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Turtle and Tortoise Newsletter, 2000, 2:4-6
© 2000 by Chelonian Research Foundation

Turtle and Tortoise Newsletter

Mysterious Terrapin Die-Off on Cape Cod, Massachusetts

Don Lewis
Founding Partner, Cape Cod Consultants, P.O. Box 1054, South Wellfleet, MA 02663

As November clouds enshrouded Cape Cod, a tragedy played out in their shadow. Nearly a hundred rare turtles, ranging from two-year-old juveniles to ancient breeding adults, died. Their bodies, hidden by murky creek waters and soon covered in a thick layer of winter ice, resurfaced in a late January thaw as mute witness to a terrapin die-off unprecedented in the annals of the Outer Cape. What killed these turtles of Cape Cod Bay?

Northern Diamondback Terrapin, Malaclemys terrapin terrapin
The northern diamondback terrapin occupies a unique niche in Wellfleet Harbor, living year-around in its rich estuarine system of rivers, creeks, coves, bays and marshes. While this subspecies can be found from Cape Hatteras to Cape Cod, Wellfleet marks its absolute northern limit, where they are subject to severe climatic variations as well as the steady advance of human activity into their once secluded salt marsh habitat. Observing an animal at the extreme edge of its range serves as an important bell-weather to monitor the health of an ecosystem. The State of Massachusetts lists the diamondback terrapin as a threatened species and recognizing its signal role within the harbor environment, the Massachusetts Audubon Society’s Wellfleet Bay Wildlife Sanctuary has organized research studies of terrapins for nearly a quarter of a century.

Diamondbacks are medium size turtles which spend their entire lives in estuaries, salt marshes and tidal creeks. Except for occasional basking on mud flats and creek banks, only female terrapins ever leave the water, twice yearly, to nest in upland landforms within fertile salt marsh habitats. Wild turtles are so sensitive to human disturbance that many residents and annual visitors to Cape Cod are surprised to learn they share the same waters with these shy creatures. For most, the only glimpse they gain of a diamondback terrapin is the mirage of a gray-green head about the size of a thumb, nearly invisible against the wave tops, snorkeling for a quick breath.

First Sign Unnoticed
Diamondback terrapins of the Blackfish Creek estuary system [see map] comprise perhaps the largest subpopulation of this threatened species within Wellfleet Harbor. Suitable upland nesting sites are abundant and each is surrounded by a rich nursery system of salt marsh creeks to protect and nourish hatchlings and juveniles. The turtles of Blackfish Creek have been under research observation since the spring of 1980.

Wellfleet tides are significant and during new and full moon periods often range nearly 15 feet. Flats are uncovered and marshes are flushed dry except for scattered tidal pools. Adult terrapins, which venture beyond the security of marsh channels, are swept in the tidal flow and join a “turtle parade” during astronomically significant daylight low tides. They paddle with the current to the mouth of the bay between Indian Neck and Lieutenant Island, only to return with the incoming tide.

In mid October, as water temperatures over the shallow tidal flats of Blackfish Creek began to dip, fewer and fewer terrapins “marched” in the parade, as they began to slip into the early stages of winter brumation. On 26 October, the last active terrapin was sighted for 1999. But fall weather continued unusually mild, with a series of storms beginning in mid-November separating long periods of warm temperatures.

The Blackfish Creek terrapins likely exploited these interludes, but no one was watching; our attention had been diverted elsewhere. As water temperatures in Cape Cod Bay slipped below 50o F and westerly winds whipped across from the mainland, cold-stunned sea turtles began to appear on our beaches in record numbers. Since mid-November, 220 Kemp’s ridleys — the rarest sea turtle in the world — were recovered, plus 58 loggerheads and five green turtles. Sizes ranged form a tiny 1200-gram ridley to a 200-pound, tagged female loggerhead.

When, on December 7th, a resident discovered a terrapin on Fox Island and brought it to the Wildlife Sanctuary, we missed this first sign of the die-off. A mature, well-nourished female of 1116 grams, she appeared dead, but once warmed in the lab, began to stir as though recovering from cold stunning. While there were no obvious signs of predation, she had sustained a serious slice on her throat and was euthanized. Unfortunately, distracted by the record sea turtle strandings, we did not follow up this early sign, which appeared then as an isolated incident.

Nearly a Hundred Dead Turtles Found
When we returned to the Fox Island system in January, we began to discover terrapin remains at an alarming rate. The weather had turned bitter cold after the December holidays and a thick ice cover formed. In late January, an unusually high tide coupled with a brief thaw cracked the ice pack over the marshes and began to expose dead turtles: four on 26 January, five on the 27th, six on the 28th, two the next day, one the following, and nine on 31 January. All were recovered from the frozen marsh in rivulets as though deposited with each new flood tide. All were well nourished; none showed any signs of predation; all were partially decomposed, suggesting deaths had occurred in the fall before the freeze would retard the process.

