Research - Population size and structure

Reliable assessment of how many individuals exist in a population is perhaps the most basic data need for managing wildlife populations; however, it is not always easy to determine. Diamondback terrapins return to the water surface to breathe about once every 10 minutes¹; thus, counting individuals at the water surface is one method that can be used to estimate population sizes over large areas, particularly with the assistance of citizen scientists². However, terrapin behavior such as basking or burying in mud varies seasonally³, which in addition to water temperature⁴, can affect the probability that terrapins will be seen at the water surface. Consequently, visual surveys in South Carolina have primarily been conducted in localized areas with limited total annual observation effort^{2, 5}.

In contrast to visual counts, physical capture of terrapins ensures accurate counts of individuals as well as confident distinction between male and female terrapins which are sexually dimorphic as adults but not also easy to distinguish at overlapping sizes as juveniles. With the exception of study sites where population declines have been attributed to crab trap mortality due to selective removal of smaller males⁶, male diamondback terrapins are typically captured twice as often as females in South Carolina estuaries^7-11. In addition to monitoring potential population change as a result of differential survival between males and females, examination of sex ratio data also provides insight into potential mating areas (a CWCS Conservation Action recommendation) given seasonal aggregation of males for breeding¹².

When captured individuals are marked and/or tagged and a sufficient number are subsequently recaptured, annual survival rates and population size estimates can also be determined. Annual survival rates for diamondback terrapins in South Carolina are reported from two studies, with >75% annual survival generally associated with diamondback terrapins captured at Kiawah Island during 1983–1999¹³ and at North Inlet-Winyah Bay between 2006 and 2010¹¹. The population at North Inlet-Winyah Bay was estimated to support 310 diamondback terrapins, whereas the Ashley River was estimated to support nearly 10 times as many diamondback terrapins during a similar timeframe (2008–2009)¹⁰.

Genetic population structure within South Carolina is not detected; however, subtle genetic differences are reported in diamondback terrapins captured between Texas and New York¹⁴.