We evaluated apparent survival of headstart hellbenders released in three Ohio streams for three years. While overall survival was relatively low across the study period (Table 4), the annual survival matched our expectations, with lower survival during the first-year post-release (2019) and higher survival in subsequent years (2020 and 2021). Animals recaptured by hand in subsequent years showed considerable growth and overall excellent body condition. Our population projections using survival data from this study and existing literature suggest that building hellbender populations requires successive reintroductions; three reintroductions every three years (at 100 per cohort per site) produced the fastest and highest number of individuals attaining maturity (Fig. 3). We recommend repeated releases on fewer sites than releasing smaller cohorts at many sites only once or twice when the number of available animals is limited. Overall, for our study system and other localities dealing with similar recruitment issues, headstarting can be an effective tool for rebuilding hellbender populations, provided that stream and habitat quality at release sites are amenable for natural reproduction and development. Specifically, collecting and captive rearing wild eggs resulted in up to seven times more animals reaching adulthood after headstarting efforts compared to natural reproduction and egg development (Table 5).
Survival was lowest in the first year post-release. This was expected given that animals were naïve relative to the natural environment, and corroborates findings from other amphibian release studies (Stamps and Swaisgood 2007, Germano and Bishop 2009). Some hellbender reintroductions have been ‘soft releases’ (Boerner 2014, Kraus et al. 2017, McCallen et al. 2018, Kocher 2019). For example, Eastern Hellbender releases in the Allegheny River drainage in New York (Boerner 2014, Kocher 2019) have included soft releases using both artificial nest boxes (Briggler and Ackerson 2012) and wire mesh cages. Kocher (2019) suggested that soft release techniques had no effect on post-release movement, but may improve short-term post-release survival via refuge from predators. Additionally, recent work has suggested that environmental conditioning may improve both swim performance (Kenison and Williams 2018a) and predator-recognition (Crane and Mathis 2011; Kenison and Williams 2018b) in captive-reared hellbenders.
Our results on relatively low survival over three years are conservative. The capture-recapture models used here do not differentiate between mortality and emigration; thus, headstart animals could have left the release sites and avoided subsequent detection. This is a common pitfall for amphibian translocations, and one of the main causes for failed introduction or reintroduction efforts (Germano and Bishop 2009). Wild hellbenders are sedentary, typically spending most of their time within 30–40 m of the same stream reach (Nickerson and Mays 1973, Foster et al. 2009). Headstart hellbenders fitted with telemetry units and tracked for short time periods have been shown to have lower site fidelity (60%) than their adult resident (100%) and translocate (90%) counterparts (McCallen et al. 2018). Bodinof et al. (2012) showed that Missouri headstarts dispersed within 550 m of release sites with most individuals dispersing < 50 m. Boerner (2014) and Kocher (2019) estimated average (± SE) cumulative movement distances for New York headstarts at 653 ± 138 m and 1102 ± 267 m, respectively, and Kraus (2017) documented a captive reared individual moving ~ 1.3 km from its initial release site. Additionally, headstarts typically move downstream (Bodinof et al. 2012b, Boerner 2014, Kraus et al. 2017, McCallen et al. 2018, Kocher 2019), likely due to their naivety to high flow events. In Ohio, hellbenders have disappeared from > 80% of their range since the1980s, and populations are sparse, spatially segregated, and have low abundance (Lipps 2011; Smeenk et al. 2021). As such, if animals leaving the release sites do not return, they are unlikely to contribute to the reproducing population are effectively ‘dead’ from a demographic perspective. 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.
From a temporal perspective, our population simulations show that rebuilding hellbender populations is a long-term process, and that adequate post-release monitoring of animals should be undertaken to evaluate the success of reintroduction efforts. Given the low survival post-release, we found that releasing multiple cohorts every 2–3 years represents a good alternative to single releases. Repeated releases (every 3 years) of a relatively large number of individuals (N = 100), yielded 11–36 reproductive adults 15 years after the first introduction. Thus, our findings meet recommendations of Germano and Bishop (2009) for large cohorts to be released at any given location. The population models used here, as well as the underlying assumption of establishing populations, assume that reintroduced animals will reproduce once they reach a suitable age structure (4–8 years; Peterson et al. 1983) and that the threats that affect reproductive success and survival of sensitive life stages are removed.
