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The Death Net
Don Lewis with Death Net
Bob Prescott examines netting debris
Fox Island marsh habitat
The marsh at mid-tide
Dead terrapin in the snow
Six-year-old female
Dead terrapin fully intact
Mature male
Headless terrapin
Well-nourished terrapin
Recording data
Six-year-old male
Hatchling terrapin
Update 4/27
Vito, the lone survivor
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An alarming number of diamondback terrapin carcasses were found this the winter (1999–2000) in Wellfleet’s Fox Island Wildlife Management Area, Cape Cod, Massachusetts. See the following links to news articles (which open new browser windows; close windows to return to this page):
- In the following article, Massachusetts Audubon Society volunteer Don Lewis presents good evidence for the causes of this tragic loss to Cape Cod’s terrapin population.
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Wellfleet, Massachusetts, March 27, 2000 —
We’ll probably never know for sure the exact cause of this year’s mass die-off of diamondback terrapins within the Fox Island Wildlife Management Area marsh. The loss of as many as a hundred turtles from this threatened
species’ northernmost habitat can only be described as tragic, no matter who or what brought it about. Ten of the dead terrapins were well known to Sanctuary 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 while courting another marked terrapin on 13 September.
The red star marks the location of the Fox Island Wildlife Management Area, Cape Cod, Massachusetts.
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In this aerial view of the Fox Island Wildlife Management Area, the obstructions were located within the red circle; the main sites where turtle remains were recovered are in blue.
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Our continued scrutiny of the marsh environment may have produced some important clues. As I kayaked through the flooded marsh yesterday afternoon, I discovered a long sheet of loose netting which had become anchored in debris and stretched across the channel. The material was wedged into the bottom and reached just to the surface, creating a perfect seine net to trap anything that passed through the creek. Since I was unable to dislodge these obstacles from the kayak, we decided to visit the creek at low tide this morning.
Bob Prescott and I slogged into the marsh around 10:00. We managed to locate a half dozen netting obstructions that blocked varying portion of the main and side channels, all potentially lethal to turtles.
The principal blockage spanned the entire channel that hugs the terrapin nesting sites from Wise Hill to Anawan Road and Fox Island. (See the detailed photos, above left; click thumbnails to view full size.) One end of this net, which was about 30–40 feet long and and more than seven feet high, had become entangled with a 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 stretched across to the opposite side, supporting the net tautly under water pressure. As the tide flows through the creek, the net stretches open and blocks passage for anything larger than the webbing size.
A second set of netting and debris blocked a side channel another 50 feet inside the marsh. We found several similar, but not as severe, blockages. A picture of the netting and debris is included here (click on the third thumbnail picture, above left).
A working hypothesis: Wellfleet’s terrapins are flushed through these channels as the marsh empties and refills with the ebb and flow of our 12-foot tides. Last fall as water temperatures over the flats began to plunge, turtles that had not yet entered hibernation would become sluggish, almost torpid, during these forced tidal migrations. Encountering the net and pushed against it by the violent current, these turtles would be trapped, not energetic enough to swim free against the tide, and they would drown. The same effect would occur during the outflow as terrapins were thrust into the other side of the netting. The result would be death on both sides of the nets.
While, as I said earlier, we’ll never be able to prove for certain what caused the deaths, these obstructions help explain the extreme localization of the die-off. And, since the main netting obstacles spanned the western channel nearest the Anawan Road/Wise Hill bank, it helps us understand why nearly all the remains were discovered on that side of the bank.
The real question for planners and policy makers should be: Who is responsible for clearing debris before it becomes a threat?
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One positive note — Don Lewis adds: I have one little critter (see Vito, final picture, left), a three-year-old juvenile, in my garage. He’s the lone survivor from marsh madness this winter, found cold-stunned on 21 February and waiting impatiently for release. Spring unfortunately comes late to the Great White North. [Update: Vito was released into Drummers Cove off Blackfish Creek in April 2000. So far, he has not been seen again.]
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Update
The die-off, which occurred in the Fox Island Wildlife Management Area of Blackfish Creek in the winters of 1999–2000 and again in 2000–2001, claimed the lives of more than a 100 turtles. Although carcasses were discovered throughout the winter months in this extensive salt marsh system, the event itself occurred during a narrow window each fall. Following discovery of the initial die-off, a brief but heated debate ensued. The question: Whether to act on our working hypothesis supported by the preponderance of circumstantial evidence, or wait and observe the area without change in the hope of documenting the direct cause of these deaths?
Our program is a research and conservation effort with heavy emphasis on conservation of the world’s northernmost terrapin population, already driven to threatened status by human intervention. So, the decision was easy. We did not wait for absolute confirmation of the precise cause of the die-off before taking action to stem the deaths. The Massachusetts Audubon Society’s Wellfleet Bay Wildlife Sanctuary enlisted staff, volunteers, and AmeriCorps participants to sweep the marsh channels clear of obstructions. This initiative supplemented a clean-up of the tidal flats by local shellfishermen and aquaculturists. Together, this community team cleared debris from the habitat before the winter of 2000–2001 and again for 2001–2002.
The result? Deaths dropped from over 100 to less than 20 after the first clean-up and down to zero this last winter.
Demographics of a Die-Off
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Age
2
2–3
3
3–4
4
4–5
5
5–6
6
6–7
7
8
9
9–10
10
10–11
11
12
15
Undetermined
Total |
Deaths
2
1
10
1
11
1
16
2
12
1
11
5
2
1
5
1
2
2
1
29
116 |
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In total, we found 116 dead terrapins during this two-year die-off. Assuming, as we do, that the cause was unnatural, and further assuming that these deaths were relatively random, we may be able to use this tragic event as a small, if cloudy window into the demographics of the terrapin population in Blackfish Creek. Dead turtles ranged in age from 2 years old to ancient, distributed as charted on the right.
Gender and Size
The gender distribution was 54 female, 43 male, and 19 undetermined. The smallest terrapin measured 5.7 centimeters plastron length and 6.6 centimeters carapace length. The largest was 18.45 centimeters plastron and 20.3 centimeters carapace — typically the size of the biggest females in Wellfleet Bay. We assume these deaths occurred during the twice-daily tidal migration as water in the marsh creeks ebbed and flowed around the time of fall brumation, when these turtles were already sluggishly slow. The narrowest shell width was slightly over 2 inches (5.4 centimeters), which may define the mesh size of the killer obstruction rather than the lower size limit of turtles moving with the tidal flow.
Continuing Concerns
The die-off and its cause remain a concern for our program. Even though deaths have stopped, the amount of debris (principally weather-tossed, lost, or discarded netting), which enters the marsh each season, is substantial. So, sweeps cannot be relaxed lest we risk a new die-off event. Another, and possibly related concern, is the number of turtles observed in Blackfish Creek each year with missing or sliced limbs, as though they were encountering and becoming entangled in material as they are flushed through the creeks by tidal forces twice daily. Others show scars of traumatic impacts. In 2001, 10 percent of the turtles captured and released in Blackfish Creek showed demonstrable signs of traumatic injury, 8 of which had lost or seriously scarred limbs. In 2000, nearly 8 percent had documented injuries of this type.
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