Key Turtle Spotlights Difference Between Cape Cod Bay and Buzzards Bay Terrapins

Sue Wieber Nourse:  Female (Left) and Male (Right) 

On Monday, May 7th, Sue Wieber Nourse took advantage of a light southwest breeze, bright sunshine and temperature in the low sixties to paddle Sippican Harbor in search of Northern Diamondback Terrapins.  A week of chilled temperatures and overcast skies had driven SouthCoast terrapins back into brumation. She netted two adult terrapins.  Sue snagged a 6-year-old adult male that had never been caught before.  Sue also netted a 14-year-old female terrapin that we had first observed on July 2nd, 2005 as a prepubescent, 7-year-old. 

Sue Wieber Nourse Captures Terrapin Pair from Kayak

The Sippican Harbor population of terrapins has been studied by the Turtle Journal team since spring 2003 when its existence was first confirmed.  In the ten research seasons since then, we have estimated an at risk population of under 100 adult terrapins resides in the Sippican estuary.  With those dramatically depressed numbers, few juvenile “recruits” are seen by researchers.  So, there is little opportunity to follow a prepubescent female through her lifecycle to maturity in order to gain insight into a growth model for Buzzards Bay terrapins.  Based on the significantly larger size of mature females in Buzzards Bay compared to those observed in Cape Cod Bay, Wellfleet Bay and Pleasant Bay terrapin populations, we had assumed that Sippican turtles would demonstrate a steeper growth curve.  Yet, the challenge in such a small and aging population was acquiring confirmatory data.

Male #1039 (Left) and Key Female Recapture #283 (Right)

That’s what made Sue’s capture of Terrapin #283 so special.  This turtle was first captured as a prepubescent female in 2005 when she was aged at 7 years old.  She measured 14.71 centimeters (cm) carapace length and 12.81 cm plastron length; she weighed 487 grams back then.  On Monday, now at 14 years of age, she had grown to 20.45 cm carapace length and 18.35 cm plastron length; she weighed 1400 grams.  These latter numbers would be representative of nearly a 40 year old terrapin in the Cape Cod Bay population.


Cape Cod Bay Growth Chart (Post-2002 Research Season)

As indicated in this chart from our comprehensive 2003 report on Northern Diamondback Terrapins in Cape Cod Bay, we had measurements on 100 individual prepubescent females positively aged at 7.  The average carapace length, plastron length and weight were 13.76 cm, 12.19 cm and 476.48 grams.  At 7 years old, Sippican Terrapin #283 was only a few % points larger than the Cape Cod average and well within expected deviation for a prepubescent female of that age.

For the 2003 report, we had 26 mature Cape Cod Bay females positively aged at 14.  The average carapace length, plastron length and weight were 18.17 cm, 16.30 cm and 1033.24 grams.  At 14 years old, Sippican Terrapin #283 is already more than 12.5% larger and weighs 35% more than the average Cape Cod 14-year-old female.  In fact, very few Cape Cod terrapins ever attain a 20 cm length; if they did, it would be at about 40 years of age.

Key Terrapin #283 (14-Year-Old Female)

While we had assessed for some time that mature female terrapins in Buzzards Bay were 10% to 15% larger than their Cape Cod Bay counterparts, #283 marks one of the first Sippican females to provide confirmation of the steeper female growth curve required to attain this significantly larger size.

 Sue Wieber Nourse Paddles Sippican Harbor

With a tiny, at risk population that produces few juvenile recruits, it’s not an easy chore to acquire key specimens to fill in the data gaps to confirm or refute hypotheses.  Thanks to Sue’s capture of Terrapin #283, some of those data holes can begin to be closed.  Turtle Journal will continue to search for such important finds during the 2012 season, while we try to reverse the downward trend of the Sippican Harbor population in order to stem extirpation of Northern Diamondback Terrapins from this Buzzards Bay estuary.

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