The goal of headstarting is to improve survival of animals during critical (or at risk) life stages, and there is much uncertainty about the demographic outcomes of releasing one stage or another. For example, (Germano and Bishop 2009) found that the success for translocations varied across amphibian species based on whether the life stages were eggs, larvae, or juveniles. However, the most important predictor for translocation success was the number of animals released, with relocations of > 1000 animals being most successful. One of the few studies that evaluated the demographic implications of releasing different life stages for endangered amphibians (Kissel et al. 2014) concluded that translocations involving Oregon spotted frogs (Rana pretiosa) yielded higher population growth and lower risk of extinction when captive-breeding and release of larvae were implemented as opposed to headstarting animals and releasing them as juveniles. In the case of hellbenders, captive breeding success has been low; Ozark hellbenders (C. alleganiensis bishopi) have been bred in captivity for the first time at St. Louis Zoo (Missouri) in 2011 (Ettling et al. 2013), and more recently at the Mesker Park Zoo (Indiana). In addition, there is limited information on survival of larvae post-hatching, or the pathways through which different perceived threats (e.g., low riparian forest cover (Bodinof Jachowski and Hopkins 2018), high conductivity (Pitt et al. 2017), siltation (Unger et al. 2021)) affect different life stages. Thus, efforts to captive-raise animals as long as possible and release large cohorts are likely to continue to be the most plausible recovery strategy for this species, while mechanisms of suppressed recruitment are further investigated.
Our analysis of the efficacy of captive-raising animals from wild nests for headstarting vs. allowing natural development of eggs highlights the important role of raising animals in captivity for 3–4 years. We found that to reach the same population size of adult hellbenders, captive rearing is up to 7 times more efficient than allowing eggs to develop naturally. Thus, considering the assumption of no or little mortality during captive rearing, this strategy may be useful to increase recruitment in declining populations experiencing reduced reproductive success. The analogy of “1 egg in hand is worth 7 in the stream” can be seen as optimistic, but hellbender husbandry is relatively well understood as many zoos and aquaria across North America have state-of-the-art facilities and resources devoted to amphibian reintroduction programs, including hellbender headstarting. This particular analysis is subject to assumptions related to natural survival rates of various age classes, some of which can only be inferred from prior studies (Unger et al. 2013). For example, larval survival in natural settings and survival of young animals (between larval stage and first reproduction) are notoriously difficult to assess for cryptic species (Foster et al. 2009, Diaz et al. 2022). While some progress has been made in understanding the demography of early stages via monitoring wild hellbender nests in artificial nest boxes (Briggler and Ackerson 2012, Bodinof Jachowski et al. 2020, Button et al. 2020, M. Kaunert, unpubl. data), we still lack basic natural history knowledge of this species or the threats to its persistence and their potential for mitigation.
The Eastern Hellbender is a species in need of active management and conservation throughout much of its range, even though the majority of the declining populations are not being warranted protection through the Endangered Species Act. Thus, improving the success of headstarting programs is essential for the persistence of the species across its geographic range. Our results suggest that despite relatively low survival of headstart animals following release, headstarting can be an effective recovery strategy provided it achieves the ultimate goal of re-establishing self-sustaining populations. PIT-tag monitoring offers a minimally invasive, time- and cost-effective method for long-term monitoring of large cohorts of released hellbenders (Kraus 2015), and could represent a valuable component for measuring success of future recovery efforts in regions where headstart efforts are implemented. 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 reintroduced cohorts. Re-establishing populations of a cryptic, long-lived organism such as the Eastern Hellbender presents many challenges, as the success of various stream or catchment-level management actions to reverse population declines has not been fully assessed. Therefore, improving practices for headstarting will be vital to properly allocate management resources while threats to hellbender populations are further understood and mitigated.