Comparison of Growth Rates of Soft Release and Hard Release in a Founder Population of Northwestern Pond Turtles (Actinemys marmorata)

Soft release techniques, whereby individuals are allowed to acclimate to an area while contained in protected enclosure prior to full release, are frequently employed with the intention of improving outcomes in introductions of species to parts of their historic range from which they have been extirpated. However, the effectiveness of soft release techniques is difficult to quantify. This study compared the growth rates of soft and hard released individuals in a founder population of captive-raised northwestern pond turtles (Actinemys marmorata) introduced into a restored urban lake in the Presidio of San Francisco, California. The soft release treatment group was held in the study lake in an enclosed pen for several days and provisioned with food prior to full release. The hard release group was placed directly into the lake with no acclimation period. Body mass and carapace length measurements were taken immediately prior to release, and for approximately five months following release, turtles were recaptured and measured to determine growth rates. Hard release turtles were found to grow significantly more quickly than soft release turtles both in mass and carapace length. However, maternal ID had a much larger effect on growth rates than all other measured effects combined. The results of this analysis demonstrate that soft release may not be necessary or appropriate for the release of captive-raised northwestern pond turtles.