Evaluating translocation success of wild eastern hellbenders (Cryptobranchus alleganiensis alleganiensis) in Blue Ridge Ecoregion streams using pre- and post-translocation home range sizes and movement metrics

Abstract

Translocations of freshwater species have become a widespread conservation strategy to mitigate the impacts of habitat fragmentation, yet they are not often rigorously monitored using animal movement data to determine their success. We demonstrate the value of monitoring pre- and post-translocation movements and home-range sizes of a fully-aquatic, benthic stream salamander, the eastern hellbender (Cryptobranchus a. alleganiensis) to determine translocation success. We studied the home range sizes, movements, and habitat use of individuals (n = 27) in two self-sustaining populations (S1 & S2) for one year, and then subsequently collected similar data from a subset of these individuals (n = 17) that were translocated into two nearby streams (T1 & T2) with dam-isolated, declining populations in the Blue Ridge Ecoregion of Tennessee. We collected 1,571 location data points (869 pre-translocation and 715 post-translocation) from four study sites, and evaluated effects of mass, sex, and pre-translocation home range size/sedentariness, as well as habitat covariates on home range size and movements. Hellbender home range sizes increased from pre-translocation estimates at both sites, but response depended primarily on physical characteristics of release sites. Home range and fine-scale movement metrics indicated that hellbenders translocated from S1 to T1 settled in more quickly, had greater site fidelity, and smaller home ranges than hellbenders translocated from S2 to T2. Hellbender movements were influenced by cover rock size and density rather than individual characteristics. Study-long survival rates of translocated hellbenders increased from S1 to T1 (80% to 100%) and decreased from S2 to T2 (76% to 33%). Monitoring pre- and post-translocation movements was a valuable method for evaluating short-term translocation success in a freshwater environment. For future hellbender translocations, managers should prioritize selecting suitable release sites with contiguous boulder-dense areas (1–2 per m2), adequate prey (crayfish) densities (>1/m2), and habitats with low risk of predation.

Restoring eastern hellbender (Cryptobranchus a. alleganiensis) populations through translocation of headstarted individuals

“…This is particularly relevant in situations when remnant populations are rare and isolated and animals leaving the release sites are not likely to encounter other populations in the same stream. While wild translocated adult and subadult hellbenders have been shown to travel up to 2 km from release sites (Nissen et al, 2023), the distances between extant populations within heavily impacted streams may be larger, impacting the likelihood of headstart animals leaving release sites to contribute to the broader population or metapopulation. Thus, we propose that releasing animals at multiple reaches within a stream, and, when possible, ensuring that release sites are adjacent to other suitable downstream habitat (Boerner, 2014; Kocher, 2019) is a better strategy than releasing animals at isolated reaches.…”

“…PIT‐tag monitoring offers a minimally invasive, time‐ and cost‐effective method for long‐term monitoring of large cohorts of released hellbenders (B. Kraus, 2015), and could represent a valuable component for measuring success of future recovery efforts in regions where headstart efforts are implemented. However, there are limitations imposed by passive detection technology such as not detecting animals that dispersed to other reaches (Nissen et al, 2023) or animals using thick cover outside the range of the PIT tag reader. Releasing a mix of animals with PIT tags (12.5 or 10‐mm tags) and VHF transmitters (Nissen et al, 2023) could improve monitoring as it could provide information on both the proportion of dispersing animals and of animals that remain in the release reach but are undetectable via passive methods.…”

“…However, there are limitations imposed by passive detection technology such as not detecting animals that dispersed to other reaches (Nissen et al, 2023) or animals using thick cover outside the range of the PIT tag reader. Releasing a mix of animals with PIT tags (12.5 or 10‐mm tags) and VHF transmitters (Nissen et al, 2023) could improve monitoring as it could provide information on both the proportion of dispersing animals and of animals that remain in the release reach but are undetectable via passive methods. Future work should investigate the effect of emerging conservation strategies (artificial nest boxes, soft release techniques, pre‐release conditioning) on the survival and persistence of released cohorts of headstarted individuals.…”

Translocation in a fragmented river provides demographic benefits for imperiled fishes

“…Further, given the costs or delays in providing engineered fish passage structures or in achieving dam removal (reviewed in Stanley & Doyle, 2003), we suggest translocations may provide an alternative conservation strategy. Thus, studies evaluating demographic responses to translocations (e.g., Nissen et al, 2023;Sheller et al, 2006;Yackulic et al, 2021; this study) may be valuable to managers during recovery and conservation planning.…”

“…Translocation above barriers can aid conservation by augmenting populations and mitigating the effects of habitat fragmentation and loss within a species' native range. Translocations might be useful where managers desire to accomplish multiple objectives, such as providing access to important habitats upstream of barriers for native species while precluding dispersal by invasive species (e.g., Rahel & McLaughlin, 2018), creating population redundancy for imperiled species (Healy et al, 2020; Johnson et al, 2022), expanding or augmenting a species' distribution in fragmented ecosystems (Harig & Fausch, 2002; Nissen et al, 2023; White et al, 2003), or aiding in the recovery of species in rapid decline (Hammond et al, 2022; Sheller et al, 2006). Studies of conservation translocations have focused on birds and mammals, with fewer studies on fishes (reviewed by Bubac et al, 2019), and defining success can be difficult if objectives are not explicitly stated a priori (Bubac et al, 2019; Ewen et al, 2014; Minckley, 1995).…”