Following a brief respite in mid-February, another series of remains were uncovered in the same marsh, but further south nearer to Field Point. These terrapins exhibited the same forensic characteristics as the first; however, the state of decomposition of the turtles found in March and early April indicated a series of events rather than a single incident had caused the die-off. Initially, when the numbers were still relatively small, speculation had centered on one occurrence that might have excavated brumating terrapins from their hibernacula. Perhaps a mooring had dragged along the muddy bottom; maybe winds had driven an anchored boat so that its keel or centerboard had plowed through a pod of sleeping turtles. But after examining the latter terrapins, Bob Prescott, director of the Wellfleet Bay Wildlife Sanctuary, was reported in the Cape Cod Times of 25 March as saying, “This is not a single incident. It happened a number of times through the fall.”

The loss of as many as a hundred turtles from this threatened species’ northernmost habitat can only be described as tragic, no matter what brought it about. Quoting from the Cape Cod Times of 19 March, “[Dr.] Tom French, assistant director of the state Division of Fisheries and Wildlife, and head of the state’s Natural Heritage and Endangered Species Program ... rated the Wellfleet die-off a 6 on a scale of 1 to 10 in terms of the Cape’s overall diamondback terrapin population, an 8 or 9 for the Wellfleet Bay population.”

It’s not just numbers that color the event tragic. At an individual level, ten of the dead terrapins were well know to researchers. One turtle, a six-year-old male, had last been seen on 28 August as he paddled through the channel in hot pursuit of his sweetheart. Five mature females had been observed nesting in this same marsh since 1989. And one of the last turtles found, another six-year-old male, had been captured on 13 September while courting another marked terrapin.

Nor do signs point to a natural or weather-related occurrence. Two similar habitats in Wellfleet Harbor, under the same level of research scrutiny, yielded only a single dead terrapin and six turtle remains respectively over the same period. Tom French was reported as saying these deaths in the Fox Island marsh were “probably the result of an unusual activity.”

“Probable Cause”
While definitive causation of this die-off will never be known with scientific certainty, continued scrutiny of this marsh environment produced some important and convincing clues. A kayak inspection of the flooded marsh on 26 March discovered a long sheet of loose aquaculture netting which had become anchored in debris and stretched across the main transport channel supporting the terrapin within this habitat. Nearly invisible when the creeks are empty at low tide as it lay mostly buried in the muck, and equally disguised in the murky estuarine waters at high tide, the net was only found when it snagged the kayak’s keel.

The material was wedged securely into the bottom and just reached to the surface at max tide, creating a perfect seine to trap anything that passed through the creek. Unable to dislodge it by kayak, an expedition returned to the creek at low tide the next morning and located this and a half dozen netting obstructions which blocked varying portions of the main and side channels; all potentially lethal to turtles.

The principal blockage spanned the entire channel that hugs the terrapin nesting sites in the Fox Island Wildlife Management area. One end of this net, which stretched about 30-40 feet long and more than seven feet high under tension, had become entangled with a green crab trap buried in the muddy creek bank; the other end was held by a long line with a ruptured plastic marker buoy. The rope was buried in the mucky bottom and spanned across to the opposite side, supporting the net tautly under water pressure. As the tide flowed through the creek, the net flared open and blocked passage for anything larger than its webbing size.

A second set of netting and debris blocked a side channel another 50 feet inside the marsh, along with several other similar, though not as severe, blockages.

Working Hypothesis
The Blackfish Creek terrapins are flushed through these channels as the marsh empties and refills with the ebb and flow of our substantial tides. Last fall as water temperatures over the flats began to plunge, turtles which had not yet entered hibernation, or which slipped in and out of brumation with the mild weather, would be sluggish, almost torpid, during these forced tidal migrations. Encountering the net and pushed against it by a substantial current, turtles would be trapped, not energetic enough to swim free against the tide, and they would drown. The same effect would occur during inflow and outflow as terrapins were thrust into opposite sides of the netting. The result would be deaths on both sides of the nets, dispersing turtle remains in the pattern we observed. The sharp pressure of the netting may also explain neck slashes on several turtles and slices cut deeply and cleanly into the carapace of some shells.

While we shall never be able to prove for certain what caused the deaths, these obstructions best explain the forensic and circumstantial evidence of the die-off, from the state of the terrapin remains to the extreme localization of the event and the dispersal of the dead turtles. The real question for policy makers should be: Who is responsible for clearing lethal debris before it becomes a threat to endangered species and other salt marsh wildlife?