Population reinforcement accelerates subadult recruitment rates in an endangered freshwater turtle

Abstract

Wildlife diversity and abundance are declining globally and population reinforcement with captive‐reared animals is a common intervention used to prevent extinctions. Released captive‐reared individuals may undergo an acclimation period before their behavior and success is comparable to wild‐reared individuals because they lack experience with predators, complex habitats and variable environmental conditions. Quantifying post‐release acclimation effects on fitness and behavior is important for maximizing the success of reintroduction programs and for predicting the number of captive‐reared animals required for release. Endangered Blanding's turtles Emydoidea blandingii exhibit low recruitment and may benefit from population reinforcement with captive‐reared, ‘headstarted’ individuals (headstarts). We used 6 years of data to compare survival, growth, habitat use and movement ecology between wild‐hatched juvenile turtles and headstarts reared from eggs rescued from injured females. We found strong evidence of an acclimation effect in headstarts, with lower movement, growth, and survival during the first one to two years post‐release. Following this acclimation period, headstarts had movement, growth and survival similar to wild‐hatched juveniles. Habitat use did not differ between headstarts and wild‐hatched juveniles. We hypothesize that the acclimation period occurred because headstarts were introduced directly into the wild (i.e. ‘hard release’) and that providing additional support before or after release may improve the success of headstarts. Headstarts had a monthly survival probability of 0.89 in the first year post‐release, and 0.98 after the first year post‐release. We estimated that headstarts at our sites have approximately three times higher probability of surviving to 10 years of age, compared to wild‐hatched individuals at other sites. Our results highlight that headstarts should be released into habitat individually rather than in clusters, and highlight the need to investigate whether post‐release mortality of captive‐reared animals could be mitigated by increased acclimation to wild conditions, for example through prerelease periods in outdoor pens.

Captive-rearing duration may be more important than environmental enrichment for enhancing turtle head-starting success

“…Our findings are largely in line with the "bigger is better" hypothesis, which suggests larger juvenile body size increases survival and performance (Packard and Packard, 1988;Janzen, 1993;Kissner and Weatherhead, 2005). Given nest predation can be a serious threat to chelonian population viability (Dodd, 2001;Spencer et al, 2017), head-starting might be an effective strategy for enhancing recruitment (Carstairs et al, 2019). Estimated annual adult survival for T. carolina can be very high (e.g., >0.95, Currylow et al, 2011), whereas at another study site in Michigan, Altobelli (2017) found estimated survival probability of radio-tracked hatchling T. carolina was zero at upwards of one-year in age.…”

A simple model for the evolution of temperature-dependent sex determination explains the temperature sensitivity of embryonic mortality in imperiled reptiles

“…Another common initiative undertaken by zoos, parks, conservation authorities and even private individuals involves incubation of reptile eggs in an ex-situ setting ( Eckert et al , 1999 ), followed by the release of hatchlings or juveniles. Growth of wild populations may then be enhanced by protecting eggs and embryos, which reduces embryonic and hatchling mortality, and/or rearing individuals to larger body sizes before release ( Carstairs et al , 2019 ; Tetzlaff et al , 2019 ), which may increase survival rates in the wild ( Rollinson and Rowe, 2015 ).…”

“…Yet, incubation regimes in conservation programs for these taxa are broadly unstandardized. For instance, some head-starting programs allow natural egg incubation ( Bona et al , 2012 ), some allow temperature to fluctuate with above-ground ambient temperature ( Tetzlaff et al , 2019 ) and others incubate different sets of eggs at different constant temperatures ( Carstairs et al , 2019 ). A broad and theoretically informed exploration of how incubation temperature affects fitness in embryos and hatchlings may therefore be useful in developing best practices for artificial incubation in conservation programs.…”

Benefits of Head‐starting for Blanding's Turtle Size Distributions and Recruitment

“…Blanding's turtle head‐starting has been implemented at other sites in northeastern Illinois (D. Ludwig and D. Thompson, unpublished data; S. Denham, McHenry County Conservation District, personal communication; M. Dreslik, Illinois Natural History Survey, personal communication), Wisconsin, USA (J. Kapfer, University of Wisconsin—Whitewater, personal communication), Minnesota, USA (Linck ), Michigan (Szymanski , Starking‐Szymanski et al ), Massachusetts, USA (Buhlmann et al , Green , Windmiller et al ), New York, USA (Ross and Johnson ), Ontario, Canada (Lentini et al , Ritchie , Carstairs et al ), and Nova Scotia, Canada (Arsenault , D'Entremont ). Some of these efforts have been of limited scope, but others have involved annual releases over extended periods and provide valuable information on head‐start survival and habitat use (Arsenault , D'Entremont , Green , Lentini et al , Windmiller et al ).…”

“…Blanding (Lentini et al 2016, Ritchie 2017, Carstairs et al 2019, and Nova Scotia, Canada (Arsenault 2011, D'Entremont 2014. Some of these efforts have been of limited scope, but others have involved annual releases over extended periods and provide valuable information on headstart survival and habitat use (Arsenault 2011, D'Entremont 2014, Green 2015, Lentini et al 2016, Windmiller et al 2016.…”