differently than in the mid-Atlantic, where they migrate over the continental shelf during the winter months,” he said. “In our area, they really don’t have much of a continental shelf, and some believe that they stay inshore throughout the year.”

Moore plans to attach a small, cigar-shaped tag containing a transmitter and battery to about 26 horseshoe crabs in the bay. Each transmitter will emit a distinct signal which will allow Moore to track each crab using a hydrophone hung over the side of the boat..

“We’ll monitor multiple locations and listen for specific codes and frequencies for each individual tag, so we can identify which crab we’re hearing,” he said.

Although the study group is limited, Moore said the tracking will provide a good idea of what happens to the horseshoe crabs in the winter.

“We do have the numbers to get a general idea of where the organism moves, their different habitats, what they do and where they do it,” he said. “We know so little about them, everything we learn will be interesting.”