Headstarting turtles to larger body sizes for multiple years increases survivorship but with diminishing returns

Headstarting is a conservation tool that assumes raising turtles in protected ex situ environments to larger body sizes, then releasing them back into the wild, increases their survivorship compared to wild, non-headstarted turtles. Our goal was to quantitatively test this fundamental assumption by comparing somatic growth and survivorship among 3 age classes of headstarted juvenile wood turtles (Glyptemys insculpta) monitored for 3 years (2016-2018) in Ontario, Canada. Our age classes were turtles headstarted for 2 years (n = 15), turtles headstarted for 1 year (n = 30), and turtles incubated and hatched ex situ, then released (i.e., no headstarting; hatchlings, n = 30). Both age classes of headstarted turtles were radio-tracked for 1 year after release. We released hatchlings in August and radio-tracked them for 1 month. All cohorts exhibited positive somatic growth after release. One-month post-release survival of hatchlings was 70%. Turtles headstarted for 2 years had slightly higher 1-year post-release survivorship (67%) than turtles headstarted for 1 year (47%), but there was little to no evidence for a difference (P = 0.17). Modeling these survivorship results with real-life constraints of only 100 hatchlings available for headstarting each year (biological constraint from source population), and a 100-turtle capacity in our headstarting facility (economic constraint), projected similar population growth in 2-year and 1-year headstarting programs. When removing the 100-turtle facility limit, the 2-year program projected higher population growth but required space for 200 turtles in the headstarting facility, which may not be feasible for many programs. Our results support the fundamental assumption that headstarting increases survivorship, but we observed diminishing returns if headstarting was increased for longer than 1 year. Given the growing number of turtle headstarting projects globally, our study provides data that can aid in establishing cost- and resource-effective best practices.