Comparing the Effects of Environmental Enrichment, Seasonality, and Soft Release on Site Retention and Survivorship of Captive-Reared Eastern Hellbenders (Cryptobranchus alleganiensis alleganiensis)

Abstract

Translocations of captive-reared animals are commonly used to stabilize declining wildlife populations. However, captive-reared animals are often raised in conditions dissimilar to their release sites and lacking natural characteristics, which could alter movement patterns and postrelease survivorship. These patterns can be further altered by season of release and soft-release conditions. We reared juvenile Eastern Hellbenders (Cryptobranchus alleganiensis alleganiensis) for 18 mo in captivity in one of two conditions: a control condition with low-velocity water flow (unconditioned) or a treatment condition with moving water (conditioned) that simulated natural flow velocities at their intended release site. We divided 4-yr-old Hellbenders (n = 118) into six treatment groups to determine the effects of release season (fall or summer), release type (standard soft release or enhanced soft release), and conditioning (unconditioned or conditioned) on the number of days until first movement, release site retention, and survival. In November 2017, we released 80 radio-tagged individuals (40 conditioned and 40 unconditioned) into soft-release structures in the Blue River, Indiana. In July 2018, we released another 38 radio-tagged individuals (18 conditioned and 20 unconditioned) into soft-release structures at the same site. After release, we tracked each individual one to three times weekly for 10 mo (fall release) or 12 mo (summer release). We found that treatment groups released into caged cobble beds (i.e., enhanced soft release) delayed their first movement and had higher release site retention relative to groups released under caged shelter rocks (i.e., standard soft release). We found that conditioning had a positive effect on survival but only in the treatment group released in the summer. By combining techniques and releasing conditioned individuals in the summer using enhanced soft releases, we increased annual survival of captive-reared Hellbenders from a probability of 0.50 (95% confidence interval [CI] = 0.31–0.79) to 0.74 (95% CI = 0.55–0.99). Our results provide important information about techniques that can be adopted across captive-rearing programs to help maximize the conservation success of Eastern Hellbenders.