Soft-Release May Not Enhance Translocations of Wild-Caught Eastern Massasaugas (Sistrurus catenatus)
Wildlife translocation is the human-mediated movement of living organisms from one location to another for varying purposes including conservation, mitigation, and research. Hard-release translocation, or immediate release, is arguably one of the easiest, cheapest, and most well-studied translocation techniques, but it sometimes results in high mortality rates because some relocated animals make large, erratic homing movements that may put them at risk. Soft-release is a strategy that may alleviate such negative effects, as it involves containing animals in an outdoor enclosure at the release site temporarily in the hopes that they will acclimate to the new environment, thereby limiting post-release movements and increasing survival. The usefulness of soft-release translocation relative to hard-release has seen some successes, but no studies have reported on whether it would benefit translocation efforts for wild-caught snakes. Here, I conducted an experiment that tested whether soft-release might increase survival rates and limit post-translocation movements of wild-caught, adult eastern massasaugas (Sistrurus catenatus) at a large military installation in Michigan, USA. This rattlesnake species is federally listed as threatened, but individuals are often hard-release translocated at this site after they are encountered by military personnel during training activities due to the potential for snake or human injury. The outcomes of these translocations are unknown, and natural resource managers at this site and elsewhere need to be able to make informed decisions about what translocation methods best compromise both conservation and mitigation goals for these animals. To assess this, 55 soft-released (i.e. held in outdoor enclosures for approximately two weeks prior to release), hard-released (i.e. released immediately upon relocation near the enclosures), and resident control (i.e. released at capture sites) eastern massasaugas were radio-tracked at this military installation for one active and overwintering season post-release from May 2013 through November 2017. To determine if the survival of the treatments differed and to examine what variables might influence survival, snake annual survival probabilities were estimated and compared using known-fate models that included treatment, sex, body condition index, distance translocated, time, season and their interactions as variables. To determine if the movement behavior of the treatments differed, maximum dispersal distance from the release site, mean distance moved per day, activity range size, activity range length, homing, and release site fidelity were assessed. Male soft-released snakes had significantly lower survival (0.44 ± SE 0.18) than resident males (0.72 ± SE 0.21) and died from a wider variety of causes (e.g. overwintering, predation), but soft-released males did not have significantly different survival than hard-released males (0.40 ± SE 0.20) and experienced the same causes of death. Body condition index, distance translocated, time, and season did not appear to have significant effects on the survival of any treatment of male eastern massasaugas. Male movement patterns were not significantly different between any treatments. Sample sizes for translocated females and gravid females were too small to reliably analyze survival and most movement metrics, and no significant differences between treatments were found for metrics that were estimable. My findings suggest that soft-release as implemented here may not substantially enhance translocation efforts for male eastern massasaugas in that soft- and hard-release translocated snakes did not have significantly different survival and movement behaviors, so hard-release is generally recommended given that it is more efficient and cost-effective. However, given that translocated snake survival was low and that translocations have the potential to disrupt populations, I do not advise translocations for any eastern massasaugas unless immediate harm to the snakes or to humans is extremely likely. If translocations absolutely must be used but the possibility of the snakes eventually homing near their capture locations is acceptable, trends in the rattlesnake translocation literature suggest that hard-releasing individuals short distances within their original home ranges may be less detrimental to the snakes than when they are translocated long distances outside of their home ranges. If translocations must be used but the possibility of snakes homing is unacceptable, then hard-releasing male eastern massasaugas in high-quality habitats long distances from their capture sites and from anthropogenic features such as roads might prevent homing, although survival may still be low. Further experimental research is critically needed to explicitly test these ideas and other translocation variants to develop more informed management practices for imperiled herpetofauna.
Kingsbury, Purdue University.
Wildlife Conservation|Wildlife Management|Biology